Cpk Studies for Concentricity - Dimension that is spec'd 0.0 - .03

D

dsalcido

#1
All,

Newbie to Cpk studies....so no laughing....

I am conducting Cpk study on a component and for dimensions that have USL and LSL it is not a problem. My problem or issue is a concentricity dimension that is spec'd 0 - .03. On the particular sw package, if the LSL is input as "0" I get a low (less than 1.33) versus leaving LSL "blank" yields a Cpk of 4.0.

Which is correct, both wrong or ??????
 
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ScottK

Not out of the crisis
Staff member
Super Moderator
#2
Re: Cpk Studies for concentricity

Hi - I moved you thread to a more appropriate forum to draw more answers and enhanced the title.

Can you tell us what software you're using?
Does it allow for one sided Cpk calculations?

There are threads both about one sided Cpk and measuring concentricity and TIR, if you do a quick search you'll probably find TOO much to digest. :D
 

Stijloor

Staff member
Super Moderator
#3
Re: Cpk Studies for concentricity

All,

Newbie to Cpk studies....so no laughing....

I am conducting Cpk study on a component and for dimensions that have USL and LSL it is not a problem. My problem or issue is a concentricity dimension that is spec'd 0 - .03. On the particular sw package, if the LSL is input as "0" I get a low (less than 1.33) versus leaving LSL "blank" yields a Cpk of 4.0.

Which is correct, both wrong or ??????
Let's talk about first things first. ;)

First of all, be absolutely clear on the correct Geometric Dimensioning and Tolerancing (GD&T) interpretation of "concentricity." What is often termed "concentricity" in the United States is actually either "runout" or "position." Your capability study should start with an agreement between you and the customer? on what exactly it means. Without a reference to the appropriate standard for GD&T (in the USA it is ASME Y14.5M-1994), you and others may be in for a long argument that no one can win.

Once that is clarified, agree on the appropriate inspection/verification method.

Next, the process must be in statistical control (stable) before a capability study can be conducted.

Even then, experts do not at all agree on the appropriate capability calculation. Cpk? Ppk? We let the experts here at The Cove chime in....:cool:

I wish you good luck.

Stijloor.
 

Tim Folkerts

Super Moderator
#4
Re: Cpk Studies for concentricity

Without knowing exactly what software it is, I would expect that you would want to leave the lower spec blank.

The lower limit is an absolute limit imposed by the geometry - there is no possible way to get below 0.000 for this measurement. In such cases, it makes no sense to include this limit in the calculations. Think about it - if you calculate the value using the lower spec limit, then the closer you get to 0, the worse the result will come out! (which is exactly what you saw).

If it were possible to get negative values (for example, you were trying to hold a temperature between 0C and 0.03C) then you would want to aim more for the middle of the range. Getting close to 0C would be a concern for this situation.


Tim F
 
D

dsalcido

#6
All,


THanks for the quick and hlepful replies. Leaving the "LSL" blank is the way I will go. Thanks again.
 
L

LMitschelen

#7
Since concentricity, like straightness or flatness, represents a deviation from a perfect form, it only has one maximum dimension. Cpk is calculated as Z (3 standard deviations) to the nearest specification limit.

The minimum "limit" should be unspecified and not included in the calculation.
 

Stijloor

Staff member
Super Moderator
#8
Since concentricity, like straightness or flatness, represents a deviation from a perfect form, it only has one maximum dimension. Cpk is calculated as Z (3 standard deviations) to the nearest specification limit.

The minimum "limit" should be unspecified and not included in the calculation.
Concentricity is not a deviation from perfect form.
It is not a form control. See ASME Y14.5M-1994.

Stijloor.
 

Kales Veggie

People: The Vital Few
#9
Yep, Jan is right. It is a tolerance of location (comparing two or more features).

ASME Y14.5M: concentricity is as “the condition where the median points of all diametrically opposed elements of a figure of revolution (or correspondingly-located elements of two or more radially-disposed features) are congruent with the axis (or center point) of a datum feature.”

Are you sure you measured concentricity per the GDT requirement? You might have measured radial runout.
 
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