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.
With special twist-drills, made for piercing hard Bessemer steel, rail holes, 13/16 in. deep and 29/32 in. in diameter, have been drilled at the rate of one hole in 1 minute and 20 seconds, in an ordinary drilling machine. Had the machine been stiffer and more powerful, better results could have been obtained. A similar twist-drill, 29/32 in. in diameter, drilled a hard steel rail 13/16 in. deep in 1 minute, and another in 1 minute 10 seconds. Another drill, § in. in diameter, drilled £ in. deep in 38 seconds, the cutting speed being 22 ft. per minute. The speed of cutting rather distressed the drill; a speed of 16 ft. per minute would have been better. The steel rail was specially selected as being one of the hardest of the lot.
The writer considers milling the most important system used in the cutting of metals. It is found practicable, and in most cases it is exceedingly advantageous, to finish (or as it is usually termed to "machine") almost every class of work, such as is now usually finished by planing, shaping, or slotting machines, in one or other of the numerous kinds of milling machines already in use. It may not be generally known that in this class of machine, milling cutters are being used of diameters ranging from 20 ft. used for heavy engine work, down to 1/2 in. or 3/4 in., used principally for the intricate work required in sewing machines, small arms, etc.
By the former, the work done is what is known as face-milling; the mill itself is somewhat similar to a large lathe face-plate, and the several cutting portions are steel tools inserted into and firmly secured to it by a series of set screws or keys. On the other hand, the milling cutters of the small sizes from 1/2 in. up to about 8 in. in diameter, are made from solid blocks of cast steel, or blanks, as shown in Fig. 1261.
The term milling is more generally understood in the United States than in this country. It means the cutting of metals by the aid of serrated revolving cutters, each having a number of cutting teeth. Milling cutters have been used in this country for many years, but until recently with only a limited amount of success, owing to the expense and difficulty of producing their cutting edges and keeping them in order. This was next to impossible before the introduction of a machine, with a small emery wheel, and compound slides, etc, for carrying the milling cutter whilst being re-sharpened. Hence in the old system of milling, which did not permit of the resharpening of the hard teeth, the results were, that after much expense and time had been bestowed on a cutter (including a quantity of hand labour spent upon it while in its unhardened state), the whole was as it were upset by the process of tempering; the accuracy which had previously been imparted to it being usually quite destroyed by the action of the fire and sudden cooling.
In some cases the cutter would be found slightly warped or twisted; in others it would be oval or eccentric; and most frequently, when set to work on a truly-running mandrel in the milling machine, not more than 1/3 of the number of its teeth were found to be cutting at all, the others not coming in contact with the work. This really meant that not more than 2/3 of the proper feed per revolution could be applied, and not more than 2/3 of the proper work produced. Nor was this the only drawback; the quality of the workmanship produced by such a milling cutter was not of the best, and deteriorated hourly from blunting and wear. Such a cutter would probably not work for more than 2 whole days before it would require to be again softened by being heated red hot and allowed to cool gradually. The expensive and unreliable process of resharpening by hand-filing had to be gone through again once more; then the retempering, which caused the cutter to again become warped, swelled, or eccentric; and each time it was subjected to the heat of the fire, it ran the risk of being destroyed by cracking when plunged into the cold bath.
It is necessary now to describe the modern system of making and maintaining the improved milling cutters. A cast-steel forging, or blank as it is usually styled, is bored, and then turned to its proper shape in a lathe. The teeth are then machined out of the solid to their required forms, in a universal milling or other machine. This work is so accurately produced, direct from the machine, that no expensive hand labour need be expended upon the milled cutter, which is taken direct from the milling machine to the hardening furnace, and tempered. The hole in the centre of the cutter is then carefully ground out to standard size, so that it may fit naturally and without shake both on the mandrels of the grinding machine and on that of its own milling machine.
The cutter or mill B, Fig. 12G2, is now placed on the mandrel M of the small cutter-grinding machine; the mandrel itself is adjusted by means of a worm W and worm-wheel 0 to its required angular, vertical, or horizontal position, and each tooth is ground or resharpened by passing it once rapidly forward and backward under the small revolving emery wheel H. The mandrel fits easily into the cutter which is being ground, so that the latter may be readily turned round by the thumb and finger of the operator.
The exact mode of setting such cutters is as follows: - The clearance angle J L K on each tooth is obtained and maintained by the emery wheel H, one of which is exhibited. The clearance is obtained by adjusting the centre E of the emery wheel H a short distance horizontally to the left of the vertical line through the centre 0 of the milling cutter. The shorter this distance E C the less the amount of the clearance imparted to each tooth of the milling cutter A. The lower L K is a tangent to the circumference of the milling cutter, drawn from the point of contact L; and the upper line L J is a tangent to the emery wheel from the same point. The angle formed by these 2 lines is the angle of clearance.