For many products the use of a dividing head is an absolute necessity. Many home workers, especially those without any engineering experience, regard them as most mysterious devices, almost bordering on the occult, and say without really thinking, 'Oh. I could never use one of those!'. Well, a dividing head is really no more than a headstock with a spindle on which the work is mounted, with some means of turning it through positive angular amounts, and holding it there when each movement has been made. Naturally there are many types of dividing head and over the years many designs have appeared in Model Engineer for heads which can be made in the home workshop. A great deal of quite satisfactory work can be done with a simple head of the type shown in Fig. 35. On the spindle, provision is made for mounting a lathe change wheel. A spring-loaded plunger with a conical point drops into the gap between two teeth of the wheel, and then the spindle is locked by a screw bearing on a pad inside the main bearing. It is advisable not to rely on the plunger holding the spindle against rotation when screwing on chucks of when fixing a component on an arbor by means of a nut. If the spindle turns, the teeth of the change wheel may be badly damaged. In fact when doing this sort of fixing I always disengage the plunger, then if the screwpad does not hold, no damage is done.
By selecting a suitable change wheel it is possible to get quite a lot of divisions very easily. For example a 60 tooth wheel will give 2, 3. 4, 5. 6, 10, 1 2, 1 5. 20 or 30 divisions. It will not give 8, but a 40 tooth wheel will do so. When doing dividing with this kind of device it is a good thing to have a bit of chalk handy and mark the appropriate tooth gaps where the plunger is going to have to drop in, before starting cutting, to avoid incorrect settings which would spoil the work. Many examples of machinery parts to which a simple head of this kind can be usefully applied could be given. Such items as crankcase drain or filler plugs which need hexagons, square ends on shafts, tools like taps, reamers, parallel flats for spanners on round articles, all these can be formed so very easily with an end mill, with less physical effort than filing, and with an accuracy which enhances the appearance of the article even if dimensional accuracy as such is not important.
But there are examples where accuracy is fairly important, and one which could hardly be done at all with hand tools is shown in Fig. 35. This is one half member of a dog clutch. The 1 2 teeth are being cut with a slitting saw which passes across the work right on the centre line. After each cut, the locking screw was eased, the plunger lifted out, the wheel turned five teeth, and the plunger dropped in again. The spindle was then locked and the next tooth gap cut. Really a very simple procedure. Now on the other half member of the clutch the teeth have to have parallel sides, and the gaps themselves are taper sided. This just involves setting the cutter with its bottom edge above the centre line by half the thickness of the teeth left upstanding in the first half. The same procedure of cutting right across is followed, and after six passes the job is complete. It is feasible, if you are willing to take the trouble, to make a clutch with all tooth sides tapered, so that the two halves are identical. If maximum strength was needed to transmit a lot of power this might have to be done, but it is a good deal more difficult and would rarely be worth the trouble. Unless you are using a well-established design for which drawings are available, it is advisable to lay out the tooth design on the drawing board, preferably at an enlarged scale, to verify the thickness of cutters which will produce the desired result. They may be the same thickness for both halves, but maybe not, it depends on the thickness of tooth selected. It is also a good thing to avoid an odd number of teeth, because the curve of the cutter when going through one side may be chewing into the metal which has to be left intact on the opposite side to make the tooth. If your design can arrange for an even number of teeth this risk will be eliminated. Another point is to check that the desired number of teeth can really be secured with the dividing head you intend to use.
