### BASICS OF Engineering drawing

Creating a drawing
Drawing  instruments are used to draw straight lines ,circles , and curves accurately  concisely

The drawing tools are  like drawing board, mini drafter, instrument box containing compass,divider etc,.. set squares,protractor,french curves, drawing sheet,pencils,erasers.....

DRAWING STANDARDS

• Drawing standards are set of rules that govern how the technical drawings are represented
• these are used so that every one can commonly understand the meaning of drawn picture
Drawing sheet
we have many types and sizes of drawing sheets..  like A4,A3,A2,A1,A0 etc

while in the drawing sheet .. first we have to draw border lines and title box and then we have to start drawing
lettering
lettering and numbering should be in a perfect manner.. for example in this figure.. the upper and lower case letters  are neatly draw

lines and its types
basically lines which are used in the representation of the diagrams are of mainly 4 types they are
• Visible line : represent features that can be seen in the current view
• Dimension line ;Extension line;Leader lineindicate the sizes and location of features
• Hidden line : represent features that can not be seenin the current view
• Center line: represents symmetry, path of motion, centers of circles, axis of axisymmetrical parts

In this way we use different types of lines in the drawing

# Basic Engineering Drawing – Projection

Engineers are confronted with the task of communicating the design, development and structures of machines to manufacturers and builders. The shape and size of various parts of a machine and its structure must be recorded on plane sheets in a systematic way for communication. The pictorial view of the object does not carry all the details, especially the inner details and correct shape of complicated parts. Different methods, therefore, are implied for describing the exact shape based on the ‘projectors’ drawn by engineers.

#### Principle of Projection

If straight lines are drawn from various points on the contour of an object to meet a plane, the object is said to be projected on that plane. The figure formed by joining, in correct sequence, the points at which these lines meet the plane, is called the projection of the object. The lines from the object to the plane are called projectors.

#### Projection on a Single Plane

If straight lines are drawn from the various points on the contour of an object to meet a plane, the figure obtained on the plane is called the Projection of the object. The object is said to be projected on the plane. In other words, we can say that the projection of an object on a plane is the shadow of the object on the plane showing each and every edge line of the object. The imaginary lines drawn from the object to the plane are called projectors or projection lines. The plane on which the projection of the object is taken is called plane of projection. Suppose an object is placed in front of a screen and light thrown on the object (assuming the light rays to be parallel to each other and perpendicular to the screen) then a true shadow of the object is obtained on the screen. This shadow is the projection on the object on the plane of screen showing the contour line of the object.

#### Types of Projections

The projections are classified according to the method of taking the projection on the plane. A classification of projection is shown below:

#### Factors on Which Type of Projection Depends

Different views of an object can be drawn by projections. Thus every drawing of an object will have four things on which projection depends

(a) Object,

(b) Projectors,

(c) Plane of projection, and

(d) Observer’s eye or station point.

#### Methods of Projection

In engineering drawing following four methods of projection are commonly used, these are:

(a) Isometric projection

(b) Oblique projection

(c) Perspective projection

(d)  Orthographic projection

In the above methods (a), (b) and (c) represent the object by a pictorial view as an observer sees it. In these methods of projection a three dimensional object is represented on a projection plane by one view only. While in the orthographic projection an object is represented by two or three views on the mutual perpendicular projection planes. Each projection view represents two dimensions of an object. For the complete description of the three dimensional object, at least two or three views are required. Orthographic projection comes under the category of ‘Non-Pictorial Drawing’.

#### Orthographic Projections

The word orthographic means straight description. The straight description here stands for the parallel projectors from the object to infinity. If a perpendicular picture plane is inserted between the projectors, a picture is formed having the same shape and size as that of the object. If an observer at position ‘A’  moves to infinity, the projectors to his eyes becomes parallel to the object and he observes the same shape and size as that of the object. The view so formed is known as the orthographic projection. Similarly, the parallel projectors shall form the pictures on the respective picture planes from the positions B and C. Usually two views are sufficient for simple objects, but the help of three or more views is necessary for complicated objects. These picture planes are mutually perpendicular to each other and are known as ‘Principal Planes’ of projectors, named Horizontal Plane (HP), Vertical Plane (VP) and Profile Plane (PP).

#### Methods of Orthographic Projection

The two methods of projections are:

(a) First angle projections

(b) Third angle projections

Figure 3shows four quadrants formed by the intersection of horizontal and vertical planes. The intersecting line of the planes is called the co-ordinate axis. The revolving direction of the horizontal plane shows that quadrants I and IIIare  ”open” but II and IV quadrants become “closed” when the horizontal plane coincides with the vertical plane. It is obvious that the closed quadrant has no use for the purpose of projectors as the views taken on these will overlap.

First Angle Projections. This method of projection is popular in Europe, especially in Britain. Bureau of Indian Standard has also recommended it now. Figure 4 shows an object placed in the first quadrant. Parallel projectors in the direction ‘A’, from the object, forms a picture on the vertical plane (VP) which is known as Front View or Front Elevation. Similarly, parallel projectors from the direction of ‘B’ forms the picture on the horizontal plane (HP), known as Top View or Plan. A mutually perpendicular plane to both HP and VP, known as profile plane (PP) also receives projectors from the object from the direction C. The view on the profile plane is known as Side View or Side Elevation. The three planes containing the views are then opened on a plane. The symbol of first angle is shown in below.

Although two or three views are enough to reveal an object, the projectors from six directions of the object in the first angle are shown, if necessary. The views are to be shown symmetrically. The view from the top (Direction B) placed underneath. The view from the front (Direction A) is placed in the centre. The view from the left side (Direction C) is placed on the right side of view A. The view from the right side (Direction D) is placed on the left side. The view from the bottom (direction E) is placed on the top as “E” view. The view from the rear (Direction F) may be placed on the right or left side of C or D views.

Third Angle Projections. This system of projection is known as the American system. The object is placed in the 3rd quadrant. The planes are imagined to be made of transparent material, say a glass box. The front wall of the box is assumed to be hinged to the other walls as shown in the figure. The parallel projectors in all the six directions form respective views on the walls of the box serving as picture planes. The hinged walls of the box are opened and laid down on a plane. The placement of various views are in a systematic way. The view from the top is placed above the Front View (FV). The view from the right hand side is placed on the right side of FV. The view from left hand side is placed on the left side of FV. The view from bottom is placed underneath the FV. The view from the rear may be placed on the right or the left of the side views. The symbol for the third angle is given below.

#### Comparison of First Angle Projection and Third Angle Projection method

 Sl No. First Angle Projection Method Third Angle Projection Method 1 The object is kept in the first quadrant. The object is assumed to be kept in the third quadrant. 2 The object lies between the observer and the plane of projection. The plane of projection lies between the observer and the object. 3 The plane of projection is assumed to be non- transparent. The plane of projection is assumed to be  transparent 4 In this method, when the views are drawn in their relative positions, Plan (Top view) comes below the elevation (Front view), the view of the object as observed from the left- side is drawn to the right of elevation. In this method, when the views are drawn in their relative positions, Plan comes above the elevation, left hand side view is drawn to the left hand side of the elevation. 5 This method of projection is now recommended by the “Bureau of Indian Standards” from 1991. This method of projection is used in U.S.A and also in other countries.

# Basic Engineering Drawing – Lettering

Writing of titles, dimensions, notes, and other important particulars on a drawing is called lettering. Lettering is an important part of a drawing. However accurate and neat a drawing may be drawn, its appearance is spoiled and sometimes, its usefulness is impaired by poor lettering.  Lettering should, therefore, be done properly in clear, legible and uniform style. It should be in plain and simple style so that it could be done free hand and speedily.

