Engineering Guidelines for Selecting Mechanical Design Tolerances

As a mechanical engineer in an R&D lab I frequently ask myself, what is a reasonable tolerance to set on this part? A question that was once difficult and frustrating for me early in my career, as I had received little guidance on this topic prior to being responsible for producing exact answers!

I used resort to a common strategy among newbies and request qualitative dimensional requirements on drawings. Leaving notes like ‘this dimension is critical’ without further description. Doing my best to communicate the design intent with written notes, hoping that that would stand in for a numerical answer when asked for the range of acceptable measurements.

Not being formally trained in GD&T at that time, I was definitely communicating incorrectly on my drawings, but the bigger problem was that I had deeper unanswered questions. How does the cost vary for a selected tolerance range, what is achievable, what is easily repeatable? How does it compare for plastic parts if molded, printed, machined, or otherwise?

Ultimately what are reasonable tolerances to ask for in a given situtation?

It was clear to me that getting it wrong would cost my company money unnecessarily. Requiring tighter tolerances is always more expensive, so the goal is to allow part features to vary from perfect to as loose as you can get away with.

Tolerance_Costs
The important thing to note is that costs go up exponentially as tolerances (& surface finish) tighten.    Image credit to CNC cookbook

I had the questions, but rarely could I find straight answers. Even the revered machinery’s handbook only mentions the word ‘tolerance’ 42 times in the whole book and only provides recommendations regarding the sizing of shafts/holes to achieve specific fits.

Part of the reason it’s hard to find advice is that the answer is nuanced and varies with your specific situation. But I think the following things are key to understanding tolerances.

#1. YOU have to figure out for your specific design how far off the dimensions can be from perfect and still end up with a functional part/assembly. But part of the difficulty is that small distances are not relatable in everyday life and are hard to visualize if you have never encountered such things before. (Just like listening to the news and hearing government spending numbers in the “-illions” that blend together but are vastly different.)

For that issue I’d recommend checking out the wikipedia orders of magnitude entry and this scale of the universe game. Between the two sets of examples, having comparative things in the scale you are working in helps to get a feel for the numbers. 100 microns is not immediately relatable, but it’s easy to understand that controlling dimensions within the width of a human hair (.1mm or .004″) is going to cost more than staying within the width of my pencil lead (.7mm or .028″).

#2. Understand that all measuring instruments have a positioning error associated with them that is linearly compounded over larger travels. Typical achievable part tolerances grow as a function of controlled dimension distance.

Take a look at the this set of general use tolerance selection guidelines. Notice how it has larger recommended tolerances for longer distances?

General_mechanical_Tolerances
Image credit to East West Manufacturing

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#3 You have to figure out how specific manufacturing methods & materials will affect the achievable tolerances. If you need more specific numerical lookup charts than the one above, refer to the list of manufacturing specific sources I’ve compiled below:

Note that the exact answer of what tolerances are achievable can vary between manufacturers because they set the rules based on their experience in their specific businesses! In any case always get multiple quotes to compare price, in case what’s hard for one shop is easy for another.

#4. Understand that you need to pay attention to your title block tolerances. If your cad system defaults to using 3 decimal places for all dimensions, you may be unintentionally specifying tighter tolerances in areas that don’t need it.

As a result the shop fabricating your part will spend more time and charge you more to meet those unnecessarily tight requirements.

tolerance_block
There is no standard set of title block tolerances I’m aware of. Every company decides on their own defaults. 

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#5. Understand that tolerances stack, and the way dimensions are drawn and which features selected for control will have an effect on the allowable finished geometry of the part.  The most obvious example is the accumulated variation possible when chain dimensioning.

chain_dimensioning
Image source from LinkedIN slideshow

It’s also worth noting that as the number of parts being manufactured increases, so does the need for performing variation analysis (VSA).  (A complex software tool for compiling a model of all the tolerances in an assembly and predicting if there will be tolerance stack up problems based on a specified distribution of part sizes. )

Well that’s all for now, I hope this article helps a young engineer in need!

 

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