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Horst
01-24-2018, 03:51 PM
I saw a video where a fellow cut a worm gear wheel by advancing a free spinning blank against a tap being rotated in a lathe. I inferred that the worm then consisted of a lenght of threaded rod equal to the tap. Does anyone know if this would actually work?

https://www.youtube.com/watch?v=J0o3W4_LRBw

flylo
01-24-2018, 04:37 PM
That's pretty awesome but why not use a tap? It's harder & sharper.

MikeL46
01-24-2018, 06:07 PM
That's pretty awesome but why not use a tap? It's harder & sharper.

????? He used a tap to make the gear. Or what were you thinking he should use the tap for?

It looks like it should work for light duty.

Mike

The Artful Bodger
01-24-2018, 06:16 PM
That's pretty awesome but why not use a tap? It's harder & sharper.

A tap would really shorten the life of the worm wheel.

Mcgyver
01-24-2018, 06:41 PM
Does anyone know if this would actually work?


you hear about it often enough. What I don't understand is how the change in pitch is accounted for or works it self out......i.e. when you start the taps pitch is at the addendum and when done at the dedendum.

The Artful Bodger
01-24-2018, 06:57 PM
If the teeth look a bit mashed just keep going and when they straighten out you are at the finished diameter.

velocette
01-24-2018, 08:00 PM
I saw a video where a fellow cut a worm gear wheel by advancing a free spinning blank against a tap being rotated in a lathe. I inferred that the worm then consisted of a lenght of threaded rod equal to the tap. Does anyone know if this would actually work?

https://www.youtube.com/watch?v=J0o3W4_LRBw

Works best with a spiral flute tap and then machine cut thread on the worm wheel as threaded rod lacks precise fit.

http://www.model-engineer.co.uk/forums/latest_posts.asp

search for worm wheel plenty of reading here

TGTool
01-24-2018, 09:32 PM
My recollection of reports is that using a tap on a plain free-wheeling blank isn't reliable for a particular number of teeth on the wheel. It will find a place somewhere and work, and many times that may be sufficient. If you just needed a quick and dirty gear reduction it's great. If you needed a specific pitch that you've calculated for, it helps to gash the wheel before addressing it with the tap so it registers the desired number of teeth.

BCRider
01-25-2018, 01:18 PM
I saw a video where a fellow cut a worm gear wheel by advancing a free spinning blank against a tap being rotated in a lathe. I inferred that the worm then consisted of a lenght of threaded rod equal to the tap. Does anyone know if this would actually work?

https://www.youtube.com/watch?v=J0o3W4_LRBw

It's my understanding that it works well enough for light use. But if you wanted something more durable and which needs to be able to generate more torque in the output that you want an Acme form of thread. Otherwise the angled teeth cause more separation force and wear more quickly. And in fact it may be that square cut teeth on the worm and hob are the better option.

I also think I've seen writeups where the blank was given a rounded groove in the periphery so the tap would engage more fully right from the get go. Saves a lot of the initial start up crunching and gets the blank turning more positively. Might be an aid to achieving the proper number of teeth too.

J Tiers
01-25-2018, 02:18 PM
Nothing says that the tap must be a v-form tap. You could use an Acme tap.

Then, if you do not do as is shown, but instead you set the tap to the correct depth relative to the desired worm wheel, and then advance it from the small end toward the deepest cut, it should do a good job of cutting, just as if it were actually cutting a thread in a nut.

It will first cut shallow, and as you advance it the cut will fill out until you get the correct depth and form.

You should be able to get any number of teeth that you want, set only by your accuracy in making the depth. You could set a little shallow, and adjust until it cuts perfectly, perhaps.

wierdscience
01-25-2018, 02:18 PM
you hear about it often enough. What I don't understand is how the change in pitch is accounted for or works it self out......i.e. when you start the taps pitch is at the addendum and when done at the dedendum.

I've done it a few times and best I can see because the wheel is free to move it can centralize in the cut.I think this allows it to behave like a gear hobb and correct any small errors as the cut progresses.

I do know it works better with a spiral flute tap and on larger tooth counts.

I have been itching to try it with a spiral flute Acme tap and see what happens for obvious reasons :)

The Artful Bodger
01-25-2018, 03:18 PM
I found that after the first revolution the teeth are somewhat deformed but as cutting progresses the diameter of the wheel decreases and the teeth take on a more perfect shape.

