Fig. 3. Index Plate showing Gear Changes for Threading
The Thread Tool.—The form of tool used for cutting a V-thread is shown at A, [Fig. 4]. The end is ground V-shaped and to an angle of 60 degrees, which corresponds to the angle of a standard V-thread. The front or flank, f of the tool is ground back at an angle to provide clearance, but the top is left flat or without slope. As it is very important to grind the end to exactly 60 degrees, a gage G is used, having 60-degree notches to which the tool-point is fitted. The tool is clamped in the toolpost as shown in the plan view, [Fig. 2], square with the work, so that both sides of the thread will be cut to the same angle with the axis of the work. A very convenient way to set a thread tool square is illustrated at B, [Fig. 1]. The thread gage is placed against the part to be threaded, as shown, and the tool is adjusted until the angular sides of the point bear evenly in the 60-degree notch of the gage. The top of the tool point should be at the same height as the lathe centers, as otherwise the angle of the thread will not be correct.
Fig. 4. Thread Tools and Gage for testing Angle of End
Cutting the Thread.—The lathe is now ready for cutting the thread. This is done by taking several cuts, as indicated at A, B, C and D in [Fig. 5], the tool being fed in a little farther for each successive cut until the thread is finished. When these cuts are being taken, the carriage is moved along the bed, as previously explained, by the lead-screw S, [Fig. 2]. The carriage is engaged with the lead-screw by turning lever u which causes the halves of a split nut to close around the screw. The way a lathe is handled when cutting a thread is as follows: After the lathe is started, the carriage is moved until the tool-point is slightly beyond the right end of the work, and the tool is fed in far enough to take the first cut which, ordinarily, would be about 1/16 inch deep. The carriage is then engaged with the lead-screw, by operating lever u, and the tool moves to the left (in this case 1/7 inch for each revolution of the work) and cuts a winding groove as at A, [Fig. 5]. When the tool has traveled as far as the thread is wanted, it is withdrawn by a quick turn of cross-slide handle e, and the carriage is returned to the starting point for another cut. The tool is then fed in a little farther and a second cut is taken as at B, [Fig. 5], and this operation is repeated as at C and D until a “full” thread is cut or until the top of the thread is sharp. The thread is then tested for size but before referring to this part of the work, the way the carriage is returned to the starting point after each cut should be explained.