Fig. 1335.
Fig. 1336.
The work is turned up to the required diameter, and the V-tool shown in [Fig. 1335] is applied; the lathe is run at a quick speed, and the heel of the tool is pressed firmly to the face of the lathe rest, the handle of the tool must be revolved from right to left at the same time as it is moved laterally from the left to the right, the movement being similar to that already described for the graver, save that it must be performed more rapidly. It is in fact the relative quickness with which these combined movements are performed which will determine the pitch of the thread. The appearance of the work after striking the thread will be as shown in [Fig. 1336], a being the work, and b a fine groove cut upon it by the V-tool.
The reason for running the lathe at a comparatively fast speed is that the tool is then less likely to be checked in its movement by a seam or hard place in the metal of the bolt, and that, even if the metal is soft and uniform in its texture, it is easier to move the tool at a regular speed than it would be if the lathe ran comparatively slowly.
If the tool is moved irregularly or becomes checked in its forward movement, the thread will become waved or “drunken”—that is, it will not move forward at a uniform speed;[20] and if the thread is drunken when it is started, the chaser will not only fail to rectify it, but, if the drunken part occurs in a part of the iron either harder or softer than the rest of the metal, the thread will become more drunken as the chaser proceeds. It is preferable, therefore, if the thread is not started truly, to try again, and, if there is not sufficient metal to permit of the starting groove first struck being turned out, to make another farther along the bolt. It takes much time and patience to learn to strike the requisite pitch at the first trial; and it is therefore requisite for a beginner to leave the end of the work larger in diameter than the required finished size, as shown in [Fig. 1336], so as to have sufficient metal to turn out the groove cut by the V-tool at the first trial cut, and try again.
[20] See [Fig. 253], Plate II., Vol. I.
If the thread is to be cut on brass the V-tool must not have any top rake. Some turners start threads upon brass by placing the chaser itself against the end of the work and sweeping it rapidly from left to right (for a right-hand thread), thus obviating the use of the V-tool.