A Different Kind of Linear Motion

[drummer 1]

Has anyone considered the following as an alternative motion design for x, y, z travel?
Basically a pulley and cable set up.Just taking the x axis as an example, you would attach your stepper motor at the front right hand side of the gantry, not on the front but around the corner so it sits perpendicular to the Y gantry line of travel.then attach a rod to it so that the rod travels across the entire width of the x axis. attach a pulley to either end of the rod and also have another rod and 2 pulleys at the other side of the axis. attach cables between opposing pulleys, with the cabling also anchored to the y gantry. This would be a similar arangment to what one finds in the larger photocopy machines. It seems to me that with this process one would eliminate most of the friction and energy loss and heat issues that come with acme, or threaded rods; which means more motor power spent on speed rather than overcoming threaded parts. and this would also eliminate backlash issues as the cabling is anchored and of fixed lenghth.Some simple math and Mach 2 can tune the motor to calibrate your distances. If some of you have tried this concept I would like to hear feedback as I have been considering trying it ever since I dismantled a photocopier and was impressed with how well it moved the upper tray region.Also I havent found many people attempting it, and was wondering why?

[BillTodd]

As discussed here?
http://www.cnczone.com/forums/showthread.php?t=68967

[drummer 1]

Yes , that thread does show that some have tried this idea. however it does taper off with some for and some against the idea of using cable to drive a gantry. from reading, i suppose the problem lies mostly in the capstan type and the accurracy.But with any error induced, the zero home would correct. I already have my cnc gantrys made using allthread and am now at the stage of working on limit switches etc.. I could bump up my screws to acme but before I spend time and money in any changes I want to look at all options for better (less friction, more efficiency) methods of motion. And as I was saying, a lot of photocopiers use this cabling system, and something is gnawing at me in the back of my cobweb brain saying consider this some more. Anyone out there have more experience with this type of motion to share.?

[millman52]

Originally Posted by drummer 1 Yes , that thread does show that some have tried this idea. however it does taper off with some for and some against the idea of using cable to drive a gantry. from reading, i suppose the problem lies mostly in the capstan type and the accurracy.But with any error induced, the zero home would correct. I already have my cnc gantrys made using allthread and am now at the stage of working on limit switches etc.. I could bump up my screws to acme but before I spend time and money in any changes I want to look at all options for better (less friction, more efficiency) methods of motion. And as I was saying, a lot of photocopiers use this cabling system, and something is gnawing at me in the back of my cobweb brain saying consider this some more. Anyone out there have more experience with this type of motion to share.?

I would think a capstan of fairly large diameter & more than a couple wraps of cable would work fairly well.

I thought of trying it on my table but didn't really have the time for a re-design if it didn't work out. Capstan & cable is most definately very inexpensive compaired to othe methods.

Good luck with your idea.

[drummer 1]

Yes, I agree. I am on the hunt now for a throw away photocopier, which besides some pretty neat motors and gears will produce a cable system gantry for me to experiment with. will keep a posting of how it turns out
Cheers.

[cadcamdave]

The motion method you described is in use on some large metal press break machines I have seen, but only in circumstances that call for limit-to-limit moves. Cables stretch and are not reliable for high precision moves at high speed, also a recoil effect can come into play on rapid reverse applications. The method does however provide reasonable power to be applied at speeds faster than lead srews, which makes it a good candidate for roll fed shearing.