This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
This system is useful where many screws of odd forms and pitches are required; but where there are sufficient numbers to be cut, special chasing lathes are far preferable to ordinary screw-cutting lathes, as they will do about 6 times as much chasing of V threads, or cutting of square threads, as can be accomplished in the ordinary lathe in the same time. Instead of carrying one chaser, the chasing lathes carry, in a chasing apparatus, 3 or 4 chasers : and these have their threads, whether square, v> rounded, or any other form, cut in their places by aid of a master tap. They are then tapered at the mouths, backed off, and hardened ready for work. The number of shavings cut simultaneously from a screw by this process varies from 12 to 24, according to the size, strength, and pitch of the thread. Screws up to 6 in. diameter can be very rapidly cut by this system, on which very much more might be said if space permitted. A few screws cut by this process are exhibited.
When the 2 systems - the round and the swivel tool-holder - are worked in conjunction with each other, their universality of application is so thorough that almost every difficulty is met; and it was only in the case of paring and shaping articles in the slotting machine that 2 modifications had to be made in the holders, the same cutters being still applicable.
The capstan bed chasing lathes made by the writer's firm have now become much used; and as a large amount of their work is done upon black bars of iron, steel, or other metals, each of which has to be finished at its extremities and cut or parted off, it was found advisable to make one special tool-holder, Fig. 1257, to carry tools of the correct sections to produce the desired shapes for the ends; the tedious and unreliable process of turning the ends with hand-turning tools is thus avoided. Each cutter is of absolutely the same section throughout its entire length, and the resharpening is done by grinding the end of the cutter only, so that it can only produce the same standar form as long as it lasts - that is to say till it is ground too short to be used any longer. The parting off might have been accomplished by the swivel tool-holder; but a special form, Fig. 125G, is found to be more convenient in parting off close up to the chuck or lathe spindle.
To produce a maximum amount of cutting in a minimum space of time, there are 2 main points which must be carefully attended to. These seem to be applicable to all cutters for cutting metals, whether they happen to be those fixed rigidly in tool-boxes, as in turning lathes, planers, sharpers, slotters, etc.; or those which cut while they revolve, as milling cutters, twist-drills, boring-bits, etc. These 2 important points are : .
(1) The angle of the cutting surface (or cutting angle), Fig. 1253, - i. e. that surface which removes the shavings of metal, and upon which the pressure of the cut comes, as shown by the arrow.
(2) The angle of the clearance surface (or clearance angle) - i. e. that surface which passes over the surface of the metal which has been cut, and does not come in contact with the metal at all.
To produce the best results, and to ensure the utmost simplicity, it is important that these 2 angles be correctly constructed in the first instance. The best measure for both angles has been arrived at from actual practice and a series of experiments. When once obtained and started with, they should not alter by use, but always remain constant, if the greatest amount of cutting efficiency is to be achieved. When aided by a mechanical system of regrinding, and the use of standard angle gauges, Figs. 1254,1255, there is no difficulty in maintaining the exact angles. The only changes that take place are that the cutters in tool-holders become gradually shorter and shorter by grinding, and that milling cutters during a long period of time become very gradually smaller in diameter, by the process of resharpening them on a fine emery wheel. In the ease of the tool-holders, as already explained, the cutting angle is maintained by the system of regrinding, and the tool-holder itself always maintains the clearance angle. The system is thus simplified, as will be clearly understood when it is remembered that each one of the tool-cutters (no matter of what description) is ground on its end only.