Cpk, GD&T & Minitab? Please review my methods

[David DeLong]

This is much more complex than first meets the eye. Is the feature of size round? If it is, then the tolerance in the feature control frame is a diametrical tolerance zone. Does the feature control frame shown at MMC? Is the reference datum also a feature of size? Is it in MMC? This is quite different from a coordinate tolerances to a centre of a hole(s).

Capability studies on positional tolerances is extremely difficult and what you did just doesn't make sense to me.

Good luck!

 

[bobdoering]

Capability studies on positional tolerances is extremely difficult...

If not futile and meaningless.

 

[David DeLong]

If not futile and meaningless.

You are absolutely correct here, Bob. It is so costly to do and I can't believe customers are still asking for it. Most customers really don't understand GD&T that well anyway and today, there just isn't any $$ left for training.

 

[JAltmann]

I don't even understand why your customer is bothering to ask for Cpk, since you stated the full order was for 30 pc's. I would have argued that point from the onset of the order at the contract reveiw phase.

Cpk is meaningless when the full order is complete, it is a predictor that the process will continue to remain stable over the long term of production, in this case your production is done.

But none the less the previous contributors offered some great word of wisdom to all dealing with capability studies.

 

[bobdoering]

I always say that the only person that should put that much meaning into a single number is a lottery winner. You know how well lottery winners and statistical analysis go together....

 

[WKHANNA]

The characteristic is the width of the gear.
The GD&T control frame shows the position symbol followed by the value .016, followed by datum A MMC(the journal area on the shaft), followed by datum B MMC (not shown on the drawing).

I know! We should never have even made the part until this was clarified. These were first-production 2nd Rev parts for Beta testing assys. Lager scale production will come in the near future.

The customer simply wanted parts ASAP, then afterwards, insisted on PPAP documentation.

They are instituting the QMS from their automotive division across all the other divisions. This has been an ongoing process for four years now. Meanwhile, they keep moving the bar on us relative to their documentation requirements without warning.

We never supplied Cpk data on GD&T spec’s that were called out as special characteristics before, though we certainly took them into account. Fortunately, we have V knowledgeable engineers here with years of practical experience who can infer what it is the customer is ‘trying’ to say ( I actualy used to work for this customer). These PPAP’s were always accepted previously.

But recently, they moved the bar again, rubber stamping Cpk’s requirements for everything from GD&T’s, Hardness after heat treating & even plating thickness?!?!

I swear, I think it’s caused from the Legal Dept. based on the issue of liability. If a part fails, they simply don’t want to be held responsible.

We are a 70 employee shop trying our best to keep our head above water and compete with international manufactures (i.e. China) while maintaining our good standing with a Fortune 500 mega-corp with plants all ove r the world who makes over 13 Billion each year in everything from aerospace, automotive, electrical, hydraulics to trucks.

In the past five years we went from a PPM of 11,746 to 595 in 2010 averaging 3 million piece-parts per year. Consider too, that we make nearly 800 different parts for this customer.

The only way we can stay in business with this customer who is 70% of our production volume is to give them what they ‘think’ they want. Meanwhile, we end up having to re-engineer their parts to work properly, provide reams of documentation, cut costs, improve quality & delivery AND cope with bean counters in Supply Chain who would not know the difference between multi-axis CNC and mini-series TNT, while the design engineers couldn’t figure out how to play with an erector set without first making a Pro-E model, and then go on to tell us the manufacturing engineers at the plants have the last word on what is an acceptable and functional part.

Oh well, such is life in the early 21st century for a lowly quality manager in the NE industrial rust belt of the great US of A as I watch the ways of the world change faster than oil consumption in India.

Yun's have great day now, Yah hear!

 

[Mr.Happy]

Hi Bill,

In your first post you mentioned 30 readings, later you posted only 27?
In your last post you explained the drawing spec.
"position symbol followed by the value .016, followed by datum A MMC(the journal area on the shaft), followed by datum B MMC"
Did you ad the MMC bonus in your readings?
Further I will attach a file where you can easily test the capability of your data.
Also I want to congratulate you and your collages with your PPM rates

Cheers and good luck.

Mr.Happy