# Capability study - What Values do I have to input to calculate Cp and Cpk

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#### NetScorpium

Hello all,
I’m doing a capacity study and I have a doubt concerning the value that I will input in the system to calculate de Cp & Cpk The specification is 30.00-0.70mm (Spec. 29.30-30.00mm) and the values measured are 29.60-29.94mm in the same dimension. As you can see, for the same dimension I have 2 different values. Now you are thinking “Simple, correct the dimension in order to obtain an equal value” Not so simple, the piece is functional and this is more an injection problem very difficult to correct concerning the piece geometry.
My question is:
- Should I input minimum values and I have a good Cp and Cpk?
- Should I input maximum values and I have a good Cp and a bad Cpk?
- Should I input an average values?
- None of above suggested?

Last edited by a moderator:

#### Marc

##### Fully vaccinated are you?
Capacity or capability?

#### Ron Rompen

Trusted Information Resource
IMHO, you need to understand the functional dimension. As an example, for a hole diameter, I would use the diameter of the largest pin which could pass through the hole. Although you could get (with sufficient resolution on your instruments) an infinite number of values for the 'diameter', depending on the number of points taken, etc, there is only ONE value for the maximum size pin which will pass through. This is the 'Functional Diameter', is the the actual characteristic which will (most often) be of importance to the customer.

If you can provide more information, I'd be happy to take another stab at it.

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#### NetScorpium

Hello Ron,
Yes... my opinion is the same. I think I must use the functional value, although I must analyse the other value to see if don’t interfere in piece functionality. Well in this case, one of both is good, because the piece is a roll to be applied in refrigerators. Thank you for your help.

Ron Rompen said:
IMHO, you need to understand the functional dimension. As an example, for a hole diameter, I would use the diameter of the largest pin which could pass through the hole. Although you could get (with sufficient resolution on your instruments) an infinite number of values for the 'diameter', depending on the number of points taken, etc, there is only ONE value for the maximum size pin which will pass through. This is the 'Functional Diameter', is the the actual characteristic which will (most often) be of importance to the customer.

If you can provide more information, I'd be happy to take another stab at it.

#### Bev D

##### Heretical Statistician
Super Moderator
are the two values dependent or independent of each other? (in other words, is oen always larger than the other? this woudl be dependent. If the 2 values are randomly distributed within a piece they are independent...)

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#### NetScorpium

Hello Bev,
Both values are measured in the same dimension. The minimum value, in my opinion is the real value and is always constant (good for a good Cp ), the maximum value is caused by a little burr in the split line and is randomly (sometimes high... sometimes low). This burr don’t interfere in piece functionality and is accepted by our customer.

Bev D said:
are the two values dependent or independent of each other? (in other words, is oen always larger than the other? this woudl be dependent. If the 2 values are randomly distributed within a piece they are independent...)

A

#### Arvind

Need clarification

1) You have mentioned specification band of 0.7 mm. Compared to this, you measured one part twice and it showed a variation of 0.34 mm.Is my interpretation correct?
2) Does this mean that your measurement system is eating away about 50 % of tolerance specification?
This is not good.

N

#### NetScorpium

You are right, about my measurement analysis, but I’m go eliminate the higher value, because is not a “real value”. The cause of this value is a burr in the split line, and this burr is accepted by our customer and don’t interfere in piece functionality. If this burr caused problems in piece functionality certainly we must eliminate.

Arvind said:
1) You have mentioned specification band of 0.7 mm. Compared to this, you measured one part twice and it showed a variation of 0.34 mm.Is my interpretation correct?
2) Does this mean that your measurement system is eating away about 50 % of tolerance specification?
This is not good.

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