Saturday, November 18, 2006



I wanted to come up with a descriptive name for my repstrap, but I couldn't come up with anything catchy, so I ended up calling it ±... (I couldn't find a + over a . and none of my fonts are 3D, so I settled for + over -) The basic premise of the ± is simplicity. There's no real set design pattern for the ±, other than the basic ± shape. I'm currently mixing Lego technology (in the - axis) with some 1x2 repstrapping technology (in the + axises), but I've got plans to upgrade all of it, both to prototyped material in the reprap sense, and with feature enhancements such as moving from threaded rod to rack & pinion. Everything was built imperfect, with a fair amount of human error, with the expectation that I'll be able to find ways to compensate for that as I have ± prototype replacement parts, with more accuracy than its own.

Here we have what passes for the brains of the operation. I'm pulling in a 12v feed off a pc power supply (it's designed to plug into the inside of the computer running it, but with as many times as I've had to turn it off/on to work on it, it's got its own power supply for the time being), and then reducing it to 5v on the board itself. I wanted to keep it to just 12v in case I decide I want to plug some other 12v supply into it.

The coms circuitry is isolated on the board, in the upper right corner, and is connected to the motors with 4wire connectors. It's also got a 5v LED on it so I can stop checking the fan on the power supply to see if my board has power or not! The upper left corner is the Z axis controller; isolated from the rest of the board, it plugs in with the same 4wire connectors found on the coms circuitry, and has a similar 2wire connector for the axis bus (terminated). This was my original controller, so the ULN is soldered in place, but the PIC is socketed. It's also got the 3wire limitor switches, even though the 5v line isn't being used. All the controllers on this board are for unipolar steppers.

The lower half of the circuit board (left and center) is for the X and Y; they're seperated from the rest of the board, but hardwired together. They've got the same 4wire connectors leading in one controller and out the other; the 2wire connector for the axis bus connects to the X axis (left), and is terminated on the Y axis (center). I got smart here (thankfully) and socketed the ULN chips as well (I think one of them is bad, and it may have fried one or two PIC's in the process, thankfuly I didn't have to desolder 18 pins to fix it!)

I've switched over to a simpler limitor switch connector here; I'm using a single 3wire plug to carry the two 'control' wires and the ground, which then split off to the two seperate switches. In all cases, I've got the 220 resistor wired inline with the wires, and not on the circuit board itself; I wanted to share the ground off the board, and I want each switch to have its own resistor, so they're what begin the split.

The lower right corner is reserved for the extruder head circuitry, though I don't expect it to fit on this board; I'm piecing together the plans for a cleaner version of this that I hope will fit the extruder circuitry as well.

Here we have the ± itself (and my messy basement in the background).

Very simply, it's an X and a Z axis, suspended above a Y axis.

You have the Y axis, the -, the Lego axis, sitting on the ground, moving forwards and back. Above it, you have suspended the X and Z axis, the +, the wood/metal axis. I'm using an old pair of speakers (haven't worked in over 5 years, and then the bunny started eating one of them...) to hold up the +, they make a nice level platform being the same height (and they're old enough to actualy be made of wood!) The Y axis is pretty easy, given that it sits on the ground, and I've already described it elsewhere, so I'll concentrate on the +.

It uses a pair of metal rods for supports (salvaged mine from a CD/DVD stand, and they're not that strong or straight, but they're good enough) and a threaded rod for movement (with a drive nut in the center support). This is done both horizontally for the X axis, and vertically for the Z axis, sharing the same center (wooden) piece. The end pieces are all identical, a 1x2 (cherry I believe) cut to size (about 6 inches), with 3 holes drilled through them. I used a little plastic hand held Dowel Jig to drill the holes, and while they were all straight and consistant, they are all, consistantly, a few degrees off perpendicular. This probably would work better had I used a friend's drillpress, but not everyone has a drill press (and I'm not patient enough to wait for people to wake up so I can borrow their power tools!).

The motors (one per axis) are mounted to one end of the axis, attached to the (wooden) support on that end. I used some long screws and two pair of small wooden thread spools to hold it away from the support (to give room for my messy attempt at a connector between the motor and the threaded rod). Using a dremel(ripoff, with authentic dremel cutoff wheels), I shaped one end of the threaded rod to get a better grip with the connection, then used a bunch of electrical tape to tape it to the motor. It's not perfect, and the X axis has been giving me trouble with the tape slipping and the threaded rod 'seperating' from the motor, but it'll get me by for the time being.

The center support is where all the 'magic' happens; it's got both drive nuts for the X and Z axises, and it's the most complex piece in the while thing! I basically took a piece of 1x2, cut it longer (8 inches I believe), and drilled extra holes in it. The same three for the X axis (same spacing as the 6" pieces, lined up to one side) , and then an additional three for the Z axis, drilled perpendicular to the first three holes, with the same spacing, but lined up to the other side. (The six holes end up generally evenly spaced, criss crossing in a perpendicular pattern.) I then drilled out a pair of slots (one for each axis) to slip the drive nut in; they're not perfect, but they hold the nut snug enough that there isn't any play and I didn't have to glue them or anything. I slipped an extra 1x2 piece (cut to 6" with the same hole layout as the 6" pieces) over the Z axis ontop of the center piece, and attached it with glue; this gives better stability to the Z axis and helps to compensate for some of the wobble brought about by the oversized holes.

