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Latest try
I've been back at it again.
I'm still not cutting threads of any merit so am sticking with recycling the worm thread from other components. The latest (and so far great) is the thread from a Trade Tools G clamp which sells for around $12.
The thread is not hard enough to do the initial cutting (it rounds over badly) but does seem fine for the final stages of cutting the worm gear. I've been roughing out the worm gear with a conventional tap of the same pitch and a slightly larger diameter then finishing it off with a slotted piece of the clamp thread.
I'm housing the worm in a piece of 25mm OD alloy tube with a 19mm ID and an 8mm shaft turned on at each end. Bearings and pullies from HE to suit and it's all pretty easy although I've not proven yet just how concentric the result is.
I've still got some cleaning up to do but this is looking promising.
Steve's mention of cork washers to allow for high load slippage solved the problem I'd had in working out how to do that. I had some old cork gasket material lining a toolbox which should be just right. I may still swap the tensioning screws over to grub screws.
Bob
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