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cnc (computer numerically controlled), cnc lathes, machine capability, capability study (part dimensions from a process or machine)
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Old 30th July 2003, 12:13 AM
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Question Machine Capability Studies on CNC Lathes

I'm currently conducting capability studies on some CNC Lathes. I was going to have them run me a 100 consecutive pieces. The question I have should I allow them to offset the machine during the study.

Appreciate your help

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Old 30th July 2003, 02:10 PM
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What do you mean to "allow them offset the machine" ?

Capability study main objective is to "know" common causes related to equipment; no adjustments, no operator changes, no measurement method- devices changes, material from same lot, etc.

Pieces needs to be consecutive and numered to check process stability

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Old 30th July 2003, 03:18 PM
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Agree with gal

What do you mean to "allow them offset the machine"?

Capability study main objective is to "know" common causes related to equipment; no adjustments, no operator changes, no measurement method- devices changes, material from same lot, etc.

Pieces needs to be consecutive and numered to check process stability


I am not a staistical guru but IMHO, you need 120 parts before you have a staistical sound sampling of the process. Those parts can not be influenced in any way to determine true capability.

Where did automotive get their 30 pc. therory is what I'd like to know.


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Old 30th July 2003, 04:00 PM
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Quote:
In Reply to Parent Post by DWPOZE

I'm currently conducting capability studies on some CNC Lathes. I was going to have them run me a 100 consecutive pieces. The question I have should I allow them to offset the machine during the study.
If you mean adjusting or centering the process, then NO.

If the CNC has any built-in program/cycles for automatic tool nose radius compensation etc, then that should be considered a part of the process and should be allowed.

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Old 30th July 2003, 05:18 PM
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Question

Atul.............

Any comments regarding automotive selected sample size for cap. studies?


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Old 30th July 2003, 07:05 PM
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Quote:
In Reply to Parent Post by DWPOZE

I'm currently conducting capability studies on some CNC Lathes. I was going to have them run me a 100 consecutive pieces. The question I have should I allow them to offset the machine during the study.

Appreciate your help

I agree with the others. In our shop, the term 'offset' is another name for adjust. Adjustments within a study are not allowed.
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Old 30th July 2003, 11:12 PM
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I understand that no adjustments (offsetting) should be made to the machine. But if set the machine up to the mean and run 100 consecutive pcs it will tell you how capable the machine is without operator making adjustments. But when your trying to hold a .001" total tolerance, adjustments will have to be made sooner or later during production
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Old 31st July 2003, 08:54 AM
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"I understand that no adjustments (offsetting) should be made to the machine. But if set the machine up to the mean and run 100 consecutive pcs it will tell you how capable the machine is without operator making adjustments. But when your trying to hold a .001" total tolerance, adjustments will have to be made sooner or later during production"

Two different processes.
- Initial process capability study; 100/125 pc sample size, run parts with no adjustments to set-up, measure oarts, plot on chart, evaluate, make adjustments and re-run study.

- on-going process control; evaluate charts periodically, make changes when process shows evidence of gion out of control.
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