spiral spindle cutter

From memory, part of the Quorn Tool & Cutter Grinder construction notes there are instructions on how to mill a groove (rapid thread) in the column - using a Myford ML7 and some sort of auxillary drive to the mill cutter mounted on the topslde, but that is all I can remember. Anyone with a better memory or the construction notes may be able to help.

Thanks, Keith... I feel I’m getting closer all the time.

I think which ever method I’ve looked at it’s the issue of driving the lead screw from the spindle that is the trickiest part. At it’s simplest I imagine a cog on the spindle and a cog on the lead screw in line with it and a chain joining the two?

Assuming I can choose cogs that give me the right speed ratio for a barley twist pitch, how do I vary it to vary the spiral pitches or would I just be stuck with one?

If thread cutting lathes had a bigger range of ratios we might be in business!

Thanks for that video. I think I see whats going on now. The problem this guy has ended up with is one I'm grappling with. How do you vary the gear ratio for different pitches? At least he's managed a low enough pitch to get one working. I didn't think that was possible on a small hobby lathe.

You would just change the gear ration between the lead screw and the spindle to get a fast or slow helix

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.

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.

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

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".

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.

John C, you have a PM

Martin

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...

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

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

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.

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

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

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

MichaelG.

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).

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.

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