P/T % of a gage R&R to 0 MMC Tolerance study

C

Coleman Donnelly

#1
What is the correct way to handle P/T % of a gage R&R analasys when the measurement is for a True Position with 0mmc tollerance?

What changes would be made if any when the datums also incorporate a material modifier?
 
Elsmar Forum Sponsor
C

Coleman Donnelly

#3
I can understand if nobody has a quick easy solution - perhaps we can have a discussion about the problems?

A percent to tolerance study is typically the industry standard for accpetance critaria when validating a measurement system. When inspecting True Position - the tolerance is not a constant if the maximum material condition rule applies.

This means that every part has a unique tolerance which for location that is relative to actual size. The actual size of a physical feature - minus the MMC condition of the feature gets added to the Positional tolerance.

If I have a hole that is supposed to be 10mm +/- 1mm than the MMC condition of the hole is 9mm. If my actual hole physically measures 9.5mm this means that there is a 0.5mm difference between the actual size of my hole and the MMC condition of the hole.

This 0.5mm gets added to the positional tolerance which as stated above is [email protected]

This means that if my hole is made to the smallest allowable diameter than my position has to be perfect, but as the hole gets larger my position can deviate from perfect.

The problem is how do I validate the measurement system if my tolerance is not a constant? Every part will have a different hole size which means that every part will have a different positional tolerance.

The situation becomes further complicated when you consider that the inspection of the size of the hole will also have some amount of variability. This means that every time i measure the feature on the same part I will find a new tolerance to use for my positional inspection.

I think this is a good place to stop for now :)

So - how can this situation be addressed?
 
C

cbehrens

#4
I think you need to look at it from the other direction. How big of a hole to I need to produce to acheive enough MMC tolerance to end up with an acceptable gage R & R.
 
C

Coleman Donnelly

#5
cbehrens - thanks for the response!

So if I follow through on this line - I should evaluate what my position does repeat to - then remove tolerance on the low side of my size requirement to protect the boundry of what I can inspect with confidence?

I am having a hard time believing that i am the only person in the industry who is using MMC on true position and trying to evaluate my measurement system...
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#6
The real issue here is that the P/T% is a silly measure. It's mathematically not correct and in this case only casts further confusion - and unecessarry complexity to 'do it right'.

When I get confused I back to first principles. What is the intent of the GR&R? It is to understand how much of my observed product variation is due to measurement error.

Here's what I would do:
How much variation do you normally see? select parts that span that range of variation. Select about 30 parts. Measure them twice. Plot the first and second measurement on a square scatter plot with a 1:1 line from the origin. Put your tolerance in as a square. (pick the tolerance without MMC considerations or with - it really doesn't matter.) Now look at the plot. What do you see? if the variation about the 1:1 line (perpindicular to the line not in the Y axis) is small compared to the tolerance square you are good to go. if not you may need to fix your measurement system. (this is called a Youden plot if you want to google it)

How small is small? it's a judgment call. it's always a judgment call. the origin of a "p value <5%" or "10% of the tolerance" as criteria of goodness didn't come on the stone tablets. they were emperically determined.
 
C

Coleman Donnelly

#8
Here's what I would do:
How much variation do you normally see? select parts that span that range of variation. Select about 30 parts. Measure them twice. Plot the first and second measurement on a square scatter plot with a 1:1 line from the origin. Put your tolerance in as a square. (pick the tolerance without MMC considerations or with - it really doesn't matter.) Now look at the plot. What do you see? if the variation about the 1:1 line (perpindicular to the line not in the Y axis) is small compared to the tolerance square you are good to go. if not you may need to fix your measurement system. (this is called a Youden plot if you want to google it)

How small is small? it's a judgment call. it's always a judgment call. the origin of a "p value <5%" or "10% of the tolerance" as criteria of goodness didn't come on the stone tablets. they were emperically determined.
Ok so now we have a situation where the intent of two different systems is fighiting each other...

Functionally speaking my TP tollerance should be [email protected] I can make it [email protected] but I can not measure it with enough consistancy to justify the potential 0 tolerance...

I agree with your approach, but I need some guidance on the judgment call - an auditor is not going to appreciate "look at how small the plots are"... :(

The problem seems to be that I can not do the right thing for both systems at the same time... I can steal a little bit of my size tolerance and create a situation where I have a functional part that must be rejected, or I can ensure accpetance based on functionality and create a situation where I may not be able to measure the part within an acceptable confidence limit...

