Capability of new CNC machines

bobdoering

Stop X-bar/R Madness!!
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#1
It has come to my attention that there are people out there purchasing CNC machines with contractual requirements for the machine to meet a Cpk prior to payment. Apparently, this is occurring in automotive circles (big surprise.) What an abject travesty of statistics. What is interesting is that in order to meet this incorrect application of statistics, the focus shifts immediately away from tool wear (the actual underlying process) and directly onto the algorithm for auto-compensation. It becomes the process...just like any over-controlling individual would be if they constantly adjusted their process to the target value. So, as the main source of variation, does anyone really understand their auto-compensation algorithm? Do you think you should??
 
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T

True Position

#2
Why would specifying purchasing machines capable of a target CpK and auto-compensation algorithms be related?
 

Kales Veggie

People: The Vital Few
#3
It has come to my attention that there are people out there purchasing CNC machines with contractual requirements for the machine to meet a Cpk prior to payment. Apparently, this is occurring in automotive circles (big surprise.) What an abject travesty of statistics. What is interesting is that in order to meet this incorrect application of statistics, the focus shifts immediately away from tool wear (the actual underlying process) and directly onto the algorithm for auto-compensation. It becomes the process...just like any over-controlling individual would be if they constantly adjusted their process to the target value. So, as the main source of variation, does anyone really understand their auto-compensation algorithm? Do you think you should??
Let me be the devils advocate here:

I am an automotive supplier, I spend 500,000 dollar on a CNC machine. I want this machine to produce some selected characteristics of my parts without scrap under normal operating conditions. So, a CpK of 1.67 or 2.0

I do not care what kind of algorithm the manufacturer uses inside that machine to control the slides, the angles, the tool wear. It is their job to design the machine and control the process.



So, what is wrong with my thinking?
 

bobdoering

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#4
Because the auto-compensation will mask the underlying continuous uniform distribution of tool wear with its own distribution from overcontrol. Essentially it becomes the process. So, in the total variance equation, and a CNX analysis, it will overwhelm any other cause of variation. As the major variation, it will be the one that generates the resulting capability. The point is, as the major influence on capability, it should be the variation you understand the best. But....does anyone understand their auto-compensation algorithm? Is is correct, or "close enough"? How does it react to special causes, such as start-up warm-up? Should you be comfortable treating you major cause of variation as "black box"?
 

bobdoering

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#5
I am an automotive supplier, I spend 500,000 dollar on a CNC machine. I want this machine to produce some selected characteristics of my parts without scrap under normal operating conditions. So, a CpK of 1.67 or 2.0
If you were doing true precision machining, the only variation would be tool wear. It is a continuous uniform distribution - and is non-normal. Even AIAG states that Cpk is not appropriate for a non-normal distribution in its 4th edition PPAP manual:

2.2.11.5 Processes with One-Sided Specifications or Non-Normal Distributions

NOTE: The above mentioned acceptance criteria (2.2.11.3) assume normality and a two-sided specification (target in the center).

So, essentially you are forcing your supplier to meet a statistically incorrect index for what your really would want to see. Why would you want to do that when you are spending 500,000 dollars? What do you think that is going to force them to do? Why would both you are your supplier not understand the variations in the process if you are spending that kind of money? And, once you own it, how are you going to make decisions on tool change and maintenance when you still don't understand the underlying variation?

Does any of that make you actually feel comfortable using Cpk? I agree you need to understand capability, but a single number calculated using the wrong statistics seems awfully weak for a 500,000 dollar investment. Building a machine to meet that requirement seems even less like a good idea.
 
Last edited:
T

True Position

#6
If the process is stable and capable of machining the features in question but not 'true precision machining' I don't think anybody really cares. The goal is in specification parts 100% at the lowest cost. Purity of technique isn't something worth spending money on.
 
W

WKHANNA

#7
But what happens when production is not 100% in spec?
How do you determine root cause if you can not determine the true tool wear?
 
T

True Position

#8
Even with an automatically adjusting machine you can determine your tool wear with the cycle counter. If you can turn off the adjusting as well you could perform a study just like you would on a traditional machine.
 
W

WKHANNA

#9
I see.
Thanks, I am just trying to get a grasp of this.
 
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bobdoering

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#10
If the process is stable and capable of machining the features in question but not 'true precision machining' I don't think anybody really cares. The goal is in specification parts 100% at the lowest cost. Purity of technique isn't something worth spending money on.
Kind of missing the point of understanding the most significant cause of variation, and its impact on the output - as well as ignoring the underlying variation which had been the only cause of variation until an artificial on was implemented to mask it.

Even with an automatically adjusting machine you can determine your tool wear with the cycle counter. .
A cycle counter does not control nor determine tool wear - it tells you when to change it based on a guess - not on ongoing, real data as you would with the X hi/lo-R chart.


If you can turn off the adjusting as well you could perform a study just like you would on a traditional machine. .
Actually, that would be a good start on determining how much variation the auto-compensation is generating as its participation in the total variance equation.
 
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