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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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
 
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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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.)
 
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!
 
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.
 
Here's the worm.
 

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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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In case this saga is of interest to someone, here are some next steps. I tidied up the gear blank (well, it has kind of half-teeth!) and attached it to its 80t mate. I decided against free-hobbing and instead I've been making a rack-form hob (following the Helichron website). Next I need to part it, bore an accurate 1/2" hole (work is shut so no reamers) and then harden it - the last step is new for me.

The pics show what happens next. I still don't have a dividing head, but I bought some cheap dividing plates and fitted them to the rotary table I have on long-term loan. The whole lot sits on my Frankenstein pillar-mill-drill listed elsewhere in this forum section.
 

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I found the foregoing posts mentally challenging as I had made one of these George Thomas small dividing heads but with the extra Acme thread etc to widen the scope not able to be obtained easily with the conventional number of division plates. So your posting is appealing- Thank You.
So I went not better but differently and acquired a Potts dividing head but is capable of working into the Third Dimension by being capable of the whole lot but rising and falling as desired.
Seemingly Hemingwaykits has the old Woking Precision Models drawings and stuff for the Potts but the recent catalogue shows that the GHT small dividing head etc can be similarly raised and dropped with a new 'pillar.
I merely have guessed the last description. Someone again might be intersted
 
FYI
I have notest overseas lathes will use the 63 tooth gear. In stead of 127 gear.
Most lead screws will have wear over time greater 0.001 per foot.
But most dividing heads can not dived 63 but the 37/47 can be cut on a BS 0 dividing heads witch calculation to 1.2702702702702

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
 
Ah, I didn't know that 63 was hard to do Dave. I have a 21 hole circle on my plates (luck on the ordering of a cheap-ish set from India), and this

http://www.bilar.co.uk/engineering/index.html
...tells me that on a 60:1 worm (common for div heads apparently), 63t is 20 holes. 'My' rotary table is 120:1 (3 deg per handle turn) and this means 63t is 1 rem 19, it seems.

(Edit - I played around some more. If a BS0 head does not come with a 21 plate, I imagine you can make one with it as 21 divisions seems achievable using a number of other combinations!)

I plan to put the hob in a bit of tube with ends closed, packed out with Kasenit, and leave it in the fire for a few hours. If anyone has any comments to make here, go ahead, I don't know what I'm doing!
 
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A 63 tooth gear is easy to design in a CAD package and a 3D printer doesn't care how many teeth the gear has. As long as you are not trying to transmit too much torque a 3D printed gear works fine.
 
Ah, I didn't know that 63 was hard to do Dave. I have a 21 hole circle on my plates (luck on the ordering of a cheap-ish set from India), and this

http://www.bilar.co.uk/engineering/index.html
...tells me that on a 60:1 worm (common for div heads apparently), 63t is 20 holes. 'My' rotary table is 120:1 (3 deg per handle turn) and this means 63t is 1 rem 19, it seems.

(Edit - I played around some more. If a BS0 head does not come with a 21 plate, I imagine you can make one with it as 21 divisions seems achievable using a number of other combinations!)

I plan to put the hob in a bit of tube with ends closed, packed out with Kasenit, and leave it in the fire for a few hours. If anyone has any comments to make here, go ahead, I don't know what I'm doing!
I made all my plates. It's easy to do
 
I think market for a special dividing plate for BS-0.
The plate would have 51,63,127 and 157 holes. This would cover the most command odd numbers not cover in the standard BS-0

Dave

I made all my plates. It's easy to do
 
Well, it worked. I put the hob in a small steel pot with some Kasenit and left it in the fire all evening. I re-heated and quenched it the next day - it survived fine but I was only cutting cast alloy. Maybe I'll try a bronze one, one day. I'm sure lots of you have cut gears before, but I haven't so I'm pleased it works. It's perhaps a little noisy in use, but soft enough that I hope it will bed in.
 

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I think market for a special dividing plate for BS-0.
The plate would have 51,63,127 and 157 holes. This would cover the most command odd numbers not cover in the standard BS-0

Dave

That 'bilar' link I posted above has a thing on it which will print out a paper template for whatever division plate you want. You can then 'just' centre pop through it and then drill away. I was going to do this, but I bought 3 small plates, the brass fingers which help you remember where you are, and the sprung stylus and arm which locates on the division plate, for about 45 quid. I know this is a hobby and the idea is to make things, but I had been putting it off for so long, I just bought them. I think my wallet is getting over it by now, it was a few months back :)
 

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