In a certain factory where the writer was employed, a machine was being constructed which called for a 3-inch shaft cut with a spiral groove of very fiat pitch. It was impossible to cut this groove with a screw-cutting lathe, owing to the unusual pitch. The piece was too large for the universal milling machines in the shop, and the pitch was not flat enough to be cut in a planer.

The handy man of the shop proposed that a milling attachment be used. Accordingly, a bracket A was made with bearings for two shafts B and C, lying in planes at right angles to each other, the one horizontal and the other inclined. The horizontal shaft B was fitted with a worm, which meshed with a gear D on one end of the shaft C . the opposite end of which carried a face mill E. The inclination of the shaft was such that the plane of the cutter coincided with the desired pitch of the spiral groove. The bracket A was bolted to the cross-feed slide of the lathe. The shaft B was fitted with a pulley F, which was belted to a long pulley or drum G on the countershaft above. A special gear was required to feed the carriage at the requisite speed. A bracket H was bolted to the head-stock of the lathe, and furnished bearings for a shaft which was fitted at one end with a pinion J, adapted to engage the face gear of the back drive, and at the other with a gear K, adapted to mesh with a gear L on the feed screw. By this means a 12 to 1 reduction was furnished between the face plate and the screw. The low speed of the driving pulleys was used so that a single rough cut and a finishing cut sufficed to form the spiral groove in the shaft. This idea was the forerunner of the thread milling machine.

Fig. 140 - A milling attachment for the lathe.