Trying To Build Dividing Head

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Lordbeezer

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hello..trying to make simple dividing head.want to make it 40:1 ratio.40 tooth to a 10 tooth gear would be 4:1 if I'm correct.if not please advise..to get 40:1 I would use 400:10...looks right but can't be..that's a lot of teeth..made a worm wheel(gear) worm gear was 1/2 x 20 bolt thread.took 193 cranks of handle in front of hole plates.want to gear it like normal so I'll know how,after looking it up to move pin in plates.been goggling for hours trying not to ask dumb a@& questions. Thanks for your time
 
You should only turn the input 40 turns to achieve 1 turn of the output. I will refer you to a website belonging to one of our members, Mr. Harold Hall. He has many projects on his website.

http://www.homews.co.uk/page425.html
 
Lordbeezer -

Your post was a bit unclear, but I think you were asking about how best to get the 40:1 ratio, and if you could use a ½-13 threaded rod to get there. I'll answer with that assumption in mind. If I'm off, please let me know.

Worm drives are generally the way to go to get high ratios with just one stage of reduction.

To get 40:1, you'll want a 40 tooth worm gear to work with the worm. Think of it this way ... One turn of the worm will advance the thread one "tooth" - one thread pitch - same distance as a nut would move on such a screw. Meshing a 40 tooth gear against it means you need 40 turns of the worm to get one turn of the gear.

If you want to use ½-20 threaded rod for the worm, the "pitch diameter" of a 40 tooth mating gear would be
40 teeth x 1/20" per tooth ÷ Pi = 2"/Pi ≈ 0.637" That's not a very large or robust gear, but it can be done. Of course, you'll have to calculate the OD of the gear blank from standard tables, etc etc. Then you'd have to find or make a suitable cutter for the teeth.

And, of course, your accuracy would be limited by the accuracy of the original threads on the rod.

Hope this helps.
 
Sorry I wasn't very clear..my first attempt was with 1/2 x 20 driving a gear I cut with a 1/2 x 20 tap..put together it took 193 turns of handle for one 360 circle..been googling to see what gears to use to make it 40:1 so I can use dividing plates the Normal way.. Have several gears...one set would make it 4:1...going back to mr,Halls website to look some more.can't find anywhere it says use this combo of gears to make 40:1...maybe I'm missing a simple answer but I sure don't see it..I guess I'm asking what tooth gear driven by what tooth gear to get 40:1...thanks very much for you time..Phil
 
Does it matter what final drive gear ratio it has as long as I keep up with holes in dividing plates.sorry for questions that are probably simple to y'all but wearing me out.Thanks
 
OK, I think I understand your questions a bit better now.

As for an overall gear ratio of 40:1 with standard spur gears ... yes, you do have the right idea. A 400 tooth and a 10 tooth do indeed produce a 40:1 ratio. Other combinations will do so. Just look for the "lowest common denominator." It's also possible to go in multiple stages - a 10 tooth driving a 50 tooth (5:1), with the 50 tooth on the same shaft as another 10 tooth, which in turn drives an 80 tooth (8:1). 5*8=40.

The "gotcha" in such an arrangement is backlash - each reduction stage makes its own contribution. Adding more gears can also affect overall accuracy, as each gear contributes.

As for your original test, which produced a 193:1 ratio, you might consider reducing the diameter of the worm gear by a bit more than half, and work toward a 90:1 ratio, This is also a "standard ratio" for dividing heads (4º rotation per turn of control handle). You might have to try several times to get it exact, but it should be do-able. Then find a set of dividing plates meant for a 90:1. I myself bought a Grizzly 90:1 rotab (H2677) a year or two before I found a set of dividing plates to fit it. The part number for Grizzly's plates is G9295, $80 plus whatever shipping they charge.

The dividing plate set includes setup instructions for dividing a circle into a goodly number of even patterns - all spacings from 1/2 to 1/66 of a circle, most from 1/68 thru 1/120, about half of all spacings from 1/120 to 1/180, and some others. I'm sure that commercially available dividing plate sets for 40:1 rotabs will have similar instructions.

In answer to your last question - YES, it can matter a lot. Calculating the setups from an arbitrary ratio (such as your 193:1) and an arbitrary set of dividing plates is possible, but messy. And unless the ratio is easily divided into 360º, very few exact solutions can be found. PM me if you want details of such calculations.
 
@Lordbeezer

Here's the direct link to the worm gear discussion mentioned above (and linked to from the rotary table thread above):
http://www.hobby-machinist.com/threads/make-a-worm-gear.36588/

It is a very detailed discussion of the problems with cutting a worm wheel with a tap. See especially post #29 in that discussion:
http://www.hobby-machinist.com/threads/make-a-worm-gear.36588/#post-313536
getting a repeatable tooth count is hit and miss.

However, you may be able to find a small worm-gear mechanism for cheap. Check around scarp yards for junk winches, garden tillers, lawn mowers, etc. I've even found one in an old washing machine. Try to find one with a 40:1 or 90:1 standard ration, then you can get pre-made dividing plates.

Good Luck, and please let us know how you progress.

-brino
 
Ray..yes sir I have..very nice work..that's what got me to try and make a worm wheel.used 1/2 x 20 thinking finer be better..not the case..I already have 40:1 plates.trying to match my ratio so I can use them..going through my gears to see what I can come up with..your work makes mine look like cave Man stuff..thanks Brino..thanks for info..rdeans article got me to try and make a worm wheel..I have a box of lathe gears.two winches to go through and two leadscrews so maybe I can end up with 40:1...using a old headstock off atlas lathe for my base.thanks for your time..Phil
 
Another option is to purchase the gears. . . . I bought mine off Amazon a couple years back when I built my dividing head. The ones that I used are a) worm, and b) worm-wheel. Yeah, I know--kinda pricey! It's just money, right?

Jim
 
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