What is the proper way to measure total runout 1

[sendithard]

Given the simple example picture below. Is the proper way to measure total runout to put both ends in a dead center on a lathe and indicate along the surface or is it to put the primary datum surface feature A in a v-block and rotate it to cheack the other surface? Is one method preferred over another, is one technically correct over another? Furthermore, does total runout mean runout from A's surface or A's axis? I assume that answer dictates the technical proper measurement technique.

 

[Belanger]

A datum is always a plane, axis, or point (or combination of these). Therefore, datum A is an axis, but it's established by fixturing/holding the large diameter. So mounting upon centers will not work, nor will a V-block setup. Instead, imagine a perfect cylinder that collapses around the large diameter until it touches the highest points around that surface. This can often be accomplished with a collet.
Now you've established a datum axis around which to rotate. Begin rotating the part, then use a dial indictor to essentially ride around the small diameter's surface, thus checking wobble. But because this is total runout, that dial indicator should be mounted on a rail so that it can go back and forth longitudinally (parallel to datum axis A). The result: you are spiraling around the entire surface of the small diameter, and the deviation on the dial indicator must not be larger than 0.030.

 

[TheTick]

Never a V-block.

If you want to use bell centers, you need to make them datums.

 

[sendithard]

Bel & Thetick,

Thank you for the input and feedback.