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
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.
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
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.
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.
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?
Using a 1/2 X 20 tap to make a 40 to 1 ratio worm gear will result in a very small final gear less than 0.750.
Lets try a 1/2 X 13 tap.
1" divided by 13 = 0.077 per tooth
Number of teeth + 2 =42
42 times .077 = 3.231
3.231 is the circumference of your gear blank.
Divide 3.231 by 3.14 to get the diameter of 1.029"
This is still really small to work with so lets try an 11 tooth per inch example.
1 divided 11 =0.090 per tooth
42 teeth times 0.090 = 3.818
3.818 divided by 3.14 =1.215 " in diameter. Still fairly small.
In my case I wanted 90 teeth on the worm wheel which ended up at 2.254 in diameter.
If you want to stay with the 40 to 1 then I would look for a courser thread on the worm shaft to start with.
In order to make exactly the number of teeth you want when cutting the blank you must gnash the blank at each place you want a tooth.
The cuts need not be deep (0.025 - 0.050) just a place for the tap to align with. Turn the tap by hand the first couple of rotations paying close attention that each cut slit is centered on the tap tooth. I made several worm wheels with different ODs trying to get the correct number of teeth without cutting the slits and never was able to one right.
Now for the next question how do you cut 40 gnashes around a blank accurately with out a rotary table or an indexer?
I don't know.
Thank ya'll very much for your help..have a 50t and 80t gears.gonna make 2 10t..put then together per hmans post for my 40:1...hope it goes as simple as it sounds..if I wanted to make a worm wheel with 100t cut with a 5/8 X11 tap how big should my blank be.ended up with 105 with blank at 2.890..Thanks
I made my dividing head from the Gingery book. I bought some acme ready thread and used it to cut the worm gear. I cut some gashes in the ready thread and mounted it in the lathe between centers. I mounted the gear blank on a post on the compound slide. The ready thread cut the worm teeth. It worked like a hobbing process without power to the gear blank.
You can make a worm wheel for any thread pitch by making sure you have the right diameter on the wheel you want to cut. It just has to work out to 40 teeth at the pitch line.
That said, if you use a standard fastener thread, it will have a 30degree pressure angle. That tends to force the worm and wheel apart. An acme thread has a 14 and a half degree pressure angle, and much more force is transmitted to turning the wheel rather than forcing it apart. you want polished threads too. A rough worm will eat your bronze gear.
With google sketchup it is very easy to divide a circle into any number of equal divisions. Attached is a drawing that took me less than 5 minutes to create where I divided the circle into 40 equal divisions.
Thanks for the information.took a 50t driven by 10t driving a 8t turning a 64 t to get my 40:1..finished indexer today.going to use it for awhile then take apart and paint..thanks for everybody's help..Phil..I'll post pictures of my Fred flint stone special after paint
Quick note.was buying a few items from a retired machinist I showed him picture of indexer.he said he had a small one with foot from 20-30 years ago.never used it..bought it real quick..Dayton electric .made in Taiwan.looks real nice. Thanks
I have just finished a dividing head shown in the June/July 2010 issue of the Machinist's Workshop it is easy to build and uses any gear for indexing. A worn indexing module is shown in the Oct/Nov 2010 issue it uses 3 gears and 1 index plate with 3 hole circles it will index most of the divisions that would be needed.
Sorry for all the adds on photobucket.... seems it's turned into photo bucket o crap adds.
Just scroll through the pics. It's the K model dividing head. It is rather old, but it's very tight and seems to work well.
As far as what I am looking for.
8 and 10 inch grinding wheels for surface grinder with 1.25 arbor hole
mag chuck for a 6X18 grinder
sine plate or compound sine plate
insert tooling for lathe or mill with common inserts
If you have other machining related stuff you would like to offer up feel free, there is always stuff that I am looking for that I don't realize I am looking for until I see it.