Chinh Nguyen

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The step-by-step instructions will vary depending upon the CMM software, but the basic requirements come from understanding datum target areas.

In cases where gage blocks or pins are unavailable, or the datum target area is a particularly unique shape, the target area can be simulated by probing the specified area with the stylus. Datum targets (points, lines, or areas) designate only a limited portion of the feature as acceptable for datum simulation, so the key is to produce a tangency from the points taken from this limited area.

Where multiple datum target areas are used to establish a datum (for example A1, A2, and A3), the datum plane must include a high point from each of the target areas.

Here’s an example from our Fundamentals of GD&T (’94) textbook.

Remember, where datum targets are specified there is a specific point,  line, or area from which points can be taken to simulate a datum plane. A common mistake by designers and inspectors is to think that any three points, or any 30mm diameter can be used to simulate the datum. Where targets are indicated, the drawing must specify how the target can be identified (dimensionally) so that part-to-gage repeatability issues do not impact part acceptance.

Michael Adcock
ASME GDTP Senior Level
Dimensional Engineering Mentor

Please see the attached drawing.

Note: I cannot shift the Datum A since it is a functioning surface.
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ASME Y14.5 permits a few dimensions to be omitted from the drawing because their indication would not clarify the reading of the drawing. Those dimensions are the 90 degree angle, basic 90 degree angle, zero degree angle, and linear dimensions of zero.

Per the drawing you provided, the use of 4X and the composite position tolerance would require the drawing reader to interpret that there are 4 coaxial diameters, even without seeing another drawing view.

The reference to datum A as a primary datum reference requires the reader to interpret that the 4 diameters are to be parallel to datum A, as well as located to datum A within the diameter 0.010 at MMC allowance.

The datum reference to datum B as a secondary datum requires the reader to interpret that the 4 diameters are located 2.675 from datum B within the diameter 0.10 at MMC allowance.

Not being able to see datum feature C in this drawing, I cannot elaborate on the requirement to datum C.

If the rules of ASME Y 14.5 are understood, the reader will understand the relationship to datum A that you were concerned about. This is partly why it is important to evaluate your supplier’s understanding of GD&T.

ETI has some interesting dialogue on this subject on our Linked-in group. Click here to connect with Alex and join the group.

Michael Adcock
ASME GDTP Senior Certified
Dimensional Engineering Mentor

Our standard title block instructs tolerances on dimensions according to the number of decimal places expressed by that dimension. So, according to our company standard, a 20.00 dimension provides limits of 20.25 / 19.75 but according to para 1.6.1 (b) that is illegal – we should only be saying 20 with no decimal point etc. and stating each tolerance alongside each dimension – something that we would find tiresome and time wasting.

We are very reluctant to have to show the tolerances against every whole number. It seems ironic that for imperial units additional trailing zeros can be used ‘as necessary’ (para 1.6.2(b)).

Can you suggest a legal way around it?

Look forward to your reply / suggestions; thanks for a very useful website.

Robin Davidson

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You’re correct. ASME Y14.5 1994 and 2009 both prohibit the use of trailing zeros on metric dimensions and tolerances. So, using title block tolerances that indicate something like this where the tolerance value is based on the number of decimal places is completely inappropriate:

.X        ± .5

.XX      ± .25

This practice for general tolerances originates with the expression of imperial units (inches), but in too many cases has not updated as the company changed from imperial to metric units. The correct method for displaying general tolerances (if you use them at all) for metric units is based on the dimensional value. These can use any interval (range of linear dimensions) imaginable, but one example is shown below. These tolerances are usually listed as a schedule or table near the title block.

0 >   20      ±0.1

20 >   80      ±0.2

80  > 120      ±0.4

120  > 200     ±0.6

I do tend to caution against an over reliance on general tolerances, simply because as general tolerances they are not tied to the functional requirements (failure modes) of each dimension. Considering the maximum allowable tolerance for each dimension may be more work for the designer, but manufacturing and purchasing will appreciate the difference in cost and lead times.

Michael Adcock
ASME GDTP Senior Level
Dimensional Engineering Mentor

In the past, I’ve used basic dimensions for the major and minor axis, a profile control, and a note that says, “TRUE ELLIPSE,” but some metrology labs don’t have the capability to measure ellipses with their CMM.

Answer
Generally speaking, an ellipse is not a true feature of size because it does not have one size dimension that can apply to all opposed surface elements, like a pin, hole, or plate thickness.

You can have two different size dimensions for the major and minor axis, but these size dimensions will not control the surface elements between the major and minor axes. Basic dimensions applied to the opposed surface elements at the two axes and basic radii are best, because then you can specify a profile of surface tolerance applied all around to control the overall size and shape of the ellipse. Datum references can be added to the feature control frame to also control the location and orientation of the ellipse.

