# Cg/Cgk formula and %VAR

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Ranma
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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.

I’m studying this type of gage 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.
My guess is: with 20% of the tolerance we are far away to the ambiguity zone close to USL and LSL.

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 so 90% of the real tolerance.
Using 90% of the tolerance seems extreme because it is very close to the ambiguity zone 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.

Bonus Question:
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)

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