Style of Lettering   – There are various forms of alphabets used in the art of lettering and each type is used for some particular purpose. ‘Old Roman’ style is the parent of all these styles. It is the basic standard for architects and artists. A variation of this style is known as ‘Modern Roman’ used by civil engineers, in maps and photographical drawings. The simplified forms called ‘Commercial Gothic’ are used almost exclusively for working drawings. ‘Gothic’ letters are elementary strokes of even width. ‘Roman’ letters have elementary strokes “accented” or consisting of heavy and light lines. All slanting letters are classified as ‘Italics’.

Types of Lettering – Lettering may be drawn in various types. These are given below:

(a) According to the Height

(i) Lettering ‘A’

(ii) Lettering ‘B’

In lettering ‘A’ type, the height of the capital letter is divided into 14 parts, while in lettering ‘B’ type, it is divided into 10 parts. The height of letters and numerals for engineering drawing are generally in the range of 2.5, 3.5, 5, 7, 10, 14 and 20 mm according to the size of drawing.

Table 1  Lettering ‘A’ (d=h/14)

 Characteristic Ratio Dimensions (mm) Lettering height   Height of capitals                             h Height of lower case letters            c (14/14)h   (10/14)h 2.5   - 3.5   2.5 5   3.5 7   5 10   7 14   10 20   14 Spacing between characters          a   Minimum spacing of base lines      b Minimum spacing between words  e (2/14)h     (20/14)h   (6/14)h 0.35     3.5   1.05 0.5     5   1.5 0.7     7   2.1 1     10   3 1.4     14   4.2 2     20   6 2.8     28   8.4 Thickness of lines                           d (1/14)h 0.18 0.25 0.35 0.5 0.7 1 1.4

Table 2  Lettering ‘B’ (d=h/10)

 Characteristic Ratio Dimensions (mm) Lettering height   Height of capitals                             h Height of lower case letters            c (10/10)h     (7/10)h 2.5   - 3.5   2.5 5   3.5 7   5 10   7 14   10 20   14 Spacing between characters          a   Minimum spacing of base lines      b Minimum spacing between words  e (2/10)h     (14/10)h   (6/10)h 0.5     3.5   1.5 0.7     5   2.1 1     7   3 1.4     10   4.2 2     14   6 2.8     20   8.4 4     28   12 Thickness of lines                           d (1/10)h 0.25 0.35 0.5 0.7 1 1.4 2

(b) According to the Proportion of Width and Height

(i) Normal Letters. Normal letters have usual height and width and are used for general purpose written in usual space.

(ii) Compressed or Condensed Letters. Compressed or condensed letters are those which are written narrow in their proportion of width to height, i.e. height of these letters is more than the width. These are used when the space is limited.

(iii) Extended Letters. Extended letters are those which are written wide in their proportion of width to height, i.e. height of these letters is less or equal to the width.

(c) According to the Stem Thickness. Letters also vary in the thickness of the stems or strokes. Letters having very thin stems are called light face, while those having heavy stems are called bold face.

LIGHT FACE

BOLD FACE

Light and Bold Face Letters

(d) According to the Stroke of Pencil

(i) Single Stroke Letters. The thickness of the lines of the single-stroke letters is obtained in one stroke of the pencil.  This is the reason why these are called single stroke letters.  These letters being simple are usually employed in most of the engineering drawings.  Single stroke letters are of two types: (i) vertical (ii) inclined. Both vertical and inclined letters and numerals are suitable for general use.  One can resort to any of these two, depending upon convenience.  The lines of the inclined letters are drawn at an approximate inclination of 75O to the right.

(ii) Double Stroke Letters. The thickness of the lines of the double-stroke letters cannot be obtained by a single stroke of pencil and hence, two strokes of the pencil are used.  This is the reason why these are called double stroke letters. The letters and numerals have uniform thickness in this style. The ratio of thickness and height of letters is 1:10 and 1: 14.

A good style of single stroke lettering should aim at:

(a) Uniformity of Thickness. Conical point pencil is used for uniformity. Pencil should not be too sharp as it would pierce into the surface of the drawing paper.

(b) Uniformity of Size.The width of the stroke of the pencil or pen is to be same as the width of the letter. Light guide lines for both top and bottom of letters should always be drawn with a sharp pencil.

(c) Uniformity of Style.One of the two styles, vertical or inclined, should be used. Vertical lettering is usually preferred. Both vertical and inclined letters are standard and the technicians should use them according to their practice and convenience.

(d) Uniformity of Spacing. Uniformity in spacing of letters, whether they are vertical or inclined, is a matter of equalizing the spaces between alphabets, words, lines, etc. The background area between letters, not only the distance between them, should be approximately equal.

(e) Uniformity of Shape. The shape of all the letters, irrespective of the styles or types used, should be uniform. If inclined letters are used, all of them must have the same inclination and should be inclined towards the right hand side.

# Engineering Drawing And Workshop Drawing

Engineering drawing is a language which is understood throughout the world by engineers and fabricators. Other languages may fail to describe the size, shape, physical aspects, inner details, finish, etc., but the engineers’ language known as ‘Engineering Drawing’ never fails. The most intricate assemblies with their various complicated parts can be easily represented by engineering graphics. There are various types of engineering drawings and all have one simple purpose, i.e.  the communication of ideas to others. The drawings used for communicating the ideas of the design engineer to the production engineer and technicians is known as ‘Workshop Drawing’.

Engineering Drawing – A drawing Prepared by an engineer, for an engineering purpose is known as an engineering drawing. It is the graphic representation of physical objects and their relationship. It is prepared, based on certain basic principles, symbolic representations, standard conventions, notations, etc. It is the only universal means of communication used by engineers and technicians.

Pictorial Drawing – Every person cannot understand the orthographic projection. Its execution requires a thorough understanding of the principles of projection and its reading requires a good practice of constructive imagination. We can describe the shape of a job by means of pictorial drawing also, which can be understood quite easily. Pictorial drawing is the drawing of a picture in graphic language of engineers, to represent a real thing by means of picture views. It shows the appearance of the object by one view only. Following three methods of pictorial projections are commonly used in engineering drawing:

(a) Isometric projection

(b) Oblique projection

(c) Perspective projection

Isometric Projection

A trained eye and good imagination will be able to understand the three dimensions of an object. Several orthographic views on different planes are to be drawn to understand fully an object. But in isometric projection, only one view on a plane is sufficient to represent an object in its realistic appearance. Anyone can understand by looking at a view what the job is by isometric projection.

Isometric projection is a type of pictorial projection. Isometric means equal measure. In this isometric projection, all the plane surfaces and the edges formed of these plane surfaces should be equally inclined to the metric plane. Metric plane is the same horizontal plane which is used in orthographic projection.

To represent the three dimensions (length, breadth and height) of the object, there are three axes known as ISOMETRIC AXES. To start an isometric drawing, a reference line (horizontal line) and the three axes (X, Y Z) are drawn by taking an angle of 30° from the reference horizontal line as drawn in Fig 7.2.  Z axis is a vertical line to the horizontal line drawn from intersection point of X and Y axes.

Oblique Projection.

Oblique projection may be illustrated in different ways, according to the choice of axes, length of inclined side and direction of looking the side.  As compared with isometric projection, in oblique projection, one side of the object is horizontal, second side is vertical and the third side is inclined at 30Oor 45O to the horizontal. The lengths of the horizontal and vertical sides are equal to the actual lengths, but the length of the inclined side is taken as three-fourth or half of the actual length.

In oblique projection, an  object is placed with its front face parallel to a vertical plane of projection and the visual rays parallel to each other pierce the plane of projection obliquely (Oblique means inclined). The projection represents the front face of the object in its true shape and size. The rest of the object is not projected true in its shape and size. Both the isometric and oblique projections are the methods of representing the object pictorially. But the oblique projection is preferable to the isometric projection in representing the objects of circular shapes, because the front face is in actual shape and size. It is not distorted.

