Questions regarding measurement of Roundness

[jonrya91]

My company is new to a tight tolerance ground bar. It's ISO H8 which is +0/-.0013" diameter, .00065" roundness. These bars are roughly 10-12 feet long.

I understand that the most effective way to do this is with a roundness tester. (Air bearing, stylus which plots the profile) Unfortunately, we cannot cut cross sections of bars to test randomly when it ruins the final length.

We are currently using a 60 degree v anvil micrometer. It's the largest reading minus the smallest reading. Since roundness is a radial measurement, is this the most accurate alternative? I've discovered that other mfg's use a v-block and indicator, but the readings are different. I attribute that to the 3 point mic thimble moving at a ratio, where the dial indicator is 1:1.

I just wanted some insight to what other engineers thought. I'm the only one here.

Thanks.

 

[BillPSU]

Lobing of the part being measured is one of the contributing factors to measurement errors when measuring a part with any micrometer. With a standard micrometer you are measuring the distance between the two anvils. With the 60 degree anvils you are measuring the distance between three anvils.

I would recommend you talk with an air gaging company to build air ring gage to measure the bar. The roundness tolerance is relatively large for the OD tolerance and if you are building a functional gage not an empirical measurement gage then this approach will work fine. The air gage will give you an average size reading for the bar but will not give you the roundness deviation. In most cases the OD size is really the controlling factor for the application.

Bill