Thursday, July 15, 2010
changing the game, and using kapton tape as a slide
I've been working on redesigning the design for an entirely laser-cuttable x/y stage, and have some neat progress.
This design also uses a rack and pinion system, with a bunch of changes:
1) the linear gear is mounted horizontally, rather than vertically -- the weight of the entire axis is no longer suppored by the gear itself, but rather by a structural outcrop.
2) the axis is driven at four points (two from each side), rather than only a single point. this removes a lot of the shearing/binding issues. this is accomplished using a captive system of gears sandwiched inside the axis itself to transfer power from the stepper to four points along the edges of the axis.
3) a long piece of kapton tape is used as something like a linear slide, significantly reducing the friction between the axis and the linear gears that it rests upon. the axis slides *really* well with the kapton tape.
The result is a working linear axis that can be entirely constructed with a laser cutter, using only a handfull of screws, a $5 stepper, and a few pieces of kapton tape as vitamins. That's fantastic! :)
So far it works pretty okay! There are a couple refinements to make:
4) Right now the gear on the stepper is just press fit -- I really need to key the gears and motor shaft, as the press fit wears quickly with the hardboard.
5) The gearbox is mostly just eyeballed, and there's probably an ideal solution when solving simultaneously for (a) a rack, with (b) two gears of radius R1, being driven by (c) another larger gear of radius R2; all for a given tooth size. Myself and a bunch of other physicist/math nerds at school have each had our hands at the problem, but haven't come up with a fantastic method of solving it yet (start with C, then find all the angles that B can attach to C, then determine if the distance between the centres of the two gears B modulo the gear pitch is an integer for each angle?).
6) This design seems great for the cross-axis. I have a similar design for the supporting axis with a MUCH larger captive gear system in it -- I'm not sure if the friction would be too much in that situation, or if it'd work out okay. (I stick a tiny washer under the gear, to reduce the friction a great deal -- maybe a nylon washer, or some kapton tape might also work out well?).
The alternative might be using a combination of two of these tiny "captive gear" axes, one at each end, for a supporting axis. This adds an extra stepper to the design (which is a negitive, although these ones are only $5!), but it also keeps the whole system fairly low, which would be kind of nice!
If anyone is curious to tinker, I've put the design files up on Thingiverse ( http://www.thingiverse.com/thing:3554 ). Happy tinkering, and thanks for reading! :)
[part 1] [part 2] [part 3] [part 4] [part 5] [part 6] [part 6 video] [part 7] [cogsci.mcmaster.ca/~peter]
Industrial CNC routers often use rack and pinion when they are large, because the cost and inertia of a lead screw is too high. However, backlash is then an issue.
One of the 'accepted' industrial solution is to use a helix angle on the gears, which in your case isnt feasible to keep it cuttable in 2.5 d.
But, you have the advantage that you have low loads, so perhaps you could use a coil spring in the system to automatically remove backlash?
(edited for typo)
backlash: i don't see the backlash as a huge issue right now -- i had planned on rastering the layers, which means the travel on a given axis is only from one end to the other, then back again -- so you can just use endstops. it would definitely be worth characterizing the backlash once a little better revision of the gearbox is made, to see if it's something constant (that software could easily work with), or potentially etching an encoder strip into one of the linear gears to get some inexpensive position information!
Going back to the idea of pre-heating the powder to reduce the amount energy needed by the laser.
While thinking about this I reckoned back to my days as a photocopier tech. The final stage in a photocopier and also a laser printer is to melt the toner into the paper. This is done by a roller with a quartz halogen light inside to act as a heater.
I was thinking that instead of heating the entire powder container, if a recoverd roller from a laser printer was used as the leveling drum it would rapidly heat the small amount of powder as it was transfered to the main chamber, It would take some fine tuning but I think it should help.
I don't have a work shop to play in at the moment, so maybe someone can try the idea out for me.
I am very interested in this project, but unfortunately my specialization is in finance, not mechanical engineering. I am really hoping that this affordable SLS technology will not turn out to be nylon powder in the wind.
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