Fig 35 Cutting teeth in dog clutch part
Fig 35 Cutting teeth in dog clutch part
STEADY STAND for MYFORD DivV&HEAD
The Myford dividing head is an excellent piece of equipment, with a very wide range of divisions. The main spindle has a 60 tooth worm wheel on it, and a single-start worm meshes with that. Concentric with the worm there is provision for mounting a multi-holed division plate which remains stationary and does not rotate with the worm. On the worm spindle is fitted an arm carrying a spring-loaded plunger which has a point of parallel shape that enters holes in the division plate. This arm is slotted and can be set to such a radius as will bring the plunger in the right place for any of the rows of holes that are already drilled in the plate. Having set the arm, if one turns the worm one whole turn and drops the plunger back into the same hole from which it started, the main spindle will have rotated one sixtieth of a turn. But if one moves the worm and arm five complete turns before dropping in, the main spindle will have turned one twelfth of a turn. Basically, that is all there is to getting any desired number of divisions. Having got the right division plate on the head one moves the arm so many turns, plus if necessary, a certain number of holes extra to the complete turns. A chart supplied with the head gives all the available combinations. In order to accomplish all divisions up to 100 it is necessary to have 4 plates, but two of these are needed only for some rather outlandish numbers with which few model engineers will ever have to deal, so the two normal plates will seive almost everything. There is one point of practical importance in using a
Opposite, Fig. 36 Drawing of steady stand for Myford dividing head worm geared dividing head. When moving from one position to the next, always turn the worm the same way, never go back. If by chance you overshoot the right hole, of course you have to turn back, but go well back, way beyond the hole you want by a good margin, then come up to it afresh. If you fail to do this you will have an error in your dividing and a scrapped work-piece. Our old enemy back-lash' will see to that. But it's easy enough to avoid this kind of disaster. There is provided on the head a most important aid to correct counting of the number of holes needed when turning the worm. Two brass blades are fitted around the worm shaft, above the division plate, and these can be moved relative to one another, by loosening a screw, and set to embrace the number of holes needed. Than after locking with the screwdriver, they make a mask to show just where the plunger should be dropped in. After each movement you rotate them till one blade comes against the plunger, and you are then ready (after doing the cutting of course) for the next move. In this part of the procedure the two blades move together as if they were one piece of metal.
I have found in using the Myford head that it is a convenience to be able to set it at lathe centre height when fixed on the boring table. If one wants to drill cylinder covers and similar work the radius of the row of holes can be readily obtained by the cross slide screw and the measurement is direct. So I have a packing block of the right thickness which I can place under it for this purpose.
There is one minor criticism of the Myford head which is nevertheless important from a practical point of view. The single bolt which holds it to a machine
table or vertical slide, etc. does on occasion come a long way from the point where cutting is being done, and accordingly there is danger of the work being spoilt by the head slipping. To overcome this I have made up a steady stand from mild steel bar material which bolts on the table of the miller, and clamps on the 1 in. overarm bar of the head. The stand has a vertical j in. bar set into a flat base with a slot for a table bolt. A two-way clamp slides on this vertical bar, and another f in. bar passes through it horizontally. At the end of this is a two-plate clamp gripping the f in. bar. with provision also for gripping the 1 in. bar of the head. The various clamps can be moved separately and make a pretty universal fitting. The whole thing is shown in use in Fig. 37. This fitting of my design is not on the market, but it has proved so useful to me that I am giving a working drawing of it in Fig. 36 and anybody who likes can make a unit for himself.
OTHER DIVIDING HEADS
Since the last edition of this book was printed three new dividing heads have appeared on the market. The first, of my
Fig. 39 The George H. Thomas Versatile Dividing Head
Fig. 39 The George H. Thomas Versatile Dividing Head
own design, replaces that shown in Fig. 35, long out of production after the maker died several years ago. it is essentially similar with detail improvements. It has a tailstock for supporting long slender pieces, and a pair of raising blocks which bring the centre height up to just over 3 j in. and thereby allow for rotating work up to the size of the 7" diameter Myford faceplate. It is shown in Fig. 38.
The second type is a much more elaborate and versatile appliance designed by Mr Geo. Thomas, and supplied, like the first one, by N.S. & A. Hemingway, 30 Links View, Half Acre, Rochdale. In this head a 24-hole division plate provides for simple dividing with those factors associated with 24. A 40-tooth worm wheel and worm can also be engaged, with a six-row drilled-hole division plate, giving much finer divisions. This plate can be rotated by a subsidiary worm, thereby permitting very high
Opposite, Fig. 40 The Kibbey dividing head numbers of divisions to be obtained. Most people will need some help to make the fullest use of this device and the book by Geo. Thomas himself on its construction and use, (Dividing and Graduating, Argus Books Ltd.) will be found the best source of information. This head is also available with tailstock and raising blocks, but in normal form is shown in Fig. 39.
The third head is supplied also in kit form by Model Engineering Services, and was designed by Mr Ron Kibbey. It uses standard Myford change wheels as division plates, but has a forked locking plunger which can span over a tooth as well as drop between two teeth. Thus the number of divisions increases to twice the number of teeth in any wheel. In addition it has a mounting for a wheel-pair to mesh with the spindle wheel, giving a gear ratio to add to the basic divisions. It is not at present provided with a tailstock or raising blocks. The head, with extra gear pair in position, is shown in Fig. 40.
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