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Tool Caliper with broken screw

Tool
Susquatch said:
Could be. Makes sense for an outfit like Starrett.

Holy shit. That Fkn thread is 78.5 thou x maybe 60 tpi. So probably an 80 thou #2 screw. Or perhaps a 2mm x 0.45 or or or.... Gunna need a better way to measure threads I can't see.

Clock makers? Any suggestions? My screw gauge is letting me down.

I suppose I could single point that but I'm not gunna. Just need to find a long screw whatever size that is!
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Toolmaker's microscope...works great.
 
Susquatch said:
Could be. Makes sense for an outfit like Starrett.

Holy shit. That Fkn thread is 78.5 thou x maybe 60 tpi. So probably an 80 thou #2 screw. Or perhaps a 2mm x 0.45 or or or.... Gunna need a better way to measure threads I can't see.

Clock makers? Any suggestions? My screw gauge is letting me down.

I suppose I could single point that but I'm not gunna. Just need to find a long screw whatever size that is!
Click to expand...
Per the exploded drawing & parts list in Post #5, it's 0.125-40. As I've noted elsewhere ( but maybe not on this forum until now) Starrett hasn't changed threads on these ancient tools to make them standard; in many cases the original toolmakers just used whatever they wanted, or made a tap to suit the single-pointed thread. I have a PT13820 "Upright Post, #196" that is (per Starrett) 0.231-28: I replicated it for a longer rod by using an adjustable 12-28 round die.
 
Tomc938 said:
If you have a good place you can go in person, take the thumbnut with you and try before you buy.
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I only have Fastenal here and they are unlikely to have something like that. In any event, I have not been able to get the thumbwheel off yet. So your earlier suggestion to use a bigger screw is starting to look pretty good.

Tomc938 said:
If you have to order, order a bunch of sizes and use the one that fits. (shipping will likely be the largest portion of cost)
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Yes, my feelings exactly. I'd prolly also order more than one and save them for a future job anyway.

Tomc938 said:
If none of them fit, see my post above.
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Yup, looking more and more like that's the way to go.
 
ChazzC said:
Per the exploded drawing & parts list in Post #5, it's 0.125-40.
Click to expand...

No way. Don't care what that reference says. That screw measures 0.0785 it's not even close to 0.125! It could be 40 tpi though.

Ian Moss said:
Toolmaker's microscope...works great.
Click to expand...

I don't have one of those. But I do have a nice biology microscope I could use. If all else fails, I'll count teeth - wouldn't be the first time I've done that!
 
Susquatch said:
No way. Don't care what that reference says. That screw measures 0.0785 it's not even close to 0.125! It could be 40 tpi though.
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Sorry, missed your earlier post with the diameter.

Apparently Starrett changed the design of the legs to make them beefier; here is the current model:

IMG_8920.jpeg
 
FatherWes said:
My guess is it’s cheaper to make it from 1/8” mild steel that’ll bend than from a high grade steel that snaps.
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You are prolly right, but I'm not really worried about it. It's a caliper, not a brake caliper bracket. I can't get my head around the Buffalo treatment the poor thing was subjected to. Prolly used it as a glue up clamp!
 
It looks like you might get a tap and die for a close approximation of the diameter and make your own threaded rod and thumbnut - you could even upgrade to brass for the thumnut!!

You could get a 2.5mm tap and die and that'd do the trick...
 
I have used a microscope at the lowest possible magnification. And I now believe that the screw is 60tpi. Recall that it is 0.0875" in diameter. The holes in the caliper body are 0.093 using a pin gauge. So that's the biggest screw I can use without enlarging the holes.

Now I need to figure out what that thread really is if I want to replace it without major modifications.

I should add that getting that thumb nut off was an ordeal. It wouldn't budge. I cleaned it in my ultrasonic tub. Then I soaked it in penetrating oil. No joy.

So I used a big pliers to hold the screw like a heat sink and heated the thumb screw slightly, then used a small plier to wiggle the nut. It moved! Yes! From thereon I applied lots of penetrating oil and wiggled it back and forth ever so gently about 15 degrees at a crack. With patience, I finally got it off using a tapping sequence. Plus 45, minus 30, plus 45, minus 30. It never did free up till I was all done.

Under the microscope, it looks like an ACME trapezoid thread. Kind of cool to see that so small. But I'd guess it's just deformation and not really ACME.

Any guesses?
 
Here is a photo. It's crappy, but it will have to do.

20250122_114550.jpg

As you examine it, keep in mind that the diameter is 87 thou and the thread is 60tpi.
 
It's a Starrett for sure. Thanks to the wonders of phone macros, here is what I cannot see with my eyes.

20250122_132540.jpg
 
thestelster said:
#8BA is 2.2mm OD (0.0866"), pitch 0.431mm (58.9 tpi)
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Maybe. But it is Starrett. And odd to have an OD slightly bigger by 1 thou than spec. Usually they are smaller not bigger. But I'd guess I'd never be able to feel the difference between 58.9 and 60 tpi on a short nut.
 
Do you intend on reusing the nut or making a new one? I have a pair of dividers that are missing an arched peace that goes between the 2 legs if you find you like repairing these. I think I have a pair of hermaphrodite calipers that is missing a piece also. should I sent them your way?
 
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