Dividing Heads and Tool Making

There are many occasions in tool making when dividing is necessary. Multiple edged cutting tools like taps, reamers, milling cutters, countersinks, etc. all really need dividing devices to produce the best results, even though some of the simple cutters can well be made by filing or freehand grinding. However, the form of the teeth or flutes sometimes settles that rough and ready methods just will not do, and as in the next example the physical difficulty of getting at the metal that has to be taken away more or less settles that mechanical dividing must be employed. Fig. 45 shows the fluting of a long-thread Acme tap which is held in a collet in the spindle of a simple dividing head, using change wheels for the dividing. In order to

Fig. 45 Fluting Acme thread tap

√Čar obtain the maximum swarf clearance this tap has five flutes. The head has no tailstock so the outboard end of the tap is rested on a pair of Picador blocks and the clamp rests on another pair. These are very useful accessories for milling operations. Of course each time that a flute is completed the clamp has to be released before the work can be rotated to the position for the next flute. A suitable tailstock, were it available, would obviate the need for this. The 5 flutes are obtained by moving 12 teeth at a time on a 60 tooth wheel. The cutter being used is a commercial tap-fluting cutter picked up cheaply at a sale. These cutters are made with a somewhat lop-sided rounded profile specifically for this duty, but if it had not been available, a flycutter would have been ground up to the profile of a similar tap. The profile is not desperately important and a small error would not matter.

Another example of the use of the dividing head, this time coupled with (he use of a small rotary table, is shown in Figs. 46 and 47. The workpiece to be produced was a fine tooth milling cutter with a round end for routing or hand milling on the light alloy cylinder head of a car engine. The commercially available cutters for use in electric drills had such coarse teeth that once they touched the surface of the alloy they were uncontrollable and pulled sideways so violently that damage to the cylinder head was almost a certainty. So as fine pitch cutters appeared to be not purchasable it was decided to make one. The blank, of a carbon steel similar to silver steel but somewhat lower in carbon, was made to hold in a collet and was given a small recess in the end for the fluting cutter to run into. The dividing head, with a 50 tooth change wheel on its spindle, was mounted on a steel plate so that the end

Fig. 46 Cutting teeth of ball-end cutter

Fig. 46 Cutting teeth of ball-end cutter

Fig. 47 Close-up of ball-end cutter of the work-piece was beyond the centre of the rotary table by half its diameter. In other words, the centre of the ball end was over the centre of the rotary table. The axis of the work was on the rotation centre. This Is not apparent from the photograph, but was an essential feature of the set-up. A stop block was clamped to the underside of the milling machine table with a toolmaker's clamp, visible in the photograph, to limit the table movement positively to this position. In the other direction the movement of the table brought the cutter to a part of the tool shank smaller than the diameter at the bottom of the flutes so that indexing could be done with the cutter in the clear. The cutter used was a carbon steel one made originally for producing locomotive lubricator ratchet wheels, with a 60 degree single angle. With the axis of the work parallel to the table a cut was started at the required full depth, and the table traversed along bringing the cutter into operation cutting along the cylindrical portion. When the table was arrested by the temporary stop block, the rotary table was turned by means of its worm, so the dovetail cutter continued cutting round the ball end of the work. When the cutter ran into the recess, the feed was reversed first with the rotary table, then with the main table, back to the starting point, where the cutter was clear of the work shank. The dividing head was then indexed one tooth on the wheel, and a new cut started. Eventually all 50 cuts were completed as shown in Fig. 47.

The working diameter of this tool is $ in. and there are 50 perfect teeth. The tool was hardened and tempered, and when put to use in an electric drill was found to be entirely satisfactory. It worked completely chatter-free, with no tendency to run away, and in spite of its fine and shallow teeth, removed metal at a very

Fig. 48 Gashing flutes in large countersinking too!

gratifying rate. The work on the ports in the alloy head was completed to the great satisfaction of the user, leaving a beautiful smooth surface for the gas flow.

Another example of cutting tool making is shown in Figs. 48 and 49. A large 60 degree countersink was needed for a commercial operation on steel tubes, the tool being about in. diameter. It was made with an internal form identical with the Myford lathe spindles from a carbon-manganese chrome alloy of known identity, so that subsequent hardening could be done without risk of failure in a commercial establishment with knowledge of this steel. In the picture it is shown mounted on the Myford dividing head having the flutes cut with a special angle form disc type cutter. Because of the peculiar angles which are involved it was necessary to be able to set the head with its axis at an inclination to the table, and it proved that the simplest way to do this was to use a vertical slide. There was quite a lot of metal to be taken out of the 25 flutes in this decidedly tough steel, and as usual the cutting was a long way from the anchorage point, so the steady stand was brought into use at the back of the head, as it proved too difficult to set it at the same side as the cutter. However, it served quite well in that position and there was never any suggestion of insecurity.

The main gashes for the flutes were taken out first, with two cuts down each flute. Then the head was tilted to a new angle and another series of cuts taken to accomplish the relief Because of the conical shape of the work the vertical slide had to have its base set at an angle to the miller table. All these apparently complicated settings had to be established experimentally (though possibly with a tot of effort they might have been calculated) to give the desired form of the cutting

Fig. 48 Gashing flutes in large countersinking too!

Fig. 49 Rear view showing steady stand in use

edges of the countersink and the rake angles desired in two directions. The small division plate in use is one which had been made some time previously for doing 125 division micrometer dials, for which unfortunately the standard Myford plates do not provide, or did not at that time. However, with this set-up and not too many hours work it proved possible with home workshop equipment to produce a very suitable special countersink which would inevitably have cost a small fortune if it had had to be made in a commercial factory.

It is hoped that these examples of tool making will encourage all who need non-standard tools, and who know of no firm that would take them on, or are deterred by the high cost of labour-intensive specials. Who knows, somebody in a home workshop might take on the job of helping out some tool factory that would not want to be diverted from its normal work by jobs of this kind?

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