??? for TP20 users - probe accuracy issues

[Mike N.]

I need help in solving a strange issue with the use of TP20 modules standard force and medium force. (I use a sp600 currently)

Another employer I just started working for part time is having an issue with running the cmm at "full speed". Upon my arrival I had to ask "what is with the 7bars method of execution on the hand pendant?" and got the explanation that... when probes are calibrated using a 2% touch speed and executed at the same 2% touch speed the diameters are reporting approx .0030inch larger than the pin that fits the hole. What is interesting is that when the hand control pendant is set to 7bars or 70% of max the diameters measure correctly. How could there be such a great difference with just changing the pendant speed 30%?

I have checked a ring gage at A0,B0 and found it to measure correct within .0001 to the stated value stamped on it. All the holes that measure out of tolerance are at various probe angles like A90,B90 - A90,B45 etc... none are at A0,B0 on this particular part.

Any direction on this wierd problem would be greatly appreciated.

[Hilton Roberts]

There are a lot of variables that can influence what you are seeing. It is probably not good practice to calibrate tips at one speed and measure at another speed. If approach/retract values are not adjusted accordingly, you can be taking hits before the machine "settles" after a fast approach.

I think most on the forum do not use CMMS to measure close tolerance diameters. We ( I ) use the machine for location and tight tolerance holes are checked by another method.

It does not seem to matter what one does about calibration, slow or fast, CMMS just have a problem reporting diameters that differ when measured by other means.

I think it has more to do with the actual pressure of the point touches. The numbers, if correct will always produce a correct diameter. It is the precision of the numbers that make the most difference. It seems a better "feel" can be obtained with an Intrimic or electronic bore gage.

I really do not think CMMS are there yet as far as reporting accurate diameters.

I almost always see a bias between the CMM and any other method.

HTH

Hilton

[Craig H]

What size are the holes and what are your prehit distances? I have to slow the touch speed down a lot when I get under 1mm on a prehit distance to get a good measurement on our Global 777.

[Mike N.]

What size are the holes and what are your prehit distances? I have to slow the touch speed down a lot when I get under 1mm on a prehit distance to get a good measurement on our Global 777.

Thats interesting...
I did notice that the 1mm probe is being used to check a 2mm hole and a 1.5mm probe into an approx 3mm hole. This might be the ticket I'm lookin for!

I will edit each hole for prehit speed when I visit them again next week Tuesday.

The machine is a B&S 777 FX and its only about 1yr old running ver4.0

Thanks for the info Craig H

[Craig H]

Try 0.1% for a touch speed, that seems to be the best speed on ours when well under a 1mm for a prehit. You may be able to go a little faster since you have a little more room than what I've been working with here. B&S was never able to give me an explanation for this but I got a good one from a service engineer from another place that we use to calibrate our machines. His explanation is that the machine is belt driven and belts give. Our other CMM runs on a screw type of drive system which does not give which is why I don't have to slow it down when less tha 1mm on a prehit.

[mds_toronto]

for bigger holes it shouldn't be big difference, I make it a standard that whenever I measure holes smaller than 0.200" (5 mm) I set my touch probe speed at 1% regardless of no. of speed bars.

[56flh]

It does not seem to matter what one does about calibration, slow or fast, CMMS just have a problem reporting diameters that differ when measured by other means.

I think it has more to do with the actual pressure of the point touches. The numbers, if correct will always produce a correct diameter. It is the precision of the numbers that make the most difference. It seems a better "feel" can be obtained with an Intrimic or electronic bore gage.

I really do not think CMMS are there yet as far as reporting accurate diameters.

I almost always see a bias between the CMM and any other method.

HTH

Hilton

Hilton, I am always reading that a CMM is not a good choice for accurate, tight tolerance diameters, but when I brought this up at the PC-DMIS user's group meeting I was told the opposite. Per Rick Schaffer, I measured a calibrated ring standard multiple times, varying the number of hits of each iteration to find the number of hits that I would achieve repetition in my measurement of the standard. I found that with 20 hits the CMM would consistantly measure the standard within .00005 (repeated the measurement 6 times at this number of hits).

An intrimic is not a good device to use for tight, accurate bore measurement as it has a tendancy to average and not indicate an out of round situation.

[ytilauq]

Are you at 100% speed during calibration as well?? also a 2mm pin will not fir in a 2mm hole it must b slightly smaller

[NinjaBadger]

Are you at 100% speed during calibration as well?? also a 2mm pin will not fir in a 2mm hole it must b slightly smaller

You've resurrected a thread from 2006 here!