Perspective Projection.

Perspective projection or perspective drawing is the representation of an object on a plane surface, called the picture plane, as it would appear to the eye, when viewed from a fixed position. It may also be defined as the figure formed on the picture plane when visual rays from the eye to the object cut the picture plane. Perspective is mainly used in architecture. By means of perspective, the architecture is able to show how an object would appear when constructed.

Non-Pictorial Drawing

It is not always possible to visualize the object merely by looking at the drawing. It requires the help of certain rules and convention of engineering drawing. It can be charts, maps, lay-outs, aerial photographs and orthographic projections.

Workshop Drawing

The purpose of a workshop drawing is to convey the instructions of the designer to the mechanic concerned, in such a manner that the work to be done can be completed with accuracy and as rapidly as possible. The drawing will show all necessary measurements, which are expressed clearly without the need for any calculation. The limits of accuracy or the class of fit and the raw materials from which the parts are to be made, are also indicated.

The drawing may not be of actual size, but for convenience may be made larger or smaller than the actual job, i.e. it will be drawn to scale. The scale is indicated in the title block. Owing to usage and the possibility of being torn, it is undesirable that the original drawing should be used in the workshop. Instead, a number of prints are reproduced.

Types of Workshop Drawing

There are three types of workshop drawing:

(a) Detail Drawing. This type of drawing provides all necessary dimensions and working instructions for the production of an item, e.g. one part of a hydraulic component.

(b) Assembly Drawing. This type of drawing shows the component or assembly in an assembled state, e.g. the assembly of the various parts of a hydraulic component.

(c) General Arrangement Drawing. This type of drawing shows a number of components in an assembled condition to form a self- contained unit, e.g. the complete hydraulic system of an aircraft.

Requirement of a Good Workshop Drawing

Workshop drawings are intended to convey the requirements of thedesigner to the tradesman in such a way that the intended work can be carried out accurately and rapidly. To facilitate its reading, a good workshop drawing should satisfy the following requirements:

(a) It should show all the necessary measurements without superfluous data or repetition.

(b) It should not entail any kind of calculations.

(c) Clearly indicate the raw material from which parts are to be made.

(d) Clearly indicate the limit of accuracy or class of fit as applicable.

(e) Provide a key to machining and other symbols.

(f) Any other information not provided in the above clauses that may be required for satisfactory completion of work.

Workshop Notes

The use of properly composed notes often adds clarity to the presentation of dimensional information involving specific operations. Notes are also used to convey supplementary instruction about the kind of material, kind of fit, degree of finish, etc. It is a good practice to specify information representing a specific tool operation or a series of tool operations by notes rather than by figured dimensions. Brevity in form is desirable for notes of general information or specific instructions.

### Introduction

One of the best ways to communicate one's ideas is through some form of picture or drawing. This is especially true for the engineer. The purpose of this guide is to give you the basics of engineering sketching and drawing.

We will treat "sketching" and "drawing" as one. "Sketching" generally means freehand drawing. "Drawing" usually means using drawing instruments, from compasses to computers to bring precision to the drawings.

This is just an introduction. Don't worry about understanding every detail right now - just get a general feel for the language of graphics.

We hope you like the object in Figure 1, because you'll be seeing a lot of it. Before we get started on any technical drawings, let's get a good look at this strange block from several angles.

Figure 1 - A Machined Block

### Isometric Drawing

The representation of the object in figure 2 is called an isometric drawing. This is one of a family of three-dimensional views called pictorial drawings. In an isometric drawing, the object's vertical lines are drawn vertically, and the horizontal lines in the width and depth planes are shown at 30 degrees to the horizontal. When drawn under these guidelines, the lines parallel to these three axes are at their true (scale) lengths. Lines that are not parallel to these axes will not be of their true length.

Figure 2 - An Isometric Drawing

Any engineering drawing should show everything: a complete understanding of the object should be possible from the drawing. If the isometric drawing can show all details and all dimensions on one drawing, it is ideal. One can pack a great deal of information into an isometric drawing. However, if the object in figure 2 had a hole on the back side, it would not be visible using a single isometric drawing. In order to get a more complete view of the object, an orthographic projection may be used.

### Orthographic or Multiview Drawing

Imagine that you have an object suspended by transparent threads inside a glass box, as in figure 3.

Figure 3 - The block suspended in a glass box

Then draw the object on each of three faces as seen from that direction. Unfold the box (figure 4) and you have the three views. We call this an "orthographic" or "multiview" drawing.

Figure 4 - The creation of an orthographic multiview drawing

Figure 5 shows how the three views appear on a piece of paper after unfolding the box.

Figure 5 - A multiview drawing and its explanation

Which views should one choose for a multiview drawing? The views that reveal every detail about the object. Three views are not always necessary; we need only as many views as are required to describe the object fully. For example, some objects need only two views, while others need four. The circular object in figure 6 requires only two views.

Figure 6 - An object needing only two orthogonal views

### Dimensioning

Figure 7 - An isometric view with dimensions

We have "dimensioned" the object in the isometric drawing in figure 7. As a general guideline to dimensioning, try to think that you would make an object and dimension it in the most useful way. Put in exactly as many dimensions as are necessary for the craftsperson to make it -no more, no less. Do not put in redundant dimensions. Not only will these clutter the drawing, but if "tolerances" or accuracy levels have been included, the redundant dimensions often lead to conflicts when the tolerance allowances can be added in different ways.

Repeatedly measuring from one point to another will lead to inaccuracies. It is often better to measure from one end to various points. This gives the dimensions a reference standard. It is helpful to choose the placement of the dimension in the order in which a machinist would create the part. This convention may take some experience.

### Sectioning

There are many times when the interior details of an object cannot be seen from the outside (figure 8).

Figure 8 - An isometric drawing that does not show all details

We can get around this by pretending to cut the object on a plane and showing the "sectional view". The sectional view is applicable to objects like engine blocks, where the interior details are intricate and would be very difficult to understand through the use of "hidden" lines (hidden lines are, by convention, dotted) on an orthographic or isometric drawing.

Imagine slicing the object in the middle (figure 9):

Figure 9 - "Sectioning" an object

Figure 10 - Sectioning the object in figure 8

Take away the front half (figure 10) and what you have is a full section view (figure 11).

Figure 11 - Sectioned isometric and orthogonal views

The cross-section looks like figure 11 when it is viewed from straight ahead.

### Drawing Tools

To prepare a drawing, one can use manual drafting instruments (figure 12) or computer-aided drafting or design, or CAD. The basic drawing standards and conventions are the same regardless of what design tool you use to make the drawings. In learning drafting, we will approach it from the perspective of manual drafting. If the drawing is made without either instruments or CAD, it is called a freehand sketch.

Figure 12 - Drawing Tools

### "Assembly" Drawings

An isometric view of an "assembled" pillow-block bearing system is shown in figure 13. It corresponds closely to what you actually see when viewing the object from a particular angle. We cannot tell what the inside of the part looks like from this view.

We can also show isometric views of the pillow-block being taken apart or "disassembled" (figure 14). This allows you to see the inner components of the bearing system. Isometric drawings can show overall arrangement clearly, but not the details and the dimensions.

Figure 13 - Pillow-block (Freehand sketch)

Figure 14 - Disassembled Pillow-block

### Cross-Sectional Views

A cross-sectional view portrays a cut-away portion of the object and is another way to show hidden components in a device.

Imagine a plane that cuts vertically through the center of the pillow block as shown in figure 15. Then imagine removing the material from the front of this plane, as shown in figure 16.

Figure 15 - Pillow Block Figure 16 - Pillow Block

This is how the remaining rear section would look. Diagonal lines (cross-hatches) show regions where materials have been cut by the cutting plane.

