Fig. 1815.
Fig. 1816.
[Fig. 1815] may be taken to represent a machine in which the pipe is held and the die revolved, and [Fig. 1816] one in which the pipe is revolved and the dies are held in a head, which allows them to move laterally to suit the pipe that may not run true, while it prevents them from revolving.
In the former figure the bolt or pipe is shown to be out of line with the die driving spindle, and the result will be that the thread will not be parallel with the axis of the pipe. Whereas in [Fig. 1816] the thread will be true with the axis of the work, because the latter revolves, and as the die is permitted more lateral motion it can move to accommodate itself to the eccentric motion of the work, if the latter should not run true.
If the end of a piece of pipe is not cut off square or at a right angle to the pipe axis, and the die has liberty to move, it will thread or take hold of one part, the longest one, of the pipe circumference first, and the die will cant over out of square with the pipe axis, and the thread cut will not be in line with the pipe axis.
The two important points in operating threading machines is to keep the dies sharp and to well lubricate them with oil. When dies are run at a maximum speed and continuously at work they should be sharpened once or, if the duty is heavy, twice a day, a very little grinding sufficing.
In nut tapping the oil lubrication is of the utmost importance, and is more difficult because the cuttings are apt to clog the tap flutes and prevent the oil from flowing into the cutting teeth.