Thursday, November 30, 2006


LegoStrap - RepStrapping with Legos!

So... I've given up on creating anything myself, by hand, with any sort of accuracy. Instead, I'm relying on the accuracy and low defect count of the largest tire manufacturer in the world: Lego. (And while they've got a nice large selection of tires, they seem to like to sell them in pairs, instead of fours, which makes producing level carrages a little hard...)

I've already described what has become the Z axis. I took the platform off the top of it (since it's now going to be holding the print head instead of the work surface), and I need to address the wheels not sitting flat on the board, but I haven't done any work on that axis yet.

This is the beginings of the X axis; a 4 wheel drive, no steering, carrage. I'm hopeful that the traction between the wheels and the carpet will be enough to maintain enough accuracy over the duration of a single print job. If not, I may move to the threaded rod driven technique I build into the Z axis. (I haven't secured everything here; I'm waiting to figure out how I'm attaching the Y axis to this before I worry about how much more 'support' I need to put into it. It'll also help to know the correct spacing between either side.)

You can see here where I started to build up the support for the motor; the two towers of black bricks in the center back. The motor will drive the center gear, which will transfer motion to the mid gears, and onto the wheel axels themselves. Everything is symetrical and matches end-to-end, so I'm hopefuly to get solid, flawless 4-wheel drive. I'll be mounting the Y axis above this, but I'm waiting to mount the motor before I work on that. (I've had a bit of trouble glueing legos to metal gear on my steppers, right now I'm trying something called E6000 and I'm giving it the full 72 hour cure time, and I'm only 12 hours into it!) I'm only glueing the grey 1x1 rounded socketed piece to the motor, so I can change not only the gear on it, but also the length of the shaft. I'm currently trying to use a gear the same size as the middle three in the carrage itself.

Here we have the Y axis. You'll notice the familiar 'fighter/spaceship' style flat pieces from the original Lego axis; they're the only decently sized flat pieces I've got! You can also see here the rack for the rack & pinion; I'm going to be using the smallest 1x1 gear I've got for the pinion; I figure I'll be getting enough speed just by moving to rack & pinion, I want to maintain some accuracy so I'll go with the slowest, most accurate rack & pinion I can make. The rack is actually attached to the top half of the axis, so I'll be mounting the motor onto the flat 'wing' in the lower left of the picture.

Here you can see the two halves of the axis seperated. You can see the smooth-flat pieces in the center of the bottom that I used to allow the smooth movement; I didn't have enough for full coverage, so I spaced out pieces along the outer edges (it's a 4x14 area) to distribure the weight as best I could. The 'floating flat 2x16' in the upper left of the picture is used to hold the top half of the axis down; riding on another line of 1x smooth-flat pieces.

Here you can see the top half of the axis taken apart. To the right, the flat platform that will eventually hold the work surface. On the left sits the carrage that handles the movement. You can see the rack on one side (for the rack & pinion), and the smooth-flat pieces on the other side (for the 'floating flat 2x16' to ride on). (The 2x2 smooth-flat pieces are not 'critical', I simply ran out of 1x2's and had to improvise...)

One trick here, I've got four 1x pieces side by side, all connected; but no 2x pieces in any of it (excluding the non-important 2x2 smooth-flat pieces). The secret is in the foundation; four technic 1x pieces, side by side, attached with the small 1x2 round pieces... (not the straight up + axis, but the round pieces that 'plug' into the technic hole). I've got two of the round connectors connecting each pair; 5th hole in from either end on the outer pairs, 6th hole in from either end on the inner pair. I found those to be a lot more secure & true than relying on 2x's or flat pieces, and being internal, they didn't effect the 'footprint' of the part they were strengthening.

The bottom consists of the 'frame' (two 1x16 pieces on the outside), the smooth-flat pieces for the carrage (I would have liked to have more in there, but I don't think you want to cover the entire surface; it's the old friction vs stability game...) and the 'floating flat 2x16'.

Here you can see the carrage inside the bottom of the axis. I actually got this 'action shot' backwards; if the carrage ever goes that far in that direction, it'll have driven off the end of the pinion gear! I'm placing the motor off to one side, rather than the center, for weight/stability reasions, though I'm not sure yet how far off to the side it will be.

