The process of cutting a large vee-shaped thread on a bolt is analogous to the cutting a similar thread in a nut. If the thread required is an ordinary Whitworth-shaped thread, the pitch, the diameter required for the bolt at the bottom of the thread-groove, and the requisite screwing-wheels, can be ascertained by referring to Table 5, page 177. This Table is only useful if the required screws do not exceed three inches in diameter, and the bolt-screws are to be produced previous to screw-cutting or tapping the nuts. Whenever large bolts of four, six, or eight inches in diameter are to be screwed, the nuts should have been previously screwed, and should be now available for measurement, in order that the bolts may be accurately fitted.

When the particular nut is selected for the bolt-screw to be cut, it must be measured with two gauges, if its size is not known, a pointed wire gauge, and a sheet gauge. These two must be specially made for the purpose, unless two such gauges exist to which the nut has been screwed, and the operator has access to them ; but supposing that he possesses these gauges, and also possesses the necessary outside screw-tools, he can proceed with the screw-cutting without other measuring tools, except an outside calliper with thin points.

The outer diameter required for the bolt-screw is known by the length of the wire gauge, and considering the pitch of the thread, a diagram being formed which is similar to those for nut-screws, shown by Figs. 1066,1068, and 1077. While using such a diagram, the wire is placed upon the Figure in the place occupied by the line D; or the gauge can be carefully measured with a calliper, and the calliper placed upon the line. It must be here mentioned that the diagram is not first made, and the gauge made to it, as for the screw-cutting of a nut; but the gauge is first placed upon the board, and the Figure, or at least part of it, is made to the gauge. To do this, the straight or primary line shown in Fig. 1066 or 1068 by L is first scribed, although it need not be of any exact length, if it is as long as the gauge. One extremity of this line is next selected as a centre, to which one point of the gauge is put. While one point is thus placed, the other end of the wire is moved or inclined away from the line L, until exactly as far from it as the step of the thread. As soon as the wire is thus accurately placed, it resembles or represents the diagonal D before referred to ; and an el-square is to be now put upon the board so that the pedestal shall exactly coincide with the line L, while the blade exactly touches the point of the wire, but without moving it away from its inclined position with the line L. If this is carefully done, that length of the line included between the square and the extremity of the gauge which touches the line is the diameter to which the bolt-end must be turned previous to beginning the screw-cutting.

The sheet gauge referred to is necessary to indicate the depth to which the thread-groove is to be cut, and its diameter is rather less, strictly speaking, than the diameter of the screw at the bottom of the thread, if measured with a thin calliper; but this gauge will be found quite sufficient to correctly denote the depth of any ordinary vee-thread when the nut can be tried upon the bolt previous to finishing it. The calliper is therefore adjusted to the gauge, and the bolt measured without removing it from the lathe-pivots.

The screw-cutting commences with a vee-point tool having an angle of sixty-five or seventy degrees; consequently, its point is similar to that of a tool for beginning the screw-cutting of a nut. This tool is used until the width of the thread-groove made is about five-sixths of the ultimate width, when it is removed from the tool-holder and another fixed in its place having an angle of fifty-five. The thread-groove is with this tool, deepened to the desired depth indicated by the sheet-gauge; but only a small amount can be taken off the sides of the thread, because of the broad-point tool previously used having removed so much metal from the groove's mouth.

During the use of the single point-tools, they should be caused to cut alternately at both sides of the thread, in the manner described for screwing a nut; and this process can be more conveniently adopted while screwing a bolt, than while screwing a nut, because the operator can more plainly see the condition of the thread.

The final shaping and smoothing of the bolt screw is performed with a tool having two teeth; but this should not be used until the thread-groove is deepened to its ultimate depth, in order that the finishing tool may not cut anything whatever from the bottom. It is therefore necessary to carefully measure the bolt at the bottom of the thread, by means of the calliper having thin points, and to finally deepen the groove with a single-point tool, whose extremity is properly curved to produce the requisite form. If this is properly done, the finishing tool will cut easily without being likely to tear or injure the thread, because the points of the tool cannot bear upon the bolt in any way, the entire cutting of this tool consisting in removing metal from the summits and sides of the thread, without taking any metal from the bottom.

It is to be also noticed that if a comparative large vee-thread is being cut, such as one having only three or four steps per inch, it is necessary to make the tool with two teeth cut alternately at both sides of the thread, as if a single tooth were in use, in order to avoid the considerable friction occasioned by the two teeth being in contact. But while finishing a small thread, it is seldom necessary to alter the tool after it is at first adjusted.

Outside screw-tools are represented by Figs. 1105, 1106, 1107, 1108, and 1109. The one shown by Fig. 1105 has a single point having an angle of seventy degrees, and therefore is principally used only for beginning all vee-threads in general, unless a thread is to be made the sides of which are to subtend an angle of seventy degrees, in which case, such a tool will both commence and also finish the screw-cutting, if circumstances require such a course.

Fig. 1106 denotes a tool having a point of fifty-five degrees, the extremity of the cutting

Eart being curved to the exact shape required to produce the Whitworth-threads. The tool shown by Fig. 1107 is similar to Fig. 1105, but its point has a sharp extremity, to render it suitable for a thread that requires a sharp angular bottom for the thread-groove, instead of the usual curved bottom. The actual sizes of the cutting parts belonging to these, and other slide-rest screw-tools, is dependent on the sizes of the lathes to which they belong, and the sizes of the threads to be cut.

Fig. 1108 represents a tool having two teeth, and Fig. 1109 denotes one with three teeth. Both these are used for finishing threads which have been partly shaped with single-point tools; but the tool with three teeth is employed only for very small screws, such as those having twelve, fourteen, or a greater number of steps per inch. For the finishing of all larger threads than these, tools with only two teeth each, are amply sufficient. The larger the thread being cut, the greater is the friction of the screw-tool, and the greater is its liability to dig forcibly into the bolt or other object, and do mischief.