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Thanks Kim!.....I'll just try the 5.75 threads per inch. See how it goes...maybe I'll try a sample worm and see.Dave
Hi,I'm off for a few days from work...and I wanted something that would let me putter I the shop, but still make progress on project I have been drawing up.Like Steve and Arnold, I have a vise mounted dividing head that I am drawing up, based loosely on the GHT versatile dividing head. Changes will include a center height change to suite a South Bend 9, and the incorporation of a 1 1/2-8 thread, and direct mounting 3C collets, so I can use my lathe spindle tooling.Here's a couple of pictures of what I have laid out so farYou will notice that the worm gear appears to be a spur gear...and it is. A 60 tooth SB change gear specifically.To make a worm that will engage it, the lead must have a linear pitch, that matches the circular pitch of the gear. Additionally, the worm will need to be mounted on the helix angle to match so that it engages the gear. ( 3.14 degrees in this case.)Now to do this, I need to set the lathe to cut the appropriate linear pitch. If you read Ivan Law's book, we need to calculate the linear pitch = 3.14159 / Diametral Pitch3.14159/18 = .1745 inches. Or 5.729 threads per inch.! To do this we need to add a pair of gears to the train, with a ratio of PI or Pi/2.... ( 3.14159 or 1.5707)For convenience, I'll be using Pi/2. If I put that in the train, and set the box to turn 9 threads/inch, Bingo!.......9 x 3.14159 /2 = 5.729 threads per inch!......But it's not quite that simple.........I'll type more!Dave