Turning

This is partially explained in the early portion of this book (p. 79) and also by the diagram No. 1 on p. 223, but further, the most suitable angles for the various metals are approximately as follows (though the best angle is determined by trial and the shape of the tool being varied according to the hardness of the material being cut):-

In actual practice the most efficient angle is that which cuts the sweetest and lasts the longest, but the three factors which tend towards or cause the destruction of a turning tool are :-

1. Actual wear due to use.

2. Presence in the material being cut of impurities which are harder than the tool itself.

3. The heat generated which may be sufficient to soften the cutting edge. This last factor does not operate to the same extent where high-speed steel is being used.

In round-nosed roughing down tools such as shown in Ch. XVI, f. 22, No. 6, there should be " side rake " as well as " top rake ". Side rake is not shown in No. 6, but it could be obtained by lowering the corner marked A.

Castings of iron should have the scale removed with an old file, or by pickle, or by chipping, before they are turned, as the scale ruins the turning tools owing to its hardness.

The turning tools for brass are very simple, and their cutting angles must be very obtuse or even square as, if made acute they tear the metal or "draw in," and if the job is not held firmly and without any shake it is liable to be full of ripples or striae. A quicker speed should be used than that in turning iron. Turning tools consisting of black diamonds brazed into suitable shanks are being used in America, and it is claimed by the users that they are as superior to highspeed steel tools as the latter are to carbon steel tools, and in the long run they are cheaper.

Screwing is done in many ways, and when a thread is cut on a piece of work in a lathe, the action is the same as turning, only the movement of the threading tool is sometimes different. Threads are cut by special thread milling machines, by dies which are operated by power, or by hand, by taps. Taps are illustrated in Fig. 18, p. 206. Dies are illustrated in Fig. 18 on the same page. The dies have to be held in various ways, one of which is in the stocks illustrated on the same page.

For cutting screws of different numbers of threads in a lathe the lathe has to be specially designed, and it is usually known as a screw-cutting lathe. The action of a tap cutting a thread is the same as that of a turning tool, as the cutting edge of the thread on the tap has relief made by the " backing off" and rake caused by the shape of the hollows which are milled out of the tap. It is found in practice that when the hole is drilled slightly larger than the theoretical core diameter and then tapped, nearly as good a thread is obtained as would have been if the hole had been the correct size, and this relieves the tap of a portion of the strain. During the cutting of the thread, and owing to the tap forcing its way into the metal, not only is the material removed but it is also squeezed between the threads, so decreasing the diameter of the hole. This illustrates the "flowing " power of metals. This action does not take place in cast iron. In comb or hand chasers, which are illustrated in Ch. XVI, f. 18, there is relief but no rake, and as they are usually used on soft metals it is not necessary.

Milling is shaping or cutting metal by means of a revolving cutter, and is usually performed in "milling machines,which are made in many forms suitable for various requirements. Cutters are made of almost infinite variety and with varying number of teeth. In some machines the work revolves as well as the cutter. Some work can be done very much quicker on a milling machine than it could be done on a lathe, as milling machines can be run at considerably higher speeds than other tools, as each tooth of the cutter is in contact with the work for only a small portion of the revolution and has a chance of being cooled by the lubricant used. The majority of milling cutters have the front faces of the teeth radial or without any rake, as shown on p. 223, Fig. 1, No. 9, and each tooth should act as a turning tool. Cutters for mild steel cut better with a small amount of rake, but for brass negative rake is necessary."Form"cutters are those which produce a formed surface and are "backed off "so that they retain their original shape after grinding, which is done on the face of the tooth marked A, Fig. 1, No. 9. Other cutters are sharpened by grinding at B as shown in the same diagram. Milling cutters are also made with inserted teeth, that is, the body is of soft iron and the teeth which are made of special steel are fitted into grooves and screwed into position. This makes a very durable tool, for when a tooth gets damaged it can be easily replaced.