Can you have a Cpk over 1.67?

C

CustomTS

#1
Ok so one of our customers sent us their Cap study, with out of spec dimensions, on a stamping part that we built the tool for. Their Cap Study is showing CPKs ranging from -1.059 to 81.961 is this even possible? This is a 1 piece cap study. I don't know six sigma and I have rarely dealt with Cap studies so trying to explain why this is not right is difficult. Anyone who can shed light on this for me or explain why this is ok would be greatly appreciated. Thanks.
 
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Stijloor

Staff member
Super Moderator
#2
Ok so one of our customers sent us their Cap study, with out of spec dimensions, on a stamping part that we built the tool for. Their Cap Study is showing CPKs ranging from -1.059 to 81.961 is this even possible? This is a 1 piece cap study. I don't know six sigma and I have rarely dealt with Cap studies so trying to explain why this is not right is difficult. Anyone who can shed light on this for me or explain why this is ok would be greatly appreciated. Thanks.
Do you have access to the AIAG PPAP manual? That manual spells out the requirements for an initial capability study. You mention a 1 piece capability study.... Capability studies are typically conducted on a "significant production run." That could be 300 pieces.

Have you and your customer agreed upon a capability study for validating the tool?

Stijloor.
 
C

CustomTS

#3
NO I do not have access to the AIAG PPAP Manual. Before the tool left our facility I ran 3 parts on the CMM to verify it was correct before leaving here. As for communication with the customer I just started here about 4 weeks ago so I am just getting up to speed with a new company and have not had a whole lot of customer contact. If I understand what I have read correctly a CPK of 1.67 is 99.99999% correct out of a million parts. SO how can you have a CPK of -1.059 or 81.961? I brought this up to the boss and he told me that you need a minimum of 1.67 cpk. Again from what I have read it would be really hard to get better than a 1.67.
 

jerry_Malaysia

Quite Involved in Discussions
#4
NO I do not have access to the AIAG PPAP Manual. Before the tool left our facility I ran 3 parts on the CMM to verify it was correct before leaving here. As for communication with the customer I just started here about 4 weeks ago so I am just getting up to speed with a new company and have not had a whole lot of customer contact. If I understand what I have read correctly a CPK of 1.67 is 99.99999% correct out of a million parts. SO how can you have a CPK of -1.059 or 81.961? I brought this up to the boss and he told me that you need a minimum of 1.67 cpk. Again from what I have read it would be really hard to get better than a 1.67.
Normally when we calculate Cpk, we will get 2 different values. Real Cpk Value is the lower value between these 2 values.
 
C

CustomTS

#5
Normally when we calculate Cpk, we will get 2 different values. Real Cpk Value is the lower value between these 2 values.
Right, the CP value and the CPK value. So on 1 the Cp value is 88.959 and the CPK value is 81.961. This still seems awfully high to me for a CPK value.
 
A

alspread

#6
A couple of important points:

1. A negative Cpk value means that the average of the population is outside of the specification limits (either over max or under min)
2. You have to have a standard deviation in order to properly calculate a Cpk and you have to have a sample greater than 1 to create a standard deviation. The bigger the sample (and more random) the better (the more accurate/representative) the standard deviation. The better the standard deviation, the better the Cpk ratio.

Having a Cpk range between -1 and +81 is enourmous and is pretty useless.

Look at it like this:
If I shoot a bunch of arrows at a 20 inch target and my arrows are clustered in a circle that is 5 inches in diameter but the cluster is centered 9 inches from the bulls eye. I would have a condition where my Cp was good (ratio of 5 inches to 20 inches) but my Cpk was poor (9 inches from center on a 10 inch radii target). If the center of my cluster was more than 10 inches from the center I would have a negative Cpk because my average was off the bulls eye (out of tolerance). Your ration goes from -1 (off target) to almost +82 (all of the arrows are stacked up on top of one another). Makes absolutely no sense.

If I understand your situation correctly, you have a one piece sample, so you can't have an average or any variation (no cluster). You have to have a cluster in order to compare (ratio) the size of the cluster to the size of the target and an average to compare (ratio) the center of the cluster with the center of the target.

This is all kind of elementary stuff, and I apologize if I'm going over things you already know.

Good Luck
 
R

ralphsulser

#7
There is no such thing as a 1 piece cp or Cpk that I ever heard of in any situation. Plus, how do you know how that piece was made and under what conditions.
 

Ron Rompen

Trusted Information Resource
#8
First, to answer your question, it is indeed feasible to have a Cp or Cpk greater than 1.67, or less than 0.
Both of these values are determined using the tolerance band of the dimension in question, and the differences between successive measurements of this characteristic ON DIFFERENT PARTS, PREFERABLY SEQUENTIALLY PRODUCED AND MEASURED.
If the tolerance band is tight, and the differences between the measurements is (relatively) large, then it is quite possible that your Cp is a negative value. This means that the process is not capable of manufacturing successive parts which are the same.
If the average value of the measurements falls outside the upper or lower specification limit, then your Cpk will also show as negative.

Hope this is of some help to you. Don't hesitate to ask more questions; we're all here to help if we can.
 
R

ralphsulser

#9
One question I just thought of...did they send you a report that stated the number of parts sampled for their cp study and sent you 1 piece they indicated was representative of the Cpk samples?
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#10
Ok so one of our customers sent us their Cap study, with out of spec dimensions, on a stamping part that we built the tool for. Their Cap Study is showing CPKs ranging from -1.059 to 81.961 is this even possible? This is a 1 piece cap study.
I am aware of a stamping company that calculates Cpk based on each of their 5 piece samples rather than on the run. They are using a screwdriver as a chisel. Just because they do it doesn't make it right - but they have convinced themselves of the practice. As others have said - especially with the number of parts one makes in the stamping process - it is absurd. If one had their individual values from their entire trial run and could correctly analyze the variation it would be of must more usefulness.

First of all, Cpk is the best value of Cpl = (Mean - LSL)/3*Std.dev and
Cpu = (USL-Mean)/3*Std.dev. So, one needs the standard deviation and the mean of the sample to calculate it. What is the standard deviation on one specimen? The mean would end up being the specimen data. Not sure what data manipulation they are doing to even come up with Cpk with one piece.

The closer the mean is to the center of the specification, the larger the Cpk. To get an 81 you need to have the data very close to the center of the specification and a very small standard deviation. It is difficult to give you an example with one part, as the standard deviation of one part, again, is nonsense.

As was mentioned before, to get a negative number the mean of the data needs to exceed the spec.

Capability studies are a thumbnail view of the capability of a process, and are of little value for much else. As described before, sequential data from a controlled process is far more important analytical tool. In fact, if this is precision stamping and the major variation form the process is tool wear, Cpk would not apply because the distribution is non-normal. It should never be negative, but it will be very low as the tool should be made at one end of the spec to allow it to wear to the other end before replacement. That is not normal distribution. Poor measurement and lack of understanding of distributions allows Cpk to exist in the industry, as wrong as it is. But with a little information we have here, we don't even know if the variation is a tool issue or maybe even a measurement issue. When measurement error exceed the process variaon, the data will appear normal (measurement error is a normal distribution.) That problem has fooled many, many people.
 
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