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.
The section is thus never altered, no smithing or alteration in form is necessitated, and consequently no repairing has to be done in the smiths' shops. The objects aimed at have been:
1. To produce the highest clas3 of workmanship, by providing the best known form of cutters, carefully made, and capable of having the cutting edges accurately reground, so that the surfaces of the machined work may be produced direct from the cutters so highly finished that no hand-work could possibly improve them. All the turning of wrought iron, for instance, is so perfectly finished that there is no necessity to polish it by means of emery or emery cloth.
2. To make all the cutters so free from complication, and simple to keep in order, that no difficulty or error may take place in regrinding them.
3. Since finely-polished surfaces cannot be obtained without the most perfect cutting edges, to make all cutters not only of the best steel, but with their cutting edges most carefully and accurately ground up, in almost all cases by mechanical means. The durability of the cutters, from their construction and high class of material, is very great, and they are thus capable of removing a great weight of metal in a given time.
The grinding or resharpening of all cutting edges is reduced to the greatest simplicity; and only three descriptions of machines are requisite for this purpose. They are all arranged to grind mechanically; that is to say, the cutters while being ground are carried and pressed on the grindstone or emery wheel by mechanism. The requisite forms and angles are also obtained by mechanism, it being found in practice that sufficient accuracy cannot be secured by hand grinding.
The machines are as follows: -
1. A grindstone with slide-rest, for grinding all the cutters used in tool-holders.
2. A twist-drill grinder; this also is by preference a grindstone, with mechanism for holding and guiding the twist-drills. A machine with an emery wheel in place of the stone is also used for the grinding of twist-drills, with much the same mechanism for carrying the drills. In practice, however, the stone grinds about double the number of drills per day, and with less risk of drawing the temper. Both stone and emery-wheet are run at a high speed, and used with water.
3. A small but very complete machine (Fig. 1262) for regrinding milling cutters. In this case gritstone does not answer, and the grinding wheels are obliged to be emery or corundum. They are very small in diameter, and many of them are exceedingly thin, and so delicate in form that if made of gritstone they would rapidly lose their shapes. They are run at a high speed, and are turned into form while revolving by means of a diamond. A milling cutter will work for a day, and in many cases for 2 days, without showing signs of distress.
Before the cutting edges are visibly blunted, but as soon as the sense of touch shows their keenness to be diminished, the cutter should be put into this machine; and the probability is that not more than 1/1000 in. need be ground off each tooth, before it is restored again to a cutting edge almost as fine as that of a wood chisel. Each cutting edge, or in other words each tooth of the milling cutter, is only passed rapidly once or twice under the revolving wheel, which is itself of very fine emery. It can therefore be readily understood how delicate an operation this is, and why emery alone will answer for it.
In order to maintain the correct forms of angles of all cutters for tool-holders, sheet steel angle gauges, Fig. 1254, are provided, and the process of grinding is thus reduced to a complete and exceedingly simple system. In well-regulated shops a young man is selected to work each machine for cutter grinding; and in practice each man so engaged can keep a works employing 150 men (exclusive of moulders or boiler makers) well supplied with all the necessary cutting tools from day to day. A very great saving is thus effected, as no machine need ever stand idle for want of cutters.
Take for instance an engineering works employing 250 men. The requisite number of improved grinding machines, with special mechanical appliances, is as follows : - 2 patent grindstones for resharpening cutters mechanically; 1 patent twist-drill grinder for resharpening twist-drills mechanically; 1 improved cutter-grinder with small emery wheel, for the resharpening of cutters used in milling machines.
To follow the system out satisfactorily, the man working the grindstone goes round to each machine every morning, collects together those cutters which have been blunted by use the previous day, carries them to his grindstone, resharpens them, and distributes them again to each machine; which is thus kept well stocked with an ample number of cutters, always ready for immediate use.
The cutters for tool-holders do not require any repairing in the smithy; consequently that operation, which is costly in so many ways, is avoided, and jobbing or tool smiths with their strikers are almost entirely dispensed with.
For rehardening the cutters, a ride is made that when the grinder meets with cutters which are not as hard at their cutting points as they ought to be, he puts them on one side, and periodically, say once each fortnight, he sends the lot into the smithy for the end of each to be retempered. This is a very inexpensive operation. They are placed in a small oven by dozens and very slowly heated up to a dull red; the end of each cutter is then plunged into a perforated iron box, the bottom of which is covered with the required depth of water, to harden the cutter to the proper distance from its point. The cutters are left standing in a nearly vertical position in the box of water, until they have gradually cooled down sufficiently to be removed. They are then sent to the grindstone, reground, and given out with the other cutters to be again used in the different machines. With steel of the highest qualities for the cutters it is most important to keep it out of the smith's fire entirely, if possible. That object is here attained, the cutters never going to the fire except for rehardening.
During the life of a cutter it only sees the fire probably 6 times.
As the weight of each cutter is small, not probably more than 1/15 to 1/20 that of a forged tool used for the same purpose, the outlay for best tool steel is not heavy; and the engineer is not tempted to purchase any but that of the highest quality. With such steel, especially when used in the best manner, each machine is capable of cutting at a high rate of speed, and the cuts may be coarser than those ordinarily taken. When the swivel tool-holders were first used on planing machines, cutting slots 1 in. broad into solid castings, it was found that 2 teeth of the feeder could be used at each stroke. Previously a forged tool of the same breadth, ground to form by the planer to the best of its ability, had been used in the same machines; but ho found, on trial from time to time, that it was impossible to use more than one tooth of the feed; or, in other words, the tool-holder cut a given depth into the metal in half the time of the forged tool.