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Tolerance Stack Analysis


Some of you engineers out there may have spent time calculating tolerances. Simple concept. In practice however this can become complex very quickly, depending on the number of parts and features involved. We know your pain!

Why tolerance stack analysis?
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In a perfect world everything would be made spot-on and would fit together perfectly. In reality we have to deal with imperfect parts, so the question is – how much ‘imperfection’ is allowable before things don’t fit together (and you need a bigger hammer). Tolerance stack analysis is essential to:

  • Assess the manufacturability of your parts and assemblies
  • Reduce risk! Reduce the chances of problems during production and assembly
  • Reduce manufacturing costs. Rather than tolerance everything to 0.01mm, be smart about the tolerancing scheme and still get great results.
There must be an easier way…
Fortunately there are inbuilt software solutions in several of the common CAD packages. Here is a brief overview of how TolAnalyst works in Solidworks Premium:

The steps to set it up are:

1. Define part feature tolerances and datum’s using DimXpert – this also pulls directly through onto the drawings so you only have to define the tolerances in one place
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2. Define the measurement you want to analyse. For example the concentricity of the axle bores
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3. Define the assembly order and constraints
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4. Output results. The worse case max dimensions between the bores is:
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5. Iterate the tolerances to achieve the required resultThe analysis shows you the percentage contribution of each tolerance, so you can identify the biggest factors on the tolerance stack and adjust them acoordingly
 

Very cool as it calculates both the RMS values (statistical distribution) and the maximum values (all tolerances at worst case). It also lets you see the the influence of assembly order and each feature tolerance on the overall stack.For more information about TolAnalyst checkout the Solidworks video here or read this helpful question and answer from javelin tech.At Motovated we’re big fans of making technology work for you, and this is one of those Solidworks features that many people may be unaware of. Why don’t you check it out for yourself or explore what sort of tolerance stack analysis is built into the CAD package you’re using and let us know how it works for you?

Happy tolerancing!

Graeme Howard
Design Engineer
Motovated Design & Analysis

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