Effects of CMM Probe Tip Ball Size measuring a Round Part OD - CMM Help

[Golfman25]

Any CMM experts out there who can help answer a question would be greatly appreciated.

Here is the situation. We are measuring a round part (OD). First measurement with a 2mm probe tip we get one measurement. When we try with a 1.5mm probe tip, we get a .002+ difference -- the diameter is larger. We are measuring in the same spot with the same setup. When we check both tips against a gage pin, they both check the pin accurately.

The part is draw formed so the wall is not perfect. We suspect that has something to do with the measurements. But we can't figure out why. Logically, we would think the larger ball would cause bigger diameter as there is more surface area to pick up high spots. But that is not the case. Any thoughts? Thanks in advance.

[True Position]

In Reply to Parent Post by Golfman25

Any CMM experts out there who can help answer a question would be greatly appreciated.

Here is the situation. We are measuring a round part (OD). First measurement with a 2mm probe tip we get one measurement. When we try with a 1.5mm probe tip, we get a .002+ difference -- the diameter is larger. We are measuring in the same spot with the same setup. When we check both tips against a gage pin, they both check the pin accurately.

The part is draw formed so the wall is not perfect. We suspect that has something to do with the measurements. But we can't figure out why. Logically, we would think the larger ball would cause bigger diameter as there is more surface area to pick up high spots. But that is not the case. Any thoughts? Thanks in advance.

How many points are you taking? Can you add more points? Sometimes those types of problems are related to surface finish. Smaller probes are more to fall inside surface irregularities such as rough turned parts. I usually default to a 4-5mm tip for general measuring if it won't cause issues with retract and clearance.

[howste]

Are the measurements repeatable? How many points are you taking for each measurement?

[Claes Gefvenberg]

In Reply to Parent Post by True Position

I usually default to a 4-5mm tip for general measuring if it won't cause issues with retract and clearance.

Excellent suggestion. That could very well be it.

In Reply to Parent Post by True Position

How many points are you taking? Can you add more points?

In Reply to Parent Post by howste

Are the measurements repeatable? How many points are you taking for each measurement?

My thoughts exactly. A roundness problem combined with few points could easily cause the results mentioned. In a case like this I would begin by scanning the object to find out about its shape.

/Claes

[Golfman25]

In Reply to Parent Post by True Position

How many points are you taking? Can you add more points? Sometimes those types of problems are related to surface finish. Smaller probes are more to fall inside surface irregularities such as rough turned parts. I usually default to a 4-5mm tip for general measuring if it won't cause issues with retract and clearance.

We are taking 11-12 points (around .750 OD). We also think the small probe would fall inside surfact irregularities. However, that would lead to a small measurment, not the larger measurment we are getting.

And yes the measurements are repeatable within each probe size. We just get different results with the different probe sizes.

[JRKH]

Interesting Problem - - -
I've been away from inspection for some time, and I only ever used a manual maching, so let me ask a couple of questions here.
1) Are these measures being taken manually or automatically?
2) Are the shafts on the two probes of different lengths?

It occurred to me that if the probe shafts are of different lengths and the measures are taken automatically then the measures may not be taken in the same location on the (presumably) Z axis.

3) Are the Probes "calibrated" on a ball for each use or is the dia's set in the software.

The thought here of course has to do with wear on the probes.

One other thought I had has to do with the probe design. Does the smaller probe have a tapered shaft? Is it possible that the shaft is hitting before the ball? I know it sounds rediculous but I'm just tossing out ideas.

Like I said - interesting problem.

Peace
James

[Golfman25]

In Reply to Parent Post by JRKH

Interesting Problem - - -
I've been away from inspection for some time, and I only ever used a manual maching, so let me ask a couple of questions here.
1) Are these measures being taken manually or automatically?
2) Are the shafts on the two probes of different lengths?

It occurred to me that if the probe shafts are of different lengths and the measures are taken automatically then the measures may not be taken in the same location on the (presumably) Z axis.

3) Are the Probes "calibrated" on a ball for each use or is the dia's set in the software.

The thought here of course has to do with wear on the probes.

One other thought I had has to do with the probe design. Does the smaller probe have a tapered shaft? Is it possible that the shaft is hitting before the ball? I know it sounds rediculous but I'm just tossing out ideas.

Like I said - interesting problem.

Peace
James

All things we considered. Each pc. is measured manually. We set the the Z down to the same level on each pc. So differing probe lengths are taken into account. And yes, each probe is calibrated on the ball during each change. We also looked at the tapered shaft issue. I thought that there might be contact, but it doesn't look like it (although I am much more comfortable with the larger ball).

Now here is an interesting tidbit. We checked a very similar part (the next larger size) and both probes check it within .0002 of each other.

So something strange is going on with the other part which gets us a .002 difference. Just can't figure out why.

[JRKH]

In Reply to Parent Post by Golfman25

All things we considered. Each pc. is measured manually. We set the the Z down to the same level on each pc. So differing probe lengths are taken into account. And yes, each probe is calibrated on the ball during each change. We also looked at the tapered shaft issue. I thought that there might be contact, but it doesn't look like it (although I am much more comfortable with the larger ball).

Now here is an interesting tidbit. We checked a very similar part (the next larger size) and both probes check it within .0002 of each other.

So something strange is going on with the other part which gets us a .002 difference. Just can't figure out why.

OK, let me ask another question. Are the parts checkd on the same part of the table evey time?
Have you tried checking the parts in a different location on the table?

Peace
James