Bore Diameter

[Clay_Man]

Good morning all

I have a bore dia. of 16.805 mm +/- 0.025mm, I am checking on the cmm I use 9 hits, to report dia.. The customer I measure it for uses 99 hits. I would like some feed back on how many hits poeple use to check a dia. of that size. I have the customer in tomorrow and don't want to get into a P-ing contest over the number of hits.

Thanks

[Matthew D. Hoedeman]

Well, unless your CMM has calibrated specs of +/-0.0025mm, your machine is not capable of measuring that tolerance with any kind of certainty, remember the 10% rule, the measurement device SHOULD be accurate to 10% of the tolerance you are checking with it.

[Wes Cisco]

I would use 5 hits to measure a circle that size unless I was trying to check roundness to a small value or had reason to think the circle was out of round but did not see it with only 5 hits. Even then I doubt I would take more than 15 for that size.

You could measure one 2 or 3 times,(or more depending on what you feel is necessary) taking 9 hits, and then measure the same one 2 or 3 times taking 99 hits. If needed you could then repeat this on more circles. I would think if you could show that 4 or 5 different circles in a row reported the same whether measured with 9 or 99 hits, could convince any reaonable person that the extra 90 hits are superfluous.

The other point to be made is maybe they know something you do not. Why are they wanting 99 hits? That is a lot. I do not know how large your company is or what the proper channels might be, but maybe someone should explain the difference in cost to these people. You might want to time the difference in how long it takes to measure with 9 hits versus 99 hits.

Of course in the end the engineer is always wrong. Or the right is always the customer. The key word above is "reasonable" and if the person(s) you are dealing with are not. . . That's what the beer is for. LOL :eek:

[BSchwark]

99 hits on a diameter that size is a little over kill. What are they checking for other than the diameter.
You could do a GR&R study with your CMM and that feature using your method.


BS

[Schrocknroll]

Sounds like they're scanning it, which would give 99 points in a few seconds.

[rangerboat72]

I try to follow this standard from the NPL Measurement Good Practice Guide Pg. 26...
:)

[Wes Cisco]

What does NPL stand for? I printed the document and it reads "courtesy Mitutoyo" at the bottom. I have a decent level of respect for Mitutoyo, but out of curiosity I wonder what B&S would recommend. Also who publishes this "Measurement Good Practice Guide No 41"? I would like to see the whole thing. TIA

[Vic Watson]

National Physical Laboratory (UK)

http://www.npl.co.uk/

[bob mappes]

cool stuff...am downloading "free info" now!:D

[J Temmen]

With that tolerance I would use 9 hits myself. Any variable of three is my preference.

[John Kingston]

That is a very tight tolerance for the CMM. I would do a comparison of other measuring methods. BUT..... If that is how the customer wants it, and will sign off on it, that is what you do.:rolleyes:

[dwade]

I think I would also use a bore gage to verify the size. Sometimes if you show a customer that you have 2 repeatable measurements with 2 different types of equipment, it will end the disagreement. But if you find the two vary by more than 0.005-0.010mm I would start looking at your measurement techniques. There are some good bore gages on the market that I would trust to measure to those tolerances.

Also, what algorythms are you using compared to theirs? Are you using least squares or max inscribed? What kind of results are they looking for? 0.025mm isn't much and any error will show greater results.

99 hits is over kill. Even if they are looking for max inscribed, anymore than 12-15 hits should be sufficient. I am new to PC-DMIS but some types of software calculate better with measurements made to prime numbers.

One last question. Have you measured a master ring to verify that your probe diameter is correct?