For DRIVERS MECHANICS & FOREMEN
Page 21
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Lamps Alight.
On Saturday, 26th October, light your lamps at 5.14 in London, 5.44 in Edinburgh, 5.11 in Newcastle, 5.21 in Liverpool, 5.19 in Birmingham, 5.24 in Bristol, and 6.4 in Dublin.
On Casehardening.
'The sender of the following communication has been awarded the los. prize this week.
[1910] "HIT." (West Bromwich) writes :—" Now that spare parts in general, and gears in particular, are so difficult to get hold of from the makers, many mechanics who would not otherwise think of tackling
• such Work are making their own. It occurs to me, therefore, that a few notes on casehardening may be of interest and assistance to readers of your DAC and F.' page.
"Casehardening of Alloy Steels.—When nickel steels are heat treated by casehardening, the effect of the nickel appears to be that of retarding the process, so that the case is not so hard as it is with ordinary carbon steels. On the other hand, the presence of the nickel tends to prevent crystallization of the steel at high temperatures, thus preventing the brittleness which accompanies that state. When using 2 per cent. nickel steel, the following procedure is recommended. The material should be first heated to 1830 degrees Fahr. It is then quenched ; secondly reheated to 1380 degrees Fahr., .allowed to cool to 1290 degrees Fahr., and again quenched. The object of this double quenching is to improve the tenacity of the core. Where such toughness is not of great importance the second procedure only need be adopted. It will be found in that event that a rather harder case results.
"S per cent, nickel steel should be heated to 1560 degrees Fahr., and quenched ; then reheated to 1245 degrees Fahr., and again quenched. However, as this high percentage of nickel content almost completely obviates the brittleness of core occasionally met with in carbon steel, it will very often be found sufficient merely to heat to 1290 degrees Fahr., and then quench. "When a specially hard case is required. steels with from 1 to 1.2 per cent. chromium are frequently used.
• The effect of chromium is to increase the tendency to crystallization of the core, and in consequence, a double quenching is necessary. The treatment first given above for nickel steel is then recommended. "The best material for carbonizing is a mixture of 60 per cent. wood charcoal and 40 per cent, of barium carbonate.
"Casehardened Gear Wheels—There are four general classes of steel Used for casehardened gears, namely, carbon steel, nickel steel, chrome vanadium steel, and chrome nickel steel. In each of these classes, too, various modifications are to be found. Generally, chromium steel is, in my opinion, preferable for automobile gears. The normal carbon content of all such wheels should be about 0.20 per cent., • and in no circumstance should it exceed 0.25 per cent. If too much carbon is present, the teeth of the gears will be brittle and liable to snap off. Properly carbonized and casehardened, this will result in a carbon content in the case of 0.9 per cent. The temperature of carboniiing should be about 1625 degrees Fahr. Lower temperatures do not give sufficient depth of case unless the heating. operation is greatly prolonged. Conversely, higher temperatures result in a skin of excessive carbon content, and the core does not as readily respond to subsequent heat treatnaent, which is very important, and should be as follows:—After it is considered that the work has been sufficiently long in the carbonizing ovens it is allowed to cool in the box. The gears should then be reheated from 1550 to 1626 degrees Fahr., then quenched. The object of this is to refuie the core. Next, they must be reheated to between 1350 and 1425 degrees Fahr., and again quenched. This to harden the case. Finally the temper must be drawn in oil at a temperature of not over 400 degrees Fahr. This further increases the toughness and strength of the material."
A Handy Pair of Bench Centres.
[1911] "FI.S.H." (Wood' Green) writes :—" It is not every small garage that possesses a lathe and even where such a machine tool forms part of the equipment, it is sometimes quite a small one, and in any event is generally very freely used and fully occupied. At the same time, one of the commonest repair jobs is that of straightening shafting, an operation which, particularly if the shaft is a. fairly stout one, requires a substantial pair of. centres if it is to be • successfully performed.
"The sketch which I enclose [we have had this redrawn.—En.] shows a very useful pair of centres 'adapted for attaching to the work bench. Two rough castings are required. These, as shownin the sketch, are quite simple. The pattern can be made by anyone with a knowledge of wood-working tools, and preferably they are made with clamping screws so that their positions on the bench may quickly be adjusted. Into one bracket a fixed steel centre is slipped. The other centre, however, should be screwed throughout its length so its to allow of fairly fine adjustment. It may be secured in place as shown, by means of a couple of nuts, one on each side of the boss on the bracket. This method does not involve tapping a hole in the bracket itself. A better way and involving a little more work on the part of the constructor, would be to tap the hole in the bracket and fit thg screwed movable centse, with a hand wheel and look nut. Alternatively, an old lathe headstock can sometimes be picked up at the marine store dealers for very little, and this can be adapted for the same purpose. I have had such a pair of brackets in use in the works now for some time. They are very useful indeed, and there is not the likelihood of having to hold up an urgent repair to a bent shaft owing to the fact that the only lathe is occupied on some important piece of turning which cannot be disturbed."