spiral spindle cutter

[john constableParticipant @johnconstable10725]

I understand I'd need to change the gear ratio but I'm not sure how to do it without reconstructing it every time. I could add more gear pairs with different ratios and move the chain each time but its still only one pitch per ratio and i'm not sure how I would change the chain each time if there's no slack in the system.

[Keith LongParticipant @keithlong89920]

John – have a look at the following webpage (http://www.lathes.co.uk/winfield/index.html). This is a simple (old) screw-cutting lathe. Along the front you can see the lead-screw with a hand-wheel on the right hand end and a set of gears at the left hand (headstock) end. Those gears are mounted on a slotted bracket, usually called the "banjo". The gears are located on the bracket by short axles that can be moved along the slots to accept the different size gears. The gears connect the lead-screw to the main spindle and you can change the gears around – or use additional ones to get (within reason) whatever gear ratio you like between the main spindle and lead-screw, that's how you set up to cut different screw thread pitches. Normally for cutting screw threads the gearing will be such that there is a reduction in speed between the main spindle and the lead screw. For your application you'll be looking for a speed increase from the main spindle to the lead screw so you'd assemble the appropriate gear train basically the "other way round" so that the small gear was on the end of the lead screw and the large gear on the end of the spindle. You'll see in the picture 4th from bottom of that page that the gear train isn't just a "simple" train but is a compound train – 2 gears rotating on the same axle and locked together, each connected to further gears in the train – that's how you get the flexibility in the ratios.

I'm not suggesting that the lathe on that page is the one you should look for – although it would do the job you want perfectly well, but picked up that page a showing a typical older style screw-cutting lathe so that you could see how the gearing is arranged. There are many lathes of that type knocking around and they can often be picked up quite cheaply. If you can find one with a good set of gears (change wheels) the so much the better, but gears can be adapted and modified as needed, the trick is to try and make sure that the gears that you want are a common pitch (tooth spacing) as that will mean that they are more readily available and usually cheaper. The bore of the gears can either be machined out or bushed down to suit and key-ways are pretty easy to cut (for one or two) with basic tools. Some lathes used pin drive into the face of the gear in which case you just need to drill a hole in the right place.

If you do go down the old lathe route don't be tempted by one with a screw-cutting gear box, the gear train will be completely wrong for your application and a nightmare (if even possible) to convert.

[Ian S CParticipant @iansc]

In my wood turning days (30yrs ago), I used to make candle sticks, sometimes with two twist, but usually three, these were just marked out ,then drilled free hand half way through to make a hollow stem. Now my nephew has one of these things, and it would probably do your job to. Nephew's grandfather got the lathe to make ornemental walking sticks.

Ian S C

Edited By Ian S C on 19/06/2018 15:32:57

[john constableParticipant @johnconstable10725]

Posted by Keith Long on 19/06/2018 15:04:31:

John – have a look at the following webpage (http://www.lathes.co.uk/winfield/index.html). This is a simple (old) screw-cutting lathe. Along the front you can see the lead-screw with a hand-wheel on the right hand end and a set of gears at the left hand (headstock) end. Those gears are mounted on a slotted bracket, usually called the "banjo". The gears are located on the bracket by short axles that can be moved along the slots to accept the different size gears. The gears connect the lead-screw to the main spindle and you can change the gears around – or use additional ones to get (within reason) whatever gear ratio you like between the main spindle and lead-screw, that's how you set up to cut different screw thread pitches. Normally for cutting screw threads the gearing will be such that there is a reduction in speed between the main spindle and the lead screw. For your application you'll be looking for a speed increase from the main spindle to the lead screw so you'd assemble the appropriate gear train basically the "other way round" so that the small gear was on the end of the lead screw and the large gear on the end of the spindle. You'll see in the picture 4th from bottom of that page that the gear train isn't just a "simple" train but is a compound train – 2 gears rotating on the same axle and locked together, each connected to further gears in the train – that's how you get the flexibility in the ratios.

I'm not suggesting that the lathe on that page is the one you should look for – although it would do the job you want perfectly well, but picked up that page a showing a typical older style screw-cutting lathe so that you could see how the gearing is arranged. There are many lathes of that type knocking around and they can often be picked up quite cheaply. If you can find one with a good set of gears (change wheels) the so much the better, but gears can be adapted and modified as needed, the trick is to try and make sure that the gears that you want are a common pitch (tooth spacing) as that will mean that they are more readily available and usually cheaper. The bore of the gears can either be machined out or bushed down to suit and key-ways are pretty easy to cut (for one or two) with basic tools. Some lathes used pin drive into the face of the gear in which case you just need to drill a hole in the right place.

