How would you draw this?

[Mjolinor]

And
What would you use to draw it?

I was thinking about what to use for drive belts and have tried string, bowden cable, toothed belts and screws but I thought that this:

http://www.ebay.co.uk/itm/2309588040...%3AMEBIDX%3AIT

would do the job really well and extremely cheaply.

I bought some and it is easily strong enough to shovel a head and carriage about so I now need to print some pulleys for it which is where my problems start.

I have to have a circumference that is an integer number of pitches, the balls are around 2.3 mm, the rod is 1mm so the pitch is 3.3mm or so. On the face of it it is a trivial problem.

I thought of setting up a great big spread sheet for all possible diameters but the arithmetic is not too easy.

Screenshot from 2014-11-21 18:28:27.jpg

I think there should be an easy way in Openscad but I am not familiar enough to use it. That picture is done in Freecad but it is less than ideal and involves loads of working out for each repeat.

[CaptainObvious]

No idea about how to do that in Openscad, and a bit dubious over the idea of using that for a drive, in my experience the balls have a little play in those things. On the other hand it would work on a steampunk themed printer.

[Mjolinor]

It's a chain, all chains have play until they are under tension. It takes a lot to snap it, I think more than the narrow toothed belts.

[rafbe]

look at http://www.123dapp.com/design its easy to use and free

[old man emu]

Just a cautionary word before you spend too much time designing a pulley.

Have you examined the accuracy of movement of the extruder you would expect by using this ball link chain and compared that the the accuracy against the expected accuracy of a belt or threaded screw system?

I'm not poo-pooing the experience gained through the design and drawing process, but you might be designing something that does not result in sufficiently accurate movement for good prints.

Old Man Emu

[Mjolinor]

Well I can't test it until I've drawn and printed some pulleys.

I can't see any reason why it should be any less accurate than any other system unless I am missing something. If you have anything specific that may make it suffer then please elucidate.

I don't really have a use for it anyway, just messing.

[old man emu]

I don't really have a use for it anyway, just messing.

Ah well! In that case, go for it. It will be a good exercise.

I agree that on first glance, it should be accurate enough, but I'd do the calculations.

I have a similar device which rolls a blind up and down at my front door. I was going to pull it down and photograph it, but the blind is inside the house and that's not my territory. I'll have to get approval from the wife to play with it.

OME

[Mjolinor]

I'll have to get approval from the wife to play with it.

OME

You have a wife that gives approval, where did you find that particular make/model? Mine is strictly the disapproving type, I thought wives all were.

[old man emu]

Funny about that. She was born in Bedford, raised near Burnham-on-Crouch, so she started off like yours. Must've of been the mellowing under the Southern Sun that changed things.

OME

[3dkarma]

I have built a polargraph, which uses this very mechanism to move a pen holder around on a vertical drawing surface (check it out - it's extremely cool) http://www.polargraph.co.uk/.

Handily, there is an openscad design for a parametric ball pulley, specificallly designed for use with stepper motors, on thingiverse here: http://www.thingiverse.com/thing:12403

My experience using these is that accuracy suffers as the engineering tolerances on the ball chain are not that great. YMMV, but it's worth giving it a go.