Circularity vs Circular runout

[BeginnerHeattreater]

Can anyone explain the difference between the two and can they be used on a print interchangeabilty? I have a customer that is telling me that I have a problem with the roundness of a inner diameter. He wants to put a circular runout back to another datum (opposite end of part - inner diameter). Any thoughts?

 

[ewh]

Circularity simply means how round something is. Circular runout is how round relative to another feature(s).

 

[dingy2]

Roundness (circularity) is to itself and never to another diameter. Roundness is the difference between the inner and outer boundary on the cylindrical surface from a centre line and must be checked or confirmed 90 degrees to the axis. It is best checked on a divider head (chuck), digital indicator with stand. No micrometers here.

Circular runout is also placed on a cylindrical feature whose centre line is also shared by another diameter which would be the datum. One would place the datum diameter in a divider head (chuck)while the digital indicator is place on the highest point on the cylindrical feature. Rotate the part and report the TIR or FIM (depends where you live). It is a bit trickier than that but it oes confirm a combination of roundness and off centre condition. Roundness is a component of circular runout.

Dave D.

http://www.qmsi.ca

 

[PaulJackson]

Circularity is a form control. Its measurement has no relation to any datum reference other than the one constructed from its own surface of revolution.

Circular runout requires an axis derived by a feature or feature(s) for its measurement. Therefore Circular Runout controls the variation of the surface of revolution for circular form and location simultaneously and since there are infinite places along the axis that must conform it controls orientation as well. It does not control size so it does not control cylindrical form (a cone can have perfect circular runout).

Just cautions: If the two cylindrical surfaces are nearly coplanar and one has sufficient depth to establish a stable and repeatable axis for measurement then the circular runout callout may work well but if the two cylindrical surfaces are relatively shallow and widely seperated from each other (as I imagine from your post)then there are better ways to dimension and tolerance these surfaces relative to each other.

Paul

 

[Heckler]

Do you have a copy of ASME Y14,5M-1994? In the spec their are some good explainations of the useage of Circular Runout and Circularity. It really depends how this part is going to interface in the assembly and without knowing those specifics it's really hard to stear you in a direction other than give you the definition of both the terms.

Heckler
Sr. Mechanical Engineer
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