Gage R&R on the Center Deflection of Composite Tubes

D

dsrsdr

#1
I want to measure gage R&R on the center deflection of composite tubes. The gage is pretty simple. It supports the tube on both ends and then a fixed weight deflects the center. The gage measures the deflection in mm with a digital indicator.

Can I pick tubes that are designed with different stiffness values and test together in a gage R&R? All tubes have the same tolerance spec for deflection but some will be stiff (low deflection) and some will be soft (high deflection).

This would be like measuring OD with calipers, but measuring parts in the gage R&R that have different spec OD's (some small and some large).

The reason that I ask this question is that only using 1 design won't test the entire range of the gage.....plus I don't have 10 tubes of the same design.

Thanks for your input.
 
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K

keithljelp

#2
dsrsdr,

The objective of a Gage R&R is to determine if the measurement system is capable of measuring a product being produced given the specifications for that product. You are not checking the measurement range of the test instrument. The first rule is to select 10 or more parts of product "XYZ" that have "true values" for the parameter that you will be measuring in production that covers the the entire tolerance range for the parameter you plan to measure.

The parameter you plan to measure is center deflection of a composite tube. You have 10 parts, but, they are part "XYZ", "ABC", "JKL", etc. The tolerance may be the same, but, the specification for the center deflection for each of the part types is different. You can not perform a Gage R&R using parts with different center deflection specifications. You need 10 pieces of the same design. This is one of the reasons that Gage R&R testing is so expensive.

Check the production schedule and see if that are going to be 10 parts produced of the same design in the next four or six weeks. There is nothing that says all 10 parts have to be available for measurement in the Gage R&R at the same time. Doing it this way you will effect the correlation of the readings because you can not do a true randomization between measurements. You just have to wait to get the results till all 10 parts are measured. Your company and your customer have to agree to wait the four or six weeks and accept the reduction in randomization. The only other option is to have a special production run of 10 of the same design and put put them into inventory when you are finished. You just have to look at the cost and constraints.

I hope this helps,

Keith
 
D

dsrsdr

#3
Keith,

Thanks for your explanation. This is all new to me and I wanted to make sure that I was not wasting time measuring parts that would not give us meaningful information. This is a new process and we want to get started in the right direction. I now understand that I need to use samples from the same design for gage R&R studies.

I was able to get 10 parts made of the same design. I did a 3x3x10 gage R&R with the following results.

% process variation (TV)

% Equipment Variation 38.2%
% Appraiser Variation 24.8%
% R&R 45.6%
% Part Variation 89.0%

Based on the above results, I am assuming that the current gage is not adequate. Is the next step to take a close look at what can cause variation in the gage, make adjustments, upgrade, etc. and repeat the gage R&R until the % R&R is <30%.

If this is not the best approach, what should be my next step?
 

Miner

Forum Moderator
Staff member
Admin
#4
Do you have any reference material on MSA? If not, my blog will give you a good overview. The MSA, 4th edition reference manual published by AIAG will give you good reference in the the most commonly used approach promoted by the US auto industry. Another good reference is EMP III written by Donald Wheeler. This one is less widely accepted, but is technically correct unlike the AIAG manual/method.

Regarding your particular study, it is hard to say without seeing the MSA graphs. Do you have adequate gauge resolution? This is a very common reason for a bad MSA. My blog explains how to interpret the graphs, and in the correct sequence.

If you would attach your data, we can provide more specific guidance.
 
K

keithljelp

#5
Dsrsdr,

Your test design is the most critical element. If it is not up to the task everything else is meaningless. Does ASTM D790 apply for your product?

Here are several things I would suggest that you check:

1. How long between tests did you allow the tubes to relax so that the composite matrix could get back into its "normal" equilibrium? Did you check with the design engineer(s) on how long to allow the composite matrix to regain equilibrium? I would have doubled their estimate.

2. Did the you ask the design engineer(s) what the maximum number of deflections that each tube could withstand without taking a permanent set? If it is less than nine you have a problem or if it is close to nine on the high side you will need to reevaluate your test design.

3. Did the temperature and/or humidity change between tests?

4. How stable was the mounting fixture for the instrument you used to measure the deflection? The mounting fixture becomes part of the instrument and any instability in the mounting fixture will show up as equipment variation.

5. Did you use a "point" tip on the deflection instrument or did you use a one inch diameter flat tip? More equipment variation with the "point" tip.

6. Did you mark the center of each of the tubes longitudinally and athwart the tube, and did you mark the locations on each end where the supports where to be places?

7. Where the tubes mounted so that they would not move as the load was applied?

8. Was the load always placed in the same spot by each of the operators? How quickly did each operator apply the load?

9. How much preload did each operator put on the deflection instrument before it was zeroed? Did all the operators preload the instrument the same?

