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# Beginner question cylindricity GD&T

## Beginner question cylindricity GD&T

(OP)
Hello,

Very beginner question but say I were to dimension an O.D. diameter with a strict tolerance, e.g.: Ø60 +/-0,02.
Would it then be redundant to add a, for example, 0,03mm cylindricity GD&T tolerance to this diameter?

### RE: Beginner question cylindricity GD&T

Hello pantr,

I don't thing so. Since the cylindricity is also about concentricity of multiple circular cross sections, not only about the diameter dimension, it would not be redundant from my point of view. Hope this helps you. However, a question would hang in the air: why not add only a cylindricity tolerance equal to 0,02mm?

Best regards,

JeSuisPasBourbaki

### RE: Beginner question cylindricity GD&T

(OP)
Hi JeSuisBourbaki,

Thank you for the reply. The 0,03mm cylindricity tolerance was purely an example. The Ø60 +/-0,02 dimension would give absolute values of either Ø60,02 or Ø59,98. The cylindricity GD&T of 0,03 would mean the entire cylinder can have an unroundness of either Ø60,03 or Ø59,97. So my guess would then be that the cylindricity GD&T would be redundant. But I'm not sure if I'm correct in this assumption.

### RE: Beginner question cylindricity GD&T

The cylindricity reduces the range of variation of acceptable diameters; it can never be larger than the size limit. Cylindricity tolerance limits the gap within which the part surface can exist - if on element of the surface is to one side of the envelope then the farthest in the opposite side can be is 0,03; per the size tolerance that gap can be 0,04.

### RE: Beginner question cylindricity GD&T

pantr,

Cylindricity is controlled by your diameter tolerance. You apply cylindricity when you have a sloppy diameter, but require something accurately round. In forty years doing mechanical design and twenty years trained in GD&T, I have never applied cylindricity.

--
JHG

### RE: Beginner question cylindricity GD&T

pantr: The Cylindricity symbol is used to describe how close an object conforms to a true cylinder. Cylindricity is a 3-Dimensional tolerance that controls the overall form of a cylindrical feature to ensure that it is round enough and straight enough along its axis. Dimensional (i.e., diametric) tolerances are a crucial part of the design and manufacturing process. A tolerance is a numerical range of measurements assigned to a part's dimensions indicating how much a manufacturing team can drift from the nominal measurement.

What this means is that your dimensional tolerance example means the cylinder can be true, but can have a range of allowable diameters. Adding cylindricity means that the toleranced surface can only vary within the cylindricity tolerance from a true circle - over its entire length.

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### RE: Beginner question cylindricity GD&T

Depends on process. If for e.g. a shaft is centreless ground, an equidiameter geometry can be produced.
The only way this can be measured is on a roundness checking machine.
Hence circular (aka roundness) tolerance depends...
A UK 50p coin is a seven-sided equidiameter figure https://en.m.wikipedia.org/wiki/Fifty_pence_(Briti...)

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