Home-Made Screw Micrometer: a Fine-Threaded Bolt and a Paper Dial (Boy Mechanic, 1913)

"A simple, fairly accurate, and easily made apparatus of the micrometer form may be constructed."

Get an iron or brass bolt about 1/4 in across and 2-1/2 in long with as fine a thread as possible, threaded nearly to the head, with its nut. Cut a screwdriver slot in the head.

Clamp two square-cornered wood blocks (about 1 x 3/4 x 2-1/2 in) side by side and nail small pieces across the ends; together they are about 2 in wide. Bore a 1/4 in hole through the centre in the 2 in direction. Unclamp, set the nut into one block in line with the hole, then cut a piece out so the blocks form a little bench. Glue its legs to a thin board about 2-1/2 in square.

Solder a stiff wire about 2 in long to the bolt head at right angles. Cut a disc of heavy pasteboard with a radius equal to the wire, divide its edge into equal spaces, and fix it on top of the bench. Screw the bolt down through the nut.

Fix a small piece of wood on the base between the bench legs and glue a small piece of metal on it where the bolt meets it.

Count the threads in one inch of the bolt on a rule: one turn moves the tip one thread. To measure, screw down onto the object, remove it, screw down to the base, and note how far the wire travels over the dial. Turns (with the part of a turn) divided by threads per inch = thickness in inches.

From **The Boy Mechanic, Book 1** (Popular Mechanics, 1913), p. 130, credited to the Scientific American (checked on the archive.org scan boymechanic0000unse_n5a8). The 1/4-20 bolt and 50-division dial are our example.