10. Was the deflection reading in the center of the reading range of the instrument?

11. As Minor said, Was the resolution of the gage at least five times better than the deflection specification?

12. Did the operators tear down the setup after each test to remove the tube?

13. How much experience did the operators have with the test procedure? Was all the data correctly recorded by the operators and entered into the computer correctly?

14. Did you have the operators practice the test procedure? Did you emphasize that speed in the testing was very very bad?

15. Does the layup of the composite material effect your test?

If I had satisfied myself that all these questions where controlled and I had used the ANOVA method to calculate the Gage R&R then I would start looking for a new gage.

Good luck. If you have any questions I will be happy to help.
 
D

dsrsdr

#6
View attachment gage r&r Center Flex Tube 450-82B.xls

View attachment Wheeler's Honest Gauge Study - Tube 450-82B.xls

Thanks guys. I have attached the data in two Excel files that I found on this forum. I have not had any training so I am not sure what is important in the results. I am currently dong some work on the gage to take out some items in hope to make it more stable. I will also answer the questions from Keith the best I can.

Appreciate your guidance on what are the key items in this study.

1. How long between tests did you allow the tubes to relax so that the composite matrix could get back into its "normal" equilibrium? Did you check with the design engineer(s) on how long to allow the composite matrix to regain equilibrium? I would have doubled their estimate. [dsrsdr] - Parts should have been okay. They were all the same design but made over a few day span and the parts should have been stable.

2. Did the you ask the design engineer(s) what the maximum number of deflections that each tube could withstand without taking a permanent set? If it is less than nine you have a problem or if it is close to nine on the high side you will need to reevaluate your test design.
[dsrsdr] - These parts should be able to take thousands of deflections. This test applies a load that won't create a permanent set.

3. Did the temperature and/or humidity change between tests?
[dsrsdr] - No. All parts were measured in the same afternoon.

4. How stable was the mounting fixture for the instrument you used to measure the deflection? The mounting fixture becomes part of the instrument and any instability in the mounting fixture will show up as equipment variation.
[dsrsdr] - This is what I wanted to find out in the R&R. Won't the R&R tell us if the gage won't do the job.

5. Did you use a "point" tip on the deflection instrument or did you use a one inch diameter flat tip? More equipment variation with the "point" tip.
[dsrsdr] - Tubes are round so we are using a round pin that is perpendicular to the tube so we get point contact where the pin and tube touch.

6. Did you mark the center of each of the tubes longitudinally and athwart the tube, and did you mark the locations on each end where the supports where to be places?
[dsrsdr] - Fixture has end stops to locate part. Part to part location should be repeatable.

7. Where the tubes mounted so that they would not move as the load was applied?
[dsrsdr] - This could be a problem because we need to let the part rotate to find the "spine". When the load is applied for the test the center goes down and the ends go up a little. This is not an easy fixture change so I wanted to see if we could get good readings before we made any changes to this part of the fixture.

8. Was the load always placed in the same spot by each of the operators? How quickly did each operator apply the load?
[dsrsdr] - Load is applied by the gage so this is not operator controlled. Location of load could change slightly from operator to operator. This is one of the changes that we are making now in the gage.

9. How much preload did each operator put on the deflection instrument before it was zeroed? Did all the operators preload the instrument the same?
[dsrsdr] - No preload was applied.....only gravity. If load on the tube is 50 lbs, any suggestions for a preload?

10. Was the deflection reading in the center of the reading range of the instrument?
[dsrsdr] - Yes. The gage is set up so the reading is in the center of the fixture.

11. As Minor said, Was the resolution of the gage at least five times better than the deflection specification?
[dsrsdr] - digital read out measures 0.00 places in mm. We are only setting out specs and tolerance to 0.0 places in cm.....so the gage is reading to 0.01 mm (0.001 cm) and are specs are tolerances are in 0.1 cm increments.

12. Did the operators tear down the setup after each test to remove the tube?

13. How much experience did the operators have with the test procedure? Was all the data correctly recorded by the operators and entered into the computer correctly?
[dsrsdr] - all data was recorded properly. I was present and checked. Only one operator has been doing the testing (operator 3).

14. Did you have the operators practice the test procedure? Did you emphasize that speed in the testing was very very bad?
[dsrsdr] - 1 and 2 were trained by 3. Speed should not have been a factor.

15. Does the layup of the composite material effect your test?
[dsrsdr] -layup on all parts should have been the same.....but again this is what I wanted to learn from the R&R. I was trying to determine if the variation is in the parts and not the gage.

I will have to spend some time on Miner's blog but thought I could get some insights first. Looking forward to your response. :confused:
 
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