Internal Micrometer

A while back I treated myself to two internal mikes, a. 2" to 1.2" and a 1-2" one from one of two tool suppliers I usually use (China manufacture). Both are very accurate as far as I can tell but the .2-1.2' inch is confusing and error prone when reading. As you turn the thimble to set it to say .5", as the edge of the thimble approaches and splits the .5 line on the sleeve the thimble reads 15 not 0 as it should. If the thimble is then continued to be turned to set the 0 opposite the horiz. ref. line on the sleeve the .5 graduation line is completely covered and the edge of the thimble is very close to the .525 graduation on the sleeve. This I find makes it very prone to reading errors. Does any one know how (or if it is possible) to adjust the position of the thimble relative to the sleeve? On the end, where the anvil would be on an outside mike, there is a screwed cover and inside there are two lock nuts on the end of the rotating shaft that is attached to the thimble. Are these possibly for adjustment? The only tool that came with the mikes was a two ended hook pin spanner but no adjusting instructions. One end is to adj. the friction on the ratchet and the other is for fine adjustment of the scale on the sleeve to line up the '0' on the thimble to the horiz. reference line on the sleeve when setting to the standard (which was included with each mike). This adjustment works well but leaves the edge of the thimble .010 further down the sleeve than it should be.

I hope my description isn't too confusing!!!

Any help would be appreciated as I have to be very careful reading it so as to avoid a .025 reading error. The 1-2" one lines up perfectly.

John Purdy

It depends on where you are getting the correct measurement of .5"

If the correct measurement of .500", as determined by measuring a known test piece, occurs when the edge of the thimble splits the .500" line, then the thimble needs to be rotated relative to the spindle by that .015". Depends on the make of micrometer but there is usually some method to unscrew the button etc from the end of the thimble and rotate it that .015"-worth of degrees so that when the anvils are set to a true .500", the zero on the thimble lines up with the datum line and the thimble edge splits the .500" mark. This is possibly adjustable by the nuts on the end of the thimble you mention.

Some pics of what you have sure would help.

Sometimes when the mic's are adjusted to align the scales with a hook spanner the sleeve slides back a bit. It might be possible to work it back down again.

Really though a photo would help to show exactly what you mean when you refer to the nuts. The only nuts I am aware of on mic's are used to set the fit of the spindle in the actual micrometer thread.

I'd be interested in a link to this Chinese manufacturer.

John

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I'll try and download a couple of pics soonest. Can't do it right now as my daughter has the camera!

John Purdy

John

You were right, the sleeve will side horizontally, although with considerable effort. I managed to slide it back the 10+ thou. to get it aligned. It took a while as it was quite sticky and would jump either side of where I wanted it. In this case it had to be slide out. Thanks for the suggestion.

John Purdy