cable drive

[dertsap]

has anyone concidered or used stainless cables to drive their gantry system , I've tryed a few different drive methods with great success but i am preparing for another build and this one will be cable driven
Why cable ? because it is cheap and readily available , it won't stretch as belts have a tendancy to do , and it can be tightened up tight enough with turn buckles so that there wont be slop or slip

i have no question whether this will work or not because i have already prooven to myself it will , I'm just curious if anyone has or is using it

[Zumba]

Interesting to note that the Z-axis of shopbots had decals with a patent number on it (not sure about the current models). The number corresponds to an old cable drive patent that they patented in the early 90s.

[RICHARD ZASTROW]

derstap, I know it works on a welding lathe. Incorporated capstan drive for motion and as you suggested, turnbuckle pulling cable tight. Turnbuckle preloaded with a spring.

Axis readout was one of those cable/resolver type. (often used for HBM spindle Z or W)

Dick Z

[Kipper]

It's in use on Roland equipment too. I would consider it for a small router or engraver.

[BillTodd]

I tried it years ago on a homebrew A0 plotter. I used the plastic coated wire used for sea fishing. The wire was wrapped once around a 12mm motor spindle (free to walk up and down), looped around a pulley at the other end with the free ends connected to the carriage and tensioned with a miniature turn buckle (from an IBM Selectric typewriter ISTR).

It worked as a drive, but I had major problems with multiple resonances, think guitar strings on acid At times it would shake the whole machine.

If you could use less tension and a heavier wire (to keep resonances below the working frequencies) it would probably work OK . However, you would have to find a way to eliminate slippage or get position feedback from the carriage not the drive motor.

[dertsap]

i looked up the cable drive shopbot and they had some complaints about having to tweak every so often but it looked to me that the cable was inferior and I would imagine that a lot of the inaccuracies were due to the patio door rollers that they used on the original designs
I'm quite optimistic in it working well , the guitar on acid does sound like a potential issue
because the drivers being used for that particular setup do cause the motors to sound a bit ugly , but if its only noise then the router cutting will drown that out quickly .

this is a project and more or less a mod for my kids machine or a complete new build , something to keep me and the boy outa trouble

[BillTodd]

Have you seen this Bell-Everman belt drive? I wonder if you could do something similar with a cable? It should solve the resonace problem if nothing else.

[greybeard]

Hi all.
I'm currently trying the same idea, using a pair of 1.3mm bicycle cable inners.
Tandem cables are 10ft long and will do just fine on my current layout.
I was hoping that by angling the capstan, I could stop the cable walking sideways (and therefore use several turns to improve friction), but so far that doesn't seem to work
Is this angle just very critical, or is my theory wrong ?
John

[BillTodd]

Originally Posted by greybeard Hi all. I'm currently trying the same idea, using a pair of 1.3mm bicycle cable inners. Tandem cables are 10ft long and will do just fine on my current layout. I was hoping that by angling the capstan, I could stop the cable walking sideways (and therefore use several turns to improve friction), but so far that doesn't seem to work Is this angle just very critical, or is my theory wrong ? John

If you think about it, if you have one or more turns around a capstan, the cable will walk one way because each new turn lays beside the previous (and the earlier turn is removed). If the cable is allowed to slip back, you're going to lose traction.

One way to prevent walking is the technique used in mining winches. Typically, these have two drums one grooved for traction and one that shifts the rope loops to the next groove.

The rope wraps partially around one groove before leaving the groove and wraping half way around the idler, then on to the next groove for half a turn (or more) then off to the idler and so on...

(I've been searching for a picture, but google has become almost useless these days; unless you're looking to buy something!)

[greybeard]

Hi Bill, and thanks.
My idea was that if the axis of the capstan was rotated horizontally, away from the feed direction of the cable, with the angle chosen correctly, the offset of one turn would exactly equal the thickness of the cable, so the outgoing line would always be in the same position.
I did think this morning that the reason my idea was wrong was that on meeting the angled capstan, the cable encounters a slight turning force which, due to the stiffness of the cable, will result in a slight curvature being transmitted back to a point before the capstan is touched. So the cable is given a small lateral displacement before it becomes "locked" to the surface of the capstan.
Hope that makes sense.
I'm going to try angling the capstan downwards instead of horizontally(or as well as), so the effect is in the same plane as the curvature of the surface, and see if that makes a difference.
John

[BillTodd]

Like this?

It looks like it should work A quick 'pen and string' test suggests it would need an input guide roller to stop the cable pulling itself 'down' the capstan.
(Two rollers, if it is to drive an cnc axis)

I would also worry about it twisting the cable

[edit]After further pen and string tests: I'm not so sure this will work

The guide roller (my thumb nail in this case) doesn't work; in effect it just straightens the string path, so we're back to the perpendicular capstan problem.

The trouble is, without a guide the string still trys to pull/walk along the angled pen; if the string is pulled tight it can be seen to slip sideways

[der_fisherman]

I cannot see anything positive with cable drives, for it to work, the cable must slip a little or you need a heavy "thread" cut in the drum, to allow it to lay in. The same bit of cable will always lie on the same thread.....the middle of the cable will need to be bolted to the drum at the middle point.
Strech will occur with usage....
I have used toothed belts (not for CNC I hasten to add) with computers to access cassettes in large silos, that worked well, the toothed belts can be a loop or a long piece (you can buy it in almost any length you wish.
I used T5, but for a large CNC I would say go to T10. Pulleys are available in many sizes, so you can add "gearing" if you wish quite easily. The parts are not really cheap, but have a very long life, low to no maintenance, as long as you oversize and do not stress (which is not needed, even quite loose there will be no backlash!)
Its a tried and trusted method of mechanically moving stuff and there are companies that sell the parts all over the world. Sometimes a few cheap parts come onto ebay...
Best of luck.
Der Fisherman.