Caliper Calibration Standards - One for CNC and one for Sheet Metal?

[louc74]

I work for a company that has a small CNC Dept and a large sheet metal Dept. I maintain the calibration for the entire shop. Lately I have failed several digital calipers for not being within .002 of an inch. This has caused problems with the sheetmetal side. I was told by their "Engineer" that I was being unreasonable. That there has to be two standards, one for CNC and one for Sheet metal. I'm at a loss.
Is there a double standard? Can someone shed some light. I was taught that calipers were calipers no mater what you are inspecting. Please help!

 

[natecsss]

Re: Calibration standards

How did you come to the .002" standard? Are you calibrating to a standard (NIST, ANSI) or just using something random?

When calibration is done at our facility, it doesn't matter what department the calipers are used. Either they meet the standard we use or they don't and can not be approved.

 

[AndyN]

Re: Calibration standards

Sounds like a typical situation I've seen/heard before.

A caliper IS a caliper. Maybe it's the WRONG item - or maybe the TOLERANCES are the wrong ones for each type of product. Sheet metal isn't going to be held to tight tolerances, except for punched/blanked/sheared dimensions. IS this the REAL reason for the comment? Measuring over a bend with a caliper is fine - but the tol. isn't likely to be held if "close toleranced"

 

[louc74]

Re: Calibration standards

Thanks, that's what I thought. Calibration standard is the same regardless. I will hold my ground and I will give him the bill when the calibration house fails them.

 

[cbehrens]

I suspect that the part tolerances in the sheet metal side of the business are less restrictive that on the machining side. If that is the case the calibration tolerance may in fact be different from process to process. You really need to review the required accuracy of the gage against the tolerances of the items you are measuring. It may not be the same for both processes!

 

[louc74]

I was instructed to use that tolerance when trained in the Calibration Lab at the last company I was employed. The ASQ's primer also uses that tolerance. I will not work to two standards in the shop. In the course of the day, I cant be sure the tool will only be used for what dept it was calibrated for. I was curious if the there was a standard that used a greater tolerance. I thank all for the input and look forward to more. When I get the owner of the company and this other person in the same room to discuss the issue, I want to be have all the info I can get.

 

[AndyN]

I was instructed to use that tolerance when trained in the Calibration Lab at the last company I was employed. The ASQ's primer also uses that tolerance. I will not work to two standards in the shop. In the course of the day, I cant be sure the tool will only be used for what dept it was calibrated for. I was curious if the there was a standard that used a greater tolerance. I thank all for the input and look forward to more. When I get the owner of the company and this other person in the same room to discuss the issue, I want to be have all the info I can get.

You also have to consider the "shape" of the feature being measured, when considering the type of equipment. A caliper is often the "fall back" equipment, but it has limitations, which aren't just accuracy/precision related. That may also help you make the case for getting/using a cmm, for example.

 

[normzone]

Re: Calibration standards

Thanks, that's what I thought. Calibration standard is the same regardless. I will hold my ground and I will give him the bill when the calibration house fails them.

Since we left rack-and-gear equipment behind and went digital, the fail mode I see in rough use calipers is usually jaw wear.

But you are correct - the parts derived may have different tolerancing, but the tools remain the same.

 

[Jen Kirley]

I was instructed to use that tolerance when trained in the Calibration Lab at the last company I was employed. The ASQ's primer also uses that tolerance. I will not work to two standards in the shop. In the course of the day, I cant be sure the tool will only be used for what dept it was calibrated for. I was curious if the there was a standard that used a greater tolerance. I thank all for the input and look forward to more. When I get the owner of the company and this other person in the same room to discuss the issue, I want to be have all the info I can get.

With all due respect to your last employer and the ASQ Primer, they don't apply to instruments that they have not been specifically written to address. The only appropriate outside source for accuracy/tolerance standards are your customers. To my knowledge, not even NIST will specify specific acceptability tolerances.

Ideally the calipers' accuracy will be measurable in increments at least 10 times that of the part and/or material tolerances. That means for tolerances listed in thousandths, your calipers should be able to read in ten-thousandths. That is called the "rule of 10," but when asked to provide the source of that "rule" I was not able to find it. So it is an industry practice that just makes sense to help ensure accuracy.

I hope this helps!

 

[BradM]

I work for a company that has a small CNC Dept and a large sheet metal Dept. I maintain the calibration for the entire shop. Lately I have failed several digital calipers for not being within .002 of an inch. This has caused problems with the sheetmetal side. I was told by their "Engineer" that I was being unreasonable. That there has to be two standards, one for CNC and one for Sheet metal. I'm at a loss.
Is there a double standard? Can someone shed some light. I was taught that calipers were calipers no mater what you are inspecting. Please help!

Is there anything wrong with having a broader tolerance for the sheet metal side?

If the instrument satisfies their process requireme, what would be the logic in failing
Them?

Put orange and blue dots on them respectively, proceduralize it, and move on.