Machine Capability Studies on CNC Lathes

P

pinpin - 2009

#31
But, as long as you can keep the process between 75% of the tolerance, and you are capable to at least 1.33 [capability=(USL-LSL)/(UCL-LCL)].
Sir,

This 75% is at the center of the width of USL and LSL?

When you submit PPAP with the capability study, capability of 1.33 is acceptable by the customer instead of 1.67 using your method?

Using [capability=(USL-LSL)/(UCL-LCL)] in this method, we do not calculate the smaller of (USL - mean)/3s or (mean - LSL)/3s? If (UCL-LCL) is not in the center of (USL-LSL), is it acceptable by the customer?

Please teach me, thank you!

:thanks::thanx:
 
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bobdoering

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#32
This 75% is at the center of the width of USL and LSL?

Yes!

When you submit PPAP with the capability study, capability of 1.33 is acceptable by the customer instead of 1.67 using your method?

Using [capability=(USL-LSL)/(UCL-LCL)] in this method, we do not calculate the smaller of (USL - mean)/3s or (mean - LSL)/3s? If (UCL-LCL) is not in the center of (USL-LSL), is it acceptable by the customer?


It depends on the customer. Remember, the 1.33 criteria is based on a bilateral normal distribution (as stated above), not the non-normal distribution found in precision machining. If the customer demands 1.67, then set the limits at 60%. According to 4th ed PPAP, it is up to the customer to determine an alternative, although I offer them my system as the alternative. There is no need to "center" the distribution, as it is centered as 75% of the tolerance when you set up the control limits. The use of the smaller of (USL - mean)/3s or (mean - LSL)/3s is specifically for bilateral normal distribution. The normal distribution goes on to some degree forever, but we decide to "call it quits" at +/-3s. The uniform or rectangular distribution - in its perfect state - stops exactly at the control limits. That is why capability=(USL-LSL)/(UCL-LCL)].
 

bobdoering

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#33
How to tell which gage has less than 10% so that we can use it?

You need to at least have some samples of the part with enough variation that you can tell the parts apart with the measurement. You need gaging with discrimination of at least 10:1 to your tolerance. If your tolerance is .001, the gaging should read at .0001. If you are doing SPC, it is best to have 10:1 to your control limits, not the tolerance. So for precision machining, .001 is the range between your control limits (.00075) would need to read at .000075 After all, if you do not have 10 units of discrimination within your control limits, how can you tell what your process is doing? This is a starting point. Follow the MSA instructions for GR&R, and determine the true statistical discrimination (ndc). ndc>=10 when calculated using the control limits. MSA says ndc should be >=5. I find that woefully inadequate. That is about as much discrimination as a gas gage.
 
P

pinpin - 2009

#34
The normal distribution goes on to some degree forever, but we decide to "call it quits" at +/-3s. The uniform or rectangular distribution - in its perfect state - stops exactly at the control limits. That is why capability=(USL-LSL)/(UCL-LCL)].


Sir,

Sorry, what do you mean by "call it quits" ?

How to tell which gage has less than 10% for us to choose before we use for the measurement/study?

With reference to first version of AIAG SPC Manual:

1) Page 126 says "If the process variation is known....", how do you know or how can we tell?

2) What is this "process variation"?

3) Page 119 says to assess a measurement system prior to engaging in spc, if it is, we will have to wait until the production run is completed in order to select parts that represent the range of the process. Is it correct? Can we do GRR and SPC at the same time? especially when we run a new part?

4) Page 126 of MSA says "The specific gage the team using has a % GRR = 25% of the tolerance....". How do we know? This section is about Attribute gage, how to establsih %GRR for attribute gage system? , if we have not conducted a GRR for this gage using the parts understudy in variable manner, how to arrive at %GRR?

5) In your earlier post, you said to use gage that is not more than 10% of the tolerance (part or process?), how to tell this % before we do GRR? and the system has not been used on this new part yet?

Please teach me...thank you!
 

Caster

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#35
As a quality professional, even if it were true that it was more work, one should recognize that less work does not trump correct.
Bob

Thanks for the reasoned and informative response. Point well taken, I will be ordering your book as soon as I can so I can try out these ideas on our parts.

One of our most skilled machinists used to say as he did a better job holding tolerances, the SPC chart would punish him with ever tighter tolerances. I'm not sure I totally agreed with him, but I am keen to try out your approach.
 

bobdoering

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#36
Sorry, what do you mean by "call it quits" ?
The Gaussian curve - or normal curve - is a function whose values extend to +/- infinity. So, in order to deal with that they artificially consider +/- 3 sigma the "endpoints" of the function (or "call it quits" at those points - an American colloquial term).
How to tell which gage has less than 10% for us to choose before we use for the measurement/study?
You have touched on one of the problems with GR&R. To use another American colloquialism - chicken and the egg. You cannot tell ahead of time if a gage is good enough to measure the process variation you need to supply to the GR&R process. But, since that is not a precise process anyway, use the best gage you have available - best being judged as least potential for gage and measurement error (High resolution, little operator intervention, measured at the same spot on the part)
With reference to first version of AIAG SPC Manual:
1) Page 126 says "If the process variation is known....", how do you know or how can we tell?
My guess is a study of the Total Variation Equation - but it is unlikely they were thinking of anything that sophisticated. Most likely they are thinking of a capability study.
With reference to first version of AIAG SPC Manual:
2) What is this "process variation"?
It should be the portion of the total process variation specifically attributable to the process alone - that is, stripped of non-process variation, such as of operator, gage, measurement and lot variations, as examples.
With reference to first version of AIAG SPC Manual:

3) Page 119 says to assess a measurement system prior to engaging in spc, if it is, we will have to wait until the production run is completed in order to select parts that represent the range of the process. Is it correct? Can we do GRR and SPC at the same time? especially when we run a new part?
Again, this is a problem with the concept - chicken and egg, again. How do you know that the calculations you used for your capability and SPC control limits were not incorrect due to "chunky" data of statistically insignificant gage resolution? You best hope is to use a gage that had satisfactory GR&R on similar processes. Also take care not to introduce measurement error - that is different from gage error. That is using a perfectly good gage incorrectly. That allows a gage that passes GR&R to generate inaccurate data.
With reference to first version of AIAG SPC Manual:
4) Page 126 of MSA says "The specific gage the team using has a % GRR = 25% of the tolerance....". How do we know? This section is about Attribute gage, how to establish %GRR for attribute gage system? , if we have not conducted a GRR for this gage using the parts understudy in variable manner, how to arrive at %GRR?
Not sure of this reference - I only have 4th edition handy.
With reference to first version of AIAG SPC Manual:
5) In your earlier post, you said to use gage that is not more than 10% of the tolerance (part or process?), how to tell this % before we do GRR? and the system has not been used on this new part yet?
The gage has a resolution - the smallest readable increment. That should be no less than 10% of the tolerance - and even that yields very low ndc, but it is a common starting point
 
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