Thread: calibrating starrett tubular id mic

Earlier this week I found a guy on craigslist selling a starrett 1.5-32 tubular type id mic. Guy said he had started his own job shop, got a job he couldn't measure, bought the mic, then the shop closed. So it was used for 1 job, less than a year old. Up to 32 inches is really more than I typically need as most of my parts are in the 3-7 range, but the price was right and the tool is in essentially brand new condition, so why not?

Problem is, it is woefully out of calibration right now. I borrowed the shop's 40 inch verniers with me to check if it was remotely accurate before I bought it, looking for obvious problems like bent rods and whatnot, which there weren't, but after getting them to the shop and checking with od mics most of the rods are off by about 3 thousandths.

SO, question is-as far as calibrating the rods, how should one go about it? I figured the small end caps (.250) should be the starting point since they don;t have a means of adjustment themselves, so I put them on and adjusted the sleeve on the mic to zero as I checked it to my OD mic set to that size. When I went to adjust the next rod up the end cap on it was tight. Really tight. I asked a co worker about it and he said it is normal acceptable standard practice to hole the rod with a set of vise grips in order to turn the end on it? I mean, I had the same thought before I had asked him but the idea of bringing vise grips around my brand new tool made me cringe a little bit. this compunded by the fact that this particular co worker has a strong reputation around the shop for over tightening and just generally breaking the ever-living s*** out of everything prompted me to pack up the mic until I could seek out the wisdom of the PM crowd.

So, what are ya'lls thoughts and recommendations? Or should I just pony up the cash to have them calibrated somewhere?

Originally Posted by J_Mac333

So, what are ya'lls thoughts and recommendations? Or should I just pony up the cash to have them calibrated somewhere?

If the cost can be justified then sending them/it out for calibration will give you most peace of mind. Doing the calibration yourself means that you have to know that what you are using as your standard/standards is/are OK.

Look at it as an investment rather than an expense

Gordon

I have had a set of these Starrett tubular inside micrometers for years, only mine only go up to 12 inches. The need to calibrate these interchangable measuring rods (the end caps are what are adjusted) to me is an exercise if futility. I would use them as they are for inside bore measurement, and use an outside mike and standard to check them for accuracy. They surely have been accurately set at the factory, so I don't see the advantage in having some other outfit re-calibrate them. Because the end caps and rods are used in a random assembly, I don't think that a dead-nuts accuracy with them is possible, even if God calibrated them.

They are almost like telescoping gages, in that they are best used for comparison measurements in conjunction with outside micrometers and standards.
What happens if you don't have an outside mike to check them out? What, Indeed?

Lord Byron

Originally Posted by Bruce Nelson

I have had a set of these Starrett tubular inside micrometers for years, only mine only go up to 12 inches. The need to calibrate these interchangable measuring rods (the end caps are what are adjusted) to me is an exercise if futility. I would use them as they are for inside bore measurement, and use an outside mike and standard to check them for accuracy. They surely have been accurately set at the factory, so I don't see the advantage in having some other outfit re-calibrate them. Because the end caps and rods are used in a random assembly, I don't think that a dead-nuts accuracy with them is possible, even if God calibrated them.

They are almost like telescoping gages, in that they are best used for comparison measurements in conjunction with outside micrometers and standards.
What happens if you don't have an outside mike to check them out? What, Indeed?

Lord Byron

I'm not sure what a "dead nuts" accuracy is with anything but re the random assembly then the same applies to gage blocks. I'm sure Leigh can give the formula for calculating possible error when stacking. Something to do with the square root + the square root etc.

Gordon

"..normal acceptable standard practice to hole the rod with a set of vise grips in order to turn the end on it?"

Laroy Sunderland Starrett, just turned in his grave.

I bet he's one of those guys that uses a cheater bar on the chuck wrench and a hammer or mallet on the hex wrench to tighten the reversible chuck and mill vice jaws too.

I used to work for/with knuckle draggers who thought this way, I used to ignore them, repair the damage they caused and ignored their childish taunts, now I'm of the advanced age and position - just fire them and hire someone who cares.

I would use the mikes only as a comparison gauge unless I had large enough mike with standard to set them. One trick is to use a lathe collet to hold the body. There is no reason for the bars or the tips to be more than hand tight - ever! Once you get them apart lightly oil them and keep them away from the gorilla.

Most shops should own a set of gage blocks, a couple holders, and end attachments. $200 or so should get you either an import setup or a nice used set of gages. These will allow you to check and adjust your internal mics to .0001 or so.

I use padded pliers or even spark plug pullers to turn knurled nuts without damaging them, if the original adjusting wrenches are missing. Another approach is to wrap knurls with masking tape during adjustment, but this is a bit finicky with the internal mics.

There's no reason you shouldn't be able to set up your internal mics so the various bits are very close to their intended half inch or whole inch increments. The gage block setup, held in a holder with caliper-type ends will make this fairly easy. That said, a quick check before you use any particular combination is in order.

They need to be calibrated in a certain order

It matters which end of the barrel they are put on.

