Tolerance Stacks

How to Factor Bonus Tolerance into a Tolerance Stack

Posted by on Aug 9, 2008 in Tolerance Stacks

For engineers who regularly perform tolerance stacks, handling regular dimensions is pretty straightforward. And even when GD&T is involved, there is usually not much difficulty, until one encounters the MMC modifier. How can the effect of this “M” symbol be accounted for? First, a primer on what the effect of the MMC modifier is. Suppose the example shown below is applied to a pin: The “M” symbol tells us that the given position tolerance of 0.5 applies if the pin is made at its maximum size of 11.8 mm. But if the pin is made at any size less than that, then the position tolerance gets a corresponding “bonus” tolerance. Thus, each part that is made gets its own customized geometric tolerance.  (Example: a pin made at 11.5 gets a position tolerance of 0.8, and a pin made at the smallest size of 11.2 gets a position tolerance of 1.1 mm.) The advantage to this system is that some parts that are made will get more positional tolerance, while still ensuring that those pins will assemble with the mating parts. With two-column tolerance stack calculations, then, we must be careful.  Without the “M” symbol, we would simply add a line item in our stack to account for the 0.5...

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What’s the Best Tolerance Stack Method?

Posted by on Jan 16, 2008 in Tolerance Stacks

When it comes to calculating tolerance stacks, there are many different methods. Some people simply take blank paper, then make a quick sketch and scribble out some numbers. While that may work for a very simple stack, it’s obviously not a very methodical approach! A better way is to use Excel or even special software for tolerance stacks to calculate an answer. Even when using Excel, there are two schools of thought for performing a stack: Some folks prefer to use a stack method that separates everything into an absolute maximum or minimum dimension, and then create two columns to stack the max and min results.   Others insist on translating everything to a nice symmetric plus/minus tolerance, centered around a nominal. Both methods will work — it often comes down to how you were taught to perform stacks. Here’s my two cents’ worth: I prefer the max/min method, especially when GD&T is involved. Because geometric tolerancing is based on limits (think about MMC and bonus tolerance) it is usually easier to plug those into a spreadsheet. For more information check out the Tolerance Stacks course that we offer. Most of the class is devoted to this max/min method, but we also teach the plus/minus method as an alternative....

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