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Making a telescope worm wheel and gear set on the mini-lathe. Part 8, Beginning the worm gear

The first task is to get the 20mm silver steel bar from which we will make the worm gear mounted on the lathe. To do this, we hold it with a chuck on the headstock end, and a fixed steady on the tailstock end. The tailstock itself can be added on the end for extra support - although things might become a little short of space on a regular mini-lathe bed. If you do use a tailstock, it is important to have it well setup so you don't end up turning a taper.
marking up the worm gear blank on the lathe with the worm wheel blank

We need to judge the position of the steel bar, keeping in mind clearance for the cross slide mounted worm wheel in the later stages of manufacturer. I mark up the bar with four regions - T for the two turned sections for the work gear housing, W for the worm itself and H for the hob region. Check that all sections of the H and W regions can mate with the worm wheel and not cause the worm wheel to hit either the fixed steady, or more catastrophically, the chuck. On the other hand, the wider apart the chuck and steady, the more wobbly and chattery operations will become.

Parting is always a difficult operation on the mini-lathe, so you need to make sure you have a good parting tool before you start. Make sure the parting tool is properly sharpened, and easily mounted at the correct height. Arrange the top slide and cross slide such that the parting tool can reach all the zones to be parted. Make the the minimum amount of parting tool is sticking out of the holder - about 6mm should be enough.

Once you know where everything is going to sit, you need to get the steel worm gear blank securely mounted in the lathe. This involves a chuck, a fixed steady, a dead center in the tailstock and a lot of indicating.
indicating on center drill held in collet in tailstock of mini-lathe indicating on worm gear blank held in 4 jaw chuck preparing the rod for the dead center by centre drilling

First we must insert a centre drill into the tailstock with a collet - you will not have room for a chuck. Indicate on the centre drill to make sure the tailstock is horizontally and vertically aligned. Adjust until it is as near perfect as you can get (less than 2 thou). Then mount the 20mm diameter silver steel bar in the 4 jaw chuck, and indicate close to the chuck jaws until it as no runout. Then slide in the fixed steady and tighten it up. You need to make sure that the silver steel rod has no runout when the lathe turns, and also, no runout as you slide the indicator along its length with the indicator attached to the saddle.
worm gear blank mounted in lathe and ready for cutting indicating on worm gear blank held in 4 jaw chuck

Re check everything after mounting the fixed steady, and make sure the tailstock end of the silver steel bar is free of runout. Then you can carefully center drill the end of the rod. All being well, you should just have room on the bed of the mini-lathe to bolt on the tailstock.

Replace the center drill with the dead center and tighten against the bar. Recheck all the runout. Everything should turn true. The bar should be turning exactly on the axis of the spindle, and it should be exactly parallel to the ways.

Now we mount up the parting tool, and start to cut away the start and end slots for the two threads, and turn down the bearing sections. In the photograph on the right you can see I've started some of the slots, and I am actually using the parting tool to turn down the bearing sections. Keep a close eye on the tailstock and fixed steady - as the bar heats up these can create a lot of friction. This three point mounting may seem a little overkill, but is extremely rigid. Even grabbing the bar and trying to move it by hand should only allow a couple of thou of play at most.
progress on the worm gear

Now is the hard work of making chips and getting the worm gear roughed out and about the right size and shape. On the right we can see I've made a fair bit of progress. The cuts, from left to right:

  • Nearest the chuck is the finishing zone for the threading.
  • Next is the longer turned section for the mounting bearings
  • A finishing zone for the thread on the worm.
  • The starting zone for the thread on the worm
  • The other turned section for mounting bearings.

We can't simply use the turned section as the starting zone for the thread. If we did, the thread lead in could interfer with the mounting bearings. Doing it this way allows a nice flat surface for the mounting bearings. THe final operation in this section is to take a very light cut across the worm and hob sections to true them up. Although we indicated the bar into the proper position, a tiny amount of metal still needs to be removed to make the surface truely concentric.

Happy cutting!

Telescope worm wheels and gears on mini-lathe series index

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