Cylindricity : Combination of straightness + Circularity? 1

[Madhu454]

Hi All,
One basic question, if a shaft is called for circulaity tolerance of 0.1mm and staightness called out for surface elemnts which is also 0.1mm, then can I replace that with a single callout of cylindricity? Does both give the same results?.

 

[pmarc]

No, these are not equivalent callouts.

Imagine a perfect cone. All linear elements of its surface are straight, all elements in each cross-section are round (circular), but the cone is not cylindrical.

There is one aspect of cylindricity that is not covered by combination of roundness and straighntess - it is a mutual parallelism of diametrically opposed linear elements of feature's surface.

 

[fsincox]

nice example! pmarc

 

[Madhu454]

Thanks pmarc, it is clear now.

 

[leeekim]

I'm a little confused with that definition. Isn't cylindricity a composite tolerance of circularity, straightness, and taper? If so, could we not use cylindricity to tolerance a cone?

 

[Belanger]

No, Leekim -- a cylinder is defined as a circle extruded straight back into a third dimension. So by definition a perfect cylinder could never be a cone.
You are correct that a cylindricity tolerance controls circularity, straightness, and taper, but we use that last word in the sense that it prevents (or limits, within the given tolerance) the taper.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[leeekim]

Thanks for the clarification.

 

[fsincox]

One of the things I see in the standard, particularly with the form controls is a reference to the image of perfection, the perfect staight line, the perfect round circle, the perfect flat plane and the perfect cylinder. The perfect cylinder is not tapered.