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Thread: How to adjust a micrometer?
11-24-2004, 03:07 PM #1
I have an older Starrett micrometer that is about a thousandth off according to the calibration standard that I used. I have the little hook wrench for the micrometer but I'm unclear on the procedure for adjusting it so that it's dead on.
Does anyone have a link to a site explaining the procedure or can it be succinctly described?
11-24-2004, 05:07 PM #2
First, be sure the standard is good.
loosen the small set screw that stops the barrel from rotating.
with the mic locked on the standard, rotate the barrel with the hook wrench to indicate "0" in the scale.
tighten the set screw
release the standard and check another known piece or rechuck the standard an see if the mic reads "0"
Hope this helps
11-24-2004, 08:58 PM #3
One: make very sure that the mike faces and the calibration standard are completely clean before checking. Even the oil film from touching these surfaces ONE time will add a tenth to the reading. More oil and or dirt equals an even greater error.
Two: mikes are usually calibrated at the zero reading. If it is adjusted to read zero with the CLEAN jaws closed, then there is nothing further you can do with it. Of course, you could calibrate it to read properly at some other point but then it would be off at zero.
I strongly doubt that a Starrett is off that much, even an older one. More likely your problem is dirt/oil, or an improper zero adjustment, or a bad standard. Check it carefully before adjusting.
It's been a while since I adjusted mine but it went something like this:
1. Carefully clean the anvils and check the zero. Use the ratchet for closing it if one is provided. If not then you must rely on trained fingers to provide a uniform closing torque. Note how far it is off.
2. Remove the cap on the rear end of the spindle.
3. Lock the shaft on a known reading. The best setting would be the reading you got in step 1.
4. Loosen the lock nut under the end cap.
5. Adjust the spindle on the shaft to provide the zero reading. Do this by locking the shaft (step 3) and moving the spindle by the amount of the error found in step 1 or if you set it to the erronous zero reading in step 3, set it to zero.
6. Tighten the lock nut and loosely reinstall the cap.
7. Recheck the adjustment. If it is still off, repeat all of the above. A one thousanth reading mike will likely take only one adjustment but a tenth reading mike may take two or three.
8. When it checks out OK, retighten the cap and check one final time. Repeat all of above if not dead on.
I hope this helps.
11-24-2004, 10:05 PM #4
Paul...would you know if a Mitutoyo mic would adjust the same way? I've had a 1" for over 25 years and right from when it was new it read close to a thou under at zero. Just one of those things I never got around to dealing with so I never use it for anything critical. Thanks.
11-24-2004, 10:15 PM #5
Just to add to Paul A's response. Most Starrett mics have TWO means of adjustment. One changes the location of the barrell/thimble on the micrometer shaft - which is what Paul covered. The other, which is only meant for very minor adjustments (+/- .001 or less) is made by rotating the normally fixed sleeve of the micrometer. If your wrench has a small hook spanner on one end, your mic has this feature.
You'll see a small hole in the sleeve of the mic (the one with vertical lines every 25 thousands and vernier lines, if so equipped). Insert spanner in this hole, and rotate sleeve until you get a zero.
Be also advised that some mics, especially older ones in production use, are set to whatever measurement is most common. So, for example, of a .500 shaft is measured, then the mic is set to a .500 gage block. That way, any wear or non-linearities in the screw are of less concern.
11-24-2004, 10:49 PM #6
A lot of the old micrometers had their graduations on a fixed sleeve; adjusting one of these usually required that the thimble be rotated on the spindle, but some had the adjustment at the fixed anvil.
From what I've seen of these, the thimble was sometimes a close sliding fit over an unthreaded spindle end held in place with a set screw, sometimes the thimble was captured between a pair of nuts threaded onto the spindle. In either case, the adjustment required moving the thimble axially until its "lower" edge was lined up with the zero graduation on the sleeve and rotated until the thimble's zero lined up with the sleeve's index line.
There was also one fixed-sleeve micrometer (Scherr-Tumico, IIRC) than had a second sleeve on the thimble. The thimble was actually threaded onto the spindle, and rotating it moved the thimble up and down the fixed sleeve. Once the lower edge of the thimble sleeve was lined up with the main sleeve's zero graduation, the thimble was clamped in place with a lock nut. Finally, the thimble sleeve was rotated to bring its zero to the main sleeve's index line.
