Reply with QuoteInteresting 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.
cable drive
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
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A poet knows no boundary yet he is bound to the boundaries of ones own mind !! ........
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.
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
DZASTR
It's in use on Roland equipment too. I would consider it for a small router or engraver.
Keith
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 acidAt 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.
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
A poet knows no boundary yet he is bound to the boundaries of ones own mind !! ........
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.
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
It's like doing jigsaw puzzles in the dark.
Enjoy today's problems, for tomorrow's may be worse.
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.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
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!)
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
It's like doing jigsaw puzzles in the dark.
Enjoy today's problems, for tomorrow's may be worse.
Like this?
It looks like it should work
(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
Last edited by BillTodd; 12-21-2008 at 07:46 AM.
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.
Coal mines here in the UK usually use traction drive (the rope is not fixed to the drum - the other end is attached to another lift).I cannot see anything positive with cable drives,
Obviously, there's a turns indicator on the main drive drum to indicate the depth of the lift.
One I've seen, also has a chalk mark on the drum. It's there ,so the lift operator can position the lift to within 'a quarter of an inch' of the doors.
So, after several thousand feet, up and down several times a day the system is repeatable to within 'a quarter of an inch'.
Sometimes, cable drives do work
You picked the perfect explanation as to why it does not work for CNC. Thank you.
As if it was as accurate as you seem to wish for, there would have been no need for the chalk mark!! There could have been a fixed indicator, bolted to the drum or even a paint mark!!
The reason chalk was used was if any slip occurred, or the cable got longer in use, one could rub out the chalk mark and make another....
This sort of accuracy is simply not good enough for any type of CNC, we cannot have the need to check the calibration before we can use it!!! Wasting high quality wood or other material when one finds that the wire has stretched by 0.5%.......
Thanks for your input though, it always promotes great discussion on a blog.......
The main reason I am personally against wire of any form is that many, many years ago, in the early days of computer controlled Robots for industry, wire was used, but as far as I am aware, its qualities were simply not good enough in many areas. I cannot say if any are still around using wire as I am not in that industry anymore.
Some early plotters had wire drives, again I have not seen any of these around for a longtime either.....
The later industry Robots I worked on had either direct drives or toothed belts.
As an example of strength & accuracy, toothed belts have been used in cars for many many years to drive the valve drive chain and the distributor. Here an exceptional amount of power, strength and accuracy is needed, an error of even 1° would be considered a Gross error......and for often 100,000 miles, under varying conditions of cold heat and moisture.......I have replaced these belts on my own cars from time to time and they still look like new on the outside after all this time!!!!!
Everyone should be allowed to go his own way though, a convinced wire person will still go for wire, mqaybe bringing a "wire renaisence" back! I personally would not uae it, as if one breaks and you are near and without your eye protection.......I have never seen such a problem with toothed belts......
Have a great day in spite of my comments!
Funny enough, it was dertsap's original posting containing his observation of the successful use of wire in his own system that encourage me to continue with my own.
Any method can be cast aside in favour of others if its disadvantages become the criteria, rather than the advantages.
Accepting, then overcoming the problems seem to me to be the way forward, and understanding what the underlying reasons are for those problems might suggest the solution.
Bill, your mention of "twisting" led me to wonder what the cable actually does while it's in contact with the capstan surface. During any slip, I assume this is a sideways movement, and would this in fact be of a magnitude that would affect the sort of set up I'm building (ie cnc router to 0.25mm) ?
Or does it roll as it goes round ? This would tend to twist the cable up, but two capstans counter wound would solve that.
With a micro thread cut into the surface of the capstan matching the "lay" of the cable, you might even find you had solved the walking problem. A 0.5mm pitch on the capstan with an 8 start thread with a 75mm pitch is the sort of thing I have in mind. In effect, the capstan/cable would be like a worm drive, with the cable/pinion being a multi start thread back driven by the capstan/worm gear. Not sure if I've got the nomenclature right, but it works in my head
The downside is there's no way I could make it
It's like doing jigsaw puzzles in the dark.
Enjoy today's problems, for tomorrow's may be worse.
cable stretch is a given , belt stretch is also a given , for me adjustments every so often isnt too big of a deal , as far as accuracy ,any cutting will be on wood , my machines have been built with a resolution of .001/step and they work great , this is going to have the same resolution ,
i am simply using shafts that will have a v-groove cut into them for the cable to sit in , slip will not be an issue as long as the tension gets adjusted once in a blue moon , this is not a heavy build so weight and acceleration or cutting force will not cause slip
A poet knows no boundary yet he is bound to the boundaries of ones own mind !! ........
Surely, one capstan and an idler both perpendicular and parallel would be a lot easier?Or does it roll as it goes round ? This would tend to twist the cable up, but two capstans counter wound would solve that.
I am not sure about the cable accuracy aspect, but I am quite concerned about a cable drive for a kid's use. A snapped cable is an amazingly dangerous hazard.
This is a good area to dramatically oversize for the project requirements and seriously consider designing it to manage a snapped cable.
As pointed out in an earlier post, I'm not really an advocate for cable drives; I tried one, wasn't happy with it.
Actually, I think the reason for the chalk mark is because the turns indicator wasn't good enough for the operators (like many proud men they are/were perfectionists) and painting on a machine could have got them the sack! (fired)there would have been no need for the chalk mark!! There could have been a fixed indicator, bolted to the drum or even a paint mark!!
The reason chalk was used was if any slip occurred, or the cable got longer in use, one could rub out the chalk mark and make another....
If you have accurate home switches this is not a problem (the chalk mark gets re-drawn each timeThis sort of accuracy is simply not good enough for any type of CNC, we cannot have the need to check the calibration before we can use it!!!
There are problem with cable drives, but they are not insurmountable:Wasting high quality wood or other material when one finds that the wire has stretched by 0.5%.......
Stretch - Long term stretch should not be a problem (no worse than belts) Short term stretch (thermal expansion etc.) will only be a problem for long axes.
Slip - Long term slippage is not a problem, if using home switches. As long as short term slip can be kept within acceptable limit between 'homes' or zeroing the tool after a change, then slippage is not a problem.
Accuracy - Absolute accuracy is not good especially with a grooved roller (the effective capstan diameter is difficult to determine). Regular calibration would not be difficult (i.e. the distance between limit switches doesn't change much)
Resonance - I found this a major problem. However, if the cable is 'heavy' and not too tight (if they are too tight they also becomes a safety issue) , the major resonances will be at a lower frequency than the drive, so should be controllable.
I would like to see someone try pinching Mike Everman's idea; i.e. use a moving capstan and let the cable rest on a flat (rubber?) surface. This would kill resonances and allow a tighter cable, thus fewer wraps around the capstan.
No problemThanks for your input though, it always promotes great discussion on a blog...
Have a great day in spite of my comments!
Last edited by BillTodd; 12-21-2008 at 05:47 PM.
if i was to make the cables so tight that they would snap then there would be no way of moving the gantry with stepper motors , the cables i will be using will be plenty thick but not too thick , there is more danger for my son to hurt himself popping a string on his guitar than there will be with this setup
A poet knows no boundary yet he is bound to the boundaries of ones own mind !! ........