Figure 17 - Section "A-A"

This cross-sectional view (section A-A, figure 17), one that is orthogonal to the viewing direction, shows the relationships of lengths and diameters better. These drawings are easier to make than isometric drawings. Seasoned engineers can interpret orthogonal drawings without needing an isometric drawing, but this takes a bit of practice.

The top "outside" view of the bearing is shown in figure 18. It is an orthogonal (perpendicular) projection. Notice the direction of the arrows for the "A-A" cutting plane.

Figure 18 - The top "outside" view of the bearing

### Half-Sections

A half-section is a view of an object showing one-half of the view in section, as in figure 19 and 20.

Figure 19 - Full and sectioned isometric views

Figure 20 - Front view and half section

The diagonal lines on the section drawing are used to indicate the area that has been theoretically cut. These lines are called section lining or cross-hatching. The lines are thin and are usually drawn at a 45-degree angle to the major outline of the object. The spacing between lines should be uniform.

A second, rarer, use of cross-hatching is to indicate the material of the object. One form of cross-hatching may be used for cast iron, another for bronze, and so forth. More usually, the type of material is indicated elsewhere on the drawing, making the use of different types of cross-hatching unnecessary.

Figure 21 - Half section without hidden lines

Usually hidden (dotted) lines are not used on the cross-section unless they are needed for dimensioning purposes. Also, some hidden lines on the non-sectioned part of the drawings are not needed (figure 12) since they become redundant information and may clutter the drawing.

### Sectioning Objects with Holes, Ribs, Etc.

The cross-section on the right of figure 22 is technically correct. However, the convention in a drawing is to show the view on the left as the preferred method for sectioning this type of object.

Figure 22 - Cross section

### Dimensioning

The purpose of dimensioning is to provide a clear and complete description of an object. A complete set of dimensions will permit only one interpretation needed to construct the part. Dimensioning should follow these guidelines.

1. Accuracy: correct values must be given.
2. Clearness: dimensions must be placed in appropriate positions.
3. Completeness: nothing must be left out, and nothing duplicated.
4. Readability: the appropriate line quality must be used for legibility.

### The Basics: Definitions and Dimensions

The dimension line is a thin line, broken in the middle to allow the placement of the dimension value, with arrowheads at each end (figure 23).

Figure 23 - Dimensioned Drawing

An arrowhead is approximately 3 mm long and 1 mm wide. That is, the length is roughly three times the width. An extension line extends a line on the object to the dimension line. The first dimension line should be approximately 12 mm (0.6 in) from the object. Extension lines begin 1.5 mm from the object and extend 3 mm from the last dimension line.

A leader is a thin line used to connect a dimension with a particular area (figure 24).

Figure 24 - Example drawing with a leader

A leader may also be used to indicate a note or comment about a specific area. When there is limited space, a heavy black dot may be substituted for the arrows, as in figure 23. Also in this drawing, two holes are identical, allowing the "2x" notation to be used and the dimension to point to only one of the circles.

### Where To Put Dimensions

The dimensions should be placed on the face that describes the feature most clearly. Examples of appropriate and inappropriate placing of dimensions are shown in figure 25.

Figure 25 - Example of appropriate and inappropriate dimensioning

In order to get the feel of what dimensioning is all about, we can start with a simple rectangular block. With this simple object, only three dimensions are needed to describe it completely (figure 26). There is little choice on where to put its dimensions.

Figure 26 - Simple Object

We have to make some choices when we dimension a block with a notch or cutout (figure 27). It is usually best to dimension from a common line or surface. This can be called the datum line of surface. This eliminates the addition of measurement or machining inaccuracies that would come from "chain" or "series" dimensioning. Notice how the dimensions originate on the datum surfaces. We chose one datum surface in figure 27, and another in figure 28. As long as we are consistent, it makes no difference. (We are just showing the top view).

Figure 27 - Surface datum example

Figure 28 - Surface datum example

In figure 29 we have shown a hole that we have chosen to dimension on the left side of the object. The Ø stands for "diameter".

Figure 29 - Exampled of a dimensioned hole

When the left side of the block is "radiuses" as in figure 30, we break our rule that we should not duplicate dimensions. The total length is known because the radius of the curve on the left side is given. Then, for clarity, we add the overall length of 60 and we note that it is a reference (REF) dimension. This means that it is not really required.

Figure 30 - Example of a directly dimensioned hole

Somewhere on the paper, usually the bottom, there should be placed information on what measuring system is being used (e.g. inches and millimeters) and also the scale of the drawing.

Figure 31 - Example of a directly dimensioned hole

This drawing is symmetric about the horizontal centerline. Centerlines (chain-dotted) are used for symmetric objects, and also for the center of circles and holes. We can dimension directly to the centerline, as in figure 31. In some cases this method can be clearer than just dimensioning between surfaces.

# Engineering drawing abbreviations and symbols

Engineering drawing abbreviations and symbols are used to communicate and detail the characteristics of an engineering drawing. This list includes abbreviations common to the vocabulary of people who work with engineering drawings in the manufacture and inspection of parts and assemblies.

Technical standards exist to provide glossaries of abbreviations, acronyms, and symbols that may be found on engineering drawings. Many corporations have such standards, which define some terms and symbols specific to them; on the national and international level, ASME standard Y14.38[1] is probably the most widely used.

Jump to:   0-9 • A • B • C • D • E • F • G • H • I • J • K • L • M • N • O • P • Q • R • S • T • U • V • W • X • Y • Z • see also

