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Geometric tolerance: Circularity and Circular runout Question
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Posted by: Harsimran

02/04/2009, 07:31:40

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Hi GDT experts

How do I measure Circularity and Circular runout tolerances in my tool room , I dont have a CMM, but I have a Granite surface and blocks and nearly all measuring instruments.
The part in question is a simple one . Its a shaft with one step in diameter. Larger dia is 50mm and 100mm long, smaller dia is 25mm and 25mm long. Circularity on smaller dia is .13 and circular runout on smaller dia wrt to larger (as datum) is also say .13
This is a hypothetical part.

Thanks
Harsimran








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: Geometric tolerance: Circularity and Circular runout
: Geometric tolerance: Circularity and Circular runout -- Harsimran Post Reply Top of thread Engineering Forum
Posted by: traingdt

02/04/2009, 09:15:47

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Circularity is one of the hardest geometric tolerances to check, because it is tolerancing the shape of the part separated from all other qualities like size, position, etc. Therefore, using traditional gage tools always comes up short. (For instance, if you use a micrometer, that only gives you the distance across two points; the part could be an octagon and the micrometer won't know that.)

The best way to check circularity is with a CMM or with a special machine dedicated to circularity called a roundness machine. You're limited to traditional tools, so I suppose you could implement a couple of methods simultaneously, and using a composite of the different results, it can be determined to some degree if the circularity is correct.

But, here's the easy answer: since your hypothetical example has circular runout and circularity on the same feature (both to .13), you can easily check the circular runout, and the circularity is automatically verified!

To check circular runout, mount the part on datum feature A -- use a chuck or collet to hold the large diameter. Then rotate the part. As it rotates, have a dial indicator mounted such that it rides the surface of the small diameter, at any randomly selected cross-section, over one revolution. The needle shall not deflect more than 0.13 from its highest to lowest reading during that revolution. Then move the dial indicator over a little, and repeat the check. (How many cross-sections to verify is not prescribed -- it's a judgement call depending on the material of the part and its length; but do as many as practical).

Hope that helps!







Modified by traingdt at Wed, Feb 04, 2009, 09:18:02


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: : Geometric tolerance: Circularity and Circular runout -- traingdt Post Reply Top of thread Engineering Forum
Posted by: RWOLFEJR

02/05/2009, 09:15:07

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For circularity a Tri-micrometer is what is typically used but they too can give you false information. A good example would be when centerless grinding. If the position of your grinding wheel to your support blade isn't right you can grind a three sided circle. It can measure to correct dimension but if you check with a ring gage that has allowance for encroachment it could still be big... or your bearing won't go on without heavy press etc.
With a window as big as you're talking on such small diameters I'm not sure you could exceed the allowance for cicularity with conventional machining methods without going out of your way.
Checking the runout can be done in your lathe etc., but you will incorporate the run-out of your chuck into reading. That run-out could improve or make worse depending on timing. Again... you have a big window to work with so probably not an issue. If you make up two cradles from a pair of precision bearings in them and nest your shaft into these then indicate you will eliminate the potential for adding machine run-out. You can check the accuracy of your cradles by indicating the bearings. You'll also be able to check concentricty fairly well by marking your highs or lows on each diameter. If you have a couple thousandths run-out in opposite directions your out four.







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: : Geometric tolerance: Circularity and Circular runout -- traingdt Post Reply Top of thread Engineering Forum
Posted by: Harsimran

02/05/2009, 09:05:05

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Thanks a lot, I never came across en engineering forum so quick and exact on the answers.
Kelly, your team is doing a great job !!!!!and u too.......

I am sure to bombard you with more questions........








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Posted by: randykimball

02/10/2009, 09:44:45

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Rotate the part in VEE blocks with indicators at the horizontal and vertical positions. This will show run out and out of round.




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