When you acquire an older micrometer, like an early 20th century Brown and Sharpe, you may find that it does not properly zero. After cleaning the measuring faces, twice, it still won't zero. It is pretty close, but not quite. The spindle turns freely and has no significant play, but it won't quite zero.
Are the faces worn or does one have a burr on it? Nope.
Is the frame bent? No indication it has been dropped or hit.The spindle lines up with the anvil, and there is no light visible between the faces when closed. Probably not.
I guess it just needs a small adjustment.
A well cared for micrometer can go decades without needing an adjustment, but due to slight mishandling, otherwise insignificant wear, and fumble-thumbed efforts to '"fix" a micrometer that just needs to be cleaned, many used units need to be zeroed.
Modern micrometers, nearly all except some digital types, specialty types, and the very cheapest standard types, adjust by rotating the barrel with a micrometer wrench. Many older mechanical micrometers, and a few modern ones, are adjusted differently. Here, we look at how to adjust older Brown and Sharpe units and older Tubular Micrometer Company (later Tumico, then Scherr-Tumico, and now ST) units. This guide applies to outside micrometer calipers, but also applies to some other types.
We see that the Brown and Sharpe #8 above is not properly zeroed. It misses by about 3/10000". Lets take a look at how this micrometer is adjusted, and then at a Scherr-Tumico 1-2" micrometer.
Step 1: Disassembly and Parts
To understand the adjustment procedure, we'll look at how the unit is constructed. It isn't usually necessary to fully disassemble the micrometer to adjust it, but sometimes it is needed, as the parts may be stuck together by crud or rust.
First, make sure the spindle is backed a few turns away from the anvil.
The unit shown has a ratchet speeder for force control. Other common options are a friction thimble and no force control. These have a plain end cap. The end cap has a small hole in it for the pin on the micrometer wrench to grab. The wrench is used to loosen the cap. Little force should be needed. Too much force will either break the pin from the wrench or damage the hole in the cap. You should be able to hold the knurled section of the thimble in one hand, and loosen the cap. If it is stuck, Don't Panic! Don't force it. A little penetrating oil in the gap between the cap and the knurling, maybe a little heat- a little heat, like a few seconds from a low power heat gun, NOT the oxy-acetylene torch that you have always wanted to use- and a little time will usually do it. A light rap on the end of the wrench in the loosening direction may help.
Once the cap is loose, we can remove it to see that there is a machined button end on the spindle and a machined annular surface inside the cap. On an older unit, these may be covered with crud or rust. They need to be CLEAN. Clean, with no burrs or roughness.
Before we continue: DO NOT try to grab the thimble with a vise. You will distort it and ruin the tool. It is a pair of thin metal shells that need to be able to move relative to each other. Any distortion prevents this.
At this point, the outer portion of the thimble should be able to slide toward the frame. If it does, Bob's your uncle. If not, then we go for the penetrating oil and time, with maybe a little heat mixed in. It may take a few days for the penetrant to work in. It should take almost no force at all to slip the outer part, but if it has been bound up, a little hand force may be needed. If it won't move after a week or so with penetrant, then decision time comes in: let it be, or up the ante a little.
To up the ante, the spindle is fully removed from the frame. DO NOT turn the lock ring or lock lever when the spindle is out. The parts often rely on the spindle for support and may be distorted into uselessness if operated without the support of the spindle.
The spindle needs to be held by the smooth portion in a soft, firm holder, such as a piece of soft wood with a hole slightly smaller than the spindle that is then split through the hole. The cap is replaced and threaded down until it touches the button, then BACKED OFF about one turn. Then comes the neanderthal action: A light, sharp tap on the cap with a soft, light mallet, such as a plastic or wooden mallet. You are trying to shock the part loose. The outside will move down the spindle. If not, try again. If it won't move after a few tries, more time and penetrating oil are needed, so give it a day or two.
After the first movement, back the cap another turn and see if it moves by hand. If not, another rap. Keep going until it loosens up enough to slip off or you have backed up 3 turns or so on the cap. At this point, remove the cap, since there aren't enough threads engaged to rap safely again. Time and hand work pretty much get it free.
Step 2: Clean the Parts and Reassemble
If we look inside the outer section of the thimble, we see that there is a lip at the end with the marks. This is what the cap works against. The large cylindrical part of the spindle is squeezed between the lip and the cap. This is also what allows us to adjust the micrometer.
The inside of the thimble and the outside of the cylinder in the spindle need to be clean and smooth, and ANY crud inside the thimble needs to be removed. Bamboo skewers, cotton swabs, and mineral spirits are good for this. No abrasives. No corrosive chemicals.
