Tip Calibration

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FialekJared2018
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Tip Calibration

Post by FialekJared2018 » Thu Jan 16, 2020 7:06 pm

Currently having some issues when calibrating tips, we have two probe assemblies for various parts. When calibrating the tips we get outcomes bigger then what the ball size is...for instance A0B0 comes out with a result of 2.019. This happens to all tips for 2mm ball and then for our 3mm ball we get big outcomes as well? Probe assembly build is correct in length for both. I have included a photo as well.
tips.JPG
tips.JPG (19.63KiB)Viewed 1554 times
If anyone can give me some ideas or what is causing this, it would be very helpful! Thank you.

Acuity_Metrology
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Re: Tip Calibration

Post by Acuity_Metrology » Thu Jan 16, 2020 8:56 pm

This is usually caused by a loose stylus, but if your form error is good and you are getting similar oversize on two different assemblies, it isn't likely the issue in this case.

When you measure the sphere, do you get the expected diametrical and form results?

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Re: Tip Calibration

Post by CMM Guru » Fri Jan 17, 2020 10:00 am

Jared,

Can you give a bit more detail of your actual probe build? PH10? PH20? TP20? Std force module? Stylus length? Extensions used?

Being oversize (at all positions) is indeed odd, and would typically be from something like a loose stylus. But you say you have two assemblies doing it which is really odd.

In the end, Acuity is correct. If it measures a standard correctly the calibration is good (usable). Although I would indeed be looking for causes!

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Re: Tip Calibration

Post by Ryan Christopher » Fri Jan 17, 2020 11:07 am

As you are investigating this, it wouldn't hurt to check your definitions too...

1. Make sure that in the Probe Assy dialog box, the stylus is defined in mm as being the nominal 2mm.
2. Find the Calibration sphere definition and make sure that it is defined as being the actual calibrated size.
RIBBON > PROBE > SETUP group >SETUP CAL. SPHERE
Ryan Tackes

FialekJared2018
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Re: Tip Calibration

Post by FialekJared2018 » Fri Jan 17, 2020 3:31 pm

We have a PH10 to a TP20 using standard force module with an extension of 10mm to a 2mm ball stylus which gives overall length at 177 for Probe assembly 0....for the second one its all the same but no extension and is a 3mm ball stylus with overall length of 167.

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Re: Tip Calibration

Post by CMM Guru » Tue Jan 21, 2020 9:53 am

The "build" lengths seem correct. And since you have two probes (two different modules) it would seem unlikely that both would be loose, and by the same amount!

I'm assuming this is a machine that has been running for some time? If so, were there any recent changes? It is possible that speeds and/or accelerations are not optimal (ie: running into the touch point way too fast!) giving bad results.

There are two things I'd like you to try.

1) After calibrating your probes, measure a "standard" such as a known ring gauge. If you do not have that, re-measure the sphere. In either case, the actual should be exceptionally close to the nominal. ie: a 25.4 mm cal sphere should measure as 25.4 within a micron or so.

2) if you have access to a *known* length bar - ie: 500mm, could you check the length? Inspect a plan on one end, then a point on the other, and ask for distance point to plane. If it is a certified or known 500mm, once again, your value should be within a micron or so (depending on temperature). This will make sure your scale factor is correct.

Although it is not unusual to have 1 or 2 "angles" calibrate slightly over nominal, to have them all over nominal, and that far over nominal is exceptionally unusual. This is indeed indicative of something not quite right in the system.

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Re: Tip Calibration

Post by FialekJared2018 » Thu Dec 03, 2020 9:02 am

Hey everyone! I know this post is quite old but were still having the same problems. The CMM was just calibated in April of this year so Im not sure why we are still having our probe assemblies measure bigger then the actual ball size.

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Re: Tip Calibration

Post by FialekJared2018 » Thu Dec 03, 2020 9:08 am

We had our CMM calibrated back in April of this year. Our Probe Assembly builds are getting calibrated bigger then what the actual ball size is. Our lengths for the builds are accurate as possible but we are still getting oversized tip calibrations. We recently did a study on some of our parts and had a third party lab measure them and they were roughly .025 smaller in diameter then what we are. This morning I just measured a 1” Master Ring that gets certified every year and it gave us a result of 25.426….do you possibly have any ideas what is causing this issue for us?

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Re: Tip Calibration

Post by CrashN8 » Thu Dec 03, 2020 12:50 pm

Almost every CMM I've encountered wills how calibrated tip diameter that's slightly smaller than the actual ruby stylus tip. This is to be expected and normally referred to as "Effective Tip Diameter"

If you're CMM's tip calibration results are larger than actual stylus tip, this is very strange. I would start by measuring length bars (gage blocks) by measuring points in the same direction - i.e. measure one end directly, then the other end by indirect measurement. Partially ring another block to the end of the larger gage block and measure the second block. For example, if the block(s) is aligned to X axis, you are measuring both ends of the block by physically moving the CMM in the same direction. This style of measurement eliminates the probe radius from measurement results and will tell you if the machine has some volumetric / linear error that's causing your results to be large.

After which... I would get your service company that calibrated the CMM to have a look. Something seems to be wrong here.

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Re: Tip Calibration

Post by FialekJared2018 » Fri Dec 04, 2020 8:37 am

We did a couple more tests using a 1 inch gage pin compared to the master ring and we measure below this time instead of above 1 inch.

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