How would you draw this?

[old man emu]

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.

I see your point about the accuracy of the ball chain. The chain that Mjolinor showed seems to be at the back end of the quality queue. The ball chain on my blind roller looks a lot better that that. Once again it's a case of paying peanuts.

Since you are both in the Old Dart, you should be able to cooperate easily on this project.

What got me with the Polargraph was the use of Polar geometry rather than Cartesian. If polar geometry allows you to work in 2D, could it be adapted to 3D? Would the adoption of polar geometry required the development of new CAD and CAM software to work with polar geometry?

Old Man Emu

[Roxy]

Note: I didn't see what 3dkarma posted until after I started making this. The files he posted are really nice. I decided to leave this post because this OpenScad code is much simpler and easier to understand. But if you are actually going to do something like this, the files 3dkarma pointed to are much better!!!!

If you plug this into OpenScad:

pully();

module pully() {
    difference() {
        cylinder(h=15, r=30, $fn=25);                    // main pully block

        translate([0,0,-.1]) cylinder(h=25, r=4, $fn=10);        // drill center hold

// add a path for the wire connecting the balls on the chain
        
        translate([0,0,(15/2)-(2/2)]) difference() {
            cylinder(h=2, r=32, $fn=20);    
            cylinder(h=2, r=29, $fn=20);    
        }

// add holes for ball chain  ---  assume 25 balls fit around the pully
// also assume the balls are 2mm in radious

        for(i=[0:360/25:360-.1]) {                
            rotate([0,0,i]) translate([30,0, 15/2]) sphere( r=2, $fn=15);
        }

    }
}

You will get something that looks like this:


Ball_Chain_Pully.jpg

[old man emu]

. I decided to leave this post because this OpenScad code is much simpler and easier to understand.

That's what I hate about OpenScad. If you read the code after reading the comments and looking at the image, OpenScad looks like a very clear and concise way to go from the mind's eye to the printer plate. However it is finding out about the syntax and terms used that slows me down.

Am I correct in saying that if I watched a few tutorial videos , it would not be long before I uninstalled my Rhino?

OME

[Mjolinor]

Excellent.

That is just perfect.

[old man emu]

Roxy, you Jezebel! You are enticing me to the Dark Side

I watched some OpenScad tutorials tonight and I think I'm getting the idea of it. It is very logical, and once you understand creating modules I think it would be a quicker way to go from a sketch on the back of a beer coaster to an stl file for slicing.

Maybe Old Man Emu's Rhino might become extinct.

OME

[Mjolinor]

It is a dog to learn is Openscad. I have had a few goes but I think it needs a few hours concentration which I never seem to get without interruption.

I have a bit more incentive now and gese that pulley making looks so simple once you have the solution, like most things in life.

[curious aardvark]

openscad is seriously easy to learn.

Admittedly it's hard to look at a really elegant script and work out what the hell is going on. But I was never in the school of elegant programming anyway.

I could easily produce a fully parametric script to make those pulleys. But it wouldn't look anything like roxy's script. That's way to elegant for me :-)

People make the mistake of thinking you have to work out all the maths first and then write the script.
Nope.
You can bodge it all together just like a drawing cad package.
Just hit the f5 button each time you do something and it'll draw what you typed.
I often move things into place by changing a number, redrawing, looking at it all ways up and then repeating.
Just like dragging things around in a gui package.

I find if I do try and work things out, I'm invariably wrong and the suck-it-and-see method is often easier and quicker :-)

One of the great things about it, is how easy it is to move bits of the script around, or copy them. With just the basic rotate, translate, hull, difference and union commands you can make pretty much anything.

Adding loops is great for repetition, but copy and paste works as well for longer less elegant scripts.

I honestly do not understand why people think it's difficult to use. The syntax is simply remembering to add the right brackets at the right place. Why some of them are there - who knows, and it doesn't matter.

And you're always learning stuff. For example, I had no idea you could vary the facet numbers within a script. Thought that was a global function only. That will come in useful :-)

@Roxy why did you use 25 facets for the pulley ? and only 20 for the spheres (well at 20 facets they're not actually spheres). I always go for the max 100 so round things are actually round.

[Duck]

One thing you will need to be mindful of is chordal action - just like chains they are susceptible to this with increasing magnitude as the pulley diameter decreases. If you have a 30 tooth "sprocket" it really a 30-sided polygon where each segment is the pitch, not a wheel with a circumference of 30 * pitch.

[Mjolinor]

That is also true for toothed belts to a certain extent.

[Roxy]

@Roxy why did you use 25 facets for the pulley ? and only 20 for the spheres (well at 20 facets they're not actually spheres). I always go for the max 100 so round things are actually round.

Mostly this was just thrown together... At the size of the pully I specified you are going to see edges unless you bump the facet number way up. Also, I was kind of thinking that each ball is connected to the next one via a straight line. So having the pully have flat edges all the way around sort of made sense to me. Like I said, it was kind of just thrown together.

Both the pully and spheres could have a big number like $fn=100 but you really don't want to do that while developing the logic. I have a really fast machine with lots of memory and that short little script would bog down with those numbers in it.

Remember, OpenScad is working with lots and lots of surfaces. If you specify $fn=100 that is way more work for it than to do things at $fn=20. Maybe its worth it when you have the logic right and are ready to render and print.