thimble adjustment on micrometer

Hi,

I've been searching the internet for information, and still cannot find what I'm looking for. I'm hoping someone can help me.

I am trying to calibrate a 0-1" Mitutoyo mic (model 202-104). At zero, it reads .016. I cannot find an adjustment for the thimble to set it to zero. If I move the barrel, the zero reading would be around the back.

Is it possible that this particular model does not allow for such adjustment?

Thank you in advance!

16 thou is a long way out! As you say the barrel or body is rotated to bring it back to zero. The scale will be around the back exactly as you say but it will be zeroed.

Steve

There should be a hole to slip an adjusting wrench in around the ratchet stop or a cap at the end of the thimble. If you can get it off, the thimble should slide off and you can turn it to the correct position and re-tighten.
You may have to gently rap the thimble on a hard surface to free it up.

Maybe you can use this technique. Stolen from an earlier post.
YouTube - Bamboo Rod Making Fitting Micro Ferrules for Bamboo Fly Rods

RJT

Marty has it right. Rotating the barrel is for minor tweaks.

If you unscrew the spindle from most Mitutoyo mics (not digital, a little more complicated for non-rotating) you can grab it in SOFT jaws and unscrew the ratchet at the back. What you'll find is that there is a taper at the back of the hardened spindle that fits a matching taper in the thimble. Either a plain screw or a ratchet keep them together.

The tapers are often at least slightly frozen. First try pulling the thimble off. Next try something like spark plug pliers or the slight tapping Marty suggested. Failing that take a piece of softwood with a hole in it to support the bottom of the thimble and insert a small brass pin punch where the screw was. Very lightly tap and all but a solidly rusted spindle/thimble will come apart.

Put it back again so zero is about where you want it. Tighten the retaining screw. Tweak the adjustment with a spanner around the barrel.

The correct way to adjust the thimble is to use the small end of your spanner wrench to unscrew the ratchet from the thimble. Loosen it a turn or two but don't remove it. Hold onto the thimble with the fingers of you off-hand and tap on the top of the ratchet mechanism with the wood handle of a hammer. One sharp tap will release the thimble from the spindle taper. Rotate the thimble to the proper position and tighten the ratchet back up. Unless you are extremely lucky it will take a couple of tries to get it cinched up in the right spot. You should only rotate the barrel when necessary to adjust a small amount, or turn it 180 degrees for the left handers.

It worked!

(No, not the youtube video! Although I have a depth mic here.....)

Thank you all for your help!
I think I'll stick around here...I like this little forum. Maybe one day I can return some advice.

Now the question is why the thimble was so far off. I'm afraid someone might have put it to use as a "precision C clamp" as seen on Ebay from time to time .

Once you get it zeroed out correctly it would be a good idea to check it against some gage blocks at various thicknesses just to have a little more confidence in what it says.

Screwmachine,

I did the full calibration check on it. They checked out pretty well, actually. I was surprised.
The faces were parallel, turned freely but not loose, etc.

The lead screw accuracy was out .0002 overall; the only reason I had to deem them "reference only".

SRuss said:

I think I'll stick around here...I like this little forum.

Click to expand...

Glad to have you with us.

We all learn something new every day. That's part of the fun.

- Leigh

Thank you for the welcome Leigh!