Micrometer calibration adjustment

[pdentrem]

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.

 

[clueless]

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.

 

[clueless]

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?

 

[clueless]

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?

 

[keeena]

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.

 

[clueless]

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.

 

[clueless]

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.

 

[francist]

Awesome! I’m glad you finally got that figured

 

[clueless]

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.