Once the parts are clean and rust free, light machine oil, like watch oil, sewing machine oil, or, believe it or not, micrometer oil, should be used to wipe the inside of the thimble and the out side of the spindle cylinder. This will allow for easy reassembly, easy adjustment, and help keep crud and moisture out so it can be adjusted again in fifty years. DO NOT use WD40 or any other substance that will get tacky or dry out. ONLY light machine oil.
If the spindle threads or the frame bore need cleaning, now is the time. Take care of the wear adjustment ring. It shouldn't be adjusted unless the spindle threads have been started a few turns, and then only snug enough to take the play out. The spindle should turn freely. Just no looseness or slop.
Slip the thimble back over the spindle and put the cap on loosely. Be sure the contact surface inside the cap has been wiped with oil and that the threads on the thimble have as well. A small drop of oil on the spindle and run it in until it is a few turn from contacting the anvil, adjusting the wear ring if needed. The spindle may not go through the spindle lock without wiggling the lock assembly a bit. There is very, very little clearance. Do not force it. Watch through the anvil end of the bore and finagle it until the spindle slips through.
Step 3: Now, We Zero
Be sure the measuring faces are clean. Carefully run the spindle into contact with the anvil. The Brown and Sharpe manual specifies this be done by grasping the spindle, not the thimble.
Now, while holding the spindle in place, loosen the cap slightly, rotate the thimble until the zero mark aligns with the index line on the barrel, and LIGHTLY snug the cap to hold position.
Back out the spindle a few turns by grasping it with your fingers, WITHOUT touching the thimble. You do not want to disturb the setting. Now, hold the thimble and snug the cap with your other hand. Do not use the wrench yet.
Bring the spindle back to the anvil using the thimble. Use the force control device (friction collar or ratchet) if present. If it is not right on zero, loosen the cap, readjust, back the spindle out, and snug the cap again. It may take a few iterations too get it dead on.
Once it is dead on with the cap snug, back the spindle out and snug a bit tighter with the wrench.
Congratulations. You are done.
Obviously, if everything is clean and free to begin with, it doesn't need to all come apart. Just loosen the cap a half turn, adjust, and snug it back up.
Now, to the Scherr-Tumico...
Step 4: Scherr-Tumico Construction
The Tubular Micrometer Company design is quite different than any other manufacturers. It has several benefits, as well as a few frustrating drawbacks.
Similar to many other makes, the thimble can be separated from the spindle by removing the screw or speeder from the end. Unusual is that the thimble screws onto the spindle thread, and the end screw is only for locking it in place. In addition, the fine adjustment is done by rotating the graduated end of the thimble relative to the rest. This combination has the advantage that the alignment of the edge of the thimble with the barrel graduations can be adjusted, allowing the user to set it to fully expose the graduations or split them, at her preference, by running the thimble further up or down the spindle thread. The position is locked by the screw or speeder in the end.
In this example, the spindle is run to its zero position on a 1" gauge block, and the thimble is positioned for the desire reveal of the graduation lines. This does not put the zero of the thimble over the index. Then, the speeder is replaced and snugged up to lock the position.
Step 5: Scherr-Tumico Adjustment
Now the drawback to the design. Final adjustment is done by rotating the graduated end of the thimble relative to the rest, and there is significant friction, even compared to the modern movable barrel adjustment most manufacturers use now. Additionally, the joint in exposed and gums up with rust, oil, and hand goo. Yes, your hand produces goo like everyone else's. It gets sticky and may be corrosive. It gets in the joint and makes adjustment a challenge.
The joint is next to the hole for the wrench, which is also a size that is difficult to find. In the absence of a wrench, a properly sized steel pin can be inserted in the hole, and a piece of wood with a properly sized hole can be cut in half to hold the pin. It works, but is a bit awkward. In the example (made for a smaller tool), I just used the end of the drill bit.
If the adjustment does not want to move, we are back to the penetrating oil and mineral spirits. The thimble should be removed from the spindle, cleaned, and penetrating oil applied. The adjustment should turn holding the knurled portion in one hand and the wrench in the other hand. It should be stiff, but should not need excessive force. If the thimble is clamped up for this, it is quite likely to be distorted and bind up forever. Once the adjustment if free, reinstall the thimble.
With the micrometer measuring faces touching (in this case, on the gauge block), hold the knurled section of hte thimble and rotate the graduated section until zero is lined up.
You are done. You now have a functional, properly adjusted micrometer, ready for calibration and use.
Calibration? That is where any errors are quantified. For many users, a zero check is all that is done. For critical users, periodic calibration is done to insure that the tool not only zeros properly, but that it is correct over the entire range, and to determine the uncertainty and errors at points throughout the range. But that is a different lesson.