Geometric Tolerancing

Geometric Tolerancing is used to specify the shape of features.

Things like:

•Straightness

•Flatness

•Circularity

•Cylindricity

•Angularity

•Profiles

•Perpendicularity

•Parallelism

•Concentricity

•And More...

Geometric Tolerances are shown on a drawing with a feature control frame.

The Feature Control Frame

This feature control frame is read as: “The specified feature must lie perpendicular within a

tolerance zone of 0.05 diameter at the maximum material condition, with respect to datum

axis C. In other words, this places a limit on the amount of variation in perpendicularity

between the feature axis and the datum axis. In a drawing, this feature control frame would

accompany dimensional tolerances that control the feature size and position.

Principles of Engineering Drawing

Thayer Machine Shop

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SECTION 3 SURFACE TEXTURE

3.1 SCOPE OF SECTION This Section provides information on the indication of surface texture on

mechanical engineeringdrawingsandsimilar applications.For a more complete understandingof surface

texture, reference should be made to AS 2536.

3.2 SYMBOLS

3.2.1 Basic symbol The basic symbol is shown in Figure 3.1. The dimensions of surface texture

symbols are shown in Figure 3.2. Sloping lines in the symbol are at 60° to the horizontal.

3.2.2 Modification to basic symbol The following modifications may be made to the basic symbol:

(a) The symbol to be used where machining is mandatory shall be the basic symbol with a bar

added, as shown in Figure 3.3.

This symbol may be used alone to indicate that a surface is to be machined without defining

either the surface texture or the process to be used.

(b) The symbol to be used when the removal of material is not permitted shall be the basic symbol

with a circle added, as shown in Figure 3.4. This symbol may be used alone to indicate that a

surface is to be left in the state resulting from a preceding manufacturing process.

3.2.3 Extension of symbols When special surface characteristics are to be indicated (see Clause

3.4), the symbols shown in Figures 3.1, 3.3 and 3.4 may be extended by adding a line of appropriate

length to the long leg, as shown in Figure 3.5.

Australian Standard

Technical drawing

Part 201: Mechanical engineering drawing

Accessed by WOODSIDE ENERGY LTD on 21 Nov 2001

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The application of welding symbols to working drawings

The following notes are meant as a guide to the method

of applying the more commonly used welding symbols

relating to the simpler types of welded joints on

engineering drawings. Where complex joints involve

multiple welds it is often easier to detail such constructions

on separate drawing sheets.

Each type of weld is characterized by a symbol

given in Table 26.1 Note that the symbol is representative

of the shape of the weld, or the edge preparation, but

does not indicate any particular welding process and

does not specify either the number of runs to be deposited

or whether or not a root gap or backing material is

to be used. These details would be provided on a welding

procedure schedule for the particular job.

It may be necessary to specify the shape of the weld

surface on the drawing as flat, convex or concave and

a supplementary symbol, shown in Table 26.2, is then

added to the elementary symbol. An example of each

type of weld surface application is given in Table 26.3.

A joint may also be made with one type of weld on

a particular surface and another type of weld on the

back and in this case elementary symbols representing

each type of weld used are added together. The last

example in Table 26.3 shows a single-V butt weld

with a backing run where both surfaces are required to

have a flat finish.

A welding symbol is applied to a drawing by using

a reference line and an arrow line as shown in Fig.

26.1. The reference line should be drawn parallel to

the bottom edge of the drawing sheet and the arrow

line forms an angle with the reference line. The side of

the joint nearer the arrow head is known as the ‘arrow

side’ and the remote side as the ‘other side’.

The welding symbol should be positioned on the

reference line as indicated in Table 26.4.

Sketch (a) shows the symbol for a single-V butt

weld below the reference line because the external

surface of the weld is on the arrow side of the joint.

Sketch (b) shows the same symbol above the

reference line because the external surface of the weld

is on the other side of the joint.

Manual of

Engineering Drawing

Second edition

Colin H Simmons

I.Eng, FIED, Mem ASME.

Engineering Standards Consultant

Member of BS. & ISO Committees dealing with

Technical Product Documentation specifications

Formerly Standards Engineer, Lucas CAV.