If you do go down the old lathe route don't be tempted by one with a screw-cutting gear box, the gear train will be completely wrong for your application and a nightmare (if even possible) to convert.

Keith – this is brilliant stuff, thanks. I've been browsing ebay and google looking for helical milling machines over the last couple of hours because that seemed the most descriptive term for what I was looking for. I ran across some old lathes with interesting looking gear arrangements, including the unimats, and I was going to come back with some more questions which you have preempted.

Do I need too use the motor or can I turn the spindle by hand? I'd prefer the extra control and overrunning the cut would not be good!

Can I ask the difference between the wheel on the tailstock end of the leadscrew and the large wheel on the slide? Dont they both turn the leadscrew and move the slide?

I thought the unimat might be suitable as there seems to be a lot of options for it which may well include these gears but they look a bit pricey. Could anyone suggest some suitable models to me please? Not too large if possible as I still only need to turn things less than a foot by 2".

[Keith LongParticipant @keithlong89920]

Hi John

No you don't need a motor on the lathe, in fact for what you what to do you don't WANT a motor on the lathe, things would happen at frightening speed! With the way the gearing would work turning the hand wheel on the end of the lead-screw will turn the main spindle as well quite easily.

The hand wheel on the slide (aka saddle) will move the saddle along the lathe bed but not under control of the lead screw. That hand wheel shaft has a gear on the far end which engages with a rack mounted on the lathe bed, if you look carefully you should be able to make out the teeth on the rack behind the lead screw. For your purposes you'll want the engage the half nuts onto the lead screw – operated on the lathe shown by the lever sticking up at approx 30degs. on the left side of the saddle. That will engage the saddle with the lead screw which will then control the movement along the bed.

If you're thinking of the Unimat with the twin parallel round bars for the bed – don't bother, it used an entirely different means of thread cutting which isn't suitable for your requirements and would require a lot of fiddling to make it work for you. Also as you've noticed they tend to go for gold plated prices generally.

[Martin King 2Participant @martinking2]

John C, you have a PM

Martin

[john constableParticipant @johnconstable10725]

Posted by Keith Long on 19/06/2018 18:01:42:

Hi John

No you don't need a motor on the lathe, in fact for what you what to do you don't WANT a motor on the lathe, things would happen at frightening speed! With the way the gearing would work turning the hand wheel on the end of the lead-screw will turn the main spindle as well quite easily.

The hand wheel on the slide (aka saddle) will move the saddle along the lathe bed but not under control of the lead screw. That hand wheel shaft has a gear on the far end which engages with a rack mounted on the lathe bed, if you look carefully you should be able to make out the teeth on the rack behind the lead screw. For your purposes you'll want the engage the half nuts onto the lead screw – operated on the lathe shown by the lever sticking up at approx 30degs. on the left side of the saddle. That will engage the saddle with the lead screw which will then control the movement along the bed.

If you're thinking of the Unimat with the twin parallel round bars for the bed – don't bother, it used an entirely different means of thread cutting which isn't suitable for your requirements and would require a lot of fiddling to make it work for you. Also as you've noticed they tend to go for gold plated prices generally.

Thanks, Keith. The unimats certainly are pricey. You're explanations are bringing my 35 year old engineering lessons flooding back to me….

So, the good news is I might even be able to save some money by finding something without a motor…

[Brian GParticipant @briang]

Hi John

The device Ian showed looks like it could perhaps be replicated using plywood plates for the ends and threaded rods and nuts to space them out in much the same way as a Prusa I3 is assembled. A plywood carriage could then carry the router, sliding along smooth rods with linear bearings (3D printer parts from eBay perhaps or just carefully bored holes in wooden blocks). The workpiece would be fixed to a drum and could rotate between screws in the centre of each end plate. As the drum is turned, the router would slide along 3.14 times the diameter of the drum for each revolution.

It is all a bit crude, so it would probably be necessary to tension the cord using two weighted cords pulling against it, one at the opposite end of the device to the drum, and one wound around the drum in the opposite direction. I suspect you would need to hold both the router and the drum to use it as it may not be that rigid.

No promises it would work, but it wouldn't cost a lot to try.

Brian

[HopperParticipant @hopper]

Posted by Ian S C on 19/06/2018 15:29:58:

In my wood turning days (30yrs ago), I used to make candle sticks, sometimes with two twist, but usually three, these were just marked out ,then drilled free hand half way through to make a hollow stem. Now my nephew has one of these things, and it would probably do your job to. Nephew's grandfather got the lathe to make ornemental walking sticks.

