# Micrometer calibration adjustment



## clueless (Dec 21, 2019)

I acquired an older set of outside mechanical micrometers made by the (now defunct) New Britain Tool Company (a brand under the Litton Tool Company), once sold through NAPA, probably in the 1970's. They appear to be in decent overall shape and still in the original storage box. There is no documentation other than the "New Britain" label on the outside of the box and on the tools themselves it says "New Britain Made in USA", but no part numbers, etc..

I'd like to adjust the calibration setting on a couple of them, but these are not made like most micrometers that I'm familiar with. They have no hole for an adjustment wrench (and no wrench), and the thimble body is a one piece type (no two-piece friction adjustment). So how do you adjust them? I searched but wasn't able to find any information about these specifically online; apparently they weren't very popular because I did not see any mention of them anywhere.

Here are some pics, anyone know how to adjust this type?


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## mmcmdl (Dec 21, 2019)

Should be a threaded adjustment nut on the back of the rods .


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## RJSakowski (Dec 21, 2019)

I suspect that the two knurled sections on the barrel unscrew to reveal the adjustment.


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## benmychree (Dec 21, 2019)

RJSakowski said:


> I suspect that the two knurled sections on the barrel unscrew to reveal the adjustment.


That would be my guess too.


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## clueless (Dec 21, 2019)

Thanks for the replies. The thimble ("two knurled sections") appears to be only one piece and not two, so the knurled areas are on the same solid piece and do not separate.

I saw something online about clamping the spindle in a soft jaw vice and unscrewing the thimble from it. However it would not unscrew. I put as much force as I felt was safe without damaging things, but nothing moved.

There must be a way to separate the spindle and thimble, or otherwise adjust it, but I have not been able to figure out how. Appreciate any further info about this design.


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## mmcmdl (Dec 21, 2019)

Hard to see by the pic , but is the nut split on the back of the rod ?


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## clueless (Dec 22, 2019)

mmcmdl said:


> Hard to see by the pic , but is the nut split on the back of the rod ?


I'm not certain where you are referring to. I will use the first illustration (below) to reference the names of the various components.




If you mean the threaded part shown by the arrow (second pic below), it is to adjust the tension between the spindle and barrel, for preferred resistance during operation of the thimble. That threaded collar is split, however this has no effect on calibration.




Also, there is no adjustment at the anvil end, as found on some micrometers.


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## kd4gij (Dec 23, 2019)

See if there is an allen head or slotted screw down where the ratchet screw came from.


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## clueless (Dec 23, 2019)

kd4gij said:


> Se if there is an allen head or slotted screw down where the ratchet screw came from.


I had the same thought. Unfortunately there is not. The ratchet is secured to the thimble by the tiny screw (shown in earlier pic). That little screw fits a blind threaded hole on the end of the thimble.

I'm thinking it is possible that the thimble is just threaded onto the spindle but has become corroded/frozen solid (why I wasn't able to move it on this one before). I'll attempt to remove it on some of the others and see if I get a different result. But I hate forcing a precision instrument to free it up.

They all work very well, just some of them are a two or three thousands off calibration. There is a fourth larger one in the lower tray beneath the ones I pictured before, so it is a nice set that I got for free. And the only other thing I have is a couple of vernier calipers, so I'd like to use them.


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## kd4gij (Dec 23, 2019)

It may be left-hand threads. There is a place for standards in the case.


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## Technical Ted (Dec 24, 2019)

Can you rotate the sleeve (the barrel in your diagram) to calibrate? It may be just a friction fit and allow you to rotate it to zero it.

Good luck,
Ted


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## pdentrem (Dec 24, 2019)

Here a page showing various makers. May be a help.








						How to Adjust an Older Micrometer
					

How to Adjust an Older Micrometer: When you acquire an older micrometer, like an early 20th century Brown and Sharpe, you may find that it does not properly zero. After cleaning the measuring faces, twice, it still won't zero. It is pretty close, but not quite. The spindle turns free…




					www.instructables.com


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## clueless (Dec 24, 2019)

kd4gij said:


> It may be left-hand threads.


