R. G. Griswold.

A bench micrometer is a very handy tool for the amateur who is doing a high class of work in which accuracy of measui-ement is essential. In the physical laboratory it is almost indispensible for comparison of lengths. While it is an instrument of precision, its construction is not difficult to those accustomed to using tools and a lathe. It should be understood that in a tool of this kind, care in the making is repaid a hundredfold in the finished article.

The capacity of the instrument about to be described is from five inches down to zero, reading in thousandths of an inch and a ten-thousandth may be easily estimated by the eye, or the index wheel may be further divided to read to this, as the circumference is sufficiently large to permit such divisions being easily read. The travel of the micrometer screw is one inch, and with the aid of distance pieces the two measuring points may be set to measure any distance within its capacity.

A Bench Micrometer 5

Considering the frame or bed first, we have two pedestals upon which the instrument stands on the bench or table. These may be made of either cast iron or brass, turned to size and polished. The under side of these pedestals should be slightly recessed, either in the pattern, or by running a light cut over the surface in the lathe. This will enable the instrument to stand more firmly upon the table. The beam passes through 3/4" holes drilled and reamed in the ends, being held in position by the set screw shown.

The beam, which is 10 3/4" long x 3/4" in diameter should be turned down from 7/8" stock and if possible, finished by grinding, so that it may be true to size and perfectly straight. If it cannot be ground conveniently, then finish it with a fine grade of emery cloth wrapped over the end of a stick and held in a tool post of the lathe. By running this to and fro over the surface with plenty of oil, a satisfactory surface may be had. The two stocks carrying the measuring points are made from the same pattern with the exception of the lug which carries the small scale along which the index wheel passes. This may be cut off the stock b when cast, thus saving an extra pattern. If a lathe is convenient the longitudinal holes in these stocks should be drilled on it in the following manner :- Make a small angle or "knee" plate which can be bolted to the face plate of the lathe, being careful to have the two sides at right angles. Secure the stock, which should previously have been finished by filing until nearly to size, to this angle in such a manner that the axis of the hole to be drilled will coincide with the center line of the lathe centers. This clamping is most readily accomplished by passing a flat piece of iron, say 1" wide by 1-4" thick, over the piece and passing two small bolts through holes in the ends into the angle plate. When securely fastened, place a drill slightly smaller than the finished size of the hole in a drill chuck held in the tail stock, and carefully drill the holes for the measuring point and the beam. By simply moving the angle plate across the face plate without loosening the clamp on the piece, the two holes may be drilled parallel, which would be difficult if either hole was drilled after removing the piece. Before passing from one hole to the other, pass a reamer through them to finish to size. It is well to state that the hole in stock a, which carries the movable screw, should be drilled to the root diameter of the screw to allow of threading, which must be done without moving the piece on the face plate after drilling the hole. Make a small thread tool which will pass into the hole and secure in tool post. Set the gears to cut 40 threads per inch. Take very light successive cuts, as this will ensure a perfect thread and avoid to a great extent the springing of the tool which would make an uneven thread.

When these holes have been finished, the piece should be split with a small saw as shown, and two machine screws provided at the top and one at the bottom to serve in clamping the spindles fast. The split in the bottom does not extend from end to end but stops about 3/16" from either end. The screw in the middle will Compress the sides sufficiently to firmly grip the beam, and the ends of the hole not being cut through makes an excellent guide. This split is put in with a small circular saw, but if one is not convenient, the split is run completely through as in the top.

The splindle c should be made of tool steel and hardened after the thread is cut, but as this process generally warps the piece, unless the amateur is familiar with this class of work it would be better to leave the body soft and only harden the end which comes in contact with the piece to be measured. Cut the thread 40 per inch and make it a snug fit in the stock a. The end upon which the index wheel fits is tapered and the wheel held thereon by a screw in the end. When this spin-dle is inserted in its nut, the binding screws should be set down with only sufficient force to make the screw turn firmly, or with a slight resistance and with no lost motion. After the end f is hardened, the spindle should be held in a chuck and the end ground perfectly square and flat. If no tool-post grinder is handy, the same result may be more slowly attained by fastening an oilstone slip to a flat piece of wood, or the flat side of a tool, and while bearing gently against the end of the rapidly revolving piece, move the slip in and out by the cross-feed screw.

The index should be turned out of a piece of steel, but brass will do and may be cast roughly to shape and then turned to size. Fit it to the taper of the spindle and counterbore for the screw in the end. When finished, lay off the divisions on the edge as follows : - Cut from a sheet of hard glazed writing paper, a strip 1/8" wide and wrap it around the circumference, cutting the ends so that they meet exactly without any overlapping or clearance. Now lay the strip on a drawing board and divide it into exactly twenty-five parts, making a light mark at each division. With a little patience each division may again be divided into ten parts, making two hundred and fifty divisions on the circumference. Now wrap the strip around the circumference again and fasten with a drop of glue or wax, allowing the edge to coincide with a fine line that was cut in the circumference while in the lathe. This will leave about 1/8" of clear metal exposed. Now coat this exposed surface with a little parafine or beeswax and when cold, scribe the divisions on the wax, being sure to cut completely through to the metal beneath. Make the 0, 5, 10, 15, 20 and 25 divisions extend from the line to edge, the other divisions being about half that length. If the smaller divisions are to be laid off, make them just a little shorter so that they will be distinct, the middle or fifth division being longer than the rest

When this marking on the wax has been completed, brush over the cuts with a camels hair brush charged with nitric acid. Be sure that the acid penetrates to the metal, otherwise the etching will not he clear. Before washing off the acid, feel the marks with the point of a needle to make sure that there is a depression there deep enough to prevent the needle point from slipping. When the divisions are etched plainly, wash off all the acid and wax, finishing by rubbing the cuts with oil to prevent rusting, which is one of the troubles with freshly etched work.

To the stock a, attach the small scale which is made of No. 20 sheet brass. One inch is laid off on its edge in fortieths, every fourth division being made double length, dividing the inch into tenths. When this scale is attached, the circumferential line on the index wheel should coincide with the 0 division when the 0 division of the index coincides with the edge of the scale. Since the number of threads is forty per inch, each revolution of the index wheel will move the point f 1/40" and the index line will have advanced one division on the scale. Now if the index is moved a part of a revolution, say one division (1/25) the spindle will move 1/25" x 1/40" = 1-1000".

To set the micrometer to measure distances from 1" down, bring the stock b up until the two spindles touch and clamp in position. If after clamping the two points do not touch exactly, as indicated by the position of the index, loosen the screws binding d and push up until perfect contact is secured. Then pieces may be measured between the two points.

For measurements beyond an inch, standard blocks must be inserted between the points and the setting thus made. The making of a set of standard pieces is a very interesting and profitable undertaking. They may be turned nearly to size from 1/4" Stubb's steel and then hardened. They are finally ground to size by hand. A set consisting of 1", 2", 3", 4" and 5" should be made for this micrometer.