Qualification-Star tool

[Hilton Roberts]

All,

A person who used to work for me at my last place of employment cannot access this web page ( or any other for that matter ) so I am going to try to post this question.

He must use a star tool to reach into the bores he must check on large engine blocks. Because of the nature of the star tool, there is always at least one tip that cannot be calibrated on sphere 1 so he must use the lower sphere and sometimes even move the calibration tool so that he can qualify the tips. He is running into problems reporting roundness determined measuring a ring gage and his contention is the qualification routine is the problem. He maintains that when you enter the qualification routine and answer "no" to the question of moving the calibration sphere, the three points that are measured determine the center of the sphere for the points that follow. For example if you have 5 points at two levels, the first three points find the center of the sphere and the remaining 5 points refine that number. I told him the three points are just points to allow the machine to go immediately into DCC and that all 8 points are used in the calculation.

After the first tip is calibrated, the remaining tips are calibrated with 5 points at two levels ( or whatever scheme you have set up). I tried to explain to him the concept of setting a part up the same way all the time and making an alignment on a single point so as to go into DCC from the start and then picking points on various geometry to make an alignment. My former co-worker thinks those first three points are blowing him out of the water and I am trying to tell him that is not the case at all.

Just so you all know, he is running a DEA 3305 110 in X 64 in. X 64 in. and a PH9 with a 300 mm extension, a tp2, a one inch stylus extension and a one inch star tool and he is concerned his roundness numbers are changing..........


I suspect he is really loosing it when he moves the calibration sphere during tool qualification and is losing something in the relationship of tools calibrated in prior position to those values being determined in current position.

I do not know how to provide any more help for him. Do you all have some words of wisdom to help me help him?

Thanks!

Hilton

[J Temmen]

First, I never like using a CMM for roundness, even a short buildup without extensions will give bad roundness results. As far as the calibration routine, the first three hits simply find the sphere, the 5 or so points that follow read to the center of the ruby and only after all hits have been taken does it calculate the center and comp each hit based on the known size of the artifact. Long winded but I hope it helps.

[Matthew D. Hoedeman]

Well, I have never had to use 2 qual spheres, so I am not 100% sure what you need to do. I believe that BOTH qual spheres MUST be manually measured using the T1 A0 B0 tip for them to both be located by Pcdmis in the correct relationship and for the calibration to relate from ball to ball. Ya know, tell it the sphere is in a new location. AND, each sphere must be defined on it's own, so you calibrate some tips to TOOL1 and some to TOOL2, but BOTH TOOL1 and TOOL2 MUST be located by manually touching with T1A0B0.

[John Kingston]

Hilton,

This thread may help. I use 2 different Star sizes, and can calibrate both on the top sphere.

http://www.pcdmisforum.com/showthrea...highlight=star


John

[Wes Cisco]

Handy document

Hilton,
I just had my first star probe experience, and that was the cause of the thread refered to. The main thing I learned is that it does not have to be tip 1 A0B0, but you DO have to calibrate both balls with one tip. It does not have to be a manual calibration. I used tip 4 A0B0 and I only used the star probe at A0B0. It worked well for me.
You say he is using a tp2, that may be his problem with roundness. The mechanical probes have 3 switches so any hit activates one or two switches. The tp200 and some of the scanning probes use hundreds if not thousands of strain gages and can therefore report roundness without the lobbing error inherent in the design of mechanical switch probes. I recently got a tp200, I conducted some tests using a brandnew calibrated ring gage. I measured the ring many times with the tp20, then unscrewed the tip and put it in the tp200, I calibrated and measured the ring many times. I repeated this process with 4 different tip builds I commonly use. I reported size and roundness to .xxxxx". The tp20 had size variation of .00010 or less for diameter but up to .00020 for roundness. The tp200 had variation of .00002 or less for diameter and .00001 or less for roundness. These tests were performed on two different days less than 2 weeks after B&S calibrated the CMM. The ring gage has factory calibration to 5 decimal places. HTH

P.S. FYI: a strain gages is a very thin piece of wire with a current running through it, when it is streched the resistance is altered. It is the change in resistance that is actually being measured. Because there are so many strain gages in one of these probes, many are streched to varring degrees with each hit.

[Rick Coleman]

Parameter sets

Hilton; If you want, check out this link. I have used this before and it works well. Maybe this is what he's looking for. You should be able to copy it and e-mail it.

http://www.pcdmisforum.com/attachment.php?attachmentid=15&d=1141391602

[Wes Cisco]

OPPS

Thank you Rick,
That link is the document I meant to post.

[KBearden]

Brown & Sharpe or Wilcox Input

You would think after all the posting you see on various ways of how to and how not to calibrate a star probe, that the EXPERTS who WROTE the software and who TESTS the software would be able to post an IRON CLAD way to calibrate a star probe on the dual artifact that they sale with the CMMs.

[Rick Coleman]

The one that I posted before and linked to in this thread did in fact come from a techie at Brown & Sharpe.

[KBearden]

Thanks Rick

I saved a copy of that .pdf and I will try it if ever I get the chance. I seldom ever use the star probe. What I did when I calibrated the #3 tip was to cal all the other tips in DCC and then come back and manually take the hits for that one particular tip. It is a pain but PCDMIS didn't complain. I think it's been over a year since the star probe I built has even been picked out of the probe changer.

Also, when I responded on this thread earlier I meant to attach a sheet which lists the positions of various probes at different head angles. This can be very helpful when selecting the correct probe to take hits with. I have a small copy of the tip table taped to my monitor for quick referencing. I have posted it before in the old forum, but I don't think I have in this new one.