Another worm gear question

I was thinking of having a go at:

**LINK**

Which for me would be a big undertaking.

The drawing calls for a Mod1 worm and 72T gear and it looks fun to try and cut my own but I don't see ay simple way of getting my Crusader lathe to cut a mod1 worm without hunting up two new gears. It can cut a 3mm pitch.

Is it reasonable/will it work to just reduce the circumference for the 72T gear by 3/Pi and otherwise hob by the drawing? Also in the interests of economy (and likely mistakes) making that 72T out of cast rather then bronze?

Assuming that you can readily adjust any other dimensions to fit: I see no problem. ...

The only valid reason for using an awkward pitch for the Worm is if it fits a convenient diametral pitch of gear. ... If you'e making both, start with whatever you can make best. [it worked for Jesse Ramsden]

MichaelG.

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Edit: O.K. I will save you the bother of searching ... here is some bedtime reading.

Edited By Michael Gilligan on 05/11/2015 22:05:05

Thanks for that and the fascinating link!

As Michael says, it should be no problem if you adjust the other dimensions...and since this design seems to have an eccentric adjustment on the worm shaft you might not even have to do that! But personally if I wanted to make a rotry table I'd buy a casting kit which would probably come with a worm and wheel; or buy a worm and wheel from HPC. Unless the challenge is to make the work and wheel of course! Actually I have two rotary tables, one small and one large, and almost never use them!

If you change the circumference of the gear wheel or the pitch of the worm screw you will change the gear ratio and you will require a different number of revolutions of the crank handle to revolve the table by 360 degrees. This may not be an even ratio between input and output revolutions. This is OK if you only need a simple rotary table but will not work if you have to index to a specific angle. A vernier gauge on the input shaft or indexing plates as shown in the linked files may not work.

Someone please correct me if my logic is incorrect. Paul.

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Paul,

The trick is to change the circumference of the wheel AND the pitch of the worm ... maintaining the intended ratio. [or some convenient alternative ratio.]

MichaelG.

Edited By Michael Gilligan on 06/11/2015 14:16:45

Indeed it is for the challenge..or rather the pleasure and stress of figuring it all out and having it fit and work. I'm guessing a cheap far east table wouldn't cost much more than a kit and supplies and cutter wear...?

pgk

Making the worm wheel seems to be a pretty straight forwards task providing it's gashed first. If not it's hard to guarantee the correct number of teeth come out. Some people have used an acme tap rather than 2 worms and gashing and hardening one of them. Other people's experience rather than mine.

My miller came with a dividing head with a couple of worm wheels that were home made. There is no back lash at all in it. One of the interesting aspect is that who ever made them kept the rim relatively narrow. The teeth are about 5mm wide tapering onto a rim round the wheel which is 7mm wide.The wheel is a 60T and the od 62.3mm. Not really big enough for a rotary table. I'd guess the cp is about 3mm probably 1/8". The worm is 15.9mm dia 5/8" dia. This isn't how most people make them. The rim of the wheel is usually a lot wider in relation to the worm dia. As the same thing has been done on the quill feed on the miller I wonder if this is the best way to go and easier to cut well with a tap. I think this head design was down to Dore or Westbury.

He also made a rotary table. On that the wheel rim is a lot thicker but also a lot bigger, the dia is something like 80mm. Pitch around 3/16". It's a 60T wheel. The arrangement on this one is interesting.

Zero shift table locking by using a cam to press the worm against the wheel. It's not something I have used much but it seems to work. The worm has to be supported a both.ends to do that. The worm is held in contact with the wheel by the spring so any slight run out is accounted for = zero backlash but I haven't checked that precisely. It aught to be because each time it's locked the worm is forced firmly into the wheel. I think this design is down to Dore Westbury as well. Could be either of them.

Come with a warning though. When I bought it the cam action wouldn't fully clear the worm from the wheel, It just touched. Not sure why. The casting might have distorted over time. So I removed the brass strap and worm plus carrier and milled a little bit more relief behind the cam bracket. 1/2 hr late the casting crack there. Most likely down to removing the strap. Just show's how much stress had been left in the casting by the supplier. This doesn't interfere with it in use. Many people reckoned that it was better to fabricate rather than buy castings especially for tooling as stress problems weren't that unusual.

It was made by Hugh Simmonds in 1988. There is a little brass plate. The same name and date is on the miller Nothing on the dividing head so I would guess the table was the first thing he made on the miller. All of the work is to a pretty high standard. That's what persuaded me to buy the miller. The "new" owner of that couldn't get on with it. Settings, crap vice and blunt cutters. Plus no clue at all I suspect.

John

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When I made my RT/Dividing head I made a worm and hob a from a single silver steel bar. I used a pitch of 0.100" which gives a pitch circle diameter of 60x0.1/PI = 1.91" Allowing for a tooth addendum to suit that pitch, that is just right for working with a skimmed 2" blank.

Neil

The sums are no different to cutting an ordinary gear but there is a catch I'll come to.

You can convert dp or mod to a circular pitch or the other way round. There are calculators about on the web so pick a worm pitch which I would suggest wants to be ideally bigger than 1/10" or 3mm especially for a rotary table convert to dp or mod and use the calculators to work out the OD etc. The gear pressure angle needs to match the "acme" angle of the worm as it's effectively behaving like a rack does and clearance as far as the gear is concerned might best be set to zero.

There is a chart in my album that converts between cp, dp and mod.

One catch that is sometimes mentioned is that the tooth count only matches the OD of the worm wheel when the "tap" that is used to make it is at full depth. So some suggest feeding it in as quickly as you can at each rev of the wheel.

Another is that the worm and wheel need to mate on the flanks so effectively the worm needs to be a some what smaller dia than the tap and be cut a little deeper to make sure that there is clearance and that they do just mesh on the flanks and that the tips and roots clear each other. It's probably easier to do it this way rather than mess with the cutting of the gear.

I'm not sure if making the rim of the worm wheel narrow helps cutting them or not. There's certainly less cutting required for for full depth teeth and having a bit more width with slowly vanishing depth wont increase the strength by much.

Cost. Fabricated way less than a kit or bought far eastern items. It might pay to remember that with a bit of care people have made things like this to the same sort of standards as items costing way way more than what model engineers generally buy.

There is one catch with putting the worm in a carrier like that rotary table. The dividing head uses one as well. The way it's done is to bore and ream always using the same reamer. Then fit a shaft by hand to carry it and pass through the bearings at each end of the carrier. It needs to be a firmish hand push fit. More or less zero clearance. On the kits the carrier would be cast iron which wont bind on a mild steel shaft. It might pay to make the carrier out of bronze instead or at least bush it. The shaft needs a centre hole in the end so that end play can be adjusted via a grub screw with a point on the end and a lock nut.

Too many ends.

Out of interest this is the dividing head with the 40T wheel on. Usual sector arms and plunger on the other side.

John

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> One catch that is sometimes mentioned is that the tooth count only matches the OD of the worm wheel when the "tap" that is used to make it is at full depth. So some suggest feeding it in as quickly as you can at each rev of the wheel.

The best solution is to gash the wheel almost to full depth first so the hob really only finishes the profile.

Neil