Thanks, I wondered the same so I tried it both directions - no movement. But I will try the other micrometers to see if anything moves.


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## clueless (Dec 24, 2019)

Technical Ted said:


> Can you rotate the sleeve (the barrel in your diagram) to calibrate? It may be just a friction fit and allow you to rotate it to zero it.


Thanks, I did try that. I applied as much force as I thought necessary (in both directions), with no movement. Maybe it just needs more force but I'd prefer to explore all other possibilities before attempting that. Honestly it isn't worth ruining them, I could just compensate my measurements by the amount they are off if nothing else works.


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## clueless (Dec 24, 2019)

pdentrem said:


> Here a page showing various makers.


Thanks. I saw that page while researching this. Unfortunately it did not have any information regarding these particular micrometers or anything like them. I'm now of the opinion these were not big sellers (I've found absolutely no mention of them on the web). It could be quite possible they did not sell because they are not adjustable??


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## keeena (Dec 24, 2019)

Watching this thread - was about to post the same question as Clueless. I have an old B&S #59 which has the same adjustment type as yours. My spindle appears seized to the thimble. Pic below. @clueless: interested to hear what ends up working for you.

I'm going to start soaking the spindle/thimble assembly in penetrant for a few days, then maybe try an ultrasonic cleaner.


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## RobertB (Dec 24, 2019)

Can you unscrew the base part of the ratchet screw from the end of the thimble?


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## clueless (Dec 24, 2019)

RobertB said:


> Can you unscrew the base part of the ratchet screw from the end of the thimble?


I believe this is what "RJSakowski" was saying previously when he posted this - "I suspect that the two knurled sections on the barrel unscrew to reveal the adjustment."

Something like "keena" shows with his B&S (I've added an arrow):




As far as I can tell the thimble and ratchet mount are one piece on mine. I was not able to separate them and I do not see a 'line' that indicates they are two parts. I suppose it's possible this actually is two pieces that are machined in a way that makes them appear as one part, and they are corroded/frozen together. If I don't have any better luck with the other micrometers then I can give it another try, just like with separating the spindle from the barrel.


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## clueless (Dec 24, 2019)

keeena said:


> I'm going to start soaking the spindle/thimble assembly in penetrant for a few days, then maybe try an ultrasonic cleaner.


Thanks for your picture. As I stated in my last post I do not think mine is exactly the same, but it helps to see the parts separated on yours for reference.
I tried some penetrant lube but no help. I am not suspecting there is corrosion internally on mine, judging by the rather clean exterior. These appear to have been well stored in the protective case with no signs of rust or oxidation (they spent their lives in a desert climate). But anything is possible and a good soak and ultrasound cleaning is worth trying. Hopefully after Christmas I'll have more time to try some of the suggestions offered throughout this thread.


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## RobertB (Dec 24, 2019)

I was referring to this piece possibly unscrewing from the thimble:


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## pdentrem (Dec 24, 2019)

I believe the thimble is the key. Hard to tell in the photos but it seems like the two knurled sections could be actually two pieces? The photo gives the impression that the diamonds don’t really line up. If I was making the thimble and it was in one piece I would knurl that whole area and use a parting tool or similar tool to make the groove at the end. Anyway that is my thoughts as Christmas is on my doorstep.


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## clueless (Dec 25, 2019)

RobertB said:


> I was referring to this piece possibly unscrewing from the thimble:


Ahhh, I misunderstood you earlier. I'll take a second look at that, but I'm pretty sure it does not come off the thimble.


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## clueless (Dec 25, 2019)

pdentrem said:


> The photo gives the impression that the diamonds don’t really line up.


I will take another try at separating the thimble into two parts. Your point about the knurling is a good one. I did not notice if they line up across the two sections or not. But your logic about how it would likely be manufactured makes sense. Besides, other than aesthetics, what functional reason would there be to divide the knurling like that?


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## clueless (Dec 25, 2019)

With all of the discussions offered in this thread and considering the suggestions for how it could be made, the idea of the thimble being two pieces seems to make the most sense at this point. Why they would not separate when I attempted earlier is a bit of a mystery. But it will be worth another try, this time with additional penetrant and a bit more effort. I believe there is less likelihood of damaging this fairly solid and large part opposed to the more delicate components.

