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Making a wormwheel by free hobbing

A wormwheel can be cut by having the workpiece held so it can freely rotate. It is driven round by the cutting tool as it is being cut. This is called free hobbing.

 

On the lathe

This can be done on the lathe. All that is required is a bar fitted with a v ertical axle. This is held in a toolholder.

fig the bar and axle fitted on the lathe

The blank for the wormwheel fits on the axle.

fig fitted with the workpiece

The cutter can simply be a tap. The tap is held in the chuck. As the chuck turns the workpiece is pushed onto the tap. The tap cuts a “thread” in the workpiece.

However, the spacing of the teeth will be that on the tap. The spacing of the teeth on the wormwheel has to be right at the pitch circle diameter of the teeth formed on the wormwheel.

If the teeth just cut the surface of the wormwheel on its surface, then when the final tooth is cut the next one must be exactly the same position as the first one. Clearly it the first cut is a light cut and then a deeper is made second time round the position of the last tooth will be closer to the first one. This means it is not possible to make a series of cuts but the whole depth of the cut must be made from the beginning.

Also for the tooth to be cut correctly, the pitch of the teeth on the wormwheel has to be the same as the pitch on the cutter, furthermore the number of teeth times the pitch must equal the circumference at the pitch circle diameter.

It is possible to do this with a tap since the depth of the cuts is small enough to be able to make the cuts in one go.

On the lathe using a hob

This is just like the example above except that instead of using a tap, a hob on an arbor is used. The difference here is that the amount to be cut is much larger. The problem is the same as above. This is interesting in that the problem described above becomes much more obvious. It the hob lightly cuts the workpiece then, at the first rotation the gaps between the teeth and the width of the teeth are equal. But if, on the second time round the workpiece moves further into the hob, it will be seen that the size of the teeth and the gaps are still the same but they have moved round slightly. Since the teeth on the wormwheel can never get wider though the gaps can, the gaps get wider and wider till the teeth completely disappear.

The solution to this is to gash the wormwheel. It might seem that this is to make cutting the teeth more easy. But it is far more likely that the gashes determine the rate of rotation of the workpiece that is now slighlt less that it would have been if it had been driven just by the hob.

Needless to say the are spaced so the pitch of the teeth on the wormwheel is the same as the pitch on the hob given the number of teeth on the wormwheel. This will only be true if the pitch circle of the workwheel is correct.

 

On the milling machine

This could be done by having the workpiece on an arbor. This is held in a chuck on a dividing head. When doing this the worm on the dividing head is not engaged with the wormwheel on the dividing head. The cutter is on an arbor in the vertical socket. The rotating cutter is pushed into the workpiece as the cutter rotates it not only cuts the workpiece but it also drives it round. The ideal sort of cutter would be a gear hob but anything with cutting edges that form a screw will work. An example of such a cutter would be a tap for cutting screw threads

For this to work the cutter has to cut to the right depth in one go. This is made possible by gashing the teeth first. This is simply a matter of cutting out most of the metal with a slitting saw. Even better is to use a gear cutter which will cut away material more closely to the final shape. The workpiece is mounted on an arbor or mandrel which is driven by the dividing head. It must be fixed rotationally when doing the cut.

The slitting saw is mounted on a stub arbor fitted into the vertical head. The vertical head is tilted so the gashes are at the helix angle required.

Fig gashing teeth for a wormwheel

When this has been done the workpiece is left as it is. But it is altered so that it is free to rotate. If it is on an arbor then all that is done is the worm on the dividing head is disengaged. If it is on a mandrel then the dog that was being used is removed. Either way it is not free to rotate.

The hob is mounted on a stub arbor and held in the vertical socket. This should already be at the required angle. The wormwheel should be moved so the center of the hob at the height of the center of the wormwheel is aligned with the center of the wormwheel.

The table should be locked along all three axes during this. The cutter rotates and the workpiece rotates.

Ideally it should be possible to cut the teeth to a specific depth. This would be necessary if the worm and the wormwheel were to fit together in a gear box. But most worm and wormwheels used by model engineers are used for driving dividing heads and usually the engagement of the worm and wormwheel can be adjusted to minimise backlash.

Mew 78 p18