Or perhaps is there a way to include a confidence interval with my result in order to ensure that if the stars might align I would not accpet a bad part? I dont really consider myself an expert on the subject of confidence intervals.

On to part 2 of this question - how do I asses process performance/capability with the call out in question?
 
Thread starter Similar threads Forum Replies Date
S GD&T Rule 1: Gage size for form check at MMC (maximum material condition)? Inspection, Prints (Drawings), Testing, Sampling and Related Topics 9
T Plug Gage Calibration Calibration and Metrology Software and Hardware 1
L Gage R&R TMV Acceptance Criteria Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4
S MSA for attribute relation gage Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
E Zero part to part variation - Gage R&R project Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 15
M Gage R&R and right way to measure Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 16
L Gage RandR on automated equipment. IATF 16949 - Automotive Quality Systems Standard 7
B Gage calibration frequency, ISO and IATF - What are the requirements Calibration Frequency (Interval) 3
D Difference between Test Method Validation and Gage R&R Qualification and Validation (including 21 CFR Part 11) 18
R Determining Uncertainty from Gage R&R Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
C Correct Calibration Method for Dial Depth Gage General Measurement Device and Calibration Topics 6
C Gage Block Wringing General Measurement Device and Calibration Topics 3
B Gage R&R with NDC=1 Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 19
S Type 1 Gage R&R or something else? Reliability Analysis - Predictions, Testing and Standards 6
G Should I perform Gage R&R only at the beginning of a new project? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 6
V Thread Plug Gage Pitch GO Diameter out of spec AQL - Acceptable Quality Level 8
G Gage R&R - Where am I going wrong? Part of a FAIR submission (Aerospace) Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
R Gage R&R Excel templates Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
K Gage R&R with more than 3 appraisers Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
Anerol C Gage R&R Template AIAG 4th Edition IATF 16949 - Automotive Quality Systems Standard 3
T Attributes SPC study - Attributive control (Go gage) Statistical Analysis Tools, Techniques and SPC 5
S Capability or Gage R&R Study for Leak Tester? Reliability Analysis - Predictions, Testing and Standards 15
N % Tolerance - Type 1 study on the gages, then a gage R&R (ANOVA) Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4
M Definitive answer on Type 1 vs Type 2 vs Type 3 Gage Study Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 0
V Gage Management and Gage R&R Software General Measurement Device and Calibration Topics 1
D Gage R&R Study on Load Cells - Large chemical blending tanks IATF 16949 - Automotive Quality Systems Standard 1
B Minitab Type 1 Gage Study on True Position Question Measurement Uncertainty (MU) 1
S Gage R&R on alloy Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 10
8 Need Help - Runout - Function Gage Inspection, Prints (Drawings), Testing, Sampling and Related Topics 7
L Gage R&R studies for identical Devices - Need to confirm the requirement to perform them Reliability Analysis - Predictions, Testing and Standards 2
G Stab Pin Size For Check Gage Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
E Functional Gage for TOP applied at RFS Design and Development of Products and Processes 5
F No reproducibility Error in Gage R&R? Reliability Analysis - Predictions, Testing and Standards 4
M Template for Attribute Gage R & R wanted Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
G Warning about Deltronic Gage Pins - The "Certification of Accuracy" document General Measurement Device and Calibration Topics 13
A Gage type 1 study on CMM Capability, Accuracy and Stability - Processes, Machines, etc. 2
Q Gage R&R for Instant Measurement Machine Manufacturing and Related Processes 6
M Gage R&R with large inter part variance - Torque Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 7
D Do we need normal data for gage r&r studies? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 5
D Do I need part variation while doing Destructive Variable Gage R&R MSA study Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 19
S Gage R&R Standard Deviations Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 6
J Informational Gage R&R: Swapping operators Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 5
S Gage R & R for large amount of inspections IATF 16949 - Automotive Quality Systems Standard 10
S Ring Gage Thermal Expansion General Measurement Device and Calibration Topics 4
L Gage R&R for test precision or uncertainty prediction? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
S Minor diameter of threaded ring gage General Measurement Device and Calibration Topics 1
A Does Torque Gage Need Validation? Reliability Analysis - Predictions, Testing and Standards 4
J Ignoring a Failed Gage R&R and upcoming external audit IATF 16949 - Automotive Quality Systems Standard 22
R Scaling a Gage R&R - 2 holding fixtures each with 10 nests Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 7
B Gage R&R Acceptable (10-30%), deduct Total Variation from Tolerance Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 12

Similar threads

Top Bottom