The shapes of the screw-tools employed for nuts, are not dependent on the nuts' sizes, but on the sizes and shapes of the threads to be produced. The tools which will cut an ordinary vee-thread into a three-inch nut, will also cut a thread of the same thickness, step, and shape into a seven-inch nut. For commencing any large nut-thread, single-tooth screw-tools should be used, because they cut much easier and remove the metal quicker than tools having two teeth. The tool or dent first used, should be one whose cutting-edges subtend an angle of sixty or sixty-five degrees. This is advanced into the metal until the summit of the thread thereby formed is but a minute amount wider than it will be when finished, in which condition it is ready for another dent, the angle of which is fifty-five. The point of this cutter is properly curved to correctly form the bottom of the thread groove; and, with this tool the bottom can be entirely finished, and the entire thread also very nearly finished, if care is exercised to take off thin cuts at the conclusion. After this the final adjusting and polishing of the thread to the exact size required, is performed with a cutter or dent having two teeth.

The tool employed for finishing must be very carefully adjusted to cause its teeth to cut equally at both sides of the thread, and the smaller the quantity which is to be taken out with this tool, the greater is the necessity for an accurate adjustment. This operation is done after the stock of the tool is tightly fastened in the tool-holder, when the top screw of the rest is gently rotated a short distance in conjunction with a similar gentle rotation of the chuck and nut, the operator observing at the same time when the teeth of the dent bear properly against each side of the thread.

The dents which are required for cutting the usual vee-threads are denoted by Figs. 1073, 1071, and 1070. The single-point tool shown by Fig. 1071, is made of small bar steel without any forging, the vee-point being ground until it subtends the desired angle. One of these is seen keyed in its stock in Fig. 1072 ; and if the same stock is to be used for holding a wider tool having two teeth similar to Fig. 1070, the slot in the stock must be wide enough to admit the widest cutter and also a key; consequently, in this case, it is requisite to use two keys for one stock, one narrow key for the cutter with two teeth, and another wider key to fasten the small cutter having one tooth.

In order to avoid the necessity of using two keys, a cutter shaped like Fig. 1073 is used.

The widest part of this one is of the same width as the widest part of Fig. 1070, so that both may be held in the same slot with the same key. But it is preferable to employ two or three stocks while screwing nuts, so that all unfixing and fixing of cutters may be avoided, except for repairs.

The method of forming a thread with two or three tools having the same angle for their cutting edges is much practised; but the author's method just indicated is a quicker process. Any vee-tool's point which is caused to advance a distance into the metal becomes soon jammed, and the capability for cutting soon ceases; and this results sooner in proportion to the smallness of the tool-point's angle. It becomes jammed also because there is no room between the extreme point of the tool and the metal in contact, so that the shavings or slices cut off" cannot easily get away when detached. To relieve a tool-point from this jammed condition while using it for screw-cutting, it should be used so that only one cutting edge can cut at one time, which will provide a relief-space between that edge which is not cutting and the contiguous side of the thread. Both sides of the vee-point are to be used for cutting, but alternately, and in conjunction with the gradual advancement of the tool into the metal. To effect this result, the operator must carefully shift the tool-point to and fro by rotating the top screw of the slide-rest.

But although the alternate cutting of the tool's two edges greatly facilitates the production of the thread, it is not the less necessary to employ a tool with a point of sixty or sixty-five degrees, because such a comparative broad point is stronger and less liable to break than a comparative sharp point of fifty-five degrees. For the making of very large vee-threads, such as those having only two or three steps per inch, the point of the cutter or dent first used should have an angle of seventy, rather than sixty-five degrees.