Abbreviation or symbol Definition Description
0-9
A
AC across corners Commonly used when measuring the corners of a hex drive, such as a hex nut.
AF across flats Commonly used when measuring the flat surfaces of a hex drive, such as a hex nut.
AISI American Iron and Steel Institute The AISI acronym is commonly seen as a prefix to steel grades, for example, "AISI 4140". The SAE steel grade system was formerly a joint AISI-SAE system.
Al or AL aluminium
ALY alloy
AMER American Referring to the United States
AMS Aerospace Material Standards Standards in materials science and engineering maintained by SAE International and widely used in the aerospace manufacturingindustries.
AN- Army-Navy A prefix for standard hardware (catalog hardware) ID numbers. Came from the era of circa 1890s-1945, when the U.S. Army and Navywere leading the way on product standardization for logistics improvement, yielding the United States Military Standards system. Today industry and ISO also do a lot of this standardization specification, freeing the U.S. DOD and military to do less of it (as explained at United States Military Standard > Origins and evolution), although many MIL standards are still current. (See also MS- and NAS.)
ANN anneal, annealed
ANSI American National Standards Institute And the many standards that it issues, for example, ANSI Z87.1.
APPROX[2] approximately
AQL acceptable quality level The threshold of defectiveness that is allowable in a group of parts. It is trivial to say that no one wants any error, and that everyone wants uniform perfection; but in the real world, it almost never happens. The intelligence behind defining AQLs is in figuring out how much error is tolerable given the costs that would be incurred by any efforts to further reduce its incidence.
AR as required An abbreviation used in parts lists (PLs, LMs, BoMs) in the quantity-per-assembly field when a discrete count is not applicable. For example, in an assembly with a bolted joint using four bolts, the PL quantity column will say "4" for the bolt PN, "4" for the nut PN, and "AR" for the liquid threadlocker that will be applied.
AS Aerospace Standards;Australian Standards 1. Aerospace Standards, technical standards maintained by SAE International and widely used in the aerospace manufacturing industries. Standard aerospace hardware sometimes has the AS- prefix in the catalog numbers. 2. Australian Standards, standards per Australianindustry.
AS, APS, APV, AV, APSL, AVL approved product supplier, approved vendor, approved-product-supplier list, approved-vendor list When only certain companies are approved by the CDA to manufacture the product (that is, to make what the drawing depicts/defines), they are called by names such as "approved supplier", "approved product supplier", "approved vendor", or "approved product vendor". The list of such companies (which usually changes over time) is called an APSL, AVL, or similar names. Vetting the companies on this list requires the CDA to audit (and possibly periodically re-audit) the companies, which incurs an overhead expense for the CDA. Therefore, smaller companies will often cite larger companies' lists in order to avoid the cost of duplicating the effort.
ASA American Standards Association Former name for ANSI (1920s-1960s).
ASME American Society of Mechanical Engineers And the many standards that it issues, for example, ASME Y14.5.
ASSY or ASY assembly referring to an assembly of parts rather than just one (sub)part ("piece part", "detail part").
ASTM Formerly the American Society for Testing and Materials; now ASTM International Maintains technical standards, especially regarding materials science and engineering and metrology.
AVG average
AWG American Wire Gauge
B
BASIC basic dimension A basic dimension is one that is the theoretical value without any tolerance range. It does not serve as an acceptance criterion. It is thus similar in some respects to a reference dimension. The reason why a basic dimension does not carry a tolerance is that its actual value will fall (acceptably) wherever it is put by other features' actual values, where the latter features are the ones with tolerances defined. A common and simple example is hole location: If a hole's centerpoint location has a position tolerance, then the centerpoint's coordinates do not need (and should not have) separate tolerances applied to them. Thus they are instead given as basic dimensions. In modern practice basic dimensions have a rectangular box around them, or sometimes the word "BASIC".
BC or B.C. bolt circle
BHCS button head cap screw Like an SHCS but with a button head.
BHN Brinell hardness number
BoM or BOM bill of materials Also called a list of materials (LM or L/M). Overlaps a lot in concept with a parts list (PL or P/L). There is no consistently enforced distinction between an L/M, a BoM, or a P/L.
BP, B/P blueprint "per B/P" = "per drawing"
BRZ bronze
BSC basic dimension See basic dimension info above.
C
CAGE Commercial and Government Entity[code] A CAGE code is a unique identifier to label an entity (that is, a specific government agency or corporation at a specific site) that is a CDA,ODA, or MFR of the part defined by the drawing. One corporation can have many CAGE codes, as can one government, because each division, department, and site (campus) can have its own CAGE code. The same CAGE code can change owners over the years. For example, a CAGE code that formerly referred to a certain Martin Marietta site will now refer to Lockheed Martin at that same site (although the buildings may have been replaced and the signage may say different names).
C-C or C-TO-C centre-to-centre; on centres Defines centre-to-centre distance of two features, such as two holes.
CBN cubic boron nitride A material from which some cutter inserts are made.
CDA current design activity The CDA is the entity (whether it be a corporation, a unit of a national military or ministry of defence, or another civilian government agency) that currently has design authority over the part design (definition). It may be the entity who first designed the part (that is, theODA), but today it is also likely to be a designated successor entity, owing to mergers and acquisitions (M&A) activity (e.g., ODA company was bought by CDA company); contract letting (e.g., an Army engineering department ODA turns over the design activity to the prime contractor that makes most or all of the parts, turning that contractor into the new CDA); privatization (e.g., a government privatizes the design and manufacture of materiel, and a state arsenal [state armory] ODA transfers design authority to a private armory [defense contractor] ODA); or patent licensing (e.g., a patent-holding inventor [ODA] licenses one or several companies to manufacture products using his intellectual property, in which case the "same" part could end up with multiple design authorities, although they may not be considered the official/nominal CDA).
CERT or cert certification For example, certification of metallurgical content and processes
CG centerless ground,centerless grinding
Center mark Defines the center of a circle or partial circle.
CH chamfer
CHAM chamfer
CI cast iron No longer a commonly used abbreviation. Better to spell out for clarity.
CL or ℄ center line; class 1. Center line, the central axis of a feature. 2. Class, for example, "paint per spec XYZ revision C type 1 class 2" may be abbreviated as "paint per spec XYZ REV C TY 1 CL 2" or even in some cases "paint per spec XYZ-C-1-2". (The latter practice is not uncommon but iscryptic for workers with minimal training and experience. The first two options are better practice.)
CNC computer numerical control
CR controlled radius Radius of an arc or circle, with no flats or reversals. This strict version of radius definition is specified in demanding applications when the form of the radius must be controlled more strictly than "just falling within the dimensional tolerance zone". It is poor engineering to specify a CR instead of an R simply on the theory of enforcing good workmanship. CR is for critical features whose performance truly requires near-perfect geometry. Like most such characteristics, its presence increases the price of the part, because it raises the costs of manufacturing and quality assurance.
CRES corrosion-resistant [steel] Largely synonymous with stainless steel, unless specific grades, specs, and distinctions are made on the drawing. Some people treat CRES as a subset of the stainless steels.
CRS cold rolled steel; on centres Defines centre-to-centre distance of two features, such as two holes.
C'BORE or CBORE or  counterbore
CSK or CSINK or  countersink
CTN, ctn carton
D
depth, deep, down Defines the depth of a feature.
[2] diameter Diameter of a circle. In a feature control frame (FCF), the ⌀ symbol tells you that the tolerance zone for the geometric tolerance iscylindrical. Abbreviations for "diameter" include ⌀, DIA, and D.
D diameter; delta Abbreviations for "diameter" include ⌀, DIA, and D. For delta usage, see for example "delta notes".
DIA[2] diameter Diameter of a circle. Abbreviations for "diameter" include ⌀, DIA, and D.
DIM dimension, dimensioning
DPD digital product definition A synonym of MBD.
DWG, dwg drawing Referring to the engineering drawing
E
EO, ECO, ECN engineering order An order from the engineering department (to be followed by the production department or vendor) overriding/superseding a detail on the drawing, which gets superseded with revised information. Also called by various other names, such as engineering change order (ECO), engineering change notice (ECN), drawing change notice (DCN), and so on. See also REV.
EQ equal, equally For example, "⌀10 4X EQ SPACED ON BC" means "drill four holes of 10mm diameter equally spaced around the bolt circle."
