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Screwed up micrometer sleeve threads

Masterjuggler

Plastic
Joined
Sep 25, 2014
Location
Nj, USA
I was helping to clean out the grandparents' house so they can move, and I found a few old Brown & Sharpe micrometers. One of them looks great, but the threaded chuck-like thing inside the sleeve is damaged. The threads themselves seem fine, but the chuck seems like it was bent and bent and opened with a screw driver at some point. I can screw the spindle in, but after about 1/4" it gets really tight.

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The threaded chuck-like thing at the end of the sleeve and the thing that screws onto it. I assume it is to get the threads snug against the spindle without any play.

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This is how far it goes in freely until it meets some resistance, then gets a lot harder to turn progressively over another 1/8".

Can anyone tell me:

1) What exactly is the purpose of the threaded-chuck, and why the heck isn't it just a threaded tube?
2) Is the threaded chuck separate from the rest of the sleeve or is it one piece? It looks like two pieces because I can see the ridge of the threaded chuck when looking from the frame side (the threads don't go all the way through). I tried removing it by tapping the ridge from inside with a flathead screwdriver, but didn't want to damage anything so I didn't do it too roughly.
3) Can I do anything to fix it? I'm going to make myself a spanner wrench at some point to remove the thimble cap and calibrate it, but it's not worth doing if the caliper is a piece of junk.

Thanks in advance.
 
Calipers are not gold, they are tools, and if messed up, get replaced. Aside from family connections, that type is maybe 10 bucks at a tag sale.

The end of the internally threaded portion is split so it can be closed up to adjust the slack out of the thread system. The collar that screws over it does the closing, by a slight taper etc, as you suspected.

The threads are the big deal, they set the accuracy and do the measuring. They have to be right to have it work acceptably.

When the split part is bent and then straightened, it will probably never fit quite right again, and the threads may be burred up at the cuts. That may be what is jamming it. there is some serious visible damage in the slots, and burring is likely. Also some bright spots seen on the threads elsewhere, which may be burrs.

It should NEVER be hard to turn, it should turn smoothly and easily, without slack. If not, it has problems.

While it may be fixable enough to fit together again, I wouldn't necessarily consider it calibratable afterwards.... although it might be fine....
 
I know they are not that expensive, but it really kills me to get rid of something if it can be fixed.

So I take it running a tap through it to get rid of the burrs would completely screw it up. Is the threaded chuck separate form the sleeve or is it one piece? If it is separate, I might still be able to take it out and fix it as a side project when I have a lot of free time.
 
+1 to Jerry's explanation of why the threads are adjustable.

The mic is probably only worth about $20 even with good threads; but there are a few things you can try to get the split threads aligned.

Straighten out the collet-like flanges as best you can. Reassemble it, putting the nut on squarely and fairly tight. This will leave you a very tight fitting thread. Use a bit of brute force both sideways and repeated screwing in and out (that doesn't sound so good) to free it up a bit. Then back off the threads a bit to see if you can get it free running. In a sense you're using the existing threaded and hardened spindle as a sort of forming tap.

This will have done some damage to the threads, but you still might have a mic good to .0005 to .001"

You could also try lapping the threads a bit -- then giving the mic to a kid as a precision play toy (what's the thickness of a hair?).
 
Well, 1/4-40 taps are available from the industrial suppliers so that's possible, assuming the thread is 1/4". I wouldn't trust one to cut threads as good as the original but it might clear burrs. If it seems like it wants to cut material out of the existing thread I'd pause and reconsider.

You might also want to disassemble the thimble (that other part with the male threads) to see if there's a booger up there somewhere inside not immediately obvious. Not sure of disassembly procedure for yours but probably a screw removes the small diameter part on the end and the outside part with markings will be on a tapered part of the screw. Hold onto it and tap the screw with a copper hammer from the short end.

Dunno if the internal threads are a separate part from the body. It looks like a join in the photo, but the sleeve with the markings on it will also be a separate part so it can be rotated for zero setting.
 
In good mics the male threads are hardened and ground and the female threads at least surface hardened. Even if re-cutting threads were feasible in hardened material, the cost of the tap exceeds the value of the mic.

In my experience, part of the problem in a situation like the OP's is that the spindle no longer lines up with the axis of the micrometer. In addition to the problem at the threads, there's often binding near front part of the frame (near the lock area). That's why forcing the spindle sideways can sometimes loosen things up. The interference could also be opened up by reaming a couple thousands larger (the frame isn't hardened there); but at this point we're moving away from "mic" and towards "precision c-clamp."

As for the threaded female being a separate part -- it often is (as in this mic). However, removing it is difficult and even if apart isn't much easier to attempt a repair. The male threaded spindle is almost always easily detached from the spindle; but not usually the source of problems unless it has rusted or acquired a layer of oxidized goo.
 
Weld a T bar to the top of it, they make great little G clamps that when used by a welder will really piss every machinist off you encounter.

