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  Capability index for one-sided specification?

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Author Topic:   Capability index for one-sided specification?
Chris_UK
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posted 30 April 2001 07:55 AM           Edit/Delete Message   Reply w/Quote
Does anybody know if there is a capability index I can use for a one-sided specification. ie +0.5mm -0mm. The distribution is normal.

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dWizard
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Posts: 20
From:Normandy, TN 37360
Registered: Mar 2001

posted 30 April 2001 08:05 AM     Click Here to See the Profile for dWizard   Click Here to Email dWizard     Edit/Delete Message   Reply w/Quote
Cpk(max) = (USL-Xbar)/3*s

Regards,
dWizard

------------------
I was better but I got over it.

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freeda
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Posts: 26
From:Jeffersonville, IN
Registered: Dec 2000

posted 30 April 2001 11:05 AM     Click Here to See the Profile for freeda   Click Here to Email freeda     Edit/Delete Message   Reply w/Quote
Chris,
I have a Word document that shows you the non-standard capabilities. If you will send me your e-mail address, I will send it to you. The formulas are a little hard to understand when you write them out here in the forum.

fhoeweler@mkmmachine.com

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Chris_uk
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posted 02 May 2001 09:53 AM           Edit/Delete Message   Reply w/Quote
Freeda
Thanks for the response
I tried to mail you my address but the message was returned as undeliverable. you can mail me on
Quality@alpressings.co.uk

Cheers
Chris

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Chris_uk
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posted 02 May 2001 10:00 AM           Edit/Delete Message   Reply w/Quote
Freeda
Thanks for the reply
I tried to mail you my address but the message was returned as undeliverable.
You can mail me at
quality@alpressings.co.uk

Cheers
Chris

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MarkR
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Posts: 4
From:Grand Haven, MI, USA
Registered: Sep 2000

posted 08 May 2001 09:33 AM     Click Here to See the Profile for MarkR   Click Here to Email MarkR     Edit/Delete Message   Reply w/Quote
Chris,

I don't think your example of +0.5 -0 illustrates a single sided tolerance. It is still bi-lateral, just not symmetrical. If you can make parts that are too big (over +.5) AND parts that are too small (under the -0) then it is bi-lateral.

A better example of single sided tolerance is flatness. If parts must be held flat within 0.030, then 0.030 is the upper spec and there is no lower spec (zero isn't the lower spec, it's the target). This is the situation where Cp doesn't apply and only the upper Cpk equation is used.

I think you still need to calculate Cp and both Cpk values to check for the possibility of making parts that are too big and too small. The only exception I can think of is if you have some natural limit on the process that simply can't be exceeded.

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V. Manea
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Registered: May 2001

posted 23 May 2001 04:21 PM     Click Here to See the Profile for V. Manea   Click Here to Email V. Manea     Edit/Delete Message   Reply w/Quote
Hello there,

I too seem to find that some confusion exists with regards to one-sided tolerances. My example of one-sided will be most of the regular GD&T callouts. For example, true position of a hole. Defined as the diameter of a circle whithin which the centre of the hole must fall. So in most cases we see a 1.0mm diameter true posistion. Zero will be the target, 1.0 will be the USL, so how do we evaluate the process?
One option will be to calculate the Cp as 1.0/6-sigma.
Then calculate Cpk based on USL-proccess average/3-sigma.

Any other ideas?

Victor M.

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Ken K.
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posted 24 May 2001 08:58 AM           Edit/Delete Message   Reply w/Quote
You're correct.

Using

Cpk = CPU = (USL - mean) / 3s

for a one-sided upper limit and

Cpk = CPL = (LSL - mean) / 3s

for a one-sided lower limit is standard AIAG SPC methodology.

If you were using MINITAB statistical software, you would also have the option of identifying a limit as being a "Boundary", which means that the product/process physically cannot exceed the boundary value.

In this case the data are treated as if there was a one-side spec limit (the bound is not considered a spec limit), AND the area beyond the boundary value is excluded from any PPM defective calculations.

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jadubar
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posted 23 June 2001 11:33 AM           Edit/Delete Message   Reply w/Quote
In response to the original question, "Does anybody know if there is a capability index I can use for a one-sided specification. ie +0.5mm -0mm. The distribution is normal."

This is not a one sided tolerance.

A one sided tolerance has a natural limit. For instance, run-out will never be less than zero.

If you have a ¯5.00 hole with a +0.5mm -0mm tolerance, you treat it as ¯5.25 ± 0.25mm and use the normal calculation methdods for a bilateral tolerance. It is still a unilateral tolerance because it does not have a natural limit.

The intent of this type of tolerance is to indicate that the low limit is more critical than the high limit.

A high cpk using USL-Mean/3s would actually mean you are producing the part toward the risky side of the tolerance.

I believe you can find information on + or - only tolerances in one of the newer GD&T handbooks.

[This message has been edited by jadubar (edited 23 June 2001).]

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