To what extent should a company comply with ASME standards on their drawings?

ASME cutSome time ago, a visitor to SolidWorks Legion asked something similar to this:  Now that we decided to use them, to what extent should my company comply with the ASME standards on our drawings versus our own internal rules?

That is a complex and difficult question.  Purist will say, “Follow the standard exactly! Otherwise, why have a standard at all?”  Internal traditionalists will say, “We already have a way that works for us.  Why change what works?”

The answer, in my opinion, is in the middle.  ASME Y14.100-2004 paragraph 1.1 states that the ASME standard establishes essential requirements for the creation and revision of drawings and BOMs.  However, paragraph 1.2 then allows for “tailoring” of the standard to exclude unnecessary requirements.  Though this is not an explicit statement that allows outright customization, it does provide a crack in the door that may be used to justify such customization of the standard.  It is important to note that the ASME standard does not take the place of internal standards; it forms their foundation.  The ASME standard still leaves options open for individual companies to define for themselves.

In a company’s internal drafting standard, I recommend including the statement, “Exclude from practice any portions of any standards (e.g., ASME Y14.100) that differ from instructions within this document.”  This formalizes the effort to employ exceptions to the ASME standard.  However, this must be used with caution.

One area that is a good case for exceptions is in how a company might handle BOMs within the context of a PLM.  In such cases, it is often considered bad practice to list BOMs in two places (on the drawing and within the PLM).  It may be advantageous to store and control the BOM within the PLM, rather than on the drawing.  ASME does not address this.  However, as long as the PLM displays the BOM in a manner consistent with the intent of ASME, I don’t personally see any issue with relying solely on that PLM for BOMs.  The internal drafting standard should address such exceptions to ASME.

An area that is bad for exceptions is in the non-standard use of established symbols or abbreviations.  This is because the symbols and abbreviations are already defined by the ASME standards.  For example, if a company allows GD&T symbols to be used in a way that is not defined by ASME Y14.5-2009, a manufacturing vendor will not know how to properly interpret the custom use of the symbology.  ASME does not allow ambiguity on drawings.  However, if a company wishes to continue the use of a few of its own custom symbols and abbreviations, these need to be fully defined on each drawing or in an internal document that is referenced by the drawing.

In my opinion, this is the bottom line: leverage the ASME standards to save time and work (both in the creation and interpretation of drawings).  Try to adhere to ASME as much as possible.  Allow deviations that are necessary, but clearly state such exceptions within the company’s drafting standard or on the drawing itself (whichever works best for the situation).

How to dimension feature patterns on drawings

This entry is part 3 of 8 in the series Dimensions and Tolerances

A couple of days ago, I briefly covered the mythical specification “non-accumulative tolerance” (or “non-cumulative”) as it is often applied to direct dimensions on feature patterns.  See the example in Figure 1 where the dimensional callout attempts to simply dimension a pattern without considering tolerance stack-up.  However, this attempt fails since any two non-adjecent holes cannot avoid accumulation of tolerance due to the dimensioning scheme.  The problem gets worse if three or more positions within the patten are compared to each other.

Non-accumulative tolerance dimension on a pattern
Figure 1

ASME repetitive feature dimensioning scheme

ASME Y14.5-2009 actually provides a linear method to detail feature patterns, called repetitive features and dimensions.  See Figure 2. Unfortunately, the standard does not provide any tolerance rules for its prescribed scheme. Presumably, this leads us to interpret a repetitive feature dimension as though it is shorthand for chain dimensioning.  Chain dimensioning accumulates tolerance as the pattern departs from the dimensioned start position.  Sometimes this is OK, but often this is unacceptable since the accumulation of tolerance can quickly lead to features that do not align to mating features on other components.

Figure 2
Figure 2

Disorganized direct dimensions

Another dimensioning scheme that I’ve seen involves a complete disregard for the fact that a pattern exists.  See Figure 3.  Directly dimensioning each of the positions within the pattern to each other may be acceptable in some scenarios, but likely isn’t a very clear choice for larger feature patterns.  The problem with this scheme is that it can be very difficult to determine the true accumulation of the tolerance stack-up.  It may also be difficult to determine design intent.

Figure 3
Figure 3

Baseline dimension scheme

To avoid the issues associated with other direct dimensioning schemes, one may choose to use baseline dimensioning, which may also be called rectangular coordinate dimensioning in some scenarios.  The advantage of a baseline dimension scheme is that it limits the accumulation of tolerances to the stake-up from just two dimensions.  This is because the total stack-up between any two positions within the feature pattern are related through a common baseline.  The problem with baseline dimensioning is obvious in Figure 4; its take up a lot of space on the drawing.

Figure 4
Figure 4

Ordinate dimensioning

A common alternative to baseline dimensioning is ordinate dimensioning, also known as rectangular coordinate dimensioning without dimension lines.  This scheme also relies on a baseline, referred to as zero (0), from which all of the features are dimensioned.  The advantage of ordinate dimensioning is that it takes up far less space on a drawing, as shown in Figure 5.  Tolerance stack-up is limited to just two dimensions between any two positions within the pattern.

