Gage R&R and right way to measure

mcpe72

Starting to get Involved
#1
I have ID dimension that for some reason when I measure with caliper, I cant get a good R&R, (its a plastic part, maybe in the process the neck get deformed) they want us a use a method with less "noise" although we are in spec, we have (lots of variation according to the costumer), i will check with a CMM ( dimension are very similar hard to get differences between parts, can i suggest in the end a go not go, so we can finish this discussion?
 
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Miner

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#2
Its a shame that it is an ID. Mitutoyo Low force caliper works well on outside dimensions. I assume that the CMM is not an option for ongoing process control? Another option of last resort is to use the average of multiple measurements as the measurement. This reduces the variation by the square root of the sample size. For example, the average of 4 measurements will have half the variation of individual measurements.

Column 7 is the standard deviation of column 5 which is the mean of the 4 measurements in columns 1-4.
Column 8 is the mean of column 6, which is the standard deviations of columns 1-4.
Note that column 7 is roughly half of column 8.

1614959871749.png
 
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mcpe72

Starting to get Involved
#3
thank you, I'm using a bore gage too, I'm going to do 3 R&R, one with a caliper one with the CMM, to compare, but the problem is that they want a dimension in the parting line and one to 90 degrees so we can get the ovality too.
in production would take way too long to do all that wit the CMM
 

Ninja

Looking for Reality
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Super Moderator
#4
I've always found CMM to be faster than caliper or micrometers, personally.

What CMM do you have? How many samples are you testing?

Using a MicroVu Vertex 120 (old one from ~2005), we measured 5 samples of each lot for length and width in 3 places (20 data points total) in about a minute.
For circles...we were measuring center to center distance...that went much faster.
For circularity of a single circle, including diameter/radius...that would be about a 2 seconds (plus part mounting time) per part including data report...call it 10sec total.
 

mcpe72

Starting to get Involved
#5
i have 2 kinds of CMMs, but the OGP is hard to use, are bottles i have constrain of size, (tall to set it up) i have one that has a touch probe that scans, that one is fast
 

Ninja

Looking for Reality
Staff member
Super Moderator
#6
Yes, I also found the OGP hard to use...thus I purchased the MicroVu.

Even with the OGP...if you take your time in proper fixturing (easy to mount the part in the right place)...and take your time in effectively arranging the measurement program, the per piece turnaround time should be measured in seconds even when including part load/unload.

(I am not trying to derail from your original question...but it seems that Miner has already answered that fully).

With a part loading/positioning/holding jig, and an automated CMM program...I would expect your GRR results to be on the order of 1.0-3.0 for most tolerances. (unless your tolerance is in microns)
 

Ninja

Looking for Reality
Staff member
Super Moderator
#9
Ummm.....Looking just at part #10, you have > .05mm variation on an ID only 26mm? On a CMM?

I would look at (in order):
- Is your program taking multiple readings as Miner describes, or just one ID? It's an automated CMM...you should be measuring literally hundreds of datum per second. Check your program. If you measure in different places each time, and your part varies that much (circularity), no wonder you're failing GRR.
- Is the part stable, or is it jiggling during measurement?

The demo I would run to show customers was to take off my wedding ring, put it on the stage, and hit "go".
In a couple of seconds, I got hundreds of ID measurements, hundreds of OD measurements, best fit circles for ID and OD and circularity for both.
Are you just using measurement #1 for your GRR?
Then I would put a penny on the stage, and we would measure the width of Abe Lincoln's knee (did you know he's in there?)...that's how a CMM should be giving you the data...not 0.05mm error.
(Note...his knee is less than 0.05mm wide...)
 
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mcpe72

Starting to get Involved
#10
fixturing is an issue for me, i m not production so i have different size bottles, for instance I'm measuring ID of Listerine bottles, so i have to use a touch probe, and all the diameters are very similar so i have only 1 category
 
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