As to running a tap through it, yeah right, might want to learn something about thread tolerance classes before you contemplate doing that!
 
toss it. There is just no way tap make would make them to micrometer thread accuracy. cripes the tap'll cost you more that a B&S, Starrett, mit in great condition off of kijiji or craigslist
 
Like I said, I figured running a tap through it would completely screw it up. Anyway, I'll just put it to the side to mess with when I have free time.
 
Ironic this came up now, as I have a uni-mic that needs repair too.

The reason for the splits there are as explained above, they allow the mic to be adjusted for wear. I've repaired a relatively large number of mics now, and find setting the "feel" of a mic can often be the most difficult part. It's easy to make them stiffer, but not as easy to make them looser. I've generally found brute force and ignorance satisfies the latter, but I'll be up at Mitutoyo in a few days and will see if they can give me a better technique that my current rapping on the end of the thread spindle :D

I'm not 100 % sure about your brand, but in Mitutoyo (and indeed all mics I've seen), that sleeve can be pressed out. The story of woe with my mic was I bought it from Germany and it arrived completely seized up. It had clearly been shipped like that and I wasn't impressed. However I'd normally disassemble and clean a mic anyway, so I just got on with the job. It took a LOT of effort to get the spindle freed, and in the end I needed to get heat on the body to break the bond. It all went back together fine and job was just about done. However I'm a bit of a perfectionist, and I like to run things like depth and uni-mics etc somewhat loose, so when I'm measuring down on a part I can feel the anvil touch the surface and not push the body off. So I thought I'd loosen it off a bit ... that's where the problems began. To cut a long story short, I finished up damaging the threads to the point the part was irreparable. Not happy as I'm normally quite careful. Most parts are available, except this part, as it's the actual "mic" part of the micrometer.

The solution for both of us? In your case I'd suggest you throw it in the bin. I was selling good used Mitutoyo 0-25 and 25-50 mics on ebay (I picked up a stack at auctions), for $10-$20 each. However I'm far too stubborn to admit defeat on a repair, and in my case I've pressed that sleeve out and will machine up a new one. Forget a tap, it's nowhere near good enough for this application, and you will need to single point the threads. I don't have a small enough threading bar to get down in that bore so will need to make one. A fiddly job in itself, but I made a smallish boring bar a couple of days ago for another job, and will make something similar for this. As mentioned above, the original material is steel but I will replicate it in brass instead. Something like a uni-mic doesn't get used all that often and I'll never wear it out, not in a thousand lifetimes, and I think brass will be the appropriate material in that case. I also won't make it adjustable for the same reason, at least not initially. I can always go back and do that later if I think it needs it. I've honestly never seen a mic that has really required adjustment for wear. All the ones I've seen have been dropped/damaged/[bozoed in various other forms] by some idiot well before they showed any form of wear. So I think I should be safe.

I'll try to remember to post the details here if anyone is interested in the repair? I don't especially like fiddly crap like this, metric mics are 0.5 mm pitch and I'm not looking forward to making up the weeny bar and poking that thread in there, but I won't be defeated :D Is it worth it? Hell no!
 
Might also consider it a parts donor if you buy another one used on off Ebay. The locks and ratchets are often missing; the carbide on the spindles chipped, the thimbles scarred by Joe Blow's engraved name.
 
@Pete F- That's exactly the kind of attitude I'm working with, haha. I however don't have the expertise or the right tools to do what you are talking about, so I'll leave that to you. I would definitely like to see any progress you make with your mic, be it good or bad. Keep us updated with your experiment.

@PeteM- I do have two other 1" mics and a 1/2" mic and all work, but they didn't look sharp like this one. The engraved numbers and lines are worn out, but they are usable so I can't justify spending money on another. Anyway, thanks for the help guys.
 
@Pete F- That's exactly the kind of attitude I'm working with, haha. I however don't have the expertise or the right tools to do what you are talking about, so I'll leave that to you. I would definitely like to see any progress you make with your mic, be it good or bad. Keep us updated with your experiment.

Unfortunately the thread got moved into the metrology sub-forum, so is effectively dead as far as I'm concerned. It wasn't btw an "experiment", rather straight forward turning, just a bit fiddly. Good luck with yours. Pete
 
Yeah, this is the second time the thread got moved, the first was to the featured threads, and I don't even know why it keeps getting moved.

Anyway, I would still like to see what happens with yours, so if you could at least post the results, that would be great.

I've been playing with mine and found that using brute force on it has actually paid off so far. I've been threading the lone spindle free of the thimble into the sleeve and prying it in different directions to free up the thread-chuck. It seems to have worked, but for some reason the frame hole in front of the lock is putting a bit of pressure on the spindle on one side and is causing friction. Maybe I clamped it strangely, but the spindle lines up perfectly with the anvil. I'll try opening up the hole a little bit with some 800 grit on a rod. I'll report the results later. Seems promising though.

I'll also take a pic later of the way this thing calibrates. I like it a lot batter than the normal taper on the spindle/turning sleeve.

As a side note, I actually have a few different buffing/stropping compounds down to a 1 micron diamond paste. I wonder if putting a little of that on the threads and chucking the thimble in a drill to run it in and out would help, or if it would be as bad a tapping it.
 








 
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