Most CMM’s are capable of measuring profile. You can reference the 3D CAD model in lieu of the 2D basic dimensions if the CMM has the capability to measure relative to the 3D model. If a metrology lab is telling you that they don’t have the capability to measure a profile tolerance, then they may be using an older CMM with outdated software or the CMM operator may not have been properly trained on how to correctly measure geometric tolerances.

Roy Cross
ASME GDTP Senior Level S-0488
Dimensional Engineering Mentor

GD&T has been around for about 50 years, yet some people resist using it. What are some of the main reasons you’ve heard from people who are against using GD&T?

If you have any input, I’d love to hear it. You can reply here or join my linkedin group.

- Alex

I am having a “discussion” with a customer about a part that was rejected because they measured a feature as out of tolerance that is not dimensioned on the print. The part is symmetrical, and the corresponding feature on the other side of the centerline is dimensioned and within tolerance. There is no indication, notation or definition on the print that states any symmetry requirement.

Is there a GD&T rule that defines or requires symmetrical features to be dimensioned? Is there an implied requirement to assume that any undimensioned feature is the same as its mirror? Would this be equivalent to scaling the drawing? By the way, there are other symmetrical non-dimensioned features on this part that they chose not to measure.

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Reply:

Let me make sure I understand. The part appears symmetrical about a center line, not a symmetry line.  (A symmetry line looks like a centerline with two short parallel lines drawn perpendicular across the centerline at both ends of the centerline.) The feature on one side of this centerline is dimensioned as an individual feature (not as a groups of features such as 2X, 4X, ALL FEATURES MARKED “A”, BETWEEN A AND B, etc.)?

I’ve made an illustration to help readers envision the drawing.

If my understanding is correct, then there truly is no specification, and the feature cannot be out of tolerance when no tolerance is indicated. It looks like your customer has let their assumptions get the better of them, a common problem when standards are not carefully followed.

Best of luck with your “discussion.”  Let me know if ETI can be of further assistance.

Michael Adcock
ETI Dimensional Mentor

The GD&T/Dimensional Engineering group was formed to discuss real world cases and practical solutions related to the topics of engineering drawing standards, GD&T usage, inspection, design analysis, and tolerance stack-ups.

Topics include ASME Y14.5M-1994, ASME Y14.5-2009, ISO GPS, tolerance stacks, gaging and measurement, and GD&T training. If we find the need, we’ll make the most-discussed topics into their own subgroups.

Please take advantage of my open invitation to my fellow GD&T professionals and connect with me on Linkedin.

Join the group.

Ipad

More than a third of the respondents to our poll here and on Linkedin said they prefer the Ipad format for reading an electronically formatted textbook. Most agreed that the larger color screen would be better for reading the detailed charts and graphics that are usually a staple of the technological textbook. The second runner up is a PC or laptop, for the same reasons.

However, many people said they still prefer the old standby, paper.

Very few people thought the usual book readers would make the grade for textbook reading, and most even thought the PC or laptop would be too tiring for the eyes.

The verdict is still out regarding ETI’s plans for digitizing our new ISO reference guide and future textbooks. Thanks to all who helped out by answering our survey.

We’ve gone from print catalogs to the etinews.com website, which enjoys more than 10,000 visits per month. We’ve upgraded from post card marketing to email marketing. Our training has evolved from self-study workbooks to videos, computer-based, web-based, and live web training. Our instructor materials moved from power point slides to digital instructor kits. We’ve created digital forums, message boards and enewsletters.

All in all, we’ve enjoyed embracing the benefits of the digital age, while we’ve kept our company and products on the cutting edge.

It’s time to decide which e-format to use for our new reference guides and textbooks. Many of the new digital readers work well for novels, but not so well for textbooks with figures, graphics, and charts. If you already read ebooks, or if you’ve been planning to start, we’d like your opinion…

A few years ago, I decided to put together some type of graphic that would organize and illustrate the many levels of workshops that ETI offers. It needed to include the prerequisites for each course and some way to easily visualize the entire  GD&T training hierarchy. The GD&T Training Pyramid was the result. (Click on the graphic above to access a larger view.)

The pyramid allows you to see all the training levels, as well as the basic courses – or the foundation – upon which your GD&T education should build. We often find that students attempt to master a higher level course without fully understanding the basics. For example, if you don’t completely understand the requirements of engineering drawings, you won’t be ready to learn GD&T fundamentals.

The pyramid also incorporates the valuable concept of mentoring employees. Once a workshop is over, mentoring allows employees to utilize their newly attained GD&T knowledge to its full potential on the job. The gray areas in the pyramid signify those areas where I advise that the mentoring be implemented.

In these tough economic times, companies often cut training budgets first, but allowing ETI to assist them in incorporating a mentoring program into the workplace can often mean valuable dollars are saved through the best use of manpower and materials.

In future blogs, I’ll go into more detail about each of the pyramid’s levels. If you’d like your own copy of the GD&T Training Pyramid, click here.