Mythical Specifications: Non-accumulative tolerance
Mythical specifications come in many forms. One realm they seem to haunt is that of repetitive features, also known as patterns. Many attempts to short-hand 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.
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.
Table of contents for Mythical specs
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Comments
Nathan, the example in the above image needs to be avoided, as it is impossible. There are effective tolerancing methods that do not take up a lot space on the drawing. You mentioned ordinate dimensions. This normally doesn’t take up a lot of room on a drawing, and is actually an achievable spec. But there’s another superior option I’ll talk about in the next article (tomorrow).
Mythical Specifications: Non-accumulative tolerance http://goo.gl/fb/9Cnfd #solidworks #asmestandards #drawing
[...] 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. Figure 1 [...]
It has been my practice to put the “non-cum” note not on the hole to hole dimension but instead on an additional overall dimension from the first pattern hole to the last.this will force the holes into proper tol. over the entire pattern. Is that acceptable practice
Well, as noted above, there’s no such thing as non-cumulative. However, your method is similar to calling out a pattern, which kinda has the same issues (ambiguity on howto apply tolerances). I cover this in more detail on a later article: http://www.fcsuper.com/swblog/?p=2440
If I were to use a pattern callout like that, I would do so instead of using baseline or ordinate dimensions for every hole in the pattern, which themselves are an attempt to reduce tolerance stack-up between holes. I have only ever used callouts similar to that one in areas where I needed to save space on the drawing sheet.