Thursday, September 20, 2007


We Strive To Improve...

Here's an idea or 3 I've been kicking around to improve the Darwin design. Maybe they'll have to wait for Mendel but here goes:

1. Y Belt gear to outside

Move the Y belt gear furthest from the Y motor to the other side of the
Y axis bearing. This probably means mounting the Y belt clamps on the X
axis square jig, but avoids filing that long flat - the gear now just
slips on the end.

2. Z motor on top

Move the Z motor to the diametrically opposite corner and take it
outside the build area. We gain another 60mm of build height - or save
700mm of steel rod & studding. Forrest suggests we also use the shaft coming out the front, not the back.

3. Beaded belt for belt drive.

The right toothed belt is hard to source and expensive. We might be able to use the
sort of stuff used for window blinds and securing vicious attack biros in the bank's foyer. Some testing will be needed, as will new methods of splicing the stuff. Here's an industrial supplier:

Right, enough brainstorm. Back to testing the new extruder temperature code.

Vik :v)

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I'm surprised there isn't a seamless way to splice that stuff. A little stamped sheet steel split sphere that can be crimped closed shouldn't be too hard to manufacture...does anyone know of that?

If not, an appropriate hook and a dab of CAPA should work OK.
This comment has been removed by the author.
OK, again with better orthographics:

Toothed belts can be scavenged from old printers. Those inkjets are so short-lived, it should be easy to obtain three of the same type at your local jumble-sale-market. And you get one or two steppers and one dc-motor for free, along with some other useful stuff.
You just need do build your reprap to the right size (for xy) and do a little redesign for the z-drive.
Right, you need three or four belts for the z-drive I´m thinking of.
Wouldn't that beaded belt have almost as much slop as the gears you're avoiding?

How about "sourcing" the belt by building the unit around an already common belt? How cheap would the belt for, oh, say, a Ford Focus be? Granted, I don't even know if the thing has a timed belt, just that it's a currently manufactured model.

I believe some motorcycles use a belt drive. Anyway, the point is: How about using a toothed belt currently in use in a current manufacture automobile of two, four, or ten wheels.

I've been wondering if a bicycle chain wouldn't be a good fit, and that stuff is designed to be spliced.

I do like the idea of moving the other things around. More space, and easier to manufacture sounds great.
A beaded belt is much like an ordinary belt in that it is very flexible, and yet has little stretch. Which ones are best will have to be determined.

If you can find a commonly available automotive belt of suitable dimensions, let us know!

Vik :v)
Forget about automotive belts, you know the size of the motors used there ... They are very big and stiff.

If you can get something like a bicycle chain, but in ~ 1/3 size, along with suitable cog wheels, that would be an option.

Remember, the cog wheels driving the belts are directly connected to the stepper. They mustn´t be considerably larger than the ones presently used, or it will cut down positioning accuracy.
Is anyone working on printing the belt? The fab@home folks are doing a lot with silicon rubber, so something with an embedded fiber strip (or the aforementioned bead chain) to control stretching, printed in segments with built in lap joints?

is a belt used for a particular model helicopter to transfer the drive to the tail rotor. I can measure the exact length, but I guess it's about 1200mm loop length.
//I'm surprised there isn't a seamless way to splice that stuff.//

This site mentions something about "endless chains" which have an open bead that can be closed by the customer.

They don't go into a lot of detail on how that's done. I'm guessing the open bead is similar to this:
Smash off one bead, drill a hole in the other, glue string in hole.

Vik :v)
Quoting from "If endless chain is to be used as a drive component, it needs to be qualified."
Another possiblity is simply to use four $15 stepping motors instead of our $60 stepping motor. Then you attach them directly to the screws.
Here's an idea.
Use three fixed motorcycle/bicycle chains. Two fixed on the "front" or "left", on the bottom, but on the "back" or "right" on the top. The third fixed to the opposite sides. All three chains crossing under the platform, passing by two cogs. The cogs would be fastened to two axles which would counter-revolve, but one would be driven by the three chains, while the other would be driven by a motor.

Chain one, fastened on the bottom left, crossing over the left side axle, (driven by the motor,) under the right side axle, and on up to the top right. Chain three doing likewise, but at the front of the cage, rather than the back. Chain two would be centered on the bottom right, crossing over the right side axle, under the left side axle, and finally being fastened to the top, middle, right.

If the chains are kept taught, possibly by putting the non-driven axle on bearings that are spring driven away from the centerline of the table, this should result in a rigid z-platform.

Just an idea. I think it'll work, but I haven't played around with it physically.

Meccano has sprockets and chain for the sprockets that just about fits that description. I'd guess it's pretty much a discontinued component these days, but it's still easily available from specialist dealers. For example: Search
this dealer's site for "sprocket" and "roller chain"
Another possiblity is christmas bead garland. The plastic beads are moulded onto a string. 35' for $7. Very cheap, and you could use the Vik Method for creating gears. The ones I saw had balls 8mm in diameter, so perhaps a "ball gear" could be extruded?
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