Using Calibrated OD mic over non-Calibrated ID mic for final dimension


Starting to get Involved
So we have a supplier who has chosen to save money by not calibrating their ID mics. The practice they have been using and will not change, is that on ID's they take the "measurement" with the ID mic that is not calibrated, remove the ID mic from the bore and then gage an OD mic over the ID mic for the reading.

This is on ID's that have total tolerances of .003-.005 metal components. I find this to be not only a poor practice, but an unacceptable one. I believe that in order to be accurately recorded final dimensions they should be taking direct measurements and not introducing variance. However, the person over supplier quality says this is a perfectly acceptable industry practice.

I am newer here, but not new to the industry (Oilfield) and I have been through enough customer audits that I would be be both embarrassed if this was my personal practice and I would also expect a corrective action from my customer. So I am concerned with this stance.

That being said, I am human and can be wrong. So, does anyone any points of clarification or industry standards they could point me towards in researching this method of inspection? The drawings do not call out a specific industry standard to use, so any insight would be greatly appreciated.

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Trusted Information Resource
It can be done in that range with reliability, but he's living on the edge. It's reliant on technique, which is a mix of training, experience, and give-a-damn.

Of course that applies to all measurement. If I wanted to fuel the fire, I'd take a known piece and do some multiple readings and record them. Then have somebody else do the same. You should be able to figure out what kind of error the process is generating.

This is no different than doing transfer measurement by any other method when you don't have exactly the calibrated tool you wish you had to do it. Piston ring grooves get measured by shade tree mechanics who only own a set of pincher calipers and a six inch vernier caliper. ID bores get measured using telescope micrometers and an OD micrometer.

They're trying to save some money on calibration - they could always calibrate them once in a blue moon and document that as the plan.


Starting to get Involved

I've always been involved with a philosophy that those are exceptions to the rules of inspection when no other options are available. I understand spring calipers get used at some places in ID grooves etc., however that is when another tool simply was not available to do the job. Using a tool that is actually meant to be calibrated as a transference method seems to introduce variability to an inspection process for no other reason than to save some bucks in calibration.

Maybe it is because I have been on a more tightly monitored side of the industry that has already adapted automotive and semiconductor industry practices and am now more exposed to the big equipment side. I guess maybe I will have to learn to loosen my parameters a bit, but that concerns me.........because these products directly relate to environmental and human safety, which is how I always want to view what is best when validating products meet design intent.


Trusted Information Resource
Well, I'd be curious about the reliability of the measurements. From what you describe the feature must be a bore, since if it was a groove the inside mic could not be removed to perform the transfer operation. And it must not be too deep, since the hand can only get such a tool in so far.

Measurements of such features are heavily dependent on skill. If the person doing this is good, I'll put money on the data being within a few tenths of the actual. If the person performing the measurement is not skilled or is running short on give-a-damn, it won't matter what tool they use or if it's calibrated or not.

It sounds as though one of those three point bore gages would be the most user friendly and reliable method, and it would only be one tool to calibrate.


Trusted Information Resource
Using an inside micrometer and then transferring it to an OD micrometer is the typical practice I followed for years (when I made parts big enough to use an ID micrometer). I never trusted the inside micrometer enough to use it as the actual measurement device. On a hole size with a tolerance of .003-.005 (total tolerance I assume not +/- .003-.005) I would be willing to accept the transfer. I might tighten up the tolerance a bit (and if I was performing the measurement I would tighten up).

Are you having quality issues with the supplier? Do you have any specific purchase order clauses that restrict this type of measurement? If no to these questions, I would not pursue it. If yes to these question then you have someplace to start.


Trusted Information Resource
[Michael M] - I don't understand your statement. No matter how you approach it, you ARE using the inside micrometer as the actual measurement device. You're just choosing to obtain your readout from another separate device.


Slightly beside the point, but does your supplier have a good calibration procedure in place to calibrate the OD mics with gauge blocks?


Quality Manager
Is it maybe a language issue? It's common for us to use telescoping gages and dial bore gages, both of which are measured to an OD mic at each reading. To the question about the standard, if I'm not mistaken as long as the tool is measured to a verified tool then it's ok. But we calibrate our ID mics and there aren't nearly as many ID mics as other measuring tools so I'm wondering if it's something else going on.


Trusted Information Resource
The dial bore gages I'm familiar with are set to nominal off of a gage block stack with end blocks in a holder, then the reading is taken from the dial on the bore gage. Why would one use an OD micrometer over that device when it contains a readout mechanism of it's own?


Quality Manager
We have mic stands instead. We just lock an OD mic into the measurement we want and keep it on a stand for checking dial bores and telescoping gages.
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