F
f finish An italic f (Latin small letter f) written on a line representing a surface was an old way of indicating that the surface was to be machined rather than left in the as-cast or as-forged state. The "f" came from "finish" in the sense of "machine finish" as opposed to raw stock/casting/forging. Later the ASA convened upon a letter V (specifically a sans-serif V) touching the surface. Soon this evolved into the "check mark" sign with accompanying number that tells the reader a max roughness value (RMS, microinches or micrometres) for the machined finish, to be measured with a profilometer.
FAO finish all over A note telling the manufacturer that all surfaces of the part are to be machined (as opposed to leaving any surfaces as-cast or as-forged). Not an obsolete usage, but not seen as commonly as it was decades ago; not least because parts that once would have been spot-faced castings are now likelier to be contoured from billet with CNC milling. But more importantly, best engineering practice today, reflectingdesign for manufacturability and avoidance of spurious cost drivers, is either to specify specific, quantifiable requirements for surfaces with specific needs (such as RMS roughness measurements in microinches or micrometres, plus any plating or painting needs), or to leave finish out of the part definition (and thus at the manufacturer's discretion) because it is not important to fit, function, or criticality. This same spirit is behind the shift in military standards from writing requirements about methods to writing them instead about performance, with the method to reach that goal being up to the ingenuity of the designer.
FCF feature control frame The rectangular box (with several cells) that conveys geometric tolerances in GD&T. It typically tells you what sort of geometric condition (e.g., parallel, perpendicular, round, concentric), followed by what size (and maybe shape) the tolerance zone is, and finally which datumsit relates to, the order of gaging against them, and what material condition applies to them (LMC, MMC, or RFS). A diameter symbol (⌀) tells you that the zone for the geometric tolerance is cylindrical.
FD or F/D field of the drawing The [main] field of the drawing, as opposed to other areas of it, such as the parts list (P/L), general notes (G/N), flagnotes (F/N or FL), title block (T/B), rev block (R/B), bill of materials (B/M or BoM or BOM), or list of materials (L/M). Rationales for drawing changes that are noted in the rev block often use these abbreviations for brevity (e.g., "DIM 14.00 was 12.50; added default TOL info to T/B; added leader lines to F/D; added alternate hardware IDs to P/L; added alternate alloy to L/M").
FL flag note, flagnote A note that is called out in specific spots in the field of the drawing. It is numbered with a stylized flag symbol surrounding the number. A general note applies generally and is not called out with flags.
FN or F/N flag note, flagnote; find number 1. Flagnote: A flagnote is a note that is called out in specific spots in the field of the drawing. It is numbered with a stylized flag symbol surrounding the number (or sometimes a delta symbol). A general note applies generally and is not called out with flags. 2. Find number: "FN" meaning "find number" refers to the ordinal number that gives an ID tag to one of the constituents in a parts list (list of materials, bill of materials). Thus "fasten using FN7" refers to a fastener that is "find number" 7 in the list.
FoS feature of size A type of physical feature on a part. An FoS is a feature that can have size associated with it, usually involving the opposition of two surfaces (e.g., the two diametrically opposite sides of a hole wall; the two opposite walls of a slot or flange). Features of size (FoSs) in reality always have actual sizes and forms that differ from their theoretical size and form; the purpose of tolerancing is to define whether the difference is acceptable or not. Thus material condition (LMC, MMC, somewhere in between, or RFS) is important in GD&T. ) A given geometric tolerance may be defined in relation to a certain FoS datum being at LMC or at MMC.
FS far side The drawing notations "near side" and "far side" tell the reader which side of the part a feature is on, in occasional contexts where that fact is not communicated using the rules of projection alone. Contexts of usage are rather limited. One example is hole locations; "3X AND 3X FAR SIDE" defines symmetrical groups of 3 holes on both sides of a part (6 total), without having to redefine equivalent hole center coordinates on two separate views, one for each group. This is not only a convenience for the designer but also a method of error prevention, because it provides a way to avoid forking geometric definition that ideally should be kept unforked to prevent discrepancies. For example, the groups defined above cannot accidentally become asymmetrically discrepant in a future revision by the revisor failing to revise both groups equally (because their definition is unified in only one place). Another example is part marking locations. An area for part identification marking can be circled on a top view but assigned to either the top or bottom of the part simply with a "near side" or "far side" notation—which obviates adding any otherwise-unneeded bottom view to the field of the drawing.
FSCM Federal Stock/Supply Code for Manufacturers An older name for "CAGE code". Also NSCM (National Stock/Supply Code for Manufacturers).
G
GCI gray cast iron
GD&T or GDT geometric dimensioning and tolerancing A standardized language for defining and communicating dimensions and tolerances.
GN or G/N general note(s) Most engineering drawings have a notes list, which includes both general notes and flag notes.
H
HBW hardness, Brinell, tungsten tip See Brinell scale. (The "W" comes from the element symbol for tungsten, W, which comes from the German Wolfram.)
HRA hardness, Rockwell, A scale See Rockwell scale.
HRB hardness, Rockwell, B scale See Rockwell scale.
HRC hardness, Rockwell, C scale See Rockwell scale.
HRS hot rolled steel
HT TR heat treat, heat treatment
H&T or H/T or HT hardened and tempered A form of heat treatment in which the metal is first hardened and then tempered. Compare N&T.
I
IAW in accordance with A common need in engineering drawings is to instruct the user to do activity X in accordance with technical standard Y. For example, "Weld all subassemblies IAW AWS XYZ.123" means "Weld all subassemblies in accordance with American Welding Society standard number XYZ.123" (the number is hypothetical in this example). The word "per" is functionally equivalent to "IAW" in such contexts; thus "rivet all sheet metal per MIL-PRF-123456" or "[...] IAW MIL-PRF-123456". Part of the motivation behind the choice of words "in accordance with" is that they do not allege that any particular activity is explicitly specified by standard XYZ.123 (which "per" could be interpreted as alleging, at least in connotation); rather, these words merely instruct the user that whatever s/he does must not contradictthe standard in any way. But this is a subtle connotative distinction, and "per" and "IAW" are denotatively equivalent.
ID inner diameter; identity, identification number
ISO International Organization for Standardization And the many standards that it specifies, for example, ISO 10303
J
K
KEY key Drawing callouts marked "KEY" define "key characteristics" that are considered especially important for fit, function, safety, or other reasons. They are thus subjected to higher inspection sampling levels.
KPSI, kpsi kilopounds per square inch, that is, thousands of pounds per square inch See discussion at synonym KSI.
KSI, ksi kilopounds per square inch, that is, thousands of pounds per square inch KSI (or ksi), also abbreviated KPSI or kpsi, is a common non-SI measurement scale for ultimate tensile strength, that is, the number of units of tensile force that a material can endure per unit of cross-sectional area before breaking. In the SI system, the unit is the pascal(Pa) (or a multiple thereof, often megapascals (MPa), using the mega- prefix); or, equivalently to pascals, newtons per square metre (N/m²).
L
LH left-hand Referring to handedness, such as the helix handedness of screw threads or the mirror-image handedness of a symmetrical pair of parts.
LM or L/M list of materials Also called a bill of materials (BoM, BOM). Overlaps a lot in concept with a parts list (PL or P/L). There is no consistently enforced distinction between an L/M, a BoM, or a P/L.
LMC least material condition A material condition in GD&T. Means that a feature of size (FoS) is at the limit of its size tolerance in the direction that leaves the least material left on the part. Thus an internal feature of size (e.g., a hole) at its biggest diameter, or an external feature of size (e.g., a flange) at its smallest thickness. The GD&T symbol for LMC is a circled L. (See also MMC and RFS.) A given geometric tolerance may be defined in relation to a certain FoS datum being at LMC or at MMC.
M
MACH machine; machined
MAJ major As in major diameter, or major characteristic (for sampling level)
MAX[2] maximum
MBD model-based definition Definition of the part via a 3D CAD model rather than via a 2D engineering drawing. Drawings may be printed (plotted) from the model for reference use, but the model remains the governing legal instrument.
MF or M/F make from When one part number is made from another, it means to take part A and machine some additional features into it, creating part B. The parts list or L/M, in the "material" field, will say "M/F PN 12345".
MFD manufactured
MFG manufacturing
MFR manufacturer May be the same entity as the CDA or ODA, or may not be.
