Diameters

[AuRules]

I know this is been ask before when measuring a circle the postion is always good the diameters is always showing small. So is the any correct setting in the misc. box under the advance tab in a auto feature to check a circle are is this a common thing. Any help are info would help me shutup the operators.

[#2#]

The diameters show small compared to what?

Are these ID or OD?

How many hits are you using?

Is this a machined part? Casting? Injection Molded?

[Underspec]

What calculation are you using for the diameter? Diameters are set to calculate to least squares by default.

[dwade]

I had a disagreement with my boss over diameters a couple of months ago and it got settled by running an R&R I ran on a XX set ring. Ring was 0.5" and using the bore to level the part then measuring the ID with 5 points using LSQ. Out of the 15 times I measured the ID the CMM showed a MAX deviation of 30 millionths and a MIN deviation of 50 millionths.

Maybe I was lucky that time. I really don't trust CMMs much for tight tolerance holes but that kept him from saying the CMM can't measure diameters accurately.

[cmmguy]

I had a disagreement with my boss over diameters a couple of months ago and it got settled by running an R&R I ran on a XX set ring. Ring was 0.5" and using the bore to level the part then measuring the ID with 5 points using LSQ. Out of the 15 times I measured the ID the CMM showed a MAX deviation of 30 millionths and a MIN deviation of 50 millionths.

Maybe I was lucky that time. I really don't trust CMMs much for tight tolerance holes but that kept him from saying the CMM can't measure diameters accurately.

That is the problem with CMM's and diameters, when the feature deviates from perfect form the diameters become unpredictable.

[William Johnson]

Dwade How close was your results to the .500xx rings calibrated size.
I have found that I can't trust the reported size of dia's, On my xcel cmm with a tp200. But on my Global with a sp600 I can.
both will repeat with about 50 millinoths or less variation, but the xcel is not accurate on the acutal size. it will be off .0003/.0005 on dia's

[Wingman]

Dwade How close was your results to the .500xx rings calibrated size.
I have found that I can't trust the reported size of dia's, On my xcel cmm with a tp200. But on my Global with a sp600 I can.
both will repeat with about 50 millinoths or less variation, but the xcel is not accurate on the acutal size. it will be off .0003/.0005 on dia's

Interesting. Is your calibration sphere calibrated / defined correctly?

[kpayne]

I know this is been ask before when measuring a circle the postion is always good the diameters is always showing small. So is the any correct setting in the misc. box under the advance tab in a auto feature to check a circle are is this a common thing. Any help are info would help me shutup the operators.

When I measure and report circular features (least square method) on CMM I always include form error when reporting diameters which will give you an indication of error in diameter measurement.

[#2#]

Dwade How close was your results to the .500xx rings calibrated size.
I have found that I can't trust the reported size of dia's, On my xcel cmm with a tp200. But on my Global with a sp600 I can.
both will repeat with about 50 millinoths or less variation, but the xcel is not accurate on the acutal size. it will be off .0003/.0005 on dia's

I am able to get more accurate results than that on my xcel with a TP20.

What size tip and what dia. are you measuring. I have found the smaller the tip usually the better the dia. results for any size dia. Also, taking care to ensure that the circle used to report dia. is measured 'square' helps a lot.

[dwade]

Tip size can be an issue. I was told to use the smallest diameter you feel comfortable using to reduce cosine error. If you use find center or remeasure you should not get any cosine error from the probe.

MAX deviation of 30 millionths FROM NOM and a MIN deviation of 50 millionths FROM NOM.

I also use TP20 on all of out Pfx's and swap out my styli about 4-5 months whether they need it or not.

[William Johnson]

Goodluck My xcel is a 12-20-10 which is fairly big. I also use alot of bigger probes up to 8mm in dia. most of the time it is 100mm long by 6mm dia ruby, with carbon fiber stems. On standard force modules.

Wingman my sphere is calibrated and defined correctly. I have also have used the same one between machines to verify. I also thought that the one that came with the xcel was off but it is not.

[Wingman]

Goodluck My xcel is a 12-20-10 which is fairly big. I also use alot of bigger probes up to 8mm in dia. most of the time it is 100mm long by 6mm dia ruby, with carbon fiber stems. On standard force modules.

Wingman my sphere is calibrated and defined correctly. I have also have used the same one between machines to verify. I also thought that the one that came with the xcel was off but it is not.

So is it safe to say your error is only present when measuring ID's but good when measuring OD's? Or do 2 dimensional OD's also measure small? (Assuming spheres measure correctly)

[John Riggins]

The relationship of tip size to hole dia has a great impact on the accuracy.

[William Johnson]

Small on id's large on od's Funny hu

[bw_bob]

What you have to remember about dmis & auto circles is this

Max insc is for ID & that dia represents the largest diameter that will fit inside the ID

min circsc is for OD & that represents the smallest circle that would fit over the stud

Least square is just the average of all points.


None measures the diameter itself....my .02