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Boxford / South Bend / 9" lathe : metric screw cutting on 8tpi leadscrew : 80/63 approximation

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MRA

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Hi folks

I like tool modifications, for the sake of it really. My Boxford (South Bend 9" clone for US readers) uses a 127/100 compound gear to cut metric threads on an 8tpi leadscrew. Original ones of these come up on ebay fairly regularly, but can get quite expensive if a couple of people want one. New plastic (delrin?) ones are also advertised there, and seem a cheaper way in though of course they need to be handled with a bit of care - no crashing the gearbox!

Because I'm a cheapskate and also because I do a bit of casting, I was excited to read somewhere that 80/63 is very very close to 127/100 - a little over a thou in a foot error, which as far as I am concerned / can operate, is 'the same'! Better still, all the Boxfords I have seen (mine included) come with an 80 tooth idler gear as standard - and this has a double-width boss which runs on an oiled bush. So making a 63 tooth gear to attach to it seems like a nice project which may work out.

I have no dividing head (well, I am making a freestyle simple one, but it is some way off). So a friend of mine who likes 3D printing, printed me two half-patterns with a small shrinkage allowance.

So here's a photo of the ingredients, so far. I only asked my friend to print the teeth, to save time and plastic - I'll make the middle of the two half-patterns from wood.

If if cast it in alloy, I hope to run it in such that the steel gear which mates with it, rubs the high spots and flash off. I suppose I could make a bronze one if I'm feeling flashy :)

It would be nice to hear from South Bend users (and perhaps those with Hercus and perhaps even S&B Sabel lathes) to find out whether they also come with an 80 tooth idler as standard.

cheers
Mark
 

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SmithDoor

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You can also use 37/47 gear set
It can be made on most dividing heads

Dave

Hi folks

I like tool modifications, for the sake of it really. My Boxford (South Bend 9" clone for US readers) uses a 127/100 compound gear to cut metric threads on an 8tpi leadscrew. Original ones of these come up on ebay fairly regularly, but can get quite expensive if a couple of people want one. New plastic (delrin?) ones are also advertised there, and seem a cheaper way in though of course they need to be handled with a bit of care - no crashing the gearbox!

Because I'm a cheapskate and also because I do a bit of casting, I was excited to read somewhere that 80/63 is very very close to 127/100 - a little over a thou in a foot error, which as far as I am concerned / can operate, is 'the same'! Better still, all the Boxfords I have seen (mine included) come with an 80 tooth idler gear as standard - and this has a double-width boss which runs on an oiled bush. So making a 63 tooth gear to attach to it seems like a nice project which may work out.

I have no dividing head (well, I am making a freestyle simple one, but it is some way off). So a friend of mine who likes 3D printing, printed me two half-patterns with a small shrinkage allowance.

So here's a photo of the ingredients, so far. I only asked my friend to print the teeth, to save time and plastic - I'll make the middle of the two half-patterns from wood.

If if cast it in alloy, I hope to run it in such that the steel gear which mates with it, rubs the high spots and flash off. I suppose I could make a bronze one if I'm feeling flashy :)

It would be nice to hear from South Bend users (and perhaps those with Hercus and perhaps even S&B Sabel lathes) to find out whether they also come with an 80 tooth idler as standard.

cheers
Mark
 

MRA

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Here's the next step in my attempt to cast a gear, having no dividing head. The larger boss will be the one through which I'll lock it to the 80t idler which I already have. The two smaller ones are there as 'why-nots' to help the thing not rock against the idler - though the bore will be machined for a good fit on the idler boss, which on a Boxford is luckily machined and cylindrical. The idler has recessed sides, and I've left the bosses a little long so I can face them down once cast to get a nice fit.
 

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awake

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Hmm ... how about just printing the gear and using that, rather than casting it? Sure, it may not last as long, but plastic teeth can be surprisingly effective for change gears, and if it fails ... just print another one. T'were it me, I'd go this route first, before risking wear on my good gears from the casting defects of a not-quite-right gear. (Actually, t'were it me, I'd cast it with the teeth over-size, and then cut it to size - but then, I have a DH, which is the point in question here.)
 

MRA

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Well, I don't gave a 3D printer either; I am relying on my friend there. I'm pretty confident that an alloy gear will make no impression on a steel one - and I'm relying on the reverse, to improve the fit of the raw-cast thing. Let's see!
 

MRA

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A penny just dropped - the cast gear will be close enough to enable 'free-hobbing' with a worm-hob. I need 18dp, which comes out at 4.43mm pitch or 5.73tpi. The standard model A gearbox will do 5.75 tpi, and this is pretty close - I had half-forgotten that I had dug this info out from the web ages ago and made a worm to engage a 60t Boxford change gear as part of my long-gestation (and incomplete) dividing head project. So if I make another one I can cut flutes and relieve for cutting faces, and harden it, and then run it against the cast gear.

I have some ancient Kasenit, and I have never used it. This might be a nice project to attempt case hardening. If anyone has experience, I'd be glad of your advice.

One interesting (and probably obvious, though not to me) oddity is that on a normal worm wheel, the teeth are angled to meet the helix of the worm. When using (or cutting) a plain spur gear, this angle has to be introduced between the axes of rotation of the worm and wheel, which are no longer at 90 degrees.
 

MRA

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I nearly didn't bother pouring this - the sand tore up so badly due to the 'grippy' teeth when removing the pattern, even though the two gear halves are only 5mm thick each. But - this is something I can work with I think. If I were really determined I could cut and file this by hand into useable form - but I think there is enough there to make free-hobbing worth a go.
 

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