=Replacing Sharpened Thread Tool.=--If it is necessary to sharpen the thread tool before the thread is finished, it should be reset square with the work by testing with the thread gage as at _B_, Fig. 1. The carriage is then engaged with the lead-screw and the lathe is turned forward to bring the tool opposite the partly finished thread and also to take up any backlash or lost motion in the gears or half-nut. If the tool-point is not in line with the thread groove previously cut, it can be shifted sidewise by feeding the compound rest _E_ in or out, provided the latter is set in an angular position as shown in the plan view, Fig. 2.
If the thread tool is ground flat on the top as at _A_, Fig. 4, it is not a good tool for removing metal rapidly as neither of its two cutting edges has any slope. In order to give each cutting edge a backward slope, it would be necessary to grind the top surface hollow or concave, which would be impracticable. When a course thread is to be cut, a tool shaped as at _B_ can be used to advantage for rough turning the thread groove, which is afterward finished to the correct depth and angle by tool _A_. This roughing tool is ground with a backward slope from the point and the latter is rounded to make it stronger.
=Use of Compound Rest for Thread Cutting.=--Another form of thread tool is shown at _A_, Fig. 7, which is very good for cutting V-threads especially of coarse pitch. When this tool is used, the compound rest _E_ is set to an angle of 30 degrees, as shown, and it is fed in for the successive cuts by handle _w_ in the direction indicated by the arrow. It will be seen that the point a of the tool moves at an angle of 60 degrees with the axis of the work, thus forming one side of the thread, and the cutting edge _a--b_, which can be set as shown at _B_, forms the opposite side and does all the cutting. As this edge is given a backward slope, as shown, it cuts easily and enables threading operations to be performed quickly. Threads cut in this way are often finished by taking a light cut with a regular thread tool. The cutting edge _a--b_ is ground to an angle of 60 degrees (or slightly less, if anything) with the side, as shown by sketch _A_.