Most newer micrometers have a loose sleeve that can be rotated on the frame extension until the zero index line on the sleeve aligns with the zero on the thimble. The rotating sleeves usually, if not always, rely on friction between the sleeve's ID and the frame extension's OD to hold them in place, and there's usually a fair amount of stick-slip when adjusting. The rotating sleeve has a pin hole in its closest-to-frame end to accept the pin of a hook-spanner adjustment wrench. (If the adjustment wrench has two hook-spanner ends, the smaller may fit the ratchet-speeder attachment or the nut that adjusts the fit of the split-nut to the spindle screw while the larger end fits the zero-adjusting sleeve.)
On these micrometers, the spindle and thimble are still separate pieces. This means that the thimble can, in some way, be rotated on the spindle end. This allows the thimble zero to be set to the usual position on the near side of the frame, but in conjunction with a rotating sleeve it allows the "reading position" to be rotated to the top of bottom of the frame as on a crankshaft micrometer.
The Starrett spindles and thimbles are usually fitted on a taper, with a small screw or the ratchet-spinner acting as a drawbar holding the tapers together. The taper is shallow enough to be self-holding. If the thimble needs to be removed from the spindle, loosen the screw a couple of turns then, while holding the thimble firmly, smack the end of the screw with a screwdriver handle to break the taper free.
Some of the Chinese-import micrometers also use a taper-fit between the spindle and the thimble, and these are released in the same way the Starretts are.
The disadvantage to the taper fitting is that there is no simple way to move the thimble axially along the spindle. The other side of that coin is that a properly-fitted spindle should stay properly fitted. The only time I've ever seen it become an issue with a Starrett micrometer is when trying to use a spindle originally fitted to one frame to another frame. With Chinese imports, though, it might be a bigger deal; I've seen these brand new with the edge of the thimble as much as 0.015 inch away from the zero line on the sleeve.
Ok, Don, I've probably over-answered your question, especially since you asked specifically about a Starrett micrometer. I'll hope that someone else can find something useful in my essay.
Did somebody ask what time it is?
11-24-2004, 11:42 PM #7
Although not as good of writer in this essay contest, I worked on mic's at a previous job. I got good enough at fixing the hammers and C-clamps of the people in the mill, that when a mic was determined to be "dead" it was sent to me discretely, so as not to offend any of the other guys that worked on them. I became the "last word" in the mic repair business.
Here are some shots of mic's that should help explain the comments.
The first shot is typically a mic with something between the jaws. Check for this before adjusting it.
In this shot, notice the small space between the barrel and the frame, on the "0" end of the barrel? Typically from being dropped, it makes the mic twenty five thousandth off, and is easily fixed. Using the wrench, turn the barrel completely around the frame, giving a little sideways pressure to "walk" it over against the frame. Recalibrate and your done
In the following one, the hole for the spanner wrench is shown on the barrel. The recessed area on the frame that looks like it holds a key is actually a small springlike piece of metal shaped like the recess. That drags on the barrel, keeping it in place, but allowing it to be moved with the wrench.
And then you have a few good mic's in with a bunch of junk that is used for parts. Amazing how fast used parts go once the supply is cut off. I used to have 10 times this many.
David from jax
11-24-2004, 11:53 PM #8
Cool stuff, David. That last shot made me laugh - as if you dumped out a bag of 1 inch mikes on the kitchen floor
did you ?
11-25-2004, 12:00 AM #9
Thanks for the information.
This mike is the type without the ratchet and has a slotted cap on the end where the ratchet knob would normally be. The barrel has no set screw and does rotate enough to zero out.
Is the cap that Paul mentioned the entire knurled end or just the slotted portion? (Sorry for the poor depth of field.)
11-25-2004, 12:18 AM #10
That slotted portion on the end of the mic is a screw which holds that part on a taper.
Actually I did dump them from a bag out of the garage, onto the floor in the foyer...
I take all of my small pictures in the same spot on the floor, there is a bright light right there, and it is just outside my computer room door.
David from jax