Diamater

[B. Jacobs]

I hope this is a simple question.
I am measuring the diamater of an OD thread. I have the auto circle selected for this measurement and also the pitch. The problem I am coming up with is that the measured diam. is smaller than when I measure it with calipers. Is there something I am not doing right? I am using a 4mm ruby tip. I have tried both pitch and no pitch.
The diam should be .618 +/-.004. It has been measuring .6143-.6147. With calipers I get .6175-.6183.
Any thoughts

Thanks

3.5MR2

[Underspec]

I would re-calibrate that tip just in case.

By default PCDMIS calculates best fit circle using lease_squares. Depending on what you want you might want to change this. Reference the help file for the definitions on the other calculations.

[BSchwark]

I have found that measuring threads can work for me as long as there is a generous tolerance. I only will use the location and measure the diameter by another means. To hard to tell if you are measureing exactly on the major or minior diameter (Small threads).

BS

[John Kingston]

Definitely not the way to measure the major diameter of a thread. You would need to know if the lead thread was in the same spot every time to get this right. Maybe use a cylindrical probe would help, but there are issues there. I mention this all the time, but the CMM is not always the best tool for certain measurements, it is just another tool like comparators, mics, height gages, indicators, etc.

[craiger_ny]

Unless you are kissing the major diameter DN you will not get the same as your caliper. Use an input dimension to report diameter and capture it with a variable then you can incorporate it into your final dimension for that feature.

Craig

[smith4]

ditto BSchwark

[bw_bob]

I use whats called a shank probe when measuring threads.

[#2#]

I use whats called a shank probe when measuring threads.

This works but if your part isn't parallel to the probe you can get some funky numbers.

[B. Jacobs]

Thanks for the replies. I will play around with it and try different tips.

[#2#]

Thanks for the replies. I will play around with it and try different tips.

If you absolutely want to use the CMM you could measure the circle with pitch at one depth, then re-measure at a little different depth, then re-measure at a little different depth...

Then pick the biggest diameter and you could get pretty close depending on how many depths you measure.

[Matthew D. Hoedeman]

Measuring the major diameter of a thread with a CMM is not a very good idea. You CAN use a barrel probe (YUCK) but you have to make **** sure you are 100% square to the shaft of the threads or your answer will not be correct. As for doing it with a ball probe, unless you can somehow get it to touch on the VERY SHARP tip of the thread for each touch, you will not get a good value for the diameter. As long as you use the pitch function, the location should be good, but the diameter, no way. The CMM is not the correct tool for measuring everything you can think of. Other tools do different jobs better. In this case, a set of micrometers is better than the CMM.