MIL- [U.S.] Military A prefix for the names of various United States Military Standards and Specifications, for example, MIL-STD-*, MIL-SPEC-*, MIL-DTL-*, MIL-PRF-*, MIL-A-*, MIL-C-*, MIL-S-*, MIL-STD-1913, MIL-STD-1397.
MIN[2] minimum; minutes; minor
MMC maximum material condition A material condition in GD&T. Means that a feature of size (FoS) is at the limit of its size tolerance in the direction that leaves the most material left on the part. Thus an internal feature of size (e.g., a hole) at its smallest diameter, or an external feature of size (e.g., aflange) at its biggest thickness. The GD&T symbol for MMC is a circled M. (See also LMC and RFS.) A given geometric tolerance may be defined in relation to a certain FoS datum being at LMC or at MMC.
MOP, MoP measurement over pins threads, splines, gears (external, male) (synonymous with MOW, measurement over wires)
MPa, MPA megapascals The common SI measurement scale for ultimate tensile strength (UTS), that is, the number of units of tensile force that a material can endure per unit of cross-sectional area before breaking. There is only one correct casing for the symbol, cap-M-cap-P-small-a, which, like any SI unit of measurement symbol, properly should be preserved even when surrounding text is styled in all caps (which latter is a frequently employed tradition in engineering drawing). But it is not uncommon to see "MPA" through carelessness. Users are not confused regardless. In non-SI terms, the unit for UTS is the KSI (or ksi), which see herein.
MRB material review board A committee that reviews some nonconforming materials which are submitted as potentially still usable/saleable (if the nonconformance does not hinder fit or function).
MS- [U.S.] Military Standard Standards established by the U.S. military and widely used in the aerospace manufacturing (military and civil) and other defenseindustries. Standard hardware sometimes uses the MS- prefix in the catalog numbers. (See also AN- and NAS.)
N
NAS National Aerospace Standards Standards maintained by SAE International and widely used in the aerospace manufacturing industries. The "National" formerly implicitly referenced the USA, but today NAS and other standards are used globally. Standard hardware for aerospace work sometimes uses the NAS- prefix in the catalog numbers. (See also AN- and MS-.)
NC National Coarse;numerical control The [U.S.] National Coarse series of pre-1949 corresponds today to the Unified National Coarse (UNC) of the Unified Thread Standard.
NCM nonconforming material(s) This abbreviation is used in a machine shop when recording nonconformances (out of tolerance, etc.). For example, "An NCM tag was tied to the scrap part."
NCR nonconformance report A report listing nonconformances (out of tolerance, etc.). Helps to analyze system weaknesses (such as worn-out equipment, operators in need of more training, or risky practices).
NEC not elsewhere classified; National Electrical Code In the sense of "not elsewhere classified", the abbreviation is well-known within certain fields, but not others; to avoid confusion, spell out. The National Electrical Code is a standard for electrical work.
NEF National Extra Fine The [U.S.] National Extra Fine series of pre-1949 corresponds today to the Unified National Extra Fine (UNEF) of the Unified Thread Standard.
NF National Fine The [U.S.] National Fine series of pre-1949 corresponds today to the Unified National Fine (UNF) of the Unified Thread Standard.
NL or N/L notes list A list of notes that appears somewhere on the drawing, often in the upper left corner.
NOM[2] nominal
NORM or NORMD normalized referring to normalization, a stress-relieving heat treatment. See also HT TR.
NPS Naval Primary Standard[3] (Not to be confused with annotating strait pipe. This should be abbreviated NPSM, NPSL or NPSH[4])
NPT National Pipe Taper A subset series of the Unified Thread Standard.
NS National Special; near side 1. National Special, a screw thread series; see Unified Thread Standard. An extensible series, covering various special threads. 2. Near side: The drawing notations "near side" and "far side" tell the reader which side of the part a feature is on, in occasional contexts where that fact is not communicated using the rules of projection alone. Contexts of usage are rather limited. See "far side" for examples.
NSCM National Stock/Supply Code for Manufacturers An older name for "CAGE code". Also FSCM (Federal Stock/Supply Code for Manufacturers).
N&T or N/T or NT normalized and tempered A form of heat treatment in which the metal is first normalized (stress-relieved) and then tempered. Compare H&T.
O
OAL overall length
OC on center(s) That is, centre-to-centre; defines centre-to-centre distance of two features, such as two holes.
OD outer diameter
ODA original design activity The entity that originally designed a part. Compare to CDA, the entity that currently has design authority over the part design (definition).
OHL over high limit This abbreviation is used in a machine shop when recording nonconformances. For example, "part scrapped because ID is OHL." See also ULL.
OPP opposite See Part number > Symmetrical parts for explanation.
ORIG original
P
pc, pcs piece, pieces
PD pitch diameter
PDM, PDMS product data management, product data manager [app], product data management system [app] A database(s) and related application(s) that facilitate all aspects of managing data files—e.g., TDPs, TDP versions, drawings, model datasets, specs, addenda, certs, memoranda, EOs, ECOs, DCNs, RFQs, quotes, POs, e-mails, faxes, photos, word processor documents, spreadsheets. See also PLM.
PH or P/H precipitation hardening, precipitation-hardened; pilot hole
PHR BRZ phosphor bronze
PL or P/L parts list A list, usually tabular and often on the drawing (if not accompanying the drawing on a separate sheet), listing the parts needed in an assembly, including subparts, standard parts, and hardware. There is no consistently enforced distinction between an L/M, a BoM, or aP/L.
PN or P/N part number
POI point of intersection A point that makes easier the layout, toolpath programming, or inspection of the part. It is the intersection point of lines that may not meet on the finished part, such as the tangent lines of a curve or the theoretical sharp corner (TSC) that edge-breaking and deburring will remove. See also SC, TSC, and AC.
PSI pounds per square inch A unit of measurement for pressure. See also KSI.
PTFE polytetrafluoroethylene Also well known by the brand name Teflon.
Q
QMS quality management system A system in place to ensure that quality of manufacture is produced and maintained; a system to prevent defective parts from being made, or, even if made, from getting into finished inventories.
QTY or qty quantity
R
R radius Radius of an arc or circle. Flats and reversals (falling within the dimensional tolerance zone) are tolerated unless "CR" (controlled radius) is explicitly specified.
RA, Ra roughness, average;Rockwell A scale See surface roughness; see Rockwell scale.
RB, Rb Rockwell B scale See Rockwell scale.
RC, Rc Rockwell C scale See Rockwell scale.
REF or ( )[2] reference The dimension or note is given only for reference and thus is not to be used as a part acceptance criterion (although it may be used as an aid to production or inspection). The dimension may also be surrounded by parentheses to signify a reference dimension. When a dimension is defined in one view but also mentioned again in another view, it will be given as reference in the second case. This rule prevents the mistake of defining it in two different ways accidentally; the "main" (non-reference) mention is the only one that counts as a feature definition and thus as a part acceptance criterion. See also basic dimensions, which are similar in some respects.
REQD or REQ'D required For example, "4 REQD" written next to a fastener means that four of those fasteners are required for the assembly.
REV revision Engineering drawings and material or process specifications are often revised; the usual revision control convention is to label the versions A, B, C, D, etc.; a revision block (rev block) is a tabular area on the drawing (typically in the upper right corner) that lists the revision letters, a brief description of the changes and reasons, and approval initials and dates. Revisions beyond "Z" start the alphabet over again with doubling, e.g., AA, AB, AC, AD, and so on. In the days of manual drafting, redrawing was expensive, so engineering orders (EOs, ECOs, DCNs, ECNs) were not always incorporated into a next-letter revision. They thus accompany the drawing as part of the TDP. With the dissemination of software usage (CAD, CAM, PDMSs), revision control is often better handled nowadays, in competent hands at least. In recent years the revision control of engineering drawings has even been standardized by ASME, in their standard Y14.35M.[5]
RFS regardless of feature size A material condition (or more precisely, freedom from such) in GD&T. Means that a given geometric tolerance is true in relation to a certain datum regardless of its actual size (LMC ≤ actual size ≤ MMC).
RH right-hand Referring to handedness, such as the helix handedness of screw threads or the mirror-image handedness of a symmetrical pair of parts.
RHR roughness height reading See surface roughness.
RMS root mean square RMS in general is a statistical technique to define a representative value for a group of data points. With regard to surface roughness, it means that the heights of the individual microscopic peaks and valleys shall be averaged together via RMS to yield a measurement of roughness. See also herein f as a finish mark.
RT or R/T rough turn, rough turned; room temperature Rough-turned means turned on a lathe but not finished to a final machined dimension and surface roughness. Can apply to bar stock or to parts in-process. Room temperature is sometimes abbreviated "RT" within tables of specs for finishing operations (plating, painting, etc.).
