Binding Delta Arms with elastic bands

[teoman]

I found these elastic bands at my wife's hair dresser.

If you put a knot right in the middle, it becomes just the right length to give a very gentle compression on the arms.

And indeed as mentioned by others it did stiffen the end effector.


However, they make a slight mechanical difference (which could also randomly happen during normal operation because of inertia). Now my infill does not exactly come to contact the shell.

What parameters of the mechanics have I changed? The only thing i can think of is the arms are now longer or shorter.

The part on the bottom (or left) is printed with the elastic bands in place. And there are obvious gaps in the shell. (Damn you black ABS! nothing is visible in the photo!).

[RegB]

I doubt that it changes the dimensions or geometry of anything.
PERHAPS in an EXTREME case it might be possible to bind an arm pair very tight at one and and very lose
at the other end - so VERY TIGHT that they bend visibly, then they would no longer be parallel.
From reading your other posts my guess is you are smart enough to not do that (-:

If not tight enough there could be a problem with the U-joint sliding out a bit on sudden starts/stops,
just briefly, but I suppose that could happen with infill starts/stops - which are done faster.

I would check that they are all same tension and the same distance from the U-joints.
Mine are about 1/2 way up the arm fork, maybe an inch or so from the U-joint, not with mine so I can't measure it now.

Another possibility is that you just overlooked a poorly seated U-joint pin ?

[teoman]

I have 6 of the elastic bands installed on each end of the arms.

The install location is the first structural "triangle" after the 'U' part of the half arm. It is not really tight so it should not create any difference, that's why i am slightly puzzled.

[626Pilot]

Your existing calibration was done when the printer had more slop, so maybe redoing it will fix the problem. If anything, it may be that the bands are making your arms bow inward a little, effectively "shortening" them. Put them as close to the very ends of the arms as possible, so they don't have any leverage to bow the arms. I am using Trick Laser's bands (rubber tubing with laser cut barbs to hold the ends together) and I have them as close to the joints as I can get them. They work wonders!

If that doesn't work, ditch the elastic stuff and use rubber bands instead. They should have "some" tension. Not too tight, but not too loose either. I lost one of my hose/barb assemblies and had to replace it with a rubber band. I didn't have a "perfect" one, so I just got one that was a bit too long and tied it together so that it had the amount of tension I wanted. It's cheap as hell and anyone can do it.

[jarek319]

I tried doing this with some cable ties, but ran into the problem above where I tightened them too much and ended up effectively shortening my arms, throwing the XY scaling off and making my parts look bad.

I'm switching to carbon arms as soon as I get a set of carriages printed

[geneb]

Give these a shot: http://repables.com/r/467#

You'll need to find some 1" or 1-1/4" rubber bands.

g.

[markwhelan]

Or you can print these to put tension on the arms.

http://www.thingiverse.com/thing:810733

[626Pilot]

You don't even need a printable clip to fasten the rubberbands. If they're long enough, you can just tie them off. How much you don't tie off will control the tension to the arms.