How does one scale a worm gear?

Hello all

I have got this idea stuck in my head about making a working 1/3rd size BCA jig borer. (Blame the SMEE stand, that rather superb 1/3rd size ML7 at Ali Paly resparked this idea!)

The only stumbling block in the design in my head stage, is the worm gears that are on the machine, The fine feed mech is a bit in the future at the moment so not too worried presently!

So the worm gear that is perplexing me is the rotary table feed, It is a 120:1 ratio, as far as I can measure a pitch of 3.63 which seems to corresponed up with a No. 22DP (is DP for worm gears? cant seem to find a definitive answer) So if I have understood previous posts about scaling gears DP is multiplied by the scale factor.

So in my case that would equal 66DP. Now I would like to screw cut this worm if possible, however having a gearbox Super 7 I will have to use the trick of switching the input gear to achive a near as damn it metric pitch. The comprehensive charts I have from this site list a DP of either 64(1.245pitch) or 68(1.173pitch). Dividing the full size pitch gives a scale pitch of 1.210.

So my main jist here I wish to ask is will this slight variation in pitch affect the 120:1 ratio?

I hope this all makes some sense! I look forward to the replys.

Any other information I can supply if required.

William

No! The ratio depends solely on the number of starts on the worm and the number of teeth on the worm gear, worm pitch accuracy does not affect the ratio as long as the gears can engage.

If the worm gear is slightly over or under pitch it will just ride a little high or low on the gear to compensate (for errors of a few percent). This should work best with involute gears as they have a profile tolerant of errors in the pitch centre distance.

Neil

Or save yourself the headache and the cost of a gear cutter and do a search on Aliexpress for "worm gear 120" .

You will find plenty of module 0 .4 worm and gear sets which are as close as dammit to 64DP and cost about 15 quid.

PS: But if you particularly want to cut your own, maybe look into the 0.4 Module option. Metric cutters are cheaper and easier to source than the old DP ones. And the worm pitch might be more compatible with your lathe gearbox.

You don't even have to hob a proper worm wheel for something like a rotary table. Cutting the gear as a straight spur gear with a conventional gear cutter, but with the blank angled at the same as the lead angle on the worm works just fine for a positioning only worm and wheel that does not need to transmit power. I did this on a fabricated version of the GH Thomas Versatile Dividing Head I made a while back and it works well. However, these days with the recent increase in the availability of a huge range of readymade worms and wheels at stupid cheap prices on Aliexpress I wouldn't bother again. It's a crazy world where its cheaper to buy the item than the material and tooling to make it yourself.

On the other hand, if you cut your own worm at the almost but not quite right pitch, then use the same screwcutting setting to cut your own silver steel hob, you could then hob the wheel to an exact match to the pitch of the worm.

Edited By Hopper on 25/06/2020 03:13:30

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What a wonderful project, William !!

I’m a little concerned about that 120:1 ratio though ...

I am almost certain that mine is 180:1

MichaelG.

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The question is obviously for William, not for us, to ponder ... but:

Is it worth bothering to make a working scale model of the BCA without incorporating a proper ‘throated‘ worm-wheel ?

MichaelG.

Yes indeed. One to ponder upon. Having just looked up the machine on Lathes.co.uk it certainly is a lovely bit of kit and worthy of a good model for sure. I hadn't come across them before, assuming the OP reference was to a run of the mill larger toolroom jigborer of the more common type. That is quite the rotary table on there so probably not something to skimp upon, just on a matter of principle.

The basic rule for worms and worm wheels is that anything goes. But it is common to specify the worm wheel in terms of a standard DP or Mod number. The downside is that the tpi/pitch of the worm is then an irrational number. On my lathe one can modify the change gears so that the gearbox will screwcut worms to approximately the correct tpi or pitch for standard DP and Mod values.

Andrew

If you really want to disappear down the wormhole (pun fully intended and all credit taken) there is a number of books that delve into worms and gear making. Ivan Law's "Gears and Gear Cutting" is a good start. Martin Cleeve's "Screwcutting in the Lathe" goes into specifics of cutting the worm thread and different thread profiles . Both are Workshop Practice Series so cheap as chips to buy. And both of George H Thomas's books have also detail in them on cutting worm threads etc and the change gears needed to cut some of the very odd pitches that result from matching DPs of gears. Not so cheap but both books are full of invaluable information. There's a lot to worms and wheels. Hence my preference these days for buying in the low cost ones that were never affordably available in the past. But it certainly is interesting to cut your own as an exercise.

