fair warning: if you’ve never heard the phrase “tolerance analysis”, you’ll likely never have to perform it and should just spend the next 5 minutes elsewhere. Otherwise…
A well-performed tolerance analysis will add years to your life.
No worrying about parts fitting together, no worries about sloppy fits. A well-done tolerance analysis both proves the design and helps communicate which dimensions are really important to the manufacturer.
It’s a good place to be.
But where to begin? The easiest place to start is the worst-case method.
the worst-case method
A worst-case tolerance analysis method is simple arithmetic (that’s right… just addition and subtraction), so lets start there.
For example, let’s say we have a block.
The block is is 1.000″ thick, and the (honest) guy selling it to you guarantees accuracy within ±0.005″ (5 thousandths of an inch). Thus, you are guaranteed that the part will be between 0.995″ and 1.005″.
Simple enough? Great. You just performed a worst-case tolerance analysis of that part.
Easy right? You bet.
But what happens if you stack, lets say, five blocks?
The five 1.000″ blocks add up to, no surprise here, 5.000″. But the tolerances stack up as well, to ±0.025″.
Now things are becoming less precise. When we had one block we knew the thickness within 0.010″ (0.995″ – 1.005″). Now that we have five blocks, our range has ballooned to .050″ (4.975″ – 5.025″).
sidebar: 0.050″ is just under 1/16th of an inch. If that seems negligible to you, just look at the sticky gas pedal problems Toyota has been having lately. This stuff is important, especially when it comes to mechanisms.
If you want to determine a window inside which every last one of your block assemblies will land, you are done. Just design everything else around the fact that this block assembly will be off by ±0.025″. Either that or go back to the block salesmen and shell out a small fortune for more accurate blocks.
If, on the other hand, you lack the small fortune for precision parts, the ambivalence to accept such a sloppy block assembly, or both, you do have a recourse:
You need to delve into the world of Root Sum Square (RSS) tolerance analysis.
Related posts:
by email:
Comments on this entry are closed.
{ 1 trackback }