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  1. #11
    Oops.. Forgot to set my render settings... This is a bigger image...

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  2. I would be concerned about slippage,
    No they can't slip; theres two cables, one from left/rear to right/front the other from right/rear to left/front.

    Basically, if say, the left side moves forward, it pulls on the cable which pull the right side forward it the right side moves it pulls the left side.

    As long as the cable tension is maintained (e.g. by a spring) the gantry cannot twist or 'rack'.

  3. #13
    The cable design looks funky! But I am still unconvinced by using an active system, I would be concerned about slippage, stretching etc..
    Dont know if this will help but here goes........
    The cable system shown looks like it is one continuous cable. I recently had the joy of repairing a Freelander electric window How ever the way it worked looked to be adaptable for CNC. (now the hard bit)

    It used 3 cables. 1 connected the two slides together (via pullys) so they always stay the correct distance apart(i.e. anti-racking) each slide then had a cable going back to the motor. These were attached in opposing directions,(as one wound on the other wound off) In effect the sytem is only ever pulling and cant slip because each end is locked to the drive pully.

    The only dificulty is that the distance of travel is governed by the diameter of the pulley and the width (no, of turns). If you increase the width to much then it will cause an arc and introduce an error.

    If circumference = pi x D then a 100mm dia. pulley with only 5 turns on it the travel would be 1570mm (if Ive got that correct ) would probably need a belt reduction to the motor to get any reasonable power.

    For a bit of stainless cable and a few pulleys its got to be worth a try

  4. #14
    Bill sorry to repeat what you said, you posted whilst I was writing

  5. #15
    Hmm.. Still sounds like a lot of cable, surely stretching is a potential issue? Also additional belt reduction is another layer of complexity?

    Bearing in mind this is an ultra cheap mdf jobby (will make the next one on the mdf machine), I was looking for simplicity and ideally a passive system..

    Still, if you have actually used this system and got accurate results from it, I will investigate it as another option...

  6. I was looking for simplicity and ideally a passive system..
    Well, that's what I was talking about. I.e. the gantry is still controlled by your lead-screw, the two cables just stabilise the gantry.

  7. #17
    Hmm.. Still sounds like a lot of cable, surely stretching is a potential issue? Also additional belt reduction is another layer of complexity?
    I would have fought that the wire would be able to rip the machine apart before streching becomes an issue, I'll have to check the tensile strengh of stainless (got an equation somewhere for calculating stretch)

    Belt reduction isnt that much of an issue, some people use it on ballscrews anyway to reduce alignment problems with direct drive.

    Sorry didnt realise it was a first machine, Im at the same stage, but converting a pillar drill instead so it can make the parts for the proper oneChicken and egg srings to mind....

  8. #18
    Alrighty... Gotta make a start on this project or will end up theorizing forever... Gonna try it my way, if there is any measurable yaw whatsoever, I will add the cable and pulley principle to it as well...! Belt & braces!!! (plus safety pins and a staple gun!).. Also a fan of over-redundancy..!

    With that last comment in mind, and a quick reminder that it has to be cheap, I am thinking of a Nema 23 for the z-axis, something a smidge more torquey for the Y, and even more torque for the x-axis.. All this on the principle that the weight driven increases with each axis.. Does that sound right?

    If so, would love low cost suggestions for the x and y motors...
    Last edited by fasteddy; 10-06-2009 at 11:19 PM.

  9. #19
    Just read through the last few posts about pulleys and cable's and must admit after using those old fashioned drawing boards a while a go that it does seem like it could work.


    Instead of all the pissing about with pulleys why not make each of your your gantry sides a little wider so that you can space your linear bearings further apart to prevent racking, yes you would loose travel distance but it may be easier, this is what I have done with mine for exactly the same reason. I have also upgraded my X rails to THK 20mm and put two bearings on each side, slightly overkill for what I am doing but better safe than sorry.

    How accurate do you want your machine to be?

    How much do you not want to spend on your X and Y motors?? and what screw are you think of using??

    I know how you feel threorizing which is why I went out and bought a load of parts to build my machine this week before I thought about it anymore, whatever you build either wont be right or you wont be happy wih it but you have to start somewhere. If you do go with the pulley and cable idea then I would be interested to see how it works out as I think it has potential.

  10. #20
    Quote Originally Posted by Kip View Post
    Knock the bottom out of a cardboard box for an idea of how to visualise/prevent/minimise this phenomenon.
    I like your theory on this one, it was what I was thinking but maybe I have had a couple of pints too many!

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