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.