Roundness woes

[Big Red]

3.7 MR2 -

I have noticed, once in a while, that the ROUNDNESS (form) reports incorrectly. Today I checked a 2.000 inside diameter with a tolerance of +.0003 and reported the ROUNDNESS (form) and got a report of .0017.

I cleaned the ruby, I calibrated the tool, I increased the # of hits on the I.D. up to as much as 30 and each time I got ROUNDNESS at .0017. I am not shanking out and my part is clean.

Using a Dial Bore Gage, I get ROUNDNESS at .0001.

Is there a reason why ROUNDNESS will report out good sometimes and bad other times?

Thanks for your help in a ROUND about way...

[cmmguy]

If your hole is tri-lobed the dial bore gage wont see the error, but the CMM will.

[Matthew D. Hoedeman]

3.7 MR2 -

I have noticed, once in a while, that the ROUNDNESS (form) reports incorrectly. Today I checked a 2.000 inside diameter with a tolerance of +.0003 and reported the ROUNDNESS (form) and got a report of .0017.

I cleaned the ruby, I calibrated the tool, I increased the # of hits on the I.D. up to as much as 30 and each time I got ROUNDNESS at .0017. I am not shanking out and my part is clean.

Using a Dial Bore Gage, I get ROUNDNESS at .0001.

Is there a reason why ROUNDNESS will report out good sometimes and bad other times?

Thanks for your help in a ROUND about way...

What probe system are you using? TP2? If so, I can see that very easy. TP2 is not a good choice for roundness checks.

[Wes Cisco]

What probe system are you using? TP2? If so, I can see that very easy. TP2 is not a good choice for roundness checks.

The same applies to the tp20 and all of the old "mechanical" trigger probes. The tp200 and the probes that use strain gages do not have the lobing problems caused by three contact points.

[BOB PARSONS]

Tp2

I use tp2 all the time and my lobing error is usually less then .0002, a scanning
probe like sp-25 is better in this application though.

[JoeRZ]

Use Text and Graphic Analysis to get a good look at the surface.
Also try getting Radial distance to center of individual probe points.

JRZ

[Big Red]

My dial bore gage is NOT tri-lobed.
My probing system is SP-600 Scanning. No TP2's for the past 2 years.
Now what do you think my problem may be?
Thanks again in advance.

[Wes Cisco]

My dial bore gage is NOT tri-lobed.
My probing system is SP-600 Scanning. No TP2's for the past 2 years.
Now what do you think my problem may be?
Thanks again in advance.

J's point was that if your bore is tri-lobed or any odd number of lobes, a two point contact method, like a dial bore gage may not show it. If you have an internal tri-mic, it might. Do the graphic analysis and examine the radial location of individual points like Joe said. HTH

[craiger_ny]

I have seen out of round parts check quite round with 2 point gage because of tri-lobing of the part like J said. Like these other guys said do a graphical analysis on a zero tol roundness callout and kick up the arrow multiplier. You'll see what they mean (if that is in fact the issue). How were the parts made and what type of workholding?

Craig