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Making a telescope worm wheel and gear set on the mini-lathe. Part 7, planning the worm gear

Telescope worm wheels and gears on mini-lathe series index
plan for the worm gear

By now you are bored of looking at the worm wheel blank, and are probably reading this as a break from gashing the 360 teeth in the worm wheel blank. Now we get to turn our attentions to the worm gear. It is possible to use a length of threaded rod as a worm gear, but this is not precise enough for our purposes.

The worm gear will be made from a length of 20mm silver steel bar. Slightly larger or smaller than this is probably fine. The bar will need two threaded sections. The first will be the actual production worm gear used in operation. This section must be adjacent to bearing surfaces on the shaft which match whatever worm gear mounting mechanism you choose. You will also need another threaded section, some distance from the first section. This will have an identical thread cut into it, but will be cut and hardened to form the hob which will cut the teeth in the circumference of the worm wheel blank.

The diagram attempts to demonstrate these features. The turned section of the bar must be designed to match whatever bearings you are going to use to mount the worm gear, but we don't want the threaded worm gear to interfere with the bearings, so you need a start and end slot for the thread separated from the turned section.

The outer diameter of the steel bar where we are going to thread does not need its diameter reduced, but does need to be turned slightly to make it concentric to the bearing surfaces.

The most critical aspect of the entire operation is to ensure the axis of rotation of the threaded section is concentric with the turned bearing surfaces. The better you get it, the less periodic error you will have.

Because we are on a wobbly mini-lathe, it is worth turning the bearing surfaces close to the required diameter, and then cutting the thread. Cutting the thread might disturb the position of the part in the lathe, so once the aggressive threading is complete, do the final cutting on the bearing surfaces.

The part must not be removed from the mini-lathe until all the cutting and threading is complete - this is very important.

Once the part is finished, it must be removed from the lathe for preparation of the hob and hardening of the hob section. Once everything is complete the hob is cut off and discarded

The worm gear section and the hob section must have identical threads - we will thread both sections at the same time to achieve this.

The thread itself is not cut with a regular 60 degree threading tool. We must grind our own tool, with a tip of 40 degrees instead. The depth of the cut must also be exact - 1.03mm or 0.86mm depending on size of the worm wheel blank. The controls on the lathe are not adequate for measuring this - I recommend mounting and dial indicator to measure the depth of the cut.

Quite a bit of the steel bar needs to be removed before you even start threading - this operating is perhaps the hardest part - it is very difficult to remove the required metal without lots of swapping between tools, and lots of parting, which is always troublesome on a mini-lathe. It is preferable to start some of the cuts with a band saw before you mount the part in the lathe.

Positioning the part in the lathe is another matter. For maximum stability and minimum chatter we need the shortest possible length of steel bar between the chuck and the fixed steady. However, we still need room to get the saddle and cutting tool in place. We also need enough room to mount the worm wheel during the worm wheel tooth cutting phase.

I will deal with all of these issues as they arise - first we need a bit of silver steel! And we have to finish the gashing job you are busy taking a rest from!

Telescope worm wheels and gears on mini-lathe series index


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