Dividing Heads and Gearcutting

The availability of a dividing head is again essential for doing gearcutting on the milling machine. Of course there are types of gears one just cannot do, but ordinary spur gears can be done perfectly well for model engineering purposes, where neither high speeds, extreme silence, nor high rates of power transmission are demanded. The design of gears Is a subject outside the scope of this book, which is intended to deal with workshop operations, but there are plenty of sources of information on gear design. The simple type of dividing head already Illustrated will serve very well if the gears to be cut have such a number of teeth as the indexing change wheels can deal with. But if the number required cannot be got from existing wheels, then a more complex head such as the Myford, will be needed. Fig. 37 shows this in use cutting the teeth of a pinion which are integral with the shaft. The blank was turned to | in. diameter on the part to be held and this was gripped in the standard Myford \ in. collet, while the other end of the component being centred was supported by a 60 degree centre in the overarm fitting. This picture shows the steady bracket described in the last chapter in use. The cutter is a simple flycutter in a boring bar held for convenience in a boring head. It is shown in close-up in Fig. 41. The profile was established by grinding to suit a wheel of the same pitch with slightly more teeth. The variation is so small as to be of no importance, especially as the pinion rotates at only a low speed.

But if several gears are to be made, and especially if duplicates may be wanted later, it can be worth while to invest in one or more proper disc type gear cutters of what is now universally known as the Brown & Sharpe' type, because they were developed by the famous firm of Brown & Sharpe in U.S.A. many years ago. They are of course now made by British firms as well, and by others all over the world, to an accuracy of international standards, far better than anything that is needed for model engineering, and are properly backed off as well as being made from high speed steel. No single cutter will properly deal with all number of teeth, so they are made in sets, eacf* cutter dealing with a limited range, and each bears an identifying number. The range runs as follows:

No. 1 135 to a rack No. 5 21 to 25 No. 2 55 to 134 No. 6 17 to 20 No. 3 35 to 54 No. 7 14 to 16 No. 4 26 to 34 No. 8 12 and 13

Fig. 41 Close-up of flycutter and pinion

Fig. 42 Gearcutting with Brown & Sharpe cutter

Fig. 42 Gearcutting with Brown & Sharpe cutter

Fig. 43 Flycutting 10 d.p . gearwheel, front view

These cutters can be bought singly at any time from regular tool merchants and I doubt if any discount would be given for buying a complete set of 8. So there is no need to go to the expense of acquiring a complete set unless it is firmly known there will be a use for every one! Fig. 42 shows one of these doing a similar job to that depicted in Fig. 37. The smoother action of the multi-toothed cutter made it possible to dispense with the use of the steady stand, though care was taken not to be too rough with the feed, especially at the start of each cut.

Going now from what might be called the sublime to the ridiculous, or at least from the miniature to the outsize, the next photograph shows the cutting of a much larger gear, actually 9.600 in. p.c.d. This is a gunmetal gear needed as part of a metal pattern from which the flywheel of Fig. 20 was made. It is only 3/16 in. thick but the teeth are 1 0 d.p., approx. 5/1 6 in. centres, so quite a lot of metal had to be removed at each tooth. The cutting was done with a flycutter, ground up by hand to match a silhouette of a 10 d.p. tooth in Machinery's Handbook, using a magnifier, this cutter being set in a boring bar of rather excessive length in a boring head. Two cuts were taken, but even then there was a good deal of spring and noise. The shape of the blank casting was arranged to provide for mounting by 8 bolts on the large Myford faceplate, (9 in. diameter) and this was fixed on the miller table so as to overhang the side. That permitted fixing a standard angleplate on the table too, just touching the back of the faceplate, which reduced the springiness of that, and provided a back-stop against the danger of slipping. It would have been asking a lot of the single bolt of the dividing head to prevent movement, under the conditions

Fig. 44 Flycutting 10 dp. gearwheel, rear view

prevailing, but using this safeguard all went well.

It is well worth keeping in mind in the home workshop that this method of back-up is widely used in the engineering industry, especially in the heavier sections, on planing machines and others where there is either high thrust or high impact, sometimes both, because it can avoid damage to machines and work, as well as possible injury. Fig. 43 shows the front of the gear disc, and in this view the dividing head is not visible. But in Fig. 44 both the angleplate and the dividing head are seen. The gear blank would only just swing in the gap of the Myford lathe so without raising blocks it represents about the largest job that can be turned.

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