# Control Limits - Can't be smaller than 3 times your measurement increment?

M

#### mboteo

For a thru hole with tolerance of +/-.005", measured with .001 incrementing gage pins, we started to apply control charts. Now after a while, and having collected enopugh data to apply control limits, we found that every time when a measurement was taken to be one meassurement unit of the mean (or .001") that it was out of stat. control.

The Gage R&R was less than 15% on the measurement method, and we are conform with the 1 to 10 rule.

Is there a certain rule that control limits can't be smaller than 3 times your measurement increment. We are getting an awfull lot of alarms doing it this way. And we are pretty sure there isn't a significant cause to it.

S.

#### Geoff Cotton

##### Quite Involved in Discussions
Assuming the hole diameter is large enough, I would gather the data using a bore micrometer with a .0001 discrimination.

You may see a diffent picture.

Geoff

J

#### JuanAgos

Re: Control Limits too narrow

Hello mboteo

I am in agreement with Geoff Cotton, when the decision is
taken to use a measurement equipment to measure a characteristic that will be controlled by control graphs, is due to take into account that the measurement equipment has a relation of minimum discrimination of 1/10(minimum increases of the equipment / tolerance or variation of the process of the characteristic)

I think that you must use a micrometer, but clear choose the
suitable one

bye

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