Fig. 275.
Fig. 276.
It is obvious that in originating either the sharp V or the United States standard thread, the first requisite is to obtain a correct angle of 60°, which has been done in a very ingenious manner by Mr. J. H. Heyer for the Pratt and Whitney Company, the method being as follows. [Fig. 274] is a face and an end view of an equilateral triangle employed as a guide in making standard triangles, and constructed as follows:—Three bars, a, a, a, of steel were made parallel and of exactly equal dimensions. Holes x were then pierced central in the width of each bar and the same distance apart in each bar; the method of insuring accuracy in this respect being shown in [Figs. 275] and [276], in which s represents the live spindle of a lathe with its face-plate on and a plug, c, fitted into the live centre hole. The end of this plug is turned cylindrically true, and upon it is closely fitted a bush, the plug obviously holding the bush true by its hole. A rectangular piece e is provided with a slot closely fitting to the bush.
The rectangular piece e is then bolted to the lathe face-plate and pierced with a hole, which from this method of chucking will be exactly central to its slot, and at a right angle to its base. The bush is now dispensed with and the piece e is chucked with its base against the face-plate and the hole pierced as above, closely fitting to the pin on the end of the plug c, which, therefore, holds e true.
The bars a are then chucked one at a time in the piece e (the outer end resting upon a parallel piece f), and a hole is pierced near one end, this hole being from this method of chucking exactly central to the width of the bar a, and at a right angle to its face.
The parallel piece f is then provided with a pin closely fitting the hole thus pierced in the bar. The bars were turned end for end with the hole enveloping the pin in f (the latter being firmly fixed to the face-plate), and the other end laid in the slot in e, while the second hole was pierced. The holes (x, [Fig. 274]) must be, from this method of chucking, exactly an equal distance apart on each bar. The bars were then let together at their ends, each being cut half-way through and closely fitting pins inserted in the holes x, thus producing an equilateral triangle entirely by machine work, and therefore as correct as it can possibly be made, and this triangle is kept as a standard gauge whereby others for shop use may be made by the following process:—