No oil they will collect dust and than get scored.

There is a little wrench that wraps around them to tighten and a wrench flat to adjust they are not interchangable if you tighten for assembly with the flat wrench it will change the calibration.

Originally Posted by Gordon B. Clarke

I'm not sure what a "dead nuts" accuracy is with anything but re the random assembly then the same applies to gage blocks. I'm sure Leigh can give the formula for calculating possible error when stacking. Something to do with the square root + the square root etc.

Gordon

"Dead Nuts" is a term that all U.S. machinists can understand. It needs no translation for a "furriner". If you can't understand it, you will have to continue to be uninformed. Your statement about your confidence that Leigh can give the formula for calculating possible error when stacking. Something to do with the square root + the square root etc. is just smoke. It has nothing to do with answering the question that the thread author asked. With assembling a stack of gage blocks, you can be assured that there won't be any error if they wring together solidly. You don't have the same assurance when you screw together the components of the Starrett Tubular Micrometer.

As I said before, you can't be assured of the measurement that you take with this random assemblage of tubular micrometer components by reading the graduations on the barrel, unless you have determined the accuracy of this reading by comparison with an appropriate outside micrometer whose accuracy is determined with a standard. If you don't have a outside micrometer of the appropriate size, you could use a stack of gage blocks as a comparison. If you can't do this, you could probably use a vernier caliper as a comparison measurement if +- .005 or so is sufficient.

Lord Byron

Originally Posted by Bruce Nelson

Your statement about your confidence that Leigh can give the formula for calculating possible error when stacking. Something to do with the square root + the square root etc. is just smoke.

Well, Bruce, it's widely-accepted smoke. I'm afraid you're the one who's uninformed, again. Look up Root Mean Square (RMS).

The RMS error calculation is used in every scientific and engineering discipline in the world. It's formal definition is:



This formula calculates an error value based on a set of individual measurements.,

Originally Posted by Bruce Nelson

With assembling a stack of gage blocks, you can be assured that there won't be any error if they wring together solidly.

Again untrue. I suggest you stop posting responses to technical questions, since they're uniformly incorrect.

All physical standards have errors. Even the standards maintained at NIST have uncertainties.

In the case of gage blocks:
Each block has a nominal accuracy. The tolerance can never be zero, although some blocks may measure nominal.
When you assemble a stack, the actual error will be the RMS value of the individual errors, as discussed above.

- Leigh

BTW... I suggest you adopt a more civil tone in future posts, lest they be deleted.

Originally Posted by The real Leigh

Well, Bruce, it's widely-accepted smoke. I'm afraid you're the one who's uninformed, again. Look up Root Mean Square (RMS).

The RMS error calculation is used in every scientific and engineering discipline in the world. It's formal definition is:



This formula calculates an error value based on a set of individual measurements.,


Again untrue. I suggest you stop posting responses to technical questions, since they're uniformly incorrect.

All physical standards have errors. Even the standards maintained at NIST have uncertainties.

In the case of gage blocks:
Each block has a nominal accuracy. The tolerance can never be zero, although some blocks may measure nominal.
When you assemble a stack, the actual error will be the RMS value of the individual errors, as discussed above.

- Leigh

BTW... I suggest you adopt a more civil tone in future posts, lest they be deleted.

Lest my posts be deleted, I respectfully ask you how this response offers any applicable use to the question posed by the author of the thread? However abrasive to you, at least my posts offered some usable insight and suggestions to solving his problem.

Lord Byron

Originally Posted by Bruce Nelson

"Dead Nuts" is a term that all U.S. machinists can understand. It needs no translation for a "furriner". If you can't understand it, you will have to continue to be uninformed. Your statement about your confidence that Leigh can give the formula for calculating possible error when stacking. Something to do with the square root + the square root etc. is just smoke. It has nothing to do with answering the question that the thread author asked. With assembling a stack of gage blocks, you can be assured that there won't be any error if they wring together solidly. You don't have the same assurance when you screw together the components of the Starrett Tubular Micrometer.

As I said before, you can't be assured of the measurement that you take with this random assemblage of tubular micrometer components by reading the graduations on the barrel, unless you have determined the accuracy of this reading by comparison with an appropriate outside micrometer whose accuracy is determined with a standard. If you don't have a outside micrometer of the appropriate size, you could use a stack of gage blocks as a comparison. If you can't do this, you could probably use a vernier caliper as a comparison measurement if +- .005 or so is sufficient.

Lord Byron

I can live with the fact that you constantly critize just about everything I write and "dead nuts" is an expression I've heard on more than one occasion. Remember though that this is a metrology forum and not "amateur corner".

Heaven only knows I don't always agree with Leigh but he knows his stuff

Gordon

Originally Posted by Bruce Nelson

Lest my posts be deleted, I respectfully ask you how this response offers any applicable use to the question posed by the author of the thread?

The post in question answers two questions posed by you, so if my answers are OT, it's because your questions were OT.

In case you've forgotten, you're not the arbiter of what is or is not on topic in this forum, so any comments you might make in that regard are irrelevant.

- Leigh