Worms

[keithmartParticipant @keithmart]

Hi

I am trying to create a 60:1 worm and wheel, for a project. I am using a 12mm X 1.5 pitch for the worm, but whatever I do I can only get 59: 1 or 62:1 never 60!

Have any of you gurus out there ever done anything similar, and could help with dimentions etc?

[keithmartParticipant @keithmart]

[John Stevenson 1Participant @johnstevenson1]

The correct formulae for worm gears is number of teeth plus 3 divided by the DP.

 

In your case because you are using a 1.5 mm pitch thread which is module its number of teeth plus 3 times 1.5 so that’s 94.5mm diameter.

 

However if you are free hobbing you will need to pregash the teeth with a slitting saw or similar to get it to follow the correct number of teeth.

 

Number of teeth plus 3 gets you an enveloping worm where the shape is carried up the side, number of teeth plus 2 gets you a worm where the teeth lie on the surface like a gear wheel.

 

John S.

[adyParticipant @ady]

I found an explanation on cnc zone.

——————————————————-

 

How to explain this: if you try hobbing a gear starting from a blank
with a tap, and feeding the work into the tap radially, you will
experience a creeping advance of the teeth along the circumference. For
a standard wormwheel (with a bit of a rounded throat on top of the
tooth), the circumference at the OD of the blank has enough length for
3 more teeth than can fit at the pitch diameter of the gear. The pitch
circle is where the worm technically contacts the teeth.

So the tap is to some degree, going to mash its way around the blank, and give you +/- 2 or 3 teeth from what you actually want

To try to avoid that, you should have a positive drive to the blank
that will force it around at the proper ratio. That is not likely in
the realm of possibility for a “one-off”.

You could try a tangential feed method, where the tap and blank are
positioned at the correct radial center distance, but the tap is
displaced axially so that it only cuts on the very end to begin with.
If there is sufficient contact to drive the blank around, you might
luck out and get the correct number of teeth.

You might need to provide some soft ‘dummy stock’ on each side of the
gear blank to create a constant engagement with the tap so that the
blank never can come clear by springing into the flute of the tap.

To generate a wormwheel on the lathe can also be done with a threading
method, and this involves some sort of indexing plate, with enough
holes to match the tooth number being sought. Basically, you build a
boring bar that runs between the chuck and tailstock, with a single
tooth flycutter sticking out in the middle. Then you simply use the
carriage half nuts and engage to thread the blank past the revolving
bar. This cuts only one tooth to full depth, and a couple of partial
cuts on either side of that one. Then stop and index the blank one
tooth, and do it again. How you index the blank is usually done with a
plate drilled full of holes in a regular pattern. You might do this on
a cnc, or you could also use a degree wheel on a small rotary table to
hold and index the blank.

 

cnc zone

 

 

[adyParticipant @ady]

I had a look at one i made for an experiment last year in aluminium.

I put the piccie in my album

 

The dia is 37.75mm

Gives us a circ of 118.60

divide by the 1.75pitch on a 12mm metric tap

 

=67.77 teeth

 

The actual cut is 70 teeth.

[adyParticipant @ady]

So if you want to try and be sneaky with a 1.5mmx12 tap which is what I would try in aluminium or delrin first.

 

57 teeth is 27.215mm dia

58 teeth is 27.692mm dia

 

and somewhere in there should give you an overcut of 60 teeth, yikes

 

I presume it will vary with the profile of the teeth on the tap, stiffness of the job, pure blind luck etc

 

 

Edited By ady on 28/06/2011 11:03:40

[adyParticipant @ady]

There’s a fancy calculator emulator called the classpad manager from casio which does all kinds of goofy stuff.

 

classpad manager v3

 

It’s a 90 day trial.

Very useful for those mafs things which make our braincells squeak

 

An interactive tutorial here lets you see what it can do, it’s banned from pretty much every exam going because it makes difficult stuff a lot easier.

It’s really a maths exploration tool, not a calculator.

 

solve(57=Pi*d/1.5) {d=27.21549526872}

solve(58=Pi*d/1.5) {d=27.69296009799}

…it makes dimwits look smart, so I just had to buy one.

Edited By ady on 28/06/2011 11:32:18

[keithmartParticipant @keithmart]

Hi

Thanks for the advice so far folks.

[QUOTE
57 teeth is 27.215mm dia
58 teeth is 27.692mm dia

and somewhere in there should give you an overcut of 60 teeth, yikes]

That is what I thought, but it just does not work.

I will try the gashing of the wheel when I next get into my workshop, BTW I am reduced to using hardwood as I used up what little delrin I had,!

Regards

Keith

Leeds UK

[adyParticipant @ady]

I had a go with a 1.75 x 12 this morning…and got a 59.

pics in album.

 

…almost went blind counting them too…lol

Edited By ady on 29/06/2011 11:46:01

[keithmartParticipant @keithmart]

Hi

[QUOTE…almost went blind counting them too…lol]

Try turning the tap round 60 times and see if it reaches the start point

Regards

Keith

Leeds UK

[adyParticipant @ady]

Yup 59…

 

I’ve got a few blanks I can play with so if I suss it out over the next week or so I’ll bump the thread.

Edited By ady on 29/06/2011 13:12:31

[keithmartParticipant @keithmart]

Hi again.

My good lady has been in hospital under the care of a surgeon, so I have only just had time to get back into the workshop.

I have tried using a fine slitting saw and cut 60 grooves to guide the tap, and IT WORKED!

I have made 2 wooden worm wheels, both with 60 teeth.

The next step will be to try it in metal.

For information, I cut the grooves 0.25mm deep, meshed the wheel with the 12mm tap, and fed it in in 0 .12mm increments up to 0.75mm deep.

Thank you all for your help

Regards

Keith

Leeds UK

[adyParticipant @ady]

Glad to see you’re getting there.

My computer cpu fan conked out this week and my row of experimental blanks have now become part of a fanless cpu heatsink.

 

Edited By ady on 10/07/2011 12:10:13

[keithmartParticipant @keithmart]

Hi

Sorry to hear that Ady.

I have now made one in brass, and that worked ok as well.

JOB DONE!!

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