[Splint]

Hi,

I need to make a 130:1 reduction worm drive and I intend cutting the gear on a lathe with a tap. This is the first time I've attempted anything like this so I was hoping someone could look over my plan and tell me if they see any issues.

The worm will be a section of 16 mm (2 mm pitch) all thread and the gear will be cut from Delrin. Being 130:1 with 2 mm pitch the circumference of the gear will need to be 260 mm (82.76 mm diameter).

The gear will be mounted neatly on a vertical shaft on the cross slide in a lathe such that it can freely rotate but have no up/down movement. With the tap in the chuck, feed the gear into the tap until it makes light contact then rotate the chuck by hand while gently feeding the gear further in so that it begins rotating from the cutting forces.

Start the lathe and continue feeding the gear in until tap has fully engages (cut depth equal to the height if the thread) around the whole gear.

Sound OK?

[dharmic]

How thick will the gear be? It'll all come down to how you mount the blank - whether you can set it up properly horizontal and fixed in everything but the rotation. Some kind of bushed mandrel might be required.

[Splint]

No problem with thickness, the round shaft I'll be machining the gear from is 100 mm by 100 mm. When the gear is finished it will be bolted to an aluminium hub which will clamp to a round shaft so their shouldn't be any issues with flexing in manufacture or operation.

The ones I've seen machined on youtube seem to work out ok just mounted on a very neatly fitting vertical shaft.

[dharmic]

Then it all sounds ok!

[handlewanker]

Hi.....this is an old DIY trick and has been done many times in the past with mixed results and not many totally successful despite wow reviews by the doers that have used taps to generate a worm wheel.

The problem is with all hobbing procedures, and this is a type of hobbing procedure, but using a tap with a 60 dreg thread form instead of a hob with a 29 deg included angle, which is not the best way to form the teeth.

That is, you first need to gash the worm wheel blank with the number of slots at the same helix angle of the worm that is going to mesh with it that represent the teeth, otherwise the 'hob" will end up cutting it's own pitch arbitrarily and leave you with half a tooth at the end and start to recut the already cut threads......that is an inescapable fact.

The slots so cut will be at an angle but straight across the blank, whereas the hob cut final form will be a curved slot with the same helix as the hob or tap.

The problem with a tap is the grooves for chip clearance are too wide and so you have a tendency for the cutting teeth to be out of the blank when the tap rotates and so hop over the grooves......that is why you gash the blank to get the hob to fully engage and generate the thread form in the worm wheel.

To do the gashing requires a simple index mechanism or dividing head......(or you "could" mark them out and manually cut them one at a time if you are masochistically inclined), to get the exact number of teeth, using a fly cutter to make the slots slightly narrower than the width of the slots in the worm wheel blank to allow the hob to cut the actual thread form.

Having ben down this path, this is purely my experience. as others have found out too.
Ian.

[dharmic]

Interesting, Iain, and good info. If you went in at full depth the issue of tooth spacing changing on the blank would be fixed, and a spiral flute tap may give you enough overlap to prevent that hopping, wouldn't it? I guess it comes down to whether the rig used to hold the thing (and the tap, and the spindle etc), had the guts to make a full depth, single pass cut...

[handlewanker]

Hi....nope, the spacing around the blank is being generated by the spiral of the hob or tap.....the fact that is pulls the blank around while it cuts the teeth is purely a matter of physics........the fact also that the spacing is not pure and exactly generated is one reason the blank worm wheel is gashed with the exact number of teeth spaces and pitch....this is what gives the hob and /or the tap a guidance as it progresses around the blank.

The hob has a thread angle of 29 deg included which is the thread form of a rack and that is what meshes with an involute curve on the worm wheel......it's a rolling contact that reduces wear as it transmits the load same two spur gear in mesh......the pointy 60 deg angle of the tap is not at all condusive to good wear characteristics, although for basic needs it will work but not for speed and load transmission.

I pondered the problem and think that an Acme thread tap would cut a worm wheel like a hob with the correct angle as it has the 29 deg thread form .....provided the blank was gashed with the required number of teeth in the beginning, but the teeth of the tap are not relieved like a real hob cutter and would cause a lot of friction as they were forced to cut on one side only.

Form relieved cutters are a completely different technology in their making.
Ian.

[dharmic]

That's what I love about this place, learn something every day. Thanks, Iain. Does it make a difference that he's using allthread instead of a rack?

[handlewanker]

Hi, no difference as the Allthread is like a rack but in a round form.

The worm wheel must be cut with the same diam tap as the Allthread diam............just needs to make sure it's the steel type Allthread and not the zinc coated one that is quite rough.

If the drive is slow and not too heavily loaded, also not expected to be precision in it's movement and well greased it should work quite well........the prior gashing will increase the accuracy of the cut thread form.

Once again, it's important that the tap teeth or thread....whatever...maintains continuous contact in the groove of the worm wheel while it's cutting.......if it's cutting brass it should be cut dry....if it's cutting bronze (not recommended as it tends to work harden with none form relieved cutters) then some coolant will smooth the way.

The more teeth in the tap or hob and a narrower tooth gap width the better it will cut, but there is a tendency for it to munch the metal out of the worm wheel grooves....like a grinding action.

I wouldn't recommend making a hob as it still needs to be from relieved and that takes some special equipment, but it can be done at home if you really get fussy........a bought in HSS hob for more than a few worm wheels will pay dividends, but you also need to make the worm and that cuts out using Allthread.......I haven't seen hobs in very small sizes, but they probably are around.

There are in depth articles in the English Model Engineer magazines, Volume 147 No. 3662, volume 142 No. 3541 and volume 138 No. 3541 for those with access to that mag from the 70's, that shows how to make form relieved cutters and gear hobbing.
Ian.