Everthing has been answered thanks John but I have just been in the workshop with a piece of Silver Steel.
Drilled and reamed an 8mm hole no problem but facing with either HSS or Carbide caused a serious 'work-hardening' issue. In the knowledge that even 1 thou is a heavy cut when I'm nearing the end of the screwcutting, what suggestions does anyone have regarding tool geometry?
The pitch is 2.356mm, Effective Dia 26.9mm, Helix 2.5°, Depth of thread
Thread milling using a tapered carbide cutter (though I've not seen any 29 degree ones)
- available in UK from the Drill Service, Horley. Possible on a manual mill with spiral atachment (and severe cranial abrasion viz-a-viz change gears), easier with basic CNC (see Tony Jeffree's excellent Taig pages).
I'd still be interested in some answers from those more wise on the hobbing question. As someone suggested, you can cut a bit deeper than standard. I guess this is the same as starting with an undersized gear blank, but stand to be corrected.
I'm not disputing, but, shall we say, would like a second opinion on your original statement that standard hobs cut 50% CP. And if you do want to alter that ratio, how would you do so? Would a narrower screwcutting tool when single-pointing the hob (wider spaces in gear) work? Might you need to very slightly alter the pitch of the hob (c.f. Cleeve's screwcutting book where he discusses leadscrew correction and how to get 999/1000 of a particular pitch)?
And, finally, particularly aimed at Sir John: if one used an electronic hobber, but _very_ slightly changed the ratio on the toothed belt drive between spindle and encoder (or by some electronic trickery the input to the diviide-by-n), what would happen? I suspect that if left long enough, you'd end up with a smooth wheel with D+f removed from the blank, but if you didn't leave it for ever, could you achieve a non-unitary tooth body:tooth space ratio on the cut gear?
That might be a posibility :- if they did a 14.5° with a 0.8mm tip dia if I had a spiral milling attachment for the Myford :)
Actually, 'hobbing' a thread (thread milling) would be the best solution but way beyond my limited machine tool facility.
I'm coming more and more to the conclusion that Mark's initial suggestion of adjusting the centre distance is the 'cleanest' solution. Primariy because this means that the gears are 'normal'. The amount of 'backlash' created by the 48/52 ratio on 0.75 MOD 14.5° PA works out at
0.09424mm which equates to a 0.3644mm increase in the centre distance.
The fact that I haven't been able to find 0.75 MOD hobs means that a re-design using the 0.7 MOD 20° PA stocked by Arc Euro is very much on the cards.
Under 'normal' conditions the ideal tooth/space ratio is 50/50 as this provides constant velocity and minimum backlash (with correct centre spacing) so 'off the shelf' hobs would be designed with that in mind.
That is _exactly_ what I had intended (and might still do if I can solve the work-hardening issues).
The tip width of single point ACME form tool for a 50/50 0.75 MOD would be 0.84mm but for a 48/50 ratio would be 0.78 - in both cases this would be to make a 'topping' hob (one which cuts to full depth)
That might have a similar effect but due to the fact that the effect would be cumulative you would have to take account of the number of passes needed to cut the full depth. I think that method would be the worst of all posible solutions.
That is why I think the modified pitch method would be a no-no.