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| Gear parameters |
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| Sea.dog:
Something like this should help, John. https://www.engineersedge.com/calculators/helical-gear-calculator.htm |
| vtsteam:
No John, it's easy. The gear blank and hob rotate together, so the hob naturally cuts that correct helix angle automatically. Just line them up perpendicular, the same as they are on the lathe. If you think about it you're just making a toothed worm, exactly the same as the existing worm in shape and size and position, and it is gradually cutting its way into the blank by rotating together, the same as the original gear train meshes. This results in exactly the proper form teeth for the gear to fit the hob (and worm). Another analogy is that this is very similar to how a tap works. Yes the threads are at an angle, but you drive the tap vertical to the work, and by rotating it cuts the angle automatically. In fact a rotating tap can be used, in a suitable fixture with a gear blank, to create a worm gear that fits a threaded rod of the tap's pitch and diameter. Then, a piece of threaded rod, end turned down to a journal, can be used as a worm for that gear. If an Acme tap and threaded stock is used, the gears are very similar in tooth form to standard worms and wheels. |
| vtsteam:
Here's a pretty good video. He gashes the wheel with a spin indexer on the mill with a fly cutter, and for that operation he does pack it up to the helix angle. But on the lathe, the hob cuts straight perpendicular to the wheel. |
| John Rudd:
Well that looked easy..... :bow: :bow: The only bit of info I am missing is the packing angle.... I guess the hob was made from Silver Steel and hardened? |
| vtsteam:
It would be the helix angle, which is the angle that the worm has when looking dead on if you hold it vertically in front of your face. You could probably approximate this, if you don't want to calculate or look it up. The reason it doesn't have to be spot on is that you don't gash the gear blank very deep. Just enough to help the hob find its way, and then the hob modifies the tooth space anyway. So "close enough" is probably good enough. But if you want to calculate it (i'm still Imperial here so forgive me these units) and you have your TPI from counting the worm teeth per measured length (let's say it's 10 TPI), and let's say your worm is 3/4" in diameter. Let's say that the length of the base of the collet spinner you want to shim is 8". Then shim height = Base length divided by (TPI X Diameter X Pi) or: Shim height= 8" / (10 X .75 X 3.14) Shim height = 0.34" This isn't using the technically correct definition of the worm helix angle, I believe, but I think it's close enough for gashing. Uh, also people, check me on this -- I just made it up. If you know how to properly calculate (or look up in a table) the helix angle of the above worm, do that, and then multiply the sine of that angle times 8" and tell me how close I got for the shim height. Thanks.....! :smart: |
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