Moving Knot

[MunchyMonster]

I read through the "Make your gantry rock solid!" thread where a "moving knot" design like those cable systems in drafting tables and pocket doors is described. Judging from the responses in that thread and having seen various "high end" fences and such that use a similar design, I'm willing to believe it will work.

One could obviously use the design to simply reduce the effect of racking, but I'm wonderng has anyone out there used such a design as the primary linear motion system. If you had a ~4" diameter "winch" you could get about 1' of cable for each wrap around the winch. If you use your stepper to directly drive the winch, and the cables are set up in a moving knot configuration, it would seem like you could get all the benefits of a double leadscrew set up without having to deal with screw whip.

Of course there may be interesting backlash issues and you may need to gear down the stepper to get a decent resolution, and half a dozen other things I haven't thought of, but it should work shouldn't it?

I'm not asking because I think it is going to be cheap, I'm curious why I haven't seen it discussed before. Or has it been and my search-fu is just weak?

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[CarbonKevin]

I read through the "Make your gantry rock solid!" thread where a "moving knot" design like those cable systems in drafting tables and pocket doors is described. Judging from the responses in that thread and having seen various "high end" fences and such that use a similar design, I'm willing to believe it will work.

One could obviously use the design to simply reduce the effect of racking, but I'm wonderng has anyone out there used such a design as the primary linear motion system. If you had a ~4" diameter "winch" you could get about 1' of cable for each wrap around the winch. If you use your stepper to directly drive the winch, and the cables are set up in a moving knot configuration, it would seem like you could get all the benefits of a double leadscrew set up without having to deal with screw whip.

I'm not asking because I think it is going to be cheap, I'm curious why I haven't seen it discussed before. Or has it been and my search-fu is just weak?

I see a few shortcomings here...

1) If the "knot" is where the drive movement is coming from - where the stepper attaches to the system, how would you overcome the need for the stepper to move with the "knot"?

2) There would quite likely be efficiency issues driving such a beast from the pulley.

3) I could see this being very hard on pulley bearings, leading to all sorts of precision problems.

BUT,

These are not fatal to the concept - there might be something to it!

[MunchyMonster]

1) If the "knot" is where the drive movement is coming from - where the stepper attaches to the system, how would you overcome the need for the stepper to move with the "knot"?

I might not have this visualized correctly But I assumed that the stepper would be mounted to the table. The "drum" of the winch would be mounted to the table as well.

Since the Moving Knot system uses the movement of one side or the other of the gantry to apply pressure to the other side (to avoid wracking), I'm simply suggesting that one drive the cable directly instead of driving the gantry and using the cable to stop the racking.

I'll have to draw up a sketch of what I'm proposing, maybe that will make it more clear and allow you to point out the problems in such a way that I'm not misunderstanding what you are saying.

[tpworks]

using 2 open ended timing belts fixed to the gantry ends and adjustments will be made there. Same principle as the Paralok fence system that I posted in the other afore mentioned thread.
Your cable idea could be adapted the same route.
I believe some of the older cnc engraver machines used a cable system utilizing the drum method, sort of like an overhead door with the torsion type springs.