Cpk for Hardness - PPAP rejected due to no Cpk value being given for a hardness

W

WKHANNA

#1
I just had another PPAP rejected due to no Cpk value being given for a hardness requirement that was designated as a Special Characteristic on the customers drawing. What I did supply was a certification & test results from the approved supplier we use for heat treating, along with our own test results.

They also said we have no documented control for this Special Characteristic. Our Control Plan specifies certification from the supplier along with recorded receiving inspection on each lot.

I am no expert on these matters by any means, but just how does one go about calculating Cpk for material hardness? :frust:


TIA

Bill
 
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howste

Thaumaturge
Super Moderator
#2
Re: Cpk for Hardness? PPAP rejected due to no Cpk value being given for a hardness

I'm not an expert either, but I don't see why it can't be done. The distribution may or may not be normal, but I'm sure that there's variation in the hardness values. I'm sure that multiple readings can be taken, analyzed, and compared against the specification.
 
W

WKHANNA

#3
Re: Cpk for Hardness? PPAP rejected due to no Cpk value being given for a hardness

Agreed, and our test measurements include the mean distribution and stdev.
But for Cpk, wouldn't the heat treater need to conduct the analysis on his process?
 
A

AdamP

#4
Re: Cpk for Hardness? PPAP rejected due to no Cpk value being given for a hardness

We used to have this with an abradable ceramic seal coating and with one major customer, we had a requirement of stating and meeting a Cpk of 1.3. We ran the hardness and abrasion tests in-house but with each run we recorded the result, plotted it on an in-house control chart and included the test results with each shipment. The capability was established not via a single sample, but through a small sample of test coupons to prove initial capability.

I expect you can have your heat treater pull up a pile of previous results to get enough data to baseline your (their) capability with respect to the hardness testing. Assuming the control limits of your/their process fit within the spec limits of your customer, you should be able to show and control via SPC and periodic capability checks.

Cheers,

Adam
 
Last edited by a moderator:

Jim Wynne

Staff member
Admin
#5
Re: Cpk for Hardness? PPAP rejected due to no Cpk value being given for a hardness

We used to have this with an abradable ceramic seal coating and with one major customer, we had a requirement of stating and meeting a Cpk of 1.3. We ran the hardness and abrasion tests in-house but with each run we recorded the result, plotted it on an in-house control chart and included the test results with each shipment. The capability was established not via a single sample, but through a small sample of test coupons to prove initial capability.

I expect you can have your heat treater pull up a pile of previous results to get enough data to baseline your (their) capability with respect to the hardness testing. Assuming the control limits of your/their process fit within the spec limits of your customer, you should be able to show and control via SPC and periodic capability checks.

Cheers,

Adam
Doing SPC retroactively is almost never a good idea in terms of statistical rigor, but something along these lines might be enough to placate the customer. Based on the OP's description of what was submitted, I can understand the customer's dissatisfaction; the process control and reporting requirements should have been passed down to the heat treater and if there were misgivings anywhere they should have been discussed and settled in advance.

The presence of special characteristics on a customer drawing--especially in automotive work--is a big red flag that's warning you that you had better understand the customer requirements and not wait until a PPAP crashes and burns before you complain that something doesn't make sense.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#6
Re: Cpk for Hardness? PPAP rejected due to no Cpk value being given for a hardness

I'm not an expert either, but I don't see why it can't be done. The distribution may or may not be normal, but I'm sure that there's variation in the hardness values. I'm sure that multiple readings can be taken, analyzed, and compared against the specification.
I would expect the variation to be normal -within a batch, since the variation comes from natural conditions. But, over many batches - and more importantly, lots of raw materials - it may become very multi-modal, which will confound standard capability calculations that assume normality. Now, if they wanted Cp or Pp, then it becomes a lot easier because the expectation that the mean is perfectly midway between the spec (which it will not be on skewed distributions) is not an issue.

The rubber stamped Cpk and Ppk requirement, and its subsequent assumption that the mean is perfectly midway between the spec for every characteristic of every process in control and capable, is key evidence of abject lack of understanding of process capability and distributions. But yet.....it is carved in stone. It is rampant normalcentricity syndrome.
 

Kales Veggie

People: The Vital Few
#7
Re: Cpk for Hardness? PPAP rejected due to no Cpk value being given for a hardness

I would expect the variation to be normal -within a batch, since the variation comes from natural conditions. But, over many batches - and more importantly, lots of raw materials - it may become very multi-modal, which will confound standard capability calculations that assume normality. Now, if they wanted Cp or Pp, then it becomes a lot easier because the expectation that the mean is perfectly midway between the spec (which it will not be on skewed distributions) is not an issue.

The rubber stamped Cpk and Ppk requirement, and its subsequent assumption that the mean is perfectly midway between the spec for every characteristic of every process in control and capable, is key evidence of abject lack of understanding of process capability and distributions. But yet.....it is carved in stone. It is rampant normalcentricity syndrome.
Be careful with "within a batch". Depending on batch size and available material from the same heat, material variation has to be taken into account.
 

Kales Veggie

People: The Vital Few
#8
I just had another PPAP rejected due to no Cpk value being given for a hardness requirement that was designated as a Special Characteristic on the customers drawing. What I did supply was a certification & test results from the approved supplier we use for heat treating, along with our own test results.

They also said we have no documented control for this Special Characteristic. Our Control Plan specifies certification from the supplier along with recorded receiving inspection on each lot.

I am no expert on these matters by any means, but just how does one go about calculating Cpk for material hardness? :frust:


TIA

Bill
To calculate Cp or Cpk the distribution has to be normal. Unlikely that the distribution will be normal.

This requirement should have been flowed to the Spec. Char. requirement to the sub-supplier, so they should have SPC in place and you can review their data to determine normality and attempt to calculate a Cpk.

The hardness is probably just a minimum requirement.

Also the question: Is it through hardened or case hardened? Then is case depth a Special Char.?
 
W

WKHANNA

#9
Re: Cpk for Hardness? PPAP rejected due to no Cpk value being given for a hardness

The presence of special characteristics on a customer drawing--especially in automotive work--is a big red flag that's warning you that you had better understand the customer requirements and not wait until a PPAP crashes and burns before you complain that something doesn't make sense.
I agree totally, however our customer has recently adopted new standards of acceptance relative to PPAP review, and simply began rejecting submissions that previously would have been accepted.

Additionally, the parts in question are for a new product that were designed with GE&T and Hardness specs being listed as Special’s for the first time.

I understand that I am responsible for insuring our processes are capable, but instead of giving advice on what their acceptance standards are, I just get emails with demands for Cpk values on things like surface profile & ‘no burrs’.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#10
Re: Cpk for Hardness? PPAP rejected due to no Cpk value being given for a hardness

Be careful with "within a batch". Depending on batch size and available material from the same heat, material variation has to be taken into account.
That's correct, when you get to long term between batches, it can become multimodal. If the material variation over time is also normal you may ultimately have a net normal distribution - perhaps over 3 to 5 years time - depending on how many batches of raw material are processed over time.

None of which is what an SQE wants to see - or ponder. They want a number to meet the ethereal specification.
 
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