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What is your companies Interval Calibration on Inspection Equipment

M.B. Naegle

Feb 7, 2011
Conroe, TX USA
Our shop isn't ISO certified and I don't think we ever will be as our industry doesn't require it, but we began documenting our measuring tool certifications a few years ago. Before then, a lot of it was based off of the tools original factory certs and which tools a machinist "liked" as opposed to which ones were provably accurate.

Our current system (which may change as time goes on) is to send out our master length gauges, and other masters that are used to check tools, to be calibrated at an accredited lab on a yearly basis. With the masters calibrated, we calibrate our measuring tools in house on the same yearly interval and have a paper trail to go along with the practice. It's up to the guy doing the calibration however, that if there is a tool that has been used very little in a years time, to skip a calibration. The main goal is just to have a paper trail of when things were last checked and to have it all link back to a known standard. Every tool has a serial number and a storage case to keep it's master, adjustment tools, and certification paper together.

In addition to having known accuracy in the shop, now no one doubts a tool because just because it's old (and we have a lot of those). The yearly calibration also helps us keep up with tool repairs and completeness. We now have an inventory of every precision tool in the building and don't purchase excess tools.

As others have said though, what the "standard" is really depends on your specific industry and the needs of your customers. I think like any other measurement in a machine shop, calibration just boils down to what you can prove and avoiding assumptions.


Jan 14, 2007
Flushing/Flint, Michigan
Many seem to forget that this is Not an ISO thing.
Requirements for gauge calibrations and documentation come from far before there was an ISO standard.
Sometimes in-house, sometimes from customer.

Gauge R&R while not calibration was in use decades before ISO began to put together a committee.
People have been checking and resetting zeros on mics and gauge stands since they became available. Is this re-calibrating? I think so.
Done without thinking and a huge problem for me to the point that I had to add a supervisor sign off required on resets above a certain limit.
We check mics and gauge stands against the master every hour or two, we do not rezero. If out of the R&R spec a supervisor gets involved and we go to other gauges for agree or disagree.
If it still checks out as off for whatever reason everything in the last hour or the last calibration check is quarantined. Then there is a mess for the inspection guys and gals.

Many factors. An optical compator or CMM obviously needs to be calibrated if moved.
Gauge block or pin sets rarely used and stored well don't need a lot of attention, sometimes send out only the common ones like .250, .500, etc.
Same with mics, here I thing an estimated number of uses to a time frame. A 0-1 mic measuring carbide parts 500 times a day is a bit different than one measuring soft steel an average of 5 times a day.

In a larger shop one can get an idea of how often things fall out of calibration and set a time well enough inside this limit.
This is the real question to address, how and why to fail calib and how to be inside it given the gauge design and usage.
One could change the oil in the car every 3 months if not driven at all, that overkill. Yet if you put 100,000 miles on a car in a year the 3 months makes more sense.

Have no problem going 10 years and way plus, some stuff and 3 months is just asking to make scrap.
Most just do one year. Why? Because it is easy to say and feels warm and fuzzy.

Working tolerance has been mentioned. If making .005 parts and a R&R at .001 one works to .004 on the floor.
Add in another thou for unknown calib (which is a lot) and make the parts to plus minus .003 and you are golden.
If you have trouble making parts to .003 I'm not sure what type of machine tool you are using.


Cast Iron
Jul 31, 2020

Thank you for the input gentlemen.
FYI I not in the business for quality certification
none the less learned a bunch
Last edited:
May 29, 2010
Many seem to forget that this is Not an ISO thing.

I'm not sure "forget" is the right word. Most probably just don't give it a thought.

Like so much "international" it helps some and doesn't really help others.

ISO - International Organization for Standardization

It is, all things considered, probably better than when all countries had their own rules.

The downside is that its often a bit like Catholic priests giving advice on sex and marriage. I suppose the intentions are good but rarely based on experience.


Even with ISO procedures in place I believe it is all part of company preference. Back in the day at the larger companies I worked at, personal inspection tools such as calipers, micrometers, and dial indicators were inspected monthly by the company QC department or an outside vendor. On top of that guys were calibrating their instruments on an as needed basis. If I am running a job with a tight tolerance, I check calibration daily at the beginning of each work day, then as I do now. Now sometimes I could check some items twice a day.


Hot Rolled
Aug 15, 2008
near Cleveland
I think the main thing with ISO is that you have records of doing the calibrations when you say you are going to do them. The actual interval does not matter. Of course it matters in practice, but it doesn't matter to whoever does the audit, so long as you're doing what you say you're doing.

The shop I worked in was first a mil-spec house, later became ISO9001- customer requirement. We tracked gage cal from the beginning, some individual gages had multi decade histories. ISO 9000 only added more paperwork. As for the cal intervals- there was a mil spec for that.