MSA on a Hardness Tester to satisfy a Customer who is requiring Hardness Cpk

W

WKHANNA

#1
I want to perform a ‘valid’ MSA on our hardness tester to satisfy a customer who is requiring hardness Cpk on the PPAP for a part we machine & have heat treated by an out side supplier.

The more I search and read, the more confused I become.

The customer’s specification for the part is 86 – 89 HR15n.
I can select between a pre-load of 3 or 10 Kg on my hardness tester.
Do I use parts from the same production, material & heat treat lot, or do I use my standard (88.2 +/- 1.0)?
How do I account for the +/- 1.0 tolerance of the standard? Does that mean the limit on my MSA is now 85 – 90 HR15n if I use the customer's parts?

I am using a very old 13.30 version of MiniTab for calculating my results.

Thanks in advance for any and all help.
 
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#2
Well, start reading the threads about hardness MSA at the bottom of the page.

You have a serious issue here.

MSA on hardness is difficult and almost never capable. First of all you can never measure the same spot more than once. You have to deal with within part variation.

Morever the HR15n is measuring with 15Kgf.

I would start with the short method using a master block (see MSA manual for details) to see what an estimate of the GR&R is.
 
W

WKHANNA

#4
The range on the customer's hardness spec is 86 - 89 HR15N.
They want a Cpk of 1.66 for the PPAP.

Even if the gage were perfect, the tolerance is so tight it will be impossible to meet their requirement.

Yet they insist they are getting 'input' that says it is possible. As per the last email, ?We have been working another supplier on this process so we are becoming well versed in the requirements at this point."

So apparently this makes them 'experts' now?

What is most aggravating is that they originally accepted the PPAP based on testing each lot as it is produced. We supplied the heat treater?s certified test data on lot samples along with our own test results.
 
S

sc00by

#5
You should use your customer required tolerance 86-89 instead of 87-89 so this will drop your R&R result by a bit for a start.

In order to perform R&R of this gauge you do not have to use customer parts to prove the gauge works OK but I would suggest some different form of samples that you can trace hardness and be sure that hardness of the sample is uniform.
This may give you more confidence in the gauge system but if it fails then you will have to look for some possible equipment upgrade.

Once you will be able to show acceptable gauge results then you can start working on your process Cpk.

You haven't mentioned the method for hardening the material but there are some ways to reduce possible spread depending on manufacturing method i.e. hardened in a chamber then extremities are typically on the outside of the load - by reducing load size you can reduce hardness spread (it will cost more though).
 
W

WKHANNA

#6
The gage MSA report was produced using my standard test block whose hardness is certified to 87 +/- 1.0 HR15n.

I have attached a test report on 50 of the customer's parts that were selected at random form a batch of 2100 parts.
 

Attachments

E

eturkan

#7
I want to perform a ?valid? MSA on our hardness tester to satisfy a customer who is requiring hardness Cpk on the PPAP for a part we machine & have heat treated by an out side supplier.

The more I search and read, the more confused I become.

The customer?s specification for the part is 86 ? 89 HR15n.
I can select between a pre-load of 3 or 10 Kg on my hardness tester.
Do I use parts from the same production, material & heat treat lot, or do I use my standard (88.2 +/- 1.0)?
How do I account for the +/- 1.0 tolerance of the standard? Does that mean the limit on my MSA is now 85 ? 90 HR15n if I use the customer's parts?

I am using a very old 13.30 version of MiniTab for calculating my results.

Thanks in advance for any and all help.
1-) I am not sure if Minitab 13 is compatible with MSA 4'th Ed. (2010)
2-) I am not sure about the equations behind the nested Gage RR (vs Crossed-Anova) but for destructive test, you need to choose nested.

3-) While experts are all here I got a few questions to ask.

3.1 For Bias and Stability we must use the real parts right?
3.2 For Linearity Gage Blocks can be used? Or do we need to pick real parts from the low end, middle and higher end of the spec.
3.3I know that we use %TOL if our CPK is over 2.5 but do we still use it just because it is higher than %GRR?

Thanks..
 
S

sc00by

#9
You have mentioned that you are using test block certified to 87?1 yet you get results of up to 89.3 and mean value at 88.96 so something doesn't add up. Have you got your hardness tester checked and calibrated recently?

By typing in your R&R data values into Cpk you achieve 1.71 however when taken real values of samples it comes out 0.68 and you are using almost 90% of your tolerance width against 30% of the tolerance in first case. So then your parts spread would have to be of a similar spread as your test block... not possible yet however... there are number of things you can consider to improve the situation.

Firstly and foremostly send the hardness tester for callibration, there are number of gauge companies on the market you will be able to talk to about trial of their hardness testing equipment and so you can compare how your equipment compares to some modern and possibly more accurate models.
Secondly how do you prepare samples prior to test, do you lap the surface and then test it? I assume you do but if not then you should consider such surface preparation.
Thirdly investigate how your batch size affects affects heat treatment by tracing samples from specific points of your chamber i.e. top level, mid level, bottom level and also corners vs centre of each level. Parts in a volume of the chamber are never the same because of slightly different conditions in certain parts of it.
There is also a possibility to consider modification to the heat treatment process i.e. time, temperature, curing method.

As a last and most radical way but also the most effective one would be to change the source material to a better quality - this obviously would affect the cost of the final product but it may be the only way to gradually improve the qulity vs required spec.
 
W

WKHANNA

#10
You have mentioned that you are using test block certified to 87?1 yet you get results of up to 89.3 and mean value at 88.96 so something doesn't add up. Have you got your hardness tester checked and calibrated recently?
On our MSA Study, the standard value is shown as 87.2 min & 89.2 max.
The standard is marked 88.2 +/- 1.0

By typing in your R&R data values into Cpk you achieve 1.71 however when taken real values of samples it comes out 0.68 and you are using almost 90% of your tolerance width against 30% of the tolerance in first case. So then your parts spread would have to be of a similar spread as your test block... not possible yet however... there are number of things you can consider to improve the situation.

Firstly and foremostly send the hardness tester for callibration, there are number of gauge companies on the market you will be able to talk to about trial of their hardness testing equipment and so you can compare how your equipment compares to some modern and possibly more accurate models.
I have ordered new USA made & certified test block statndards for HRC & HR15n scales. When the new HR15n standard arrives i will mark 10 distinct areas on it & perform a new MSA.

Secondly how do you prepare samples prior to test, do you lap the surface and then test it? I assume you do but if not then you should consider such surface preparation.
You mention a V interesting point. The parts are small threaded insert nuts & are V difficult to measure. Also, after heat treating, the parts are plated & then thermally trated for hydrogen embrittlement. Any valid testing resluts would only be obtainable from 'test' samples of the actual raw bar stock used to make the part. We use the samples to verify the lot but the customer tests the final parts.
Thirdly investigate how your batch size affects affects heat treatment by tracing samples from specific points of your chamber i.e. top level, mid level, bottom level and also corners vs centre of each level. Parts in a volume of the chamber are never the same because of slightly different conditions in certain parts of it.
The parts are .625" dia x .312" thick. the production run is roughly 2000 parts. they take up V little room in the contractors oven.
There is also a possibility to consider modification to the heat treatment process i.e. time, temperature, curing method.
The cusotmer will not modify their specification.

As a last and most radical way but also the most effective one would be to change the source material to a better quality - this obviously would affect the cost of the final product but it may be the only way to gradually improve the qulity vs required spec.
Not likely. As long as material cert is provided our plant manager will always insist purchasing order based on the best price.
 
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