Dennis E Maguire

CEng. MIMechE, Mem ASME, R.Eng.Des, MIED

Design Consultant

Formerly Senior Lecturer, Mechanical and

Production Engineering Department, Southall College

of Technology

City & Guilds International Chief Examiner in

Engineering Drawing

Elsevier Newnes

Linacre House, Jordan Hill, Oxford OX2 8DP

200 Wheeler Road, Burlington MA 01803

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EVOLUTION OF A DESIGN

Most likely you have, right at this moment, at least one machine design project in progress. Maybe you were the originator of the design, but I suspect you inherited this design from others. I further suspect that you have already identified elements of the design you feel could be improved. You might be under pressure from customer service or marketing to respond to some need for change. In responding successfully, either to your own observations for change or to those of others, the design will evolve. Recognizing that the evolutionary design process is decidedly complex, with a seemingly random sequence of steps, the primary purpose of Standard Handbook of Machine Design is to make the information you need as readily accessible and usable as possible. As an example of how a design can evolve, and to provide perspective on how the information in this Handbook has traditionally been used, let me review for you a project I was given in my first job as a mechanical engineer. It involved the positioning of a microwave feed horn for a 30-ft-diameter antenna dish.The original gn (not mine, by the way) called for a technician to climb up onto a platform, some 20 ft off the ground, near the backside of the feed horn.The technician had to loosen a half dozen bolts, rotate the feed horn manually, and then retighten the bolts. This design worked quite well until several systems were sold to a customer providing telecommunications along the Alaskan oil pipeline. Workers were not really safe going out in below 0°F weather,with snow and ice on everything. As a result of their concerns for safety, this customer asked that we provide remote positioning of the feed horn from the nearby control room. The critical design requirement was that the positioning of the feed horn needed to be relatively precise. This meant that our design had to have as little backlash in the drive mechanism as possible. Being a young engineer, I was unaware of the wide variety of different drive systems, in particular their respective properties and capa- bilities. I asked one of the older engineers for some direction. He suggested I use a worm drive since it cannot be back driven, and loaned me his copy of Joseph Shigley’s book, Mechanical Engineering Design. He said that Shigley’s book (a precursor to this Handbook) had been his primary source of information about worm drives, and a wealth of other machine design information. As it turned out, the resulting design worked as required. It not only pleased our Alaskan customer but became a standard on all antenna systems. I did not get a promotion as a result of the success of this new design, nor did I receive a raise. However, I was proud, and, as you can surmise, still am. I credit this successful design evolution to the material on worm drives in Shigley’s book. And there is more to this story. The worm drive gearbox we ultimately purchased contained a plastic drive element. This allowed the backlash to be greater than what could be tolerated in positioning accuracy and did not provide the necessary strength to break the feed horn loose from a covering of ice.The original manufacturer of the gearbox refused to change this drive element to metal for the units we would be buying. If we made the change ourselves, they said, the warranty would be voided. However, after absorbing the wealth of information on worm drives in Shigley’s book, I felt confident that we could make this substitution without endangering the reliability of the unit. Also, because of Joseph Shigley’s reputation in the mechanical engineering community and the extensive list of references he cited, I never felt the need to consult other sources. Another aspect of this story is also important to note. In addition to the information on worm drives, I also used Shigley’s book to find comprehensive design information on the many other machine elements in the new design: gear train geometry, chain drives, couplings, roller bearings, bolted joints, welds, lubrication, corrosion, and the necessary stress and deformation calculations I needed to make. All this information, and much more, was contained in the First Edition of the Standard Handbook of Machine Design, which Joseph Shigley coauthored with Charles Mischke. Now in its Third Edition, this Handbook includes the information machine design engineers have come to trust.We hope you will find this information invaluable as you constantly strive to improve your designs, whether by your own initiatives, or for other reasons.
Thomas H. Brown, Jr., Ph.D., P.E.
Faculty Associate
Institute for Transportation Research and Education
North Carolina State University
Raleigh, North Carolina
Source: STANDARD HANDBOOK OF MACHINE DESIGN
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