SPC for Single Batch Nickel Plating Process

A

AndrisSkulte

#1
Hi fellow Covers,

We have a customer print that requires the outer diameter of a nickel plated shaft (.750" ?.00025") process to meet 1.33 CPk. The nickel plating is a single batch process done by a supplier where parts are 100% inspected after the batch plating, accepted if in-size, and lapped into size if oversize.

Per our supplier, the plating operation is not capable of plating all parts to size, let alone statistically capable without the lapping step.

Is there a statistical technique we can recommend to our customer? Or just push back stating SPC isn't valid for this process and we'll provide 100% measurement results.
 
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Golfman25

Trusted Information Resource
#2
"Or just push back stating SPC isn't valid for this process and we'll provide 100% measurement results. "

That's the route I would take. If you are doing 100% inspection, then no need for SPC. Maybe ask them what they would do if cpk wasn't 1.33. Good luck.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#3
Just want to state - I highly disagree with the statement if you are doing 100% inspection you don't need SPC. The analysis of the measurement data will tell you how likely you are to making an out of spec part, and will also pick up on a trend towards a limit faster than waiting to exceed the limit.
 
A

AndrisSkulte

#4
Steve - The challenge with this part is there is no opportunity to identify a trend and make a correction. It's done in one batch, all parts plated in the tank at once, twice a year.

I could see merit in mapping out the tank, seeing which areas plate heavier than others based on currents, to improve the process (and predict which 'stations' will need lapping), but that's still not SPC per the print.

Is there a statistical way to do SPC on a single batch?

Thanks for the feedback.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#5
Is there a statistical way to do SPC on a single batch?
Yes, although there may not be a natural sequence to the parts within a batch, you can still do a data analysis that calculated the mean and standard deviation of the batch. With those results, you may still calculate the "capability" of the batch, and you could do xbar-S charts across the batches however often they occur to see if something shifts in a given infrequently made batch. If you are committed to collecting 100% of the measurement data anyway, the additional time to put them on a control chart is very minimal, especially compared to the potential benefits and avoidances.
 

Miner

Forum Moderator
Staff member
Admin
#6
You could also use SPC (or automated controllers) on the process itself, specifically temperature, chemical concentrations, time in tank, etc.
 
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