The limitor switches are taped to the four outer (wood) support pieces, and are triggered by the center piece. (I actually taped a yellow flat 2x4 piece to the center piece to help trop the switch on the top of the Z axis.) This whole contraption is mounted to the speakers through the two end pieces of the X axis, with a single screw through each. I've got them sitting ontop of some scrap 1x (from earlier attempts), to give enough clearance for the motor (with a diameter larger than the 1" thickness of the support).

There's a little bit of wobble, but I'm not still not too concerned about it. I've got a lot of imperfections in the system (as per my design), and I plan to start trying to extrude replacement parts before I worry about any of if; The wobble is reduced considerably as the speed is reduced, and if reprapping a more accurate reprap just means spending lots of time at it, that's fine with me... Once I get an extruder head, it'll be attached to the (wooden) support at the bottom of the Z axis. I'm not too worried about the axis being able to support it, it still moves pretty smoothly at 'full speed', and being the Z axis, it won't need that speed during use.

I originally tried cutting down a 1x6 into strips to use as the supports here, but the material was too unstable and too prone to splitting to be of any good. It was also slightly warping on me, giving in to the curvature of the grain. I've also done a lot of work with molding pieces out of polymer clay, but they've all turned out to be too fragile for any reliability.

Wowzie! How are you going to prevent your extruder head from behaving like a pendulum way down there. As well it doesn't look like you have much to resist torque forces centred on your z-axis threaded rod. Unless you've got some really good ideas about how to avoid eccentrically loading that z-axis I see trouble coming your way.

All that aside, you've been going great guns on your repstrap. It has been great to watch your progress!
The second piece I added to the center for stability is to help reduce the pendulum effect, I'm also expecting to have very slow extrusion speeds with the repstrap to to help counter act that as well. Once I'm able to prototype replacement parts with more accuracy, I'm hopeful that tightening everything up will increase stability as well. The Z axis already hits its maximum wobble (the second piece to the center helped reduce that) when the X axis is in motion, so I think I know what I'm working with there, and the wobble virtually disappears when I bring the speed down to below 200 or 150. I've also got the beneifit of being able to orient the piece so that the bulk of the work is done by the Y axis, which has no impact on the Z axis at all.

The Z axis itself doesn't have much problem with its threaded rod; for most of its operation the drive nut is all the way at the top near the motor, away from the the extruder head. Additionaly, I oversized the holes for the threaded rod on either end, so (ideally) the threaded rod never touches the (wooden) support pieces; it probably doesn't have enough play in the center support, but I'll be engineering at least some play into the prototyped replacement. Again, I've seen the limitations of the wobble, and I expect them to be reduced once I can prototype replacements. They're also minimal enough that they seem to disappear in the mid 100's speeds... I'll post some video of it in motion once I dig my camcorder out and get the battery charged.

I'll admit, I'm least happy with the Z axis, but it's the best compromise I've come up with given my goals. In true nature to it's name, I'm just aiming for 'close enough', and I'll tackle whatever comes up along the way...

I'm trying to keep this as reprappable as possible, so I'm avoiding all the 'overconstruction' that I'm seeing in everyone else's repstraps, and hope I can compensate with engineering and progressive accuracy. As it is, ± has a larger work area than I ever plan it to need; I'm figuring on being able to find a 'sweet spot' where the rods/holes/stars magically align and give me just the right 2" x 8" work space to replicate everything...
Maybe just sacrificing some Z axis sway is worth the gain in accuracy? I mean, for a 1st generation repstrap, does it really need anything over 6-18in of Z movement?

That aside, it looks good.
***I'm trying to keep this as reprappable as possible, so I'm avoiding all the 'overconstruction' that I'm seeing in everyone else's repstraps, and hope I can compensate with engineering and progressive accuracy.***

I really like your approach.
***Maybe just sacrificing some Z axis sway is worth the gain in accuracy? I mean, for a 1st generation repstrap, does it really need anything over 6-18in of Z movement?***

Actually, my Z axis has about 15 inches of movement; same with the X and I think the Y is a little shorter. It's based off the largest span I could make with the size support rod I started with. I'd like to eventually get the Z axis working to those 15 inches, so I don't want to cut down the rods I have. If I can dig up another one, I may cut it in half and see what a 4-5" Z axis gets me... I really only need 2"+clearence to replicate all the parts...
The XZ axis has been torn down! I've gone back to the drawing board (same idea, different approach) and am working on something new.

I initially added more support to the Z axis to stablize it more, but in the process I made it heaver than the support rods in the X axis could handle.

My woodworking/etc skills are not good enough to come up with something that accurate, so I've moved my attention towards 'mass produced parts'...

For my next trick... I'll be building my XZ axis out of a pair of keyboard tray slides (which I never installed into the desk in the first place...) They're good quality; they came from a quality low-end desk (not the really cheap 'student desk' stuff...), so I expect they'll have less sway than my last XZ axis attempt. They're also pretty sturdy and are designed to bear a fair amount of weight themselves, so I expect the X axis to perform better (even if it's now a one-sided overhang)

I'm not sure... but I think i'm building a 'fixed' crane (of some sort, maybe a boxcar loader?) over a set of railroad tracks...
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