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ExHempKnight OP t1_j92qn5d wrote

Reply to comment by A_Light_Spark in "The longest-lived micrometer than can be bought." J.T. Slocomb micrometers and stand, all fully restored by myself. Each mic is between 70-120 years old, and they're all still accurate. by ExHempKnight

Mostly true. The threads, spindle and spindle bushing, and (most importantly) the measuring faces, are all subject to wear and corrosion. This brand in particular compensates for thread wear in a unique (and in my opinion, superior) way, compared to Starrett or Mitutoyo. The bushings are also replaceable, which I'm not sure is an option with the other brands.

And as old as these mics are, and as much work as I've put into them... They can almost certainly benefit from having the faces lapped. That's really the only thing keeping them from being good as (or better than) new. But I don't have the equipment to do that, and having it done is not cheap (especially for as large a set as I have).

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TheMooJuice t1_j9302el wrote

How specifically does this brand compensate for thread wear?

Also what kind of hobbies do you have which utilise these? I have read all your other replies however am unfamiliar with much of the terminology unfortunately

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ExHempKnight OP t1_j936wep wrote

Mostly, measuring instruments like this are used in precision machining. Metalworking lathes, milling machines... Stuff like that. I have a small hobby shop in my basement with a lathe, a mill, and a couple of other machines.

Machining in and of itself is a big part of my hobby, but it also supports basically all the other things I'm interested in. I've designed and machined a custom hotend for my 3d printer. I've machined mounts and pulleys to mount a supercharger on my mid-90s Saturn. Brackets for valves on my beer brewing rig. Custom feet for my bed, which house scales, which I use to tell my home automation system when I'm in bed. I could go on.

As for the thread wear compensation... I'll preface this by saying that Starrett and Mitutoyo are fine instruments, have been around for a long time, and are basically industry standard as far as I know. Thread wear in a micrometer is not a huge issue, and plenty of those mics are still kicking, and still accurate, with decades on the clock.

So, quick and dirty micrometer terminology:

  • Thimble: the part you actually turn with your fingers.
  • Spindle: rod that sticks out of the of the thimble. Has external threads on the end inside the thimble, the other end is a very flat face, which is one half of the measuring surfaces
  • Barrel: cylindrical part of the micrometer frame, has internal threads that the spindle screws into
  • Frame: the main body of the instrument.
  • Anvil: sits in the frame, directly opposite the spindle. This is the other half of the flad measuring surfaces.

Starrett and Mitutoyo compensate for thread wear using a tapered, threaded collar at the far end of the barrel. There's slits in the end where the anvil screws in, and the tapered thread is on the outside. As you tighten the collar, it squeezes those slits smaller, which tightens the internal threads.

The problem with that, is it's only those first couple of threads that are taking up the slack.

The Slocomb micrometers use a 2-piece threaded portion. The 2 pieces have serrations that mesh. If the thread wears, you simply tighten the outer collar one or two serrations, and all the slack is gone. The difference here is that by clocking the threads differently, it causes all the threads to be engaged, rather than just a few at the very end.

This distributes the wear over more surface area, which will make it wear slower. Plus, you can clock the threads infinitely, whereas you can only tighten a tapered collar so much.

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