spiral spindle cutter

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?

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.

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

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?

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.

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

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

Whoops, double post

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

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.

Thanks for those kind word, Larry.

Sometimes these enforced life changes are a blessing in disguise. In 2011 I left hospital in a wheelchair after having a brain haemorrhage, my wife left me and a few weeks later I was made redundant. Looking back its difficult to see how I survived all that or see anything positive in it. However, I didn't like working for that company and I probably still would be (although last year they went under!) and I am clearly better off without my wife. Friends have disappeared into the woodwork but they've been replaced with a better class of people who are worth knowing. And, best of all, its given me time to take up things I've always loved or wanted to do like my woodworking. And, to be honest, the idea of working in small scale actually appeals to me. Boxes and clocks were always on my project wish list. So, it could be worse!

like that - like that a lot. What would it take to get a lower pitch?

I'd need to line up the nuts very carefully so tnuts might be a bit tricky to get perfectly square to the rods.

If you used separate blocks to mount each T nut you could thread them onto the rods spaced the right distance apart, then screw them to the carriage. Position wouldn't be critical as long as the blocks laid flat against it. (I used to work in quality, it is often cheaper and more reliable to use a design that absorbs errors than to try and eliminate them - either that or I am just a bodge artist).

Brian

do you mean a higher pitch where the pen moves along the work even more, If so then More gears and bigger ones too though there comes a time when they simply won't fit on the bracket (banjo) so a longer one would be needed.

Lower pitch becoming more like a screw would be easy just by altering the gears supplied with the machine.

If you want a very low pitch spiral then maybe forget this method and get a wood lathe and Sorby spiral tool, the coarse wheel gives a nice shallow spiral. In engineering terms this is a bit like free hobing in reverse.

yes higher pitch - less thread-like. someone mentioned that you could reverse the wheels on the banjo to go higher.

I'm having trouble posting a jpeg. The little paste window is popping up but it still wont let me paste....

Might this be an instance where CNC is the best way to go? There have been plenty of standard lathes fitted with "digital leadscrews" using CNC gear to move the carriage along in relation to the main spindle to form a precision thread. Sure it's been covered in MEW some years back. All this application would need would be to make the thread a coarser pitch. Presumably just a matter of programming in the right numbers? And you would not run into the problems with very large gears not fitting the banjo and very high gear ratios straining the little plastic gears of a mini lathe.

Just a thought.(From someone who knows nothing much about CNC.)

.

Sit quietly with your beverage of choice, and read this: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=103028&p=1

MichaelG.

That is what I did for the video, quickly swapped the fine feed gears the opposite way round, had I spent a bit more time I could have got about 40mm pitch but no more with the supplied set of gears.

One other thought crossed my mind about using a Dremel particularly now that you have confirmed you want a slow twist and that is you will only be able to use a small 1/8" shank tool in the dremel so will have to make a number of passes with it set at different depths and also either side of the first pass to make the curved cut in a series of steps. This will require a means to move the Dremel bot sideways and in/out with reasonable accuracy which you may not get with a simple wooden carrage moved along by bits of studding.

This is what an off the shelf barley twist cutter looks like, one crest will blend into the next so the spacing or pitch between cuts is equal to the cutter diameter, you would need a good size router to hold them and make that sort of size cut.