Cutting a worm for changewheel dividing

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Cheshire Steve

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I want to make a worm drive to add to a simple dividing attachment that uses a changewheel and plunger on the end of a spindle. Ideally I would like this to work for a range of changewheel sizes. I assume the worm should be angled at the helix angle, but I am struggling to work out exactly what pitch or shape to cut - or even whether the shape is important. My indexed spindle takes gears off my Winfield lathe - which are 20DP, and is used on my Centec mill.

Reason is that I wanted to index 125ths, and couldn't accomodate a 125 tooth wheel even if I could find one. I am open for other cheap and cheerful ideas to acheive 125ths indexing, but I am favouring a worm at the moment.

Grateful for any pointers.
 
No club or library here - do the publishers offer these mags on line ? Worth a quid or two to access the right article when you need it.

Well I am keeping an eye out for a cheap dividing head, but if worm and wheel is easy enough I could apply the same approach on my IXL lathe, which also has changewheel based indexing on the headstock spindle. I only got it recently, on that the changewheels are mod 1.5, which could support the idea that these were european imports. I want to make a couple of index collars for 8tpi thread, hence 125 divisions, though not sure if that is too fine.
 
I have no idea what you mean. 62.5 is half of 125, 62*5 is 310.

The index collars are for 8tpi cross-slide and top-slide, the leadscrew is 4tpi. The diameter has to be right, so I though to make my own. I do have lathes with 8tpi leadscrew and finer down to a Unimat, but I don't have indexing on that.

And, yes, there is not a secondhand model engineering magazine shop in our village, or a library, or other model engineers as far as I knoe. But even out in the sticks there is the Internet, with some helpful websites. Clearly some are more helpful than others.
 
If I understand correctly, what you want to do is mount a change wheel on a spindle and index it with a worm gear.

With 20 DP gears, the worm gear thread TPI will be 20/Pi = 6.37 TPI or 0.1571 inch pitch.Not much chance of having change gears to cut that, but that is so close to 4mm (0.1575 inch) that you or I could not measure the error. So if you can cut metric threads you are 1/2 way there.

You should cut the thread to match the angle of the gear teeth, which will probably be either 14.5 or 20, most likely 14.5. The included angle on your cutting tool would be twice this. For hand cranked operation it will not be too critical, but will affect backlash if it is not correct.

The worm will have to be offset by the helix angle as you said. This angle will be dependent on the diameter of the worm.
Gail in NM
 
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How about...
Assuming you have a 50 tooth gear you can attach to the spindle (or change-gear train, 1:1 to spindle rotation) mesh that with a 20 tooth, then index on another 50 tooth pinned or keyed to the 20. Each tooth on the 50 indexing gear will rotate the 20 (and so the 50t on the spindle) through 2/5 of a tooth, so you'll get 125 divisions stepping one tooth at a time?

The exact gears used don't have to be 50,50 and 20, as long as you can get the factors of 125 in - this won't work for prime numbers though ( but can get pretty damn close!).

For instance, you could use 60 on the spindle, 24 meshed, 80 and 32 - with a 50 index and still get 125, if I've worked it out right! I too have 8tpi feedscrews, and had to make a tailstock handwheel collar....
 
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Thanks, a least a couple of understandable replies. I have no idea what goldstar is going on about. Maybe someone else can decode his posts.

I thought making a worm and holding it in register would be easier and more accurate than using a banjo to mount two gears, as changewheels tend to be sloppy - which I think is by design to stop them going bang if a bit of swarf gets in (good tip is always to set them up with a bit of paper between in old open gear lathes).

I think I have worked out the thread for the worm now and will have a go, with the pitch of the worm reduced from the pitch of the gear a tad by the helix angle. It is a small change, and it is a short engagement so not too critical.

I am using old lathes because I want to learn how they were used to make such fine models before CNC came along, and my grandfather was a cracking model engineer on them back in the 30s. He made his own tools, he even made his own scale taps and dies, and he made one heck of a lot of models. So I would rather make my own indexing than buy one. I reckon that even if you try and fail, you still learn something.
 
Steve,

Get hold of a copy of 'Workshop Techniques' by Geo. H. Thomas

This has a complete section devoted to making divider attachments for use with lathe changewheels etc, which will do exactly what you are trying to achieve.
He gives full details on how to cut the necessary wormwheel and how to mount it.
It also gives changewheel setups to obtain the correct worm pitch for lathes with 8TPI leadscrews.

The book is well worth getting, since it has a wealth of other stuff in it applicable to model engineering workshop equipment for haome manufacture... including screw cutting and a very nice dividing head.

It should be available from most good On-Line model engineering suppliers or possibly 'Amazon'

You might even get your local library to get it for you, if they don't have a copy already.

Hope this is of help.

Best regards.

SandyC.

Edit... Should have also said, the book is published by 'Tee Publishing' of Leamington Spa, Warwickshire... their tele. no. is 01926 614101.
 
I've already suggested that he buys that. Clearly, he has no intention-- and I have therefore deleted all my posts- save the remark about the Albatross.
 
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