# Question: Cg/Cgk formula

#### Ranma

##### Registered
Hi everyone, I’m performing Type one Gage on some control we have in the machine.
The first part of the machine has two control that do the same type of control so I will be performing the Gage R&R with the two controls as appraiser.
All the product that comes from the first part is controlled in the second part of the machine in which there is only one control. So, I’m thinking to do the type one Gage on this control (as suggested here).

I’m studying this type of gage (very thank to the cove!) and there is one thing I cannot understand well:

Why, as guidelines, we should calculate Cg/Cgk using 20% of the tolerance? Because to me it seems very narrow, but I don’t know if I’m missing some important concepts that don’t let me understand well.

Here an example of our process:

The control I’m studying is on the total length of the product. We have 10mm +/- 0,5 as specification limits. In the machine we set narrower limit +/- 0,45. Our range is basically 0,90 à 90% of the real tolerance.
Using 90% of the tolerance seems extreme but, to me, also the 20%.

So, I would like some more detailed explanation to “customize”, the better I can, this parameter to our case, please.
Thank you for the support in advance and sorry for my English, hope it is clear.

Edit: cannot really understand what add the two %Var. Since they are calculated as K/Cg or K/Cgk i don't know what they can possible add (K from the Cg formula ((K/100)*toll)/6Sd)

Last edited:
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#### Bromus

##### Registered
I don't fully understand your measurement process, but I can explain where the 20% came from in the formula for Cg. The idea is to be able to compare the obtained Cg result with the expected process capability (Cp or Pp depending on the client). For example, if you expect the process to have a capability of 1.33, then when you get a Cg of 1.33, then you can say that the observed repeatability of the measurement is no more than 20% of the total process variation (which, again, is used for Cp/Pp calculation).

This means that this statistical assumption (20% of tolerance) cannot be compared with the narrowed specification limits (also 20%) that you apply to product qualification. These are 2 different things.

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