Edited By Hopper on 25/06/2020 11:28:18

Agreed the rotary table is the definitive thing about a BCA so it really needs to be right.

Sensible answer to the worm and wheel conundrum is to define the scale by whatever worm and wheel you can get close to your desired scale size. Its not as if anyone will notice, or even care, if it comes out as 1/3.2 scale rather than 1/3 rd. You will be dealing with lots of tricky dimensions regardless.

Or cheat by choosing a Mk 2, Mk 3, or Boley version as your prototype which have a spiffy cover plate to hide any unauthentic worm and wheel details. Makes for a boring appearance tho'.

Getting the sliding, rough setting, rulers to look right might be an issue as they will be very thin. The ones on my Ultra badged version were barely 1/8" thick. Expect some over-scale juggling on thickness to get the appearance right.

Although I was distinctly underwhelmed by mine as a practical workshop tool it will make a nice model.

Clive

Unless my sums are wrong, a 66 DP 120T gear will be 1.8485 inches in diameter.

+1 for "Gears and Gear Cutting" as an aid.

Being me, and bone idle, once the pitch of the worm had been calculated, it would be cut as a straight sided screwthread.

But making the cutter to cut a 66DP gear would frighten me silly!

But don't let my cowardice dissuade you.

Howard

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It would seem more appropriate to make an extra length of worm, and convert some of it to a hob.

[and also to check that ratio before cutting any metal]

MichaelG.

Just found some photos of my worm housing, which might illustrate how nicely built the BCA was:

The arrangement of the worm in its housing is a delight:
Plain bearings are bored directly in the cast iron, and thrust bearings are fitted both sides at the handle end ... leaving the distal end to float. The preload/backlash is adjusted by the conical nut and hexagon locknut that retain the index and ball-handle. [Both sliding on a keyway].

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MichaelG.

Hello all

Thanks for all the replys. Neil that is straight to the point and just what I needed to hear, so thanks, I really couldn't see the wood for the trees when looking at the various websites!

Hopper; yes I had considered purchasing some proprietary gears but would like to follow the original table as closely as possible, its machined out of 1 casting (an extremely heavy one at that!!) also wouldn't mind having a go at hobbing! hence the reason for screwcutting.

Micheal; yes you are quite correct its 180 turns so therefore 180 teeth on the actual table. Thats working on to many machines thinking I know the machine at home well! Yes a definite undertaking but something I have always had in the back of my mind ever since acquiring the machine.

Andrew; Thats again just the answer I needed!

Clive; yes starting with a fine machine and then making it smaller isnt going to be a five minute job, not to worried about the scales/rulers, more racking my brain on how to hold cutters in an 1/8" spindle through hole. Its going to be a mk3, I quite like the idea of fighting a piece of sheet metal in many directions to form that guard!

Howard; Its a 180 tooth gear I decided wrongly that it was a 120. the calculation that allowed you to work the diameter out sounds handy to know though.

A quick question about hobbing is it always necessary to gash a gear prior to hobbing?

Thanks for all your help much appreciated

William

Assuming free hobbing (hob and blank are not geared together externally) then it depends upon if you need a set number of teeth. One can free hob using a tap and a blank with no gashing:

But it's a crap shoot as to how many teeth you end up with; in the case above it didn't matter. If a set number of teeth is important then you need to gash the blank at the helix angle of the worm -

- to give the hob something to engage with and drive the embryo worm wheel:

The DP of the worm wheel shown was approximately 6.281, although I used a commercial 6DP involute cutter for the gashing. The hob is home made.

Andrew

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It does seem reasonable to assume that 180 teeth are required.

MichaelG.

Easier than going outside to photograph mine ... Here are some photos that confirm the table has a throated worm-wheel: **LINK**

http://www.myford-lathes.com/milling_acc5.html

Despite his caution ... Yes it does look just like the table on my BCA.

MichaelG.

It's single enveloping, ie, the worm wheel wraps partly around the worm giving line contact. Double enveloping is where the worm also wraps around the worm wheel. Which means that the thread on the worm is cut on a circular path on a plane which also contains the axis of the worm.

Andrew

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Agreed, Andrew

a.k.a. throated wheel, if you can forgive the plebeian terminology.

MichaelG.

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https://en.wikipedia.org/wiki/Worm_drive#Types

Edited By Michael Gilligan on 25/06/2020 23:00:09