Ian S C

Edited By Ian S C on 19/06/2018 15:32:57

That looks like it would suit the OP's needs better than jury-rigging a model engineer's lathe. Does it have a brand name?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Hopper on 20/06/2018 10:29:24:

That looks like it would suit the OP's needs better than jury-rigging a model engineer's lathe. Does it have a brand name?

.

Yes … probably several, but the one most of us mentioned was the Trend RouterLathe.

[see p1 of this thread]

MichaelG.

Edited By Michael Gilligan on 20/06/2018 11:30:10

[JasonBModerator @jasonb]

You only need to put Router Lathe into google and look at the images to see many home made versions some simple some not quite so simple to see how one can be knocked up or a wood lathe converted.

[Brian GParticipant @briang]

The Trend machine has been out of production for 10 years or so, but they still turn up on eBay, typically selling for a little north of £100. I had assumed that as the OP had one before and didn't simply buy another he was after either a cheaper (hence my sketch) or a more "engineered" solution.

Brian

Edit

Another way to do this might be to use up a copy lathe, with the pattern on a second spindle driven 1:1 from the lathe spindle.  It would however need to be very heavily built.  A vintage machine I once saw turning barley twists shook as if it would tear itself apart.  The fact that in 100 or so years it hadn't done so was a testament to its builders.

Edited By Brian G on 20/06/2018 11:46:54

[Michael GilliganParticipant @michaelgilligan61133]

I've just spotted an interesting-looking upgrade to the RouterLathe: **LINK**

http://www.notjustround.com/Tools.html

MichaelG.

[john constableParticipant @johnconstable10725]

Yes – essentially you're all correct. I had a trend router lathe but it was lost in a house move before I could use it and I had to stop woodworking for a while so I didn't replace it.

More recently I've ended up in a wheelchair with mobility problems and a full sized set-up is beyond me to use so I decided to put together a miniature workshop and concentrate on small decorative things like boxes and clocks – hence my desire to make barley twists from maximum 2" x 8" stock but usually smaller.

The trend router lathe will be too large for me and also I think not accurate for the scale I want to work at.

There are two routes that seem to have opened up to me. Firstly getting a mini lathe which has change wheels and a disconnectable motor. That's not cheap but on the other hand offers me the joy of other things a lathe can do.

Or home-made,like Brian's sketch. That's similar to the pen wizard but I don't want to use cords because I think they'll be inaccurate. The part I'm finding difficult at the moment is an arrangement to sync the travel of the tool along the work and the rotation of the work and do it in such a way that it can be changed easily for different pitches (in effect it's some kind of simple gear box needed).

[Michael GilliganParticipant @michaelgilligan61133]

To be honest, John … I think the best bet would be to build a scaled-up copy of the Pen Wizard [probably with something other than the simple mandrel].

There is a lot of information on their website !!

… and nothing wrong in making one for your own use.

MichaelG.

[Brian GParticipant @briang]

Posted by john constable on 20/06/2018 14:53:19:

.. The part I'm finding difficult at the moment is an arrangement to sync the travel of the tool along the work and the rotation of the work and do it in such a way that it can be changed easily for different pitches (in effect it's some kind of simple gear box needed).

Going back to my Heath Robinson lash up, how about using a leadscrew (from DIY store studding) connected to the spindle by Delrin chain like this? **LINK** You could get 6:1 reduction with 8 and 48 tooth sprockets, so two pairs would give you 36:1 and a 36mm pitch using M6 studding for the leadscrew. Unlike gears, it would be easy to accomodate the spindle-leadscrew distance and a third wheel acting as a movable tensioner could adjust the chain when you changed sprockets.

Meccano chain and sprockets would be a little more sturdy, but only offer 4:1 reduction, although it would be simple to combine this with gears and would allow a massive range of spiral pitches.

Brian

[john constableParticipant @johnconstable10725]

Posted by Brian G on 20/06/2018 15:59:45:

Posted by john constable on 20/06/2018 14:53:19:

.. The part I'm finding difficult at the moment is an arrangement to sync the travel of the tool along the work and the rotation of the work and do it in such a way that it can be changed easily for different pitches (in effect it's some kind of simple gear box needed).

Going back to my Heath Robinson lash up, how about using a leadscrew (from DIY store studding) connected to the spindle by Delrin chain like this? **LINK** You could get 6:1 reduction with 8 and 48 tooth sprockets, so two pairs would give you 36:1 and a 36mm pitch using M6 studding for the leadscrew. Unlike gears, it would be easy to accomodate the spindle-leadscrew distance and a third wheel acting as a movable tensioner could adjust the chain when you changed sprockets.