I'll either have 1 motor ready to play with this weekend, or it'll be back to the drawing board for attaching the axis to the motor! If the E6000 does work, I'll set a second motor up to cure, and start attaching the first to one of the axis (haven't decided which yet, and I've got to restrain myself from getting sidetracked and making some sort of 4 wheel drive mobile launch platform out of it...)

I love this!

I do some agressive cleaning of metal parts before using strong glues on them. A bit of sanding and acetone is good, but any solvent that doesn't leave a residu will work well. I tend to favour epoxies. They cure faster and a bit of heat both accelaretes cure times, and increases strength.
I wonder is there something that would function as rack for such a Lego set up.. something that's out there that's easier (& cheaper) to get than lots of Lego 'racks'...

if all the axes were racked, then the precision has a baseline; as it is, the tyre/tire-carpet interface looks a source of future frustration..

Have you thought about a substitute for Lego racks? I won't research it if you (or someone else) has already..
If you can live with 12 inch racks this lot has good prices on nylon racks and gears to go with them.
***if all the axes were racked, then the precision has a baseline; as it is, the tyre/tire-carpet interface looks a source of future frustration..***

I don't know how much I'd trust a Z axis made with rack & pinion at this stage; maybe in a full reprap environment, but I don't even feel comfortable enough with legos to try it with them. I'm definatley sticking with threaded on the Z axis...

I am looking into trying to order extra 1x4 rack pieces from lego... (you can order just about everything else BUT the rack pieces off the web site) and see if I can't make a proper axis to replace the 4x4...

At this point, I don't want to move beyond legos (but at the same time, I don't want to be buying $150 kits for 4 inches of lego rack...)
Why are you concerned about the Z-Axis rack? Is it simply the force of gravity? Would putting the Z-Axis on an angle help this? (I.E, pitch it with a 45-60 degree angle rather than a 90 degree angle) Software can compensate for the horizontal axis shift as the Z axis is changed.
I can get away with a single rack & pinion setup for the X and Y axis, but moving to the Z axis would require a more robust rack & pinion setup for stability. Additionally, having it move in two directions at once (Z and X (or Y)) adds to the complication of alignment and magnifies any flaws in the system...

While I am trying to stick to Legos as much as possible, I don't actually work for Lego, nor do I have enough Legos to make it '100% Lego'...

I've also never seen the benifits of a rack & pinion for the Z axis anyhow. You don't need that kind of speed for the Z axis, the screw drive works fast enough for it... I also don't like how much force it puts on the motor; if your motor slips or skips, or if the power suddenly cuts, you could easily have your extruder head come crashing down on what you're making... The screw drive uses the nut, not the motor, to hold its position. The rack & pinion system uses the motor to hold its position, usualy gravity helps by pressing the rack against the pinion, but in the Z axis, gravity works against you, forcing the motor to exert effort to hold its position. Putting it at an angle would transfer some of the force of gravity towards pushing the rack and pinion together, but the more you compensate for gravity, the lower your angle, and the more you have to compensate in the appropriate axis. You're also going to have to make sure you always park the head at the bottom before you shut the unit off, or, again, the extruder will come crashing down... Maybe you can find some stepper motors that are 'resistant enough' to hold up a light Z axis, but I know mine won't even hold up the axis on their own, let alone the extruder head on the end of it. I'm also not attaching the Z axis to the XY axis, so currently, I don't have to worry about calibrating the two together, using an angled Z axis, I would have to worry about that.
Just as an idea for RepStrapping, there is a cheap clone brand of faux LEGO out there called MegaBlocs I think.

Plenty good enough for big baseplates, lots of regular blocks, and low cost. I believe (the ones I've got anyway) they are mostly made of styrene, -excelent- for glueing into semi-permanent modules after basic testing.

Oh, and they are very compatible with LEGO, so all parts can be swapped at will.
Ok... so if the Z axis is light enough, and is just holding up the worksurface, then you could get away with rack&pinion for it... (if it crashes, it crashes away from the extrusion head, and if it's light enough, the motor might be able to hold it without power...)

I'm using the Lego Digital Designer to redesign everything (with more parts than I have), and I'm going to look into a Z axis ontop of the XY axis, and it might end up being rack&pinion...

I looked at the megablocks, and they might be good for the bulk of the stages, but I didn't see any rack&pinion on there either!!! I ended up going to ebay and finding some expensive/cheap rack (12 for $9, but that includes shipping from uk, and I've never seen a $9 that has any rack, let alone 4 to 12 of them...)
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