Low Process Capability - Cpk - Man-made part - What to do?

Z

ZLYDCOM

#1
I confused that how to handle the low cpk.
Sometimes we have to "prepare" a good data for auditor to meet requirement.
But some process such as man-made part.
Can anybody help me on this?
 
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W

wmarhel

#2
You could go a couple of different routes:

1) Perhaps the specification limits are unreasonable for a hand-made part. Engineers have been known to make these types of mistakes, regardless of their comments to the contrary.

2) The tools being used, or the level of skill of the person performing the duties is inadequate.

If you could expand on the problem (process, results, etc.) maybe some more light could be shed on the subject.

Keep in mind that process capability is just the measurement of a process as compared to the specification limits. It doesn't necessarily have anything to do with the actual quality of the end product.

Wayne
 
B

bramanta

#3
Low cpk mean there is still variation of the ouput which is big if it is compared with specification range. knowing that cpk is low is the first step to improve the process. maybe the material contribute significantly the variation. or may be the machine, tool, the operator as wmarhel said. What we have to do is find the source of output variation and try to reduce it.
hope this help.
 

Statistical Steven

Statistician
Staff member
Super Moderator
#4
ZLYDCOM said:
I confused that how to handle the low cpk.
Sometimes we have to "prepare" a good data for auditor to meet requirement.
But some process such as man-made part.
Can anybody help me on this?
Only 3 things can be done.

1. Change the specifications

2. Reduce the variability

3. Sort and Suffer

Reduction in variation is the quest of most quality professionals, but it is the hardest. Changing the specification requires customer buy off, so that can be difficult. So you can always inspect every unit and throw away the units that fail.
 
A

alfawei

#5
Statistical Steven said:
Only 3 things can be done.

1. Change the specifications

2. Reduce the variability

3. Sort and Suffer

Reduction in variation is the quest of most quality professionals, but it is the hardest. Changing the specification requires customer buy off, so that can be difficult. So you can always inspect every unit and throw away the units that fail.
pls check Ca large ?
if is large>0.125, adjust the process

pls review your control chart ,make sure stable
 
M

Murphys Law

#6
Another "It depends" answer.

Low Cpk potentially suggests that you are producing parts out of spec and you need to be 100% testing/inspecting each product.

Why you are fudging it for an audit? It is a bad practice. Your management needs to be seeing this and reacting to it.

Low Cpk could also come from having outlier readings. The distribution may be bimodal. Take a different view on your data not just on the Cpk chart.

Low Cpk may also be because it is too tight a spec as was mentioned earlier.
You could negotiate a relaxation with your customer. You never know, the thing you are taking a yield on is not a customer careabout. (I have seen this many times). On the otherhand, if it is critical and you are taking considerable scrap on this, you need a continual improvement action. Either improve process or your company needs to live with the scrap. (Your business manager should be aware of this as it if it is not profitable, your company should exit that product line or make business case to invest in new process).
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#7
What is the volume? When I think of 'man-made part', I think of very low volume. How does the volume come into play here?
 
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