Build rotary table

[JBV]

I'm thinking of building a rotary table, mostly for the fun but also the saving.

I have 2 taper roller bearings with 35 mm ID, so i'm thinking of making a hollow shaft and a 25mm hole trough the rotary table and chuck.

The problem is the worm gear. I have found a source for pretty cheap worm gears and worms, but the stock model (100 tooth, Module 1) has only a 8 mm axle hole. Would it be hard to bore up the hole to 35+ mm and drill a bolt circle around it to match a flange on the axle? Even if they are "cheap" i don't want to take a risk of making the gear eccentric from the axle. How much offset can i live with?

Think it would be a good idea to combine the T-slot table and 4 jaw individual chuck?

/Jay

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[RotarySMP]

Here are two good write ups how to make a worm and wheel.

http://bedair.org/Worm/Worm.html
http://easyweb.easynet.co.uk/~chrish/worms.htm

[JBV]

Yeah i have been thinking of that. Do you really need to "notch" the gear blank? I would need a rotary table to build a rotary table then hehe

[ESjaavik]

Even if they are "cheap" i don't want to take a risk of making the gear eccentric from the axle. How much offset can i live with?

Not much.

What I did was to mount the gearwheel on a well fitting axle stub. Then I turned it to a press fit on a stub on the underside of the table. Of course it was not possible to turn down more than a step about 1/2 the width of the gear. But once that was done I took it off of the axle stub, turned a new stub with the large diameter identical to the one under the table. Smacked the wheel onto this (tigth fit), and turned out from the other side until only a thin ring gear remained. This I press fitted on the stub under the table. The runout was exactly the same as the original hole. Since the ring had an inside diameter of appx 90mm, there is no way normal forces will move it. I think it will rip teeth out before that happens.

And guess what! Just a few weeks later I found a new Emco Maier rotary table w. division plates for a song, and that work just gathered dust. Not complaining though.

Einar

[Pat2000]

Yeah i have been thinking of that. Do you really need to "notch" the gear blank? I would need a rotary table to build a rotary table then hehe

I believe (not tried it yet!) that you can get away with out the notching, but if you want a lot of teeth on your wheel then I guess it becomes even more of a problem... you might end up making a few mind LOL.

I wonder.. if Acme type thread forms are used, whether the worm can be sprung-loaded against the worm... theres going to be a lower limit on the force transmitted mind, no good for milling. I'd wondered about using that idea in a stepper drive system...

a large tap can also be used as a kind of 'hob'

the other thing that you're probably aware of, is to use an eccentric mounting for the worm so that some backlash/wear can be adjusted out

[JBV]

The problem is that i don't have access to a lathe :P I belive you could make the gearing on a mill, but the gear blank is a bit harder. Especially the grove on the outer face.

Yeah i know about the eccentric thing Have also been thinking of making some kind of spring loading of the worm that push it into the gear. But that probably means more wear :P

[JBV]

This is what i have in mind so far... You can see what i was talking about with the gear, to bore up the axle hole to 35 mm and drill a bolt circle around it.

1 - Here the chuck/t-slot table is screwed onto the shaft to a stop (to get the gear to the correct position).

2 - Here a nut tightens the axle assembly over the taper roller bearings.

I haven't made the hole for the worm and its bearings yet, but thats pretty straightforward!

[Pat2000]

Don't have a lathe..ah.. well you have to do you're turning on your mill... don't tell me you havent a mill...

you need real slow speeds on your mill... nice piccys!

[JBV]

Actually i don't have a mill yet Its on the way.. I just do some planning of the things i want to make, to stand the waiting time hehe I have made plans for a screwless vise so far...

[HuFlungDung]

With regard to precutting the teeth (called gashing), I think it would be necessary if you wish to control the tooth number. Either that, or you must have a positive drive of the blank in the proper ratio to generate the proper number of teeth.

The explanation is that the OD of a gear has enough extra circumferential length to add in two more teeth. This is the rule for calculating the OD of a gear blank: add two to the required tooth number, divide by the diametral pitch and you get the blank OD. Actually a standard wormwheel is shaped with a bit of a hollow around the OD to improve the wrap of the wormwheel tooth around the worm, so the OD of such a worm wheel is calculated as the (required tooth number +3)/diametral pitch.

By the time that the cutter has descended to full depth, then it should cut exactly the required number of teeth on the pitch diameter circle. So where and how do those 2 extra teeth have to disappear to?

So if you start out with your hob gashing the smooth blank, as it progresses to full tooth depth, the hob must tolerate this mismatch between the pitch diameter and the number of teeth it started with.

If you are clever, you might be able to figure a way to position your hob at full depth to begin with, and feed it tangentially into the blank, but most likely you will get some slipping of the blank going on, and it may spoil it unless you have a positive drive of the blank in time with the hob rotation.

