Fig. 1834.
[Fig. 1834] represents a bolt threading machine having two heads each of which is capable of threading bolts from 1⁄2 up to 11⁄2 inches in diameter.
The levers for operating the clutch rings are here placed horizontal, so that they may extend to the end of the machine and be convenient to operate, and a pump is employed to supply oil to the dies.
The capacity of a double machine of this kind is about one ton of railroad track bolts per day of 10 hours’ working time.
In American practice it is usual to employ four cutting dies, bits, or chasers, in the heads of bolt threading machines, while in European practice it is common to employ but three. Considering this matter independently of the amount of clearance given to the teeth, we have as follows:—
Fig. 1835.
If a die or internal reamer, the cutting points of which were all equidistant from a common centre, were placed over a piece of work, as a bar of iron shown in [Fig. 1835], and set to take a certain cut, as shown by the circle outside the section, it is evident that if revolved, but left free to move laterally, or “wabble,” the cutter would tend to adjust itself at all times in a manner to equalize the cutting duty—that is, if the die had two opposite cutting edges or points, and the piece operated upon were not of circular form, then, when one cutter reached the part that was not round, it would have either more or less cutting to do than before, and hence, the opposite cutter having the same amount, the tendency would be for the two cutting edges to travel over and equalize the cuts, and hence the pressure. With three cutting points, no two being opposite, the tendency would all the while be to equalize the cuts taken by all three; with four, spaced equally, the tendency would always be to equalize the cuts of those diametrically opposite; with five, the tendency would be to equalize the duty on each, and so on. Thus it will be noticed that there is a difference between the acting principle of a die having an even or an odd number of cutters, independent of the difference in the actual number of cutting edges, or points, as we are now considering them.