Meccano chain and sprockets would be a little more sturdy, but only offer 4:1 reduction, although it would be simple to combine this with gears and would allow a massive range of spiral pitches.

Brian

Thanks, Brian. That looks like the best type of hardware to use. I never came across Delrin before. I wanted to use meccano but as you say the choices are limited although someone mentioned a robotics supply site.

How would you arrange the gears for different ratios? Would you use a pair for each different pitch you wanted and then just connect your chain to whichever you needed for each job? How would a moveable tensioner work? Would it need to move in a slot to take up the slack and then be held there with a nut or something?

[john constableParticipant @johnconstable10725]

I'd also considered some kind of cartridge arrangement with 2 or more gears inside for a given pitch and then the cartridges can be changed for each change in pitch required and no need to fiddle with gearwheels and chains.

I couldn't quite work out how to put the cartridges on though.I'm not explaining myself very well…I can feel some sketches coming on.

[Brian GParticipant @briang]

Only problem with the cartridge arrangement is that you would have to use pairs of gear that had the same centre-centre distance, which limits your choices. As any useful twist would need double reduction (or more) perhaps the easiest thing to to with either gears or chain is to copy the "Banjo" of a lathe. Mounting the idler on a slotted part that pivots around the leadscrew would let you slide the gear/sprocket up and down the slot and lock it with a nut to set the distance to the leadscrew gear/sprocket, and rotate the whole arrangement to set the distance to the spindle. Otherwise you might just use a gear, pulley or even just a slipper on the slack side of the chain, sliding it along a slot and as you say, locking it with a nut – a bit like adjusting a cam chain.

Lots of Meccano gears and sprockets here **LINK** (Never used them but I have been looking for gears for another project).

Incidentally, if using chains you could use two screwed rods linked together to both move and support the carriage, probably cheaper and more reliable than smooth rods and linear bearings which probably wouldn't like sawdust.

Brian

[john constableParticipant @johnconstable10725]

I was thinking I could maintain the same centre-centre distance by having more than 2 gears in the cartridge.They could be as complex as needed as they wouldn't need to be fiddled with once set up.

By two screwed rods, you mean that a chain will engage a cog on each whereas fixed cogs will only allow one?

[larry phelan 1Participant @larryphelan1]

John,

I cannot be of any help to you,since my idea was not suitable,but I have to say I admire your guts,that you are still working away,from a wheelchair. I say GOOD FOR YOU,long may you continue,I hope I will be able to do the same if I ever get to that stage. No matter that the workload is smaller,so what,to be able and willing to tackle it is what matters. Two of my old friends are in wheelchairs,but without your positive attitude,so,hang in there !

Remember,you will get all the help you need from this site.

[JasonBModerator @jasonb]

I think an M6 lead screw is going to be too fine. A 2" dia typical barley twist needs a pitch of around 3xD so that is 6" or 150mm pitch which would mean some big sprokets or a longer compound gear train to get 150:1 ration. Even with M16 studding that has a 2mm pitch you would be wanting 75:1.

Typical stair spindle of 41mm dia has a 2 start rope twist with 60mm pitch so 120mm tool movement per rev of the work which equates quite well with the 150mm pitch on a 50mm spindle.

I had a quick play with my Warco lathe and got 32mm pitch which on 1" stock would have been about OK for a single start. If the video comes out OK I will post in a while.

J

[Brian GParticipant @briang]

Yes John, I was suggesting linking the two threaded rods together (regardless of whether the first rod is connected to the spindle by gears or chain) so that they turned in sync when one was turned with a handle. That way, you could just fix nuts to the carriage and not need to bother with any kind of sliders. The real cheat's method would be tee nuts (**LINK**) which could just be hammered into both ends of wooden blocks.

I'm with you on the difficulties of working from a wheelchair, but I am lucky in that my paralysis is intermittent, so although I keep a few hand tools by my desk, I generally work on drawings (or 3d models for printing) whilst paralysed and make things when I can move. Generally I talk my son through the machining and stick to the bench work.

Brian

[Brian GParticipant @briang]

Whoops, double post

Edited By Brian G on 20/06/2018 19:20:01

[JasonBModerator @jasonb]

Not the best video but much the same as the mini lathe one I posted earlier. 5:1 using the banjo gear train and then putting it through the gearbox brought it upto 10:1 so 10 x 0.125" = 1.25" per turn of the work. Does need a lot of effort to turn the leadscrew so a decent handle would certainly be needed.

[Author]

Posts