[JBV]

Seem to be to much trouble to make my own gears when i can get a worm and gear for about $40

[HuFlungDung]

You've got it, JBV. My job is done

[JBV]

Hehe Back to the origional question then: Can i modify it from A to B without making it eccentric of the axle?

[HuFlungDung]

I dunno, can you? I could, but that's me.

Are you sure the new commercially made gear is a solid design, or is it webbed or spoked?

[JBV]

I guess i just have to try when i get the mill converted

This is the gear i have in mind http://www.kuggteknik.se/lager2.htm
Perhaps i should ask for a quote for a custom gear... They seem to have good prices!

[Pat2000]

Interesting Hu, have a look at
http://www.atmsite.org/contrib/JSAPP.../wormgear.html
if you're further intrigued, its just a site I found with a 'google search, I haven't read the last bit, it seems the guys who are into this are the 'telescope' guys wanting to position their 'scopes.

I'm thinking (out loud somewhat!) that these folks 'get away' with this technique because they have fairly large dia gears and cut fairly fine teeth. so long as in the first rev of the wheel the cutter begins to bite/match into the first shallow teeth cut, then will not further cutting take place on both sides of the tooth form as the wheel is fed towards the cutter/tap/canIcallITaHob!! ?

would be interesting to see if a smaller wheel having ACME thread form and reasonably coarse pitch could be cut like this mind, my guess is your thinking would prevail in that case!

when I retire in 20 years I give it a whirl LOL

With regard to precutting the teeth (called gashing), I think it would be necessary if you wish to control the tooth number. Either that, or you must have a positive drive of the blank in the proper ratio to generate the proper number of teeth.

The explanation is that the OD of a gear has enough extra circumferential length to add in two more teeth. This is the rule for calculating the OD of a gear blank: add two to the required tooth number, divide by the diametral pitch and you get the blank OD. Actually a standard wormwheel is shaped with a bit of a hollow around the OD to improve the wrap of the wormwheel tooth around the worm, so the OD of such a worm wheel is calculated as the (required tooth number +3)/diametral pitch.

By the time that the cutter has descended to full depth, then it should cut exactly the required number of teeth on the pitch diameter circle. So where and how do those 2 extra teeth have to disappear to?

So if you start out with your hob gashing the smooth blank, as it progresses to full tooth depth, the hob must tolerate this mismatch between the pitch diameter and the number of teeth it started with.

If you are clever, you might be able to figure a way to position your hob at full depth to begin with, and feed it tangentially into the blank, but most likely you will get some slipping of the blank going on, and it may spoil it unless you have a positive drive of the blank in time with the hob rotation.

[HuFlungDung]

I was one of those telescope making guys, years ago. I tried the hobbing with a tap, but wasn't overly pleased with what I ended up with. You are correct, a large tooth number with a fine pitch tooth is going to average out a lot better than a small gear with only a few teeth, for the reasons I described.

Amateur astronomers taking astrophotos fully expect to sit at the guidescope and make fine adjustments to keep a star centered on a crosshair. So if their large gear is plus or minus a tooth or two, it really doesn't matter to them. Inherent inaccuracy of the gear is discounted by too many other factors, like the aberration of starlight and atmospheric refraction, so even a perfect gear still needs to be actively tweaked to keep the scope on track.

For a rotary table though, a person is going to want a fairly strong tooth, and this precludes cutting the thing with a thread tap. And most certainly, he will want a common 40, 60 or 90 to one tooth ratio, not a tooth more or less. And he would like the gap to be the same between all the teeth without a catchup glitch between #40 and #1

I built the first rotary table that I ever owned, using a standard Browning worm and gear. I had enough fun building all the rest of the table, I was quite happy with the results, using the commercially made gear.

[JBV]

A dude mailed me and wanted the plans for my rotary table! I guess i could make 2D plans and upload them, but they would not have been tested and you would have to be modify some dimensions to fit your gears. Anyway i finished my solid model assembly. The T-slot table is 150mm (~6") diameter with a 25mm (~1") trough hole.

I have not made any coupler in the assembly, but i was thinking of a oldham coupler so you easily could mount the motor without having the motor mount in the way for setscrews or so.

I didn't make any way to mount the rotary table to the mill table, as it would be best to solve it for each different machine.

Comments? Improvements? Would someone want the plans if i make them?

/Jay

[HuFlungDung]

Nicely drawn up.

Have you considered the need for a spindle brake of some sort? Ideally, this would add some degree of stablizing force to the table platter, maybe pushing up from underneath or something.

You might also consider some kind of large diameter bearing beneath the outer diameter of the table platter. It could be a plain bearing or an anti-friction type, even a track filled with loose steel balls or something. This is because the torque of milling some distance from the rotary table can apply a lot of torque to the platter, and you are currently counting on a lot of support from the threaded on connection, on a relatively slim spindle.

[Ken_Shea]

Jay,
Have you given any thought to backlash adjustments?

I would imagine many would like the drawings, both in 2D and solids while you are feeling generous
It looks super.

Ken