J Tiers
01-25-2018, 03:39 PM
If you start with the "front end" of the tap, where the partly ground away teeth are, and advance the tap slowly by one tooth for each revolution of the blank, it should cut progressively deeper teeth in the blank, without deformation. It sould behave much as if it were cutting a thread inside a hole.

I think that was actually shown here years ago, but it might have been somewhere else.

JCHannum
01-25-2018, 06:21 PM
The past couple of issues of HSM have revisited the Phil Duclos dividing head. I built the original version over twenty years ago. Too frugal to purchase the worm & gear, I free hobbed my own, turning an approximation of an Acme thread for the worm long enough to cut off a section to case harden and use to free hob the gear with.

I don't recall all the details of calculating the blank dimensions at this point, but ended up with a 40 tooth gear on the first pass. I might have gashed the blank, but with no means of indexing, I can't recall how I managed to accomplish that.

J Tiers
01-25-2018, 07:40 PM
Mebbe with a 40 or 80 tooth gear?

Gears make really good index references.

JCHannum
01-25-2018, 08:38 PM
Mebbe with a 40 or 80 tooth gear?

Doubtful, way back then my shop was very basic. Using a gear to index would have involved fabbing up some means of holding the gear and blank and indexing them to gash. That would have become a shop fixture, which I did not have. Best recollection is that I did not gash, but simply fed the cutter in.

Brett Hurt
01-25-2018, 10:09 PM
that was great I now know to do it.

Ian B
01-26-2018, 09:50 AM
The idea of using a tap as a hob to make a worm wheel is a foundation stone of the 3D printing fraternity - they all seem to make their own worm wheels for the extruder heads.

As to whether the tap will cut an exact number of teeth; they sort of do, in the same way that knurling cuts a nice pattern. With tap hobbing, if the result doesn't look good, just feed a bit deeper until it does.

The last time I used this method was when I was 16. HSM'ers didn't have spiral flute taps back then. Or HSS taps...

Ian

Sun God
01-26-2018, 10:54 AM
Funnily I have exactly this task sitting on my bench for the weekend - was given a 90, backdriveable worm and wheel gearbox to repair, steel worm with a custom form, plastic wheel with about a quarter of the teeth stripped off.

Parts not available, the ratio needs to be matched to a second transmission so I really don't want to have to 'approximate' the pitch and make a new worm and hob lest I end up having to make a set for both sides. I think it's a 2-start worm as well but I haven't looked closer, I'm not even sure my lathe could generate a high enough helix angle.

Remembering free hobbing, and those that have made plastic gears by feeding a free-spinning round plastic blank against a driven steel gear, I'm going to try free hobbing a new wheel using the existing worm as a 'forming tap'. I'll feed a plastic, probably LDPE wheel blank on the toolpost into the worm spinning on the lathe, and try and use high feed pressure alone to form a set of teeth.

It might be a total disaster, it might not work at all, but trying it is a hell of a lot less work than the alternative.

As an addendum: shame on any company that deliberately removes or defaces OEM part numbers or identification markings just so they can lock you into their replacement parts chain - because when they inevitably go out of business, their equipment which *should* last decades becomes all but unrepairable - no local agent to back up the product with parts, no manufacturer to contact for parts directly, no published parts list or maintenance manual, and no way to identify what I *frustratingly* know is an off the shelf OEM part I can't identify amongst the hundreds of other similar parts, all to serve petty greed.

The Artful Bodger
01-26-2018, 02:31 PM
Hi Sun God, I posted on this forum about forming plastic gears in a lathe.

The principle is that the gear runs against the blank until friction heats the plastic enough to begin to form then you increase the pressure and form just a little deeper. You do not need a really high pressure.

For a worm wheel there may not be enough friction especially with slippery plastics so for your experiment may I suggest warming the edge of the blank with a hot air gun.

JoeLee
01-27-2018, 09:20 AM
Well that was different...... threads aren't gear teeth. Looked like maybe a 1/2-13 tap ?? I suppose if you had a custom project and both gears meshed perfectly they might last for a short time.
So with this method you can only end up with a concave gear. What about the mating gear???
How many different diameter gears will work out with that tap?

JL.............

Sun God
01-27-2018, 10:17 AM
It's not for making gears, it's for making worm wheels.

Thanks for that Bodger. I could remember the pictures, but none of the details. Will give the heat gun a go.

J Tiers
01-27-2018, 01:10 PM
I have seen a considerable number of V-thread worms and wheels. Most of them have been for adjustments, as with transits and that sort of device, where there is no real force transmitted. The more usual acme-like worm is designed to transmit force, and will clearly work better for any device that transmits force, and is operating often or for longer times.