If the thimble does come apart to reveal the inside of the thimble, how are things typically made in there? The spindle must separate from the thimble somehow. So I assume there would be a locking nut or screw holding them together? And with that loosened I assume the two can be rotated relative to one another for adjustment of the calibration?


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## keeena (Dec 26, 2019)

From my understanding the spindle is a very snug fit in the thimble. Tightening the cap (in my case) is what fixes the 2 parts together to maintain the calibration. So far I have not been able to free up the spindle.

I found this thread (link) which indicates that the screw on the ratchet is what is used to adjust the mike; if so would mean @pdentrem is right. But you'll still need to get the spindle freed up from the thimble.


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## clueless (Dec 27, 2019)

keeena said:


> I found this thread (link) which indicates that the screw on the ratchet is what is used to adjust the mike


Thanks. I saw that reference previously. However on mine it appears the ratchet screw does not do anything other than retain the ratchet knob. But if I can manage to separate things further then I'll be able to tell more. Looks like I should have a chance tomorrow to try again.


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## clueless (Dec 27, 2019)

Due to the holiday today was my first opportunity to give this another attempt since we've been discussing it. Following the comments everyone offered here, I decided the most likely approach would be to try and separate the main thimble body into two parts at the division between the knurled sections. With another application of penetrant and a little heat from a hot air gun, I clamped one end of the thimble in a soft jaw vice and gripped the other end with soft faced pliers. I managed to break it loose; the thimble is indeed two parts that screw together. Inside there was evidence of some rust that must have been holding them stuck. It took a fair amount of force to break it loose. The way they are machined there is a shoulder that overlaps where the two halves join, which was giving the illusion that the whole thing was a solid one piece component.

With the thimble halves separated it became obvious how the spindle is held into the thimble. There is a tiny set screw on the side of the thimble that goes directly against the spindle (see pics below). Even that little set screw was difficult to loosen. And with it retracted the spindle was still frozen in position. I had to clamp the exposed end of the spindle in the soft jaw vice and work the thimble with the soft faced pliers to loosen it. Then adjusting the calibration was a matter of some trial settings and checking against a calibration block.

I cleaned and lubed things, reassembled, and repeated everything on the other three micrometers in the set. One of them was much more problematic than the rest - and as luck would have it that was the one I was first attempting to work on last week (featured in my first post).

Here is what it looks like with the thimble separated and exposing the locking set screw:




If it helps anyone else, here are a few points with this design of micrometer:

1) The ratchet mechanism has nothing to do with the calibration adjustment or the spindle/thimble interface. It is a separate item screwed onto the end of the thimble's cap.
2) There is no friction/interference fit on any of the components. Everything either slides freely or is threaded.
3) The little set screw is what anchors the spindle in place in the thimble. The thimble threads into the barrel, with a tension adjustment. The barrel is pinned solidly into the frame.

Thanks to everyone for all of the advice and support.


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## francist (Dec 27, 2019)

Awesome! I’m glad you finally got that figured


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## clueless (Dec 29, 2019)

keeena said:


> I have an old B&S #59 which has the same adjustment type as yours. My spindle appears seized to the thimble.


Keeena, not sure if this will help you any. But on mine I discovered something once I finally got the thimble separated. The spindle actually passes completely through the main body of the thimble. It extends a little out the other end of the thimble, below where the ratchet mechanism and cap go. You can see the tail end of the spindle in my picture below (arrow):




On your picture I cannot see that area of the thimble to tell. But if the spindle sticks through on yours like it does on mine, then maybe that will give you something to work from both ends of the spindle in order to free it up. Although if yours does not have a set-screw type adjustment like mine does, then maybe the spindle won't go all the way through the thimble like that.

Have you tried some heat to help free it? I used a heat gun with hot air and focused the heat on the thimble only. I don't see how that could damage the tool steel of a micrometer, it's not like putting a torch to to it. Combined with penetrant and a soft jaw vice to hold the spindle, you might have some luck getting them to move.


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