Monday, February 05, 2007

 

Lego Brick Z Axis!

After many failed attempts at vertical rack & pinion for my Z axis, I finally gave up on it and re-approached it from a different angle; horizontally. I had such good luck with my horizontal rack & pinion on my XY axis, so I pulled out my original prototype and went to work on it. I built up the sides of the base, and unbuilt the moving axis, so that I could attach something to the top of the base, without interfering with the moving axis, and placed my 'box in a box' Z axis on top of that.

I'd tossed around the idea of using a pulley system before, but I was never happy with the inaccuracies of winding the 'carpet thread', nor was I willing to accept the possibility of slippage in a 'loop' system. Instead, I'm using the sliding axis to pull and 'push' the thread back and forth, in a linear motion, to ensure that I get steady, controlled movement with it.

Attaching the thread to the far end of the 'control axis', I ran it under the Z axis, around a U turn consisting of two pulleys, into the bottom of the outer box, through a technic piece (acting as a pulley), up to the top of the outer box and through a second technic pulley, and back down to the bottom of the inner box, where I secured the other end of it. Below is a rough drawing of what I've done:



The Control Axis is in green/yellow, and is constructed generally the same as the XY axis I've blogged about before (the main difference being that I've lowered the height of the moving part, and raised the height of the base, so that I can attach the Z axis on top of it without interfering with the movement). The first orange pulley, off to the right, is attached to the base of the control axis, and is actually a pair of pulleys (lego axle with a small, thin round piece that holds the thread nicely). I used two to help properly guide the thread out the end of the control axis, and up to a point where it lined up with the holes in a technic piece in the base of the red outer box. I originally didn't include the pulleys, but I felt there was too much strain on the motor and wear on the thread.

From that point, it runs along the bottom of the outer box, and through a second technic piece attached to its bottom. I may eventually replace that with another pulley, but that's going to take a lot more reconstruction than simply adding the technic piece to the bottom. The thread goes through that and heads up to the top of the inner box, running through a slit in the bottom of the inner box. At the top of the outer box, yet another technic piece acts as yet another pulley, to reverse the direection 180 degrees, and sends it back down to the bottom of the inner box, where it attaches to the inner box.

The outer blue box is also 'slotted' so that it straddles the technic piece at the top of the outer box, and moves freely up and down. It's actually a lot more open that that, for weight and material reasons, but the key is to allow the free movement around the upper pulley. The inner box fits snugly inside the outer box, so there is little play there. There is a slight bit, and if it causes any problem, I'll build up some of the sides with Teflon tape to fill it in and reduce friction; it's currently greased with petroleum jelly to ease movement.

As the control axis moves to the left, it pulls the thread, causing the Z axis to rise. As the control axis moves back to the right, it gives slack to the thread, which allows gravity to pull the Z axis back down. The axis itself is heavy enough to cause this movement, and adding the glass deposition surface and extruded material on top will only increase that ability. Due to weight restrictions, the fact that I refuse to spend $10+ on a stepper motor and stability, I'm placing this Z axis on top of my existing XY axis. Conveniently enough, when I position the axis parallel to the Y axis it sits on, and place the motor opposite the Y axis's motor, the wires for the Z axis motor hang just behind the little Lego controller that sits on the X axis, controlling the whole machine!

I'm planning on revising my Lego Digital Designer plans to separate the XY axis from the Z axis, and incorporate the slight change I had to make to tighten them up to prevent wobble, and then creating my current Z axis as best I can (given the fact that not all pieces are available in the Lego Digital Designer). I've also got some pencil sketches of what I'm going to put into AoI for the RepRap version of my model, and hope those won't be far behind. I'm hoping to make the majority of the XY & Z axis parts be interchangeable, but I don't want to sacrifice stability/integrity to do so.

And now for the video:



I've also been thinking about the limiter switches; I'm not a big fan of the whole optical stuff (especially with dust and cost/etc), so I think I've come up with a great 'mechanical' set of switches that I'll build into a future version of the RepRap version. I'm going to embed them into the axis itself! I'm going to run three 'pads' down the center of the base of the axis, with the center being the ground and the edges being the control for the two limiter switches. On each side of the moving part, I'm going to place a second 'pad' to bridge the ground to the appropriate control pad. As the axis moves to the edge, it'll reach the end of the base pads, thus breaking the connection. Of course this all relies on quality metal deposition, but we'll get there! (even if I have to paint it on by hand!)

Comments:
That is SO cool! I just LOVE your project!
 
Yes, I second that. I've got a huge pile of lego... and so little time. It could make for a quick repstrap...

BTW, have you looked here:

http://www.bricklink.com/storeList.asp

You can find countless rare lego parts, in small quantaties. :)
 
Hmm. Metal deposition, especially stuff painted on by hand, is going to wear out fast. How about something optical, or maybye Hall-effect sensors?
 
I'm leery of optical sensors... dust... smoke... stuff in the air... I see a point where those will lose their effectiveness...

I know painted on stuff will wear off, but if I can do some solid deposition on the bottom, runners if you will, that should last a lot longer...

Even though they've never heard of Lego bricks (at least the guy I work with hasn't), I'm envisioning my model running in a grass shack somewhere in India or something... I know it's going to get a little dusty running in my basement, I hate to think how much it could get caked in a place like that...
 
I can see your point. Optical can be annoying when the sensors are off by just a little, too. But I've tried to make painted-on metal tracks before, and never got the characteristics of metal, just somewhere between enamel paint and play-doh. It's just not meant to take mechanical stress. Hall-effect sensors on the other hand don't require contact and will go through normal crud like it's not even there.

On the third hand, if you know a way to get real metal plating on a chunk of plastic, I'll be very interested to know how! :)
 
I don't recall where I saw it, probably somewhere from my youth, but I'm thinking of pieces of thin copper (or other conductive metal), that are cut into strips, with the ends bent at 90 degree angles, so that they slip into slits in the plastic.

Essentially:

======plastic======
==|=============|==
***---copper----***

(== for plastic, -- for copper, *** so the page doesn't cut off the spacing i tried to put in!, just ignore the *** and pretend the ---'s line up!)

so it slides into place and is just held by friction/etc. This also makes the copper pieces easy to remove/replace/clean...

The above example is just flipped upside down (and turned sideways) to mount onto the lower surface that it rides along...
 
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