RTP release to production The issuance of a drawing from the engineering/design activity to the production activity. In other words, the event when a draft becomes a completed, official document. A stamp on the drawing saying "ISSUED" documents that RTP has occurred.
RTV room-temperature vulcanizing; return to vendor 1. RTV sealants, a way to seal joints. 2. Return to vendor, send parts back to a vendor for rework or refund because they are nonconforming. Such RTV often requires an RMA.
RZ, Rz roughness, mean depth See surface roughness.
S
SAE Formerly the Society of Automotive Engineers; now SAE International And the many standards that it issues, for example, the SAE AMS and SAE AS standards series.
SC or S/C sharp corners Dimensions may be given as "across sharp corners" although the corners get radiused. In other words, distances may be given from intersection points where lines intersect, regardless of edge breaks or fillets. This is usually implied by default, so "S/C" often need not be explicitly added. But in some cases it clarifies the definition. See also TSC, POI, and AC.
SF or S/F spotface
SFACE or S/FACE spotface
SHCS socket head cap screw A cap screw with a socket head (usually implying a hex socket, driven with a hex key.
SHN shown See Part number > Symmetrical parts for explanation.
SHSS socket head set screw A set screw with a socket head (usually implying a hex socket, driven with a hex key.
SI Système international [d'unités] [International System of Units] The metric system in its current form (latest standards).
SN or S/N serial number
SOL ANN solution anneal, solution annealed
SPEC or spec specification
SPHER ANN spheroidize anneal
SPOTFACE Spot facing
SS or S/S stainless steel; supersede 1. Stainless steel, see also CRES. 2. Supersede/supersedes/superseded, refers to when one document (specification, standard, drawing, etc.) replaces (supersedes) another (see also revision control).
SST stainless steel A somewhat unusual abbreviation; spell out for clarity. "SS" or "CRES" are more likely to be recognized with certainty.
STD Standard
STEP Standard for theExchange of Product Model Data A standard format defined by ISO 10303 for MBD data generation, storage, and exchange.
STA solution treated and aged
STL steel
STK stock A nominal dimension for the stock material, such as bar stock
T
TAP Tapped hole Usually implies drilling a hole if the hole does not already exist.
TB or T/B title block An area of the drawing, almost always at the bottom right, that contains the title of the drawing and other key information. Typical fields in the title block include the drawing title (usually the part name); drawing number (usually the part number); names and/or ID numbers relating to who designed and/or manufactures the part (which involves some complication because design and manufacturing entities for a given part number often change over the years due to mergers and acquisitions, contract letting, privatization, and the buying and selling of intellectual property—see CDA and ODA); company name (see previous comment); initials/signatures of the original draftsman (as wells as the original checker and tracer in the days of manual drafting); initials/signatures of approving managers (issuance/release-to-production information); cross-references to other documents; default tolerancing values for dimensions, geometry, and surface roughness; raw-material info (if not given in a separate list/bill of materials); and access control information (information about who is authorized to possess, view, or share copies of the information encoded by the drawing, e.g., classification notices, copyright notices,patent numbers). Drawing revision (versioning) information is not always included in the title block because it often appears in a separate revisions block.
TDP technical data package The complete package of information that defines a part, of which the drawing itself is often only a subset. It also includes engineering orders (drawing change notices), 3D model datasets, data tables, memoranda, and any special conditions called out by the purchase order or the companies' terms-and-conditions documents.
THRU Through Optionally applied to a hole dimension to signify that the hole extends through the workpiece. For example, THRU may be stated in a hole dimension if the hole's end condition is not clear from graphical representation of the workpiece.[6]
THRU ALL Through all Similar to THRU. Sometimes used on hole dimensions for clarity to denote that the hole extends through multiple open space features as it goes through the whole workpiece.[7]
Ti titanium
TiN titanium nitride[plating]
TIR total indicator reading; total indicated run-out For measurements of eccentricity and other deviations from nominal geometry
TOL tolerance, tolerancing
TSC theoretical sharp corner(s) See discussion at SC and POI.
TY type For an explanation of "type" abbreviated as "TY", see the example given at "CL" meaning "class".
TYP[2] Typical Other features share the same characteristic. For example, if the drawing shows 8 holes on a bolt circle, and just one is dimensioned, with "TYP" or "(TYP)" following the dimension label, it means that that hole is typical of all 8 holes; in other words, it means that the other 7 holes are that size also. The latest revisions of Y14.5 deprecate "TYP" by itself in favor of the specifying of a number of times, such as "2X" or "8X". This helps avoid any ambiguity or uncertainty. TYP or Typical was describe in Mil-Std-8, the directing body prior to adoption of the dimension tolerance interpretation Y14.5 series. Its last revision was C in 1963, but can still be found in many older aircraft drawings.
U
UAI use as-is One of the possible MRB dispositions. Others include scrap and rework.
ULL under low limit This abbreviation is used in a machine shop when recording nonconformances. For example, "part scrapped because OD is ULL." See also OHL.
UNC Unified National Coarse A subset series of the Unified Thread Standard.
UNEF Unified National Extra Fine A subset series of the Unified Thread Standard.
UNF Unified National Fine A subset series of the Unified Thread Standard.
UNJC Unified National "J" series Coarse A subset series of the Unified Thread Standard, with controlled root radius and increased minor diameter. For applications requiring maximum fatigue resistance amid chronic vibration (such as in aircraft).
UNJF Unified National "J" series Fine A subset series of the Unified Thread Standard, with controlled root radius and increased minor diameter. For applications requiring maximum fatigue resistance amid chronic vibration (such as in aircraft).
UNS Unified National Special; unified numbering system Unified National Special is a subset series of the Unified Thread Standard. It is an extensible series, covering various special threads. Theunified numbering system is a vaguely named standard for naming alloys by principal element percentages.
UON unless otherwise noted A little-used (thus not well recognized) abbreviation. To avoid confusion, spell out.
UOS unless otherwise specified A fairly well-known abbreviation, but to avoid confusion, spell out.
USASI United States of America Standards Institute Former name for ANSI (1966–1969).
USS United States Standard; United States Steel U.S. Standard threads became the National series (e.g., NC, NF, NEF), which became the Unified National series (e.g., UNC, UNF, UNEF); see Unified Thread Standard. As for U.S. Steel, it was once the largest steel company on earth, often an approved supplier, and not infrequently a sole source; hence its mention on drawings.
UTS ultimate tensile strength; Unified Thread Standard
V
v finish A letter v (Latin small letter v) written on a line representing a surface is a way to indicate that the surface is to be machined rather than left in the as-cast or as-forged state. The older symbol for this was a small script (italic) f (see herein f). Later the ASA convened upon a letter V (specifically a sans-serif V) touching the surface. Soon this evolved into the "check mark" sign with accompanying number that tells the reader a max roughness value (RMS, microinches or micrometres) for the machined finish, to be measured with a profilometer.
W
WC tungsten carbide The "W" comes from the element symbol for tungsten, W, which comes from the German Wolfram.
WI wrought iron Both the material and the abbreviation are obsolete, or nearly so. Spell out the words if this material is to be mentioned at all in modern drawings.
W/I, w/i within A little-used abbreviation. Better to spell out for clarity.
W/O, w/o without Better to spell out for clarity.
X
_X_ used to indicate the word “by” When the letter X is preceded by a space, this means "by". For example, a chamfer may be called out as 12 X 45º
X[2] or ( ) number of places—for example, 8X or (8) When a dimension is used in multiple places either of these prefixes can be added to the dimension to define how many times this dimension is used. This example signifies eight places. There should be no whitespace between the numeral and the letter X. (Note oncharacter encoding: Although in typography (including Unicode) the letter X and the multiplication sign (×) are distinct characters with differing glyphs, it is a longstanding tradition in engineering drawing that the letter X is interchangeable with the multi sign, unless otherwise specified by the CAx systems used.)
Y
Y14.X Calls out the drawing standard that this drawing is following. For example, ASME Y14.5 and Y14.100 are commonly used standards that define all of the symbols and drafting conventions used.
YS yield strength
Z