Maintaining Micrometer Accuracy

Posted By Richard Barker on 12 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No26. The published material is accurate as of 1940. This article concerns itself with techniques in use within Precision Engineering at the time.

Some owners go to a great deal of trouble with their micrometers trying to keep out dust and grit. Many have two micrometers, one for the less accurate work and the other for checking and fine limits; but sooner or later the time comes when wear becomes apparent. The faces of the anvil and spindle become slightly rounded, and so give a very different reading on the edges to that given in the centre. Instead of an absolute check, some allowance has to be made, with misgivings, no doubt.

Surface grinders and universal grinders experience this anvil wear quite a lot owing to the amount of grit about, however carefully a job is cleaned, and also due to the need for constantly measuring sharp corners and numerous other places where the whole of the anvil cannot be used. The fixture to be described in this article will, if carefully made, provide a means of maintaining the lin. micrometers in the whole of the workshop for an indefinite period, and perfect accuracy can be obtained by any skilled engineer without the slightest risk of spoiling his best micrometer.

A Suitable Fixture

The fixture itself is simple and sturdy and is designed especially for this job. The outside sleeve is made of mild steel, turned, bored and recessed to the dimensions shown, leaving allowances for grinding after hardening. Two flat grooves should be milled to accommodate the clamp, two drilled and tapped holes should be made in the grooves to hold the clamp, and finally a series of four tapped holes should be provided to take the holding screw. The four holes should be on the centre line and of good size. The sleeve is then ready for case hardening; this should be to a depth of 1/16in. if possible.

The last operation is to grind all over, making sure that the bore and the   two ends are quite square with each other. The centre piece is also of mild steel and should be turned on centres. The length should be the same as the sleeve before hardening, and the diameter large enough to be a press fit in the bore of the sleeve after hardening and grinding. The V should next be cut, taking away about one-third of the periphery of the centre piece. It will facilitate the grinding of the V if a spigot is turned on each end of the centre piece, about 3in. long. Hardening is the same as for the sleeve. When ground all over—paying attention to the tiny slot at the point of the V for clearance— press into the sleeve, taking care to put the centre of the V in line with the four tapped holes.

The clamp is of mild steel, fitted to the grooves in the sleeve with clearance holes for the fastening screws. The boss should be drilled and tapped for a set screw, and the clamp when in place should be set so as to press directly over the centre of the micrometer head. All the set screws should have brass tips.

Lapping the Test Bar

The complete fixture is used in conjunction with a lapping plate about 6in. square and 2in thick. Grooves are cut in the surface, and these should be diagonal across the plate and should be fine, the reason for this being that wide grooves, cut square, tend to jar the micrometer spindle and prevent absolute flatness. To test the V for being square with the top and bottom, obtain a piece of silver steel approximately the diameter of the micrometer spindle. Harden and cylindrically grind it, remove the clamp from the fixture and place the silver steel in the V, holding it firmly with the holding screw. Allow a little to stand above the fixture, grind the end on a surface grinder, carefully turning the fixture round on the grinder table to give a good result.

The silver steel should be reversed in the fixture after first removing any burr from the ground end. Then the clamp should be replaced and the clamp screw carefully adjusted to press gently on the top of the silver steel while the holding screw keeps the steel square on the V. Charge the lapping plate with lapping paste or fine carborundum, remembering that the surface of the plate should be kept as dry as possible. Move the fixture in small circles and cover the whole surface of the plate. When the end of the test bar has been lapped, rotate the test bar half a turn and rub it carefully over the lap two or three times; the new lapping position will show up very distinctly against the first. If the new lapping is not uniform over the surface of the test bar, the V is not square with the base of the fixture.

The treatment for this is to obtain a hardened and ground mandrel, hold one end in a hardened toolmaker’s angle block, invert the fixture and place it on the other end of the mandrel. While gripping it firmly against the V the surface of the fixture should be skimmed, again turning it round on the grinder table. When one end has been ground the other end must also be ground from it.

Method of Use

The treatment for the micrometer is simple; first remove the complete head, spindle, thimble and ratchet in one piece, and place it in the fixture as illustrated. Using the same method as for the silver steel test bar, proceed to lap the spindle end until it is completely cleaned up. Remove it from the fixture, wash it thoroughly with benzine and dry it; an air gun will facilitate this. Put a little clock oil on the thread and put it back into the micrometer frame.

Take up any slack in the thread at this point by using the means provided on the micrometer. Place the micrometer upright in a vice fitted with fibre jaws. Obtain two pieces of lin. diameter cast iron of good close grain about Jin. thick and surface grind the faces, putting on a good finish. One of these pieces is for roughing, the other for finishing. Charge the roughing one on one side only with fine carborundum and place it between the anvil and spindle faces, the charged side on the anvil. Gently adjust the micrometer until the lap can just be moved smoothly. Rotate the lap, keeping the micrometer adjusted until the whole face of the anvil has been cleaned up. Carefully clean the anvil and spindle faces, first with a clean cloth and then by bringing the faces on to a slip of clean paper. It will be seen that when the micrometer is brought to zero it will measure "down" the amount that has been lapped off; therefore, the next job is to adjust the reading by the method recommended by the makers of the micrometer until the zero measuring is about .0002in. "up."

The last operation brings into use the other or finishing disc. Charge this on one side with fine finishing compound, again putting the uncharged side against the spindle face. Proceed as before until the anvil face has a good polished surface.

If reasonable care has been taken with the rough lapping, the anvil face will lap to a beautiful finish within .0002in. When a satisfactory finish has been obtained, thoroughly clean the anvil and spindle faces and, using the ratchet, bring to zero and carefully note the reading. If it is still "up," which in any case should not be more than .0001in., continue the lapping until the reading is "spot" on. If the reading is "down" readjust the micrometer to read .0001in. "up" and lap back to zero.

Although this seems to be a long procedure it must be remembered that a micrometer deserves special treatment if satisfactory service is expected of it. The average time taken to recondition a not-too-bad micrometer is half an hour; a really bad one may take an hour. The spindle face must always be considered the true one, therefore the spindle must be tackled first and all the other work must be done from it. If after lapping it is found necessary to re-adjust the anvil it must be lapped again until it is perfectly square. Prove this by bringing the faces together and holding them before a strong light while using an eyeglass. Keep the charged faces of the laps free from surplus lapping compound.

The reason for having the four tapped holes when apparently one would do is that the fixture may be used on a surface grinder for grinding small pins, punches and numerous other small jobs of varying lengths, and it is better to put the holes in while making the fixture than to wish they were in afterwards.