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I am familiar with the basic concepts of fits and tolerances, and I am aware of the various online calculators for determining them. However, I don't understand how to apply those numbers to my actual CAD design for the parts that I want to fit together. None of the calculators seem to account for the actual manufacturing tolerances of the shop I'm working with (in this case +/- 0.1 mm).

So, to be more specific, let's say I have a shaft and hole and I want to work with a nominal size of 6 mm. I want a "locational clearance" fit between them. The calculators tell me the maximum and minimum hole sizes, etc., but I don't know how to gather from this information what I should make the nominal sizes of the hole and shaft in my CAD model. And how can a calculator tell me that anyway, if it doesn't know what size my manufacturing tolerance is?

Can anybody elucidate?

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    $\begingroup$ You didn't specify the type of fit: clearance, transition, or interference? $\endgroup$ – Jules Manson Dec 28 '15 at 0:05
  • $\begingroup$ Locational clearance is a clearance fit. $\endgroup$ – Ryan Norbauer Dec 28 '15 at 9:00
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I think you may be getting a little confused here. There is a fit tolerance, which determines how easy it is to insert and rotate a pin in a joint, and then there is machining tolerance, which is how accurately a shop can match your nominal values.

If your shop can't meet your printed fit tolerances, you're using the wrong shop.

That said, if you must go with a particular vendor (3d printing, for example), and you have no other choice, then make your nominal value the mean of your MIN and MAX values and then change your tolerance values.

So, for an easy example, say your hole should be 6mm +/- 0.05mm. Find the MIN (6-0.05 = 5.95mm) and the MAX (6+0.05 = 6.05mm), and average them - 6mm. Tolerances remain unchanged and this was a trivial problem.

Consider though 6mm +0.1mm -0mm. Now do the same, MIN=6mm, MAX=6.1mm, new nominal value of 6.05mm, and new tolerance of +/- 0.05mm.

It's functionally the same spec, but the "new" format would get you some dirty looks at a machine shop. However, now you can compare your needs: +/-0.05mm, to the shop capability: +/-0.1mm, and can see that they cannot meet your tolerancing needs and are thus unlikely to produce a satisfactory product.

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  • $\begingroup$ Thanks! If I'm inferring correctly from what you're saying, the proper approach when the shop is making both parts is simply to ignore the shop's machining tolerance and to design both shaft and hole at a nominal size in CAD and then specify via some other communication means to the machinist the type of fit one wants between two parts? But what if you're designing for a situation where you have a pre-manufactured shaft with measured dimensions from it and you want to design a hole to accommodate it with a certain fit? Give it a nominal size that is an average of the MIN and MAX values? $\endgroup$ – Ryan Norbauer Jan 4 '16 at 14:37
  • $\begingroup$ @RyanNorbauer - Correct; the machine shop should be able to meet whatever tolerance you give them, but you have to give them one. If you don't they'll default to whatever their general tolerances are, which may or may not be acceptable. If you think they can't meet your requirements, be sure to ask them specifically in writing or over the phone if they can when you get the quote. $\endgroup$ – Chuck Jan 4 '16 at 15:21
  • $\begingroup$ Regarding the pre-purchased part, I would use the published information on the part rather than trying to take measurements off of it. You can't expect to use a general purpose shaft in a high precision, low tolerance application. The shaft could be eccentric (not round), and your measurements could be bad, or it might not be straight and the bends don't fit through your hole, or it might have poor dimensional tolerance along the length of the shaft and you didn't measure where it was a different dimension. If you need a specific fit from a general purpose shaft, get the shaft machined. $\endgroup$ – Chuck Jan 4 '16 at 15:23
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Use nominal hole sizes in CAD for both the shaft and hole.

I'll also commonly used shaft and hole tolerance using English units because in the U.S. English sizes are still more common for industrial materials. Standard cold drawn 1/4 shafting has a tolerance of +0/-.002: http://www.mcmaster.com/#standard-steel-rods/=10hs6l4

For easy assembly, the hole tolerance should be .250 +.005/-.000. These can be converting into nominal metric values by 25.4.

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