Figure 5
Figure 5

Using GD&T for best results

The best way to avoid accumulation of tolerances is to use a methodology that does not rely on any form of direct dimensions.  ASME Y14.5 actually suggests that GD&T should be used instead of direct dimensions to locate features.  I have discovered the hard way that many individuals in the engineering field have an irrational fear of GD&T.  Even still, GD&T provides a far superior method for the location of positions within a feature pattern. The example in Figure 6 shows a less cluttered drawing.  With the addition of MMC to the feature control frame, this method could provide even better results since it would make use of bonus tolerance.  The position of each feature within the pattern has an optimal tolerance zone that more closely matches design intent.  One more added benefit is that all features controlled by a single feature control frame are automatically considered as a pattern.

Using GD&T to locate features
Figure 6

Since the tolerance zone is optimized, using GD&T may help reduce costs by allowing the manufacturing process to vary in a way that is more in line with design intent.  In turn, this can reduce the number of unnecessary part rejections.

Conclusion

When detailing feature patterns, one may wish to avoid the use of direct dimensioning methods or shortcuts like the mythical “non-accumulative tolerance”.  The best choice to detail a feature pattern is GD&T.  However, if GD&T is not desired, the next best method is prolly an ordinate dimension scheme.  It should be noted that for each of the dimensioning and tolerancing schemes shown within this article, there are a variety of ways to implement them.  This article is meant to present general examples.  Actual tolerancing requirements are guided by design intent and other considerations per individual cases.

DraftSight ends Beta; general release announced!

The big news from Dassault Systemes today is that they just announced the general release of DraftSight for Windows.  DraftSight is a no-cost 2D CAD application for CAD professionals, students and educators which allows them to create, edit and view .dwg files.  The very long beta release of the Windows version has ended with an impressive 400,000 450,000 downloads (as of 5:00PM ET on February 22, 2011) from the DraftSight website.  In a conversation I had with Aaron Kelly, Senior Director of DraftSight at Dassault Systemes, he stated that this number is “a lot more than we expected.”  He added that the total number of full DraftSight activations is over 95,000.

Language support

Also according to Kelly, since the beta 3 release, the DraftSight user interface has simultaneously supported 14 languages at once (English, German, French, Japanese, Simplified Chinese, Traditional Chinese, Spanish, Portuguese, Italian, Turkish, Korean, Polish, Russian, Czech).  As of today’s general release, DraftSight will now also support these languages in the Help file, meaning that DraftSight is now fully localized (all 14 versions will be released at once for each update).

DraftSight is community driven

Free support, training and enhancement requests may be conducted through the DraftSight online community, based on Dassault Systemes’ SwYm online collaboration and social innovation platform.  SwYm communities have profiles, blogs, micro-blogging, “iQuestions”, wikis, media sharing spaces (data, audio, video, and even 3D), status updates, and more all within one online user interface.

Other updates in the general release

API support for DraftSight is now available, for a fee.  This enables users to write add-on programs for DraftSight in C++ and other supported programming languages.  The addition of Command Variables Enhancement allows users to set and change system variables directly from the command line.  Aaron Kelly adds, “we fixed a bunch of bugs that people reported”.  He also stressed the value the user base in improving DraftSight.

Service options

DraftSight offers a variety of support options, including no-cost community support (mentioned above), as well as fee-based Premium Services that may include telephone and email support, network licensing and access to DraftSight APIs.  There is now the Education Premium Service for educators which includes curriculum materials, network licensing and telephone and email support.

SWUGN reaches 200

Old SWUGN headerRichard Doyle announced today that the number of SolidWorks User Groups  in the SolidWorks User Group Network (SWUGN) has reached 200 worldwide.  His announcement may be found on the SolidWorks Community Blog in the SolidWorks Forums.  In a forward looking statement, Doyle predicts, “There are at least 2 more groups coming on line in the next few days.”

Mythical Specifications: Non-accumulative tolerance

This entry is part 4 of 4 in the series Mythical Specifications

Mythical specifications come in many forms.  One realm they seem to haunt is that of repetitive features, also known as patterns.  Many attempts to shorthand pattern callouts are continuously made.  Bad habits die hard as old mistakes are passed down from one generation of engineers to the next.  One particularly bad habit is the use of linear dimensions with the term “NON-ACCUMULATIVE TOLERANCES”, or something similar.  There is no such thing.  Trying to use this shorthand leads to tolerance issues.

Pattern non-accumulative tolerance callout

In the example above, the dimensional callout attempts to simply dimension a pattern without considering tolerance stack-up.  However, this attempt fails since any two non-adjecent holes cannot avoid accumulation of tolerance due to the dimensioning scheme.  Tolerance stack-ups on linear dimensions have accumulation.  There’s no way to avoid it without dumping linear dimensions.

I had originally planned on a short article about this topic.  However, once I started delving into it, I found out that there is a lot of ground to cover.   So, this topic will be addressed in detail within a future article (Feb 23, 2011) where examples of different pattern dimensioning schemes will be explored.

Also see How to dimension feature patterns on drawings.