SPC for Dummies - Measurements by cavity in a program called QcCalc

foxwilds

Starting to get Involved
#1
Hello,

We currently track individual measurements by cavity in a program called QcCalc. I'd like to group the cavities (say a 6 cavity mold) and XBar R chart the single shot of 6 Cavities. 16 would be the maximum amount for any group.

Someone talk me down from the ledge.

Perry
 
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bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#2
Re: SPC for Dummies

We currently track individual measurements by cavity in a program called QcCalc. I'd like to group the cavities (say a 6 cavity mold) and XBar R chart the single shot of 6 Cavities. 16 would be the maximum amount for any group.
Before you do that you need a convincing argument that the variation between cavities is statistically insignificant, so that when you are analyzing your variation and out of control events, you know that it is a process issue (material, pressure, temperature, humidity, etc.), and not a cavity issue, flow or mold issue, and can respond accordingly.

Of course, if your molds are that good, why not just do one cavity? Feeling lucky? :notme:

Some short cuts never get you to where you were trying to go the long way...
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
STEP AWAY FROM THE LEDGE

seriously, the fundamental concpet of the Xbar R or Xbar S chart is that the within subgroup variation is homogenous and representative of the full spread of the process (otherwise known as independent).

If your individual cavities have systemic differences (which most do) lumping them all together will create a 'dumb' looking chart.

there are ways to lump them together using a median individuals chart

you might consider posting the data for oen or more molds and we can help walk you thru a couple of approaches that might work for your needs
 
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