CMM Accuracy - Formula for Excel or a spreadsheet

J

John Bumgardner

#1
IS there a formula for Excel or a spreadsheet floating around here that will allow me to report on the accuracy of my CMM? We have a customer wanting a GR&R on all families of gaging that will be used to measure the PPAP parts with. I know that a CMM should have a very good GR&R, but I dont think that really shows what the machine is capable of, using fixed fixtures and I as the lone operator. Thanks for any help.
 
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J

justncredible

#2
Re: CMM Accuracy

Send the calibration cert, it works for some big companys. Some will ask for a GR&R, then ask what size for the study, 10 parts 3 operators you need to make them pay market price for the time. Which varys from $60 a hour to $130.
 
Z

zancky

#3
Re: CMM Accuracy

can you wait for tomorrow?.
we have done it for a customer, but the file is on my office and I'm at home now.
 
P

prototyper

#5
I have attached a spreadsheet which replicates the forms from the MSA manual third edition complete with data from the manual which validates the calculations.

As it replicates the manual, no customer or auditor could question the results, only your interpretation of the results.

I have used this to conduct R&R studies on CMM programmes. Select 2 or 3 SC's to study.

I have found that the largest source of variation is usually due to the way different operators locate or clamp parts.
Different clamping pressure distorting the part.

Make sure that the samples you select for the study reflect the full process range.

Good luck!
 

Attachments

Z

zancky

#6
Hi Prototyper,
do You use same procedure when You are using a CMM or a manual device?
Our customer request different procedures: one when an authomated machine is involved, one when is the operator performing manually the measure.
the form I have attached came from our german automotive customer for CMM

I agree with You that how You fix the part is very important expecially for very thin parts. Location should not be a problem if the program is well written.
Anyway operator/appraiser should have no effect at all if the procedure and the program work well.
 
T

True Position

#7
There is a lot of good advice in this thread. Especially the people who mention on a DCC machine that there shouldn't be any variation since all the operator does is put the part into the holding fixture and presses start.

Generally when I'm asked for a GRR on my CMM I'll offer repeatability of a few samples, even with multiple operators if they like as well as copies of cert showing the machines accuracy.

Many times people ask for GRRs just out of habit not questioning if it's really necessary, push the cost onto them and they start caring a lot more.
 
Z

zancky

#8
There is a lot of good advice in this thread. Especially the people who mention on a DCC machine that there shouldn't be any variation since all the operator does is put the part into the holding fixture and presses start.

Generally when I'm asked for a GRR on my CMM I'll offer repeatability of a few samples, even with multiple operators if they like as well as copies of cert showing the machines accuracy.

Many times people ask for GRRs just out of habit not questioning if it's really necessary, push the cost onto them and they start caring a lot more.
Well I prove first that there is no influence of operators. If I find any difference I start to think that I have made a mistake into the program or on the procedure.
Generally CMM suppliers provide evidences on standard features not on real parts (I have a new part every month so I write a new program each time).
If You write the program (i.e. typing the script) it doesn't matter location because You can write the alignement subroutine starting with a manual point.
For my R&R we perform the check putting the part each time in different location by different people so I'm quite sure that during production everybody will have the same result.
may be I'm :topic:
 
T

True Position

#9
I used a Rayco style fixture plate for most of my fixturing and I am the only operator, so I have fewer issues related to moving of the parts and multiple operators.

I have about 200 part numbers that I cycle through checking and I keep detailed fixturing notes on each to help make sure that everything stays as similar as possible. This helped when there was a second operator quite a bit.
 
Z

zancky

#10
I used a Rayco style fixture plate for most of my fixturing and I am the only operator, so I have fewer issues related to moving of the parts and multiple operators.

I have about 200 part numbers that I cycle through checking and I keep detailed fixturing notes on each to help make sure that everything stays as similar as possible. This helped when there was a second operator quite a bit.
Hi Hawat and John.
Well it will be very helpfull to know what John is measuring (soft, gib etc parts). That's will change the fixture philosophy.
Using touch probe my best results it is when I can use a magnetic V-block, as we measuring very thin metallic parts and sometime any pressure will deform the parts, but the parts must be magnetic too:(.
Sometime we use a v-block and a double self-adesive tape and so on...
we have more then 200 parts too, and some parts have different program to check different manufacturing phase.
One question do You record every program and every changes in the program? When auditors look at the program they say "ok let's go over" everytime, no question on our ability to write programs or naythingelse.
Going back to John question only a programmer will check Your program and look for mistakes most of the other will ask just for the R&R. the file I have previously attached was for one of the biggest company in the world famous to stress suppliers. There was only a minor problem, they asked to perform the analysis on a part certified by external laboratory with an accuracy better then 0.1 micronmeter:mad:. (John that will be Your biggest problem too:bonk:)
Well at that time my answer was "As You are a iso TS certified company and You perform R&R analysis on every device and method, it should not be a problem for You to give me Your certified reference part for a while every Year, so I can complete my R&R analysis":lmao:.
No answer and still waiting for the certified reference.
Currently we perform our R&R analysis with production parts and after we send the same parts to the customer to check the correlation.
I have never used a accreditaded external laboratory certified/calibrated part.
 
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