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Homemade rotary table dividing head

As part of my ongoing mission to making an equatorial telescope mount, I need to make some worm wheels and gears.

The worm wheels which I am aiming for are roughly 6 inch diameter with 360 teeth. Part of the routine for making these wheels is to make rough indexing marks around the edge of the worm blank with a slitting saw or similar. One cut per degree. 360 cuts all together. A Process known as gashing. It helps ensure the correct tooth count.

To rotate the worm blank through steps of 1 degree it is usual to use a rotary table or dividing head. Both these items cost rather more than I am prepared to pay for a tool that I will only use occasionally. Also, a "proper" dividing head or rotary table is designed to allow you to do fairly heavy milling - I don't need this facility. I decided to make my own homemade DIY dividing head for light work.

In my capacious parts box I had a small (2 inch diameter) 60 tooth worm wheel gear set. I had been thinking about making this into some kind of rotary table / dividing head / worm wheel gashing gadget. One weekend I decided to see what I could come up with.

Like my of my creations, I'm never sure how they are going to turn out. I'm hopeless at designing things, I need to work them out as I go!

First, I found a bearing in my parts box. It was 15mm inner diameter and 32mm outer diameter. This seemed ideal to support the working table end of the gadget. A length of 15mm diameter silver steel (drill rod) fitted it well.

Obviously the overall design consists of a supported worm gear, which can be turned, this turns the worm wheel. The other end of the worm wheel axis is connected to a flat working surface, onto which you can bolt parts to be worked upon. So, the first part I tackled was the support structure for this axis.

I used a 2 inch long piece of round aluminium round bar stock (2 inch diameter). I bored this out to fit the 15mm silver steel axle. I bored it a couple of thou' oversize. One end of the bore was enlarged to fit the 32mm od bearing. With the bearing in one end, and the axle turning in what is basically a long bush, the end play was pretty minimal, and within requirements.

Next I tackled the axle itself. The surface is supplied ground, and requires no further work. One end I drilled and reamed out to 5.5mm, and the other end I drilled and tapped M6.

The inner bore of the worm wheel is an annoying 5/8 inch. Slightly bigger than my 15mm axle. So I made an aluminium plug that was a light push fit into the worm wheel, and a light push fit into the 5.5mm hole in the end of the axle. A stainless steel set screw holds the plug to the axle, and the plug is held into the worm gear with high strength retainer.

Then I made the work table. This is simply a 20mm long length of 3 inch o/d aluminium barstock. The centre is bored out to be a press fit for the 15mm o/d axle. I made it a very tight fit. I heated the aluminium in the oven to 250C and the frozen steel axle slipped in easily. Now it has cooled, you'll not get them apart! It always amazes me how much heat the aluminium retains for some time after it has come out of the oven.

It is very difficult to measure inner bores to get a nice fit in this sort of heating/cooling press operation. Telescoping bore gauges, micrometers and a lot of practice. I usually cheat and aim to get the inner diameter of the bore equal to the o/d of the bar I am inserting. Normally I end up with it measuring a few tenths under. Leave the aluminium part in the oven on maximum for a good 20 mins or so. Leave the steel bar in the freezer wrapped in cling film. Make sure there are no burrs on the end of the steel bar. Take the aluminium out of the oven and place it on a sturdy wooden plank. Do not be hesitant about slipping the bar into the bore. Soon it will be heated up and you'll not be able to move it. Have a copper headed hammer nearby to give it a whack if needed.

Next I made the holder for the worm gear. This started life as some 2inch o/d aluminium barstock. I bored it out 5/8 inch, a couple of thousandths oversize. The 5/8 o/d shaft turns in this nicely. I didn't bore all the way through. I then milled a flat on the side with the micromill and then slit the whole thing in two.

Everything is held together with M5 bolts. The micromill will quite happy tap holes M5 after drilling out 4.2mm. I've made all untapped parts of the bolt holes 6.5mm drill. This means there is room for adjustment - especially important on the worm gear shaft bushes..

A dividing plate was made out a thin slice of 2.5" o/d aluminium barstock. Once the unit was assembled, one turn of the worm gear moves the work table end by 6 degrees. Therefore the dividing plate was bolted to the worktable. For ten turns of the worm gear, the table moves one sixth of a circle (60 degrees). I used an centre drill to make 6 equally spaced holes in the dividing plate using this method. This plate was then attached to the worm gear. Now I can (fairly accurately) turn the worm gear through 1/6th of a turn, which will move the worktable by 1 degree - which was the intention!

I drilled a hole in the main bodywork behind the hole in the dividing plate. I can now use a transfer punch of the correct size as my indexing spike.

The images below show various shots of the equipment.