# Help with fit tolerance



## birdtrick (Jul 23, 2020)

I'm designing two cylindrical parts that fit together tightly with hand force. My understanding is this would be a Locational Clearance fit. The nominal diameter is 94mm, and I am finding that a tolerance of 0.35mm is what should be expected for this diameter. If the smaller cylinder has an OD tolerance of 94mm +0.0/-0.35, then my calculation tells me the ID of the larger cylinder needs to be 94mm +0.35/-0.0, resulting in a fit range of +0.70 to 0.0 so that the lower end of the range isn't a negative value which would enter the light press fit category. But if the end result is near the upper limit of 0.7mm, wouldn't that be a loose fit?

I'm hoping somebody can give me some clarity on this... thanks..


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## mmcmdl (Jul 23, 2020)

Pick up a book on Geometric Tolerancing and Dimensioning .


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## birdtrick (Jul 23, 2020)

Yeah, I asked the wrong question..

I’ve actually done quite a bit of reading on GD&T and already know that a 0.7mm clearance on a 94mm diameter will be a looser fit than what I am looking for.

What I am really trying to figure out is how it’s possible to assure a Locational Clearance fit when the total interference/clearance range spans multiple fit ranges. So far I haven’t been able to grasp that in what I’ve read. As of now it seems not possible, but I know that can’t be true.


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## extropic (Jul 23, 2020)

I don't understand the nature of the problem. Maybe you can provide additional information.
Are you trying to work in conformance with a GD&T standard? If yes, specify the standard.
Since you capitalized "Locational Clearance" I'm assuming the term has special meaning to you.
It has no special meaning for me. Please clarify.
The easiest way to communicate the issue is to provide a graphic.

Since we don't have a graphic (yet), let's call the smaller diameter part the rod and the larger diameter part the cylinder. You say they are both cylindrical parts, so there are no additional interfaces that would constrain the radial fit. All you have to do is tolerance the diameters to achieve your desired fit (and within your time/cost constraints). "Locational Clearance" with regard to what?
What is the length of engagement of the rod into the cylinder?
What materials are you planning to use?


Welcome to TH-M.


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## benmychree (Jul 23, 2020)

All you really need to know is what clearance do you need or want, and machine to that fit; toleranceing is only necessary for mass production.


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## pstemari (Aug 15, 2020)

I wouldn't say that. The tolerancing is needed whenever you're having somebody else make a part. It's the contract that you're living with.

A different way to phrase the OP's question would be, "How do I write a contract to ensure that particular class of fit?" I don't remember the actual names, but if you put "ISO Class blah-bidi-blah fit" on the print, that would suffice. 

Alternatively, just build in an allowance and tighten the specs on either side. If you want a 0.40±0.20mm gap, spec the ID at 92.8±0.1mm, and the OD at 93.2±0.1mm. 

Really, though, the specs given in the standard should be fine, even if there's some overlap between adjacent classes of fit. The question is what is your real requirement: the actual gap between the two pieces, or how the two pieces feel when you put them together?

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## addertooth (Aug 16, 2020)

I would machine it a bit on the tight side, and then stone the ID until it had the amount of "press fit force" you desire (if it is a one-off prototype).  I wouldn't sweat the details unless you are planning to make a dozen or more. Both halves need to be at the same temperature when you do your test fit.  A freshly worked part will be hotter.


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## RJSakowski (Aug 16, 2020)

Writing a tolerance spec fir mating parts was something that I struggled with when I was designing parts.  Your two worst case scenarios come when the parts differ from nominal in opposing directions.  Using the rod and cylinder example above, if the rod is on the high side and the cylinder is on the low side, You will have a tighter fit.  The other way around, and the fit is loose.  This is particularly troublesome if the design is to have an interference fit where tenths of thousandths can make a difference.  Trying to control this can place undue requirements on the machinist and raise production costs.

My design philosophy was place my desired dimension as the nominal dimension and use assymetrical tolerancing. e.g 1.000+.0005/-.0000.for a bore and .9995 +.0000/-.0005 to indicate that my target was a .0005 clearance fit but up up to .0015 was acceptable.  It was interesting to see the reactions from various machine shops.  Some just said they would just go half way between the toreance extremes and call it good.  Others would respect your design intent and aim to hit the nominal.  

One particularly challenging part had a diameter of .2509" +.0002"/.0000.  It was made of PEEK plastic and had to make a pressure tight seal to a mating part good to several hundred psi but it had a restriction on the maximum insertion force.  The machinist was able to hold that tolerance, much to my admiration.

Here is a good reference source for various classes of fit.  https://www.engineersedge.com/general_tolerances.htm


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## birdtrick (Aug 17, 2020)

These last few posts have really hit the nail on the head of what I am struggling with. I went with the original values I posted and feel I should end up with the majority of the parts fitting correctly. We'll see...


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## pontiac428 (Aug 17, 2020)

At least you'll be able to hand-match your fits from the lot.  If you batch them for tolerance (A tight and B loose), you can arrange tight pairss as A-A and loose ones as B-B to get your desired fit.


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## extropic (Aug 17, 2020)

Since birdtrick is either unwilling or unable to answer any questions . . .
Does the term "Locational Tolerance Clearance fit" mean anything to anyone? If yes, please explain.

Edit: Clearance, not Tolerance

Edit2: Locational Clearance is a class of fit defined in ANSI B4.1, Standard Fit Designations. "Locational clearance fits are intended for parts which are normally stationary, but which can be freely assembled or disassembled. They range from snug fits for parts requiring accuracy of location, through the medium clearance fits for parts such as spigots, to the looser fastener fits where freedom of assembly is of prime importance."


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## birdtrick (Aug 17, 2020)

I didn't indicate Locational Tolerance fit. That wouldn't make any sense. I'm speaking of Locational Clearance, which can be found in the link that RJ included above.


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