Repeatability issue in Gage R&R study for inner diameter of a bushing

mstfbakr

Starting to get Involved
#1
Dear cove members,

Sorry for my baby english and i kindly request your valuable comments for

My company has a discussion with supplier regarding a gage issue.

this supplier provides us bushings. Tolerance is 0,009 µm. As a customer, we request from them to control this feature with a inner diameter micrometers has a resolution 0,001 µm.

To be honest this gage does not comply with 1/10 rule.

Supplier offers to use go/no gage and has studied a Gage R&R study to proof us inside diameter micrometer is not capable to measure this feature. Please find attached screen shots for R&R study results.

There is template spreadsheet that we received from supplier and with same measurements our minitab analysis.

Is there anything wrong with supplier measurements. don't randomized parts or same readings for same parts from all operators like etc.

Reproducebility is ok but repeatability is not ok. What does it means.

Operator's skill, training are ok to use tihs gage or measure this part ?
Or they saw previous operator measurement result.

repeatability is not ok means that there is a big variation within choosed samples or process does not under control.

NDC result is 0. Is micrometer not capable to measure 0,009 tolerance ?
If a not good result added to this results NDC becomes greater then 5.

Please advice

Best Regards,
 

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Bev D

Heretical Statistician
Staff member
Super Moderator
#3
I would add that your repeatability is very poor and so you can make NO conclusions about the reproducibility. although the spreadsheet will calculate a number for you it is essentially meaningless until you get a decent repeatability.

looking at the Range chart operator 3 appears to have less repeatability error than the other two operators.
also the maximum range for both operator 1 and operator 2 is close to your tolerance (,0009)

Since this is a diameter measurement, I'm going to suggest that you investigate the possibility that this is really within piece variation: you may have an oval tube instead of a round one. so the first measurement is of the smaller diameter, then the next one is in a different location around the diameter - a slightly larger dimension...try marking the tube(s) and measuring the same location multiple times...
 

Miner

Forum Moderator
Staff member
Admin
#6
Since this gage is used solely for inspection, the metric to use is % Tolerance. Ignore %Study Variation, %Process Variation and ndc as they are irrelevant for an inspection-only device.

BevD's comments are right on target. Since the resolution of the gage appears to be adequate, I would start by investigating the possibility of Within-part Variation (WIV). This MSA blog entry discusses how to assess the impact of WIV.

After you have made a determination of the impact of WIV, update us on your progress and we can advise further.
 

Golfman25

Trusted Information Resource
#7
Since this gage is used solely for inspection, the metric to use is % Tolerance. Ignore %Study Variation, %Process Variation and ndc as they are irrelevant for an inspection-only device.

BevD's comments are right on target. Since the resolution of the gage appears to be adequate, I would start by investigating the possibility of Within-part Variation (WIV). This MSA blog entry discusses how to assess the impact of WIV.

After you have made a determination of the impact of WIV, update us on your progress and we can advise further.

If it is a machined ID, I would be hard pressed to suspect within part variation (unless the machine is completely fubar). Worth checking. We do stamped parts and variation is a major issue for our measuring capabilities.
 

Miner

Forum Moderator
Staff member
Admin
#8
If it is a machined ID, I would be hard pressed to suspect within part variation (unless the machine is completely fubar). Worth checking. We do stamped parts and variation is a major issue for our measuring capabilities.
All features of size display some degree of variation in form. While machined holes may have less, they also have tighter tolerances, so the variation, though small, may consume a lot of the tolerance.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#9
in addition, I've seen machined dimensions exhibit relatively large within piece variation due to fixturing problems. Every technology has it's own sources of variation that the technology promoter overlooks and/or trivializes. :cool:
 
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