The class of screw-forming tools to be here mentioned are hand tools, and named combs, through having a resemblance to hair combers. An outside comber for making a screw upon the outside of a bolt-end, rod, spindle, or any other cylindrical piece while in the lathe, consists of a tool in a wood handle, and is denoted by Fig. 407 or 408. Fig. 407 shows the form for a tool that is to be repaired after being worn, by merely flattening the end and making fresh teeth, without any forging. The tool shown by Fig. 408 is shaped for lightness, and when the screwing part is worn away, the tool must be upset or rejected for another one. While an outside screw tool is in use, it is held by the workman so that the length of the tool shall be at right angles to the length of the lathe, and therefore at right angles to the length of the spindle or bolt to be screwed. The preparation of a bolt for screwing consists in smoothly turning it to a cylindrical shape, by means of slide-rest tools, heel tools, or gravers; and when ready for the screw, the screwing is commenced with a graver's point. For this purpose, a graver is held to the part to be screwed so that the graver point shall be in contact with the work and make a small groove into it during its rotation in the lathe. While the workman thus holds the graver, he gives a twisting movement to the tool handle, and makes the tool point travel quickly across the work, if he desires a large thread for the screw; but, for a small thread he moves the graver slowly, the precise speed of movement depending on the pitch of the intended thread. The direction in which the graver's point travels, depends on the character of the intended screw; for a right-hand screw, the graver point moves to the left hand, and for a left-hand screw, the graver point moves to the right hand. After the screw is commenced with a graver, a comb is applied, and if a small screw is being made, no other tool is needed for beginning and completing the work.

The making of large screws is greatly facilitated by the use of vee-groovers. An outside groover is shown by Fig. 410, which is a quick and easy cutter for purposes of screwing. Such a tool is applied to screws having threads an eighth or a quarter of an inch thick, for the purpose of deepening the thread-groove in an easy manner, the comb being a slow cutter when compared with a vee-groover. A tool of this sort should not be used at the beginning of a screw-making process, but after a comb tool has begun the screw, a groover is applied until the thread-groove is exactly the depth which it is required to be when the screw is finished; after this, a comb tool is again used to finish the work. The angle of a groover for screw forming should be less than the angle of the thread to be made; consequently, a groover cuts at its point until the thread-groove is of a proper depth, and leaves the thread thicker than the finished thickness; while in this condition the comb finishes the thread by travelling along its entire length and cutting the whole of it to a proper thickness. This finishing process does not in any way deepen the thread-groove, because the proper depth of this groove is attained by the vee-groover. If a comb tool is thus properly used, its teeth do not wear at their points, and a great quantity of grinding for sharpening the tool is avoided. The groover indicated by Fig. 411 is a tool for inside screws, which is employed to deepen thread-grooves of inside threads instead of outside ones. This groover is supported while in use by the short end of the hook tool shown by Fig. 412.

For making screws into the boundaries of holes an inside comber is employed, which is denoted by Fig. 409. Such a tool is held in the hole so that the length of the tool is parallel to the length of the lathe, and screws are made with inside tools much easier than with tools for outside screwing, through the inside tool sustaining but little friction while sliding along the tool supporter, and also through the tool being wedged into its place by the forward motion of the work which is being screwed.