Thursday, April 23, 2009


#include "repstrap.h" repstrap+=1;

Hi all,

I thought I'd take a moment, while folks are thinking about a design for the next generation machine, to post the design that my father and I have been working on. My Dad is an electrical/mechanical engineer with about 25 years of designing and building all sorts of industrial CNC machines under his belt, and I'm a PhD student with a mixed background in physics and artificial intelligence, working in neural computation. Together, we figured we could come up with something interesting. :) Our goal was (and is) to build something inexpensive, with off-the-shelf or scrounged parts as much as possible.

We initially set out to make a little CNC mill that we could attach a dremel to, and so we constructed a 3-axis CNC mill with a stationary table and a moving gantry. We made two iterations -- the first using inexpensive (and just bad) lead screws made with ready rod and nuts from Home Depot, and the second with belts. Our lead-screw version was overly precise, fairly slow to move, and had a lot of issues with the cheap lead screws binding. The current version, using a belt drive for the X and Y axes, runs quite well -- it's both speedy, and probably as precise as it will need to be for something that extrudes a liquid. :)

The Z axis does use a lead screw. We tried a few different options before settling on probably the most unlikely (and, hillarious) choice. My dad found some car jack's on sale for about $10, with heavy duty lead screws in them. He figured for $10 you couldn't go wrong. The nut on this axis does normally have quite a bit of backlash, but since it's mounted vertically the weight of the tool pretty much negates that, and you have a pretty good lead screw for not too much. This one is definitely over precise (I think around 4000 steps/inch), but it actually runs quite fast if you're just zipping up half a millimeter to your next extrusion level.

The extruder has a typical extruder heater assembly made from parts from the RRRF, with a simple pinch-wheel extruder made from little bits we had laying around. The plastic can /POUR/ out of this thing, but we would like to replace the toothed gear to something with a little more bite, because it slips at certain speeds.

The controller is very simple -- it's essentially a dsPIC33FJ256MC710 connected to a few darlington arrays (one for each axis), a few power supplies for the various voltages required by the steppers (4V and 6V), logic (3.3v), and heater (12V, but variable -- we usually run it a little sweet, since the darlington on the PWM board seems to drop the voltage by about 0.5v). The dsPIC is definitely overkill for this situation (256k of program space, 30k RAM, ~100 I/O pins, and various peripherals -- SPI, I2C, ADC, UART, etc.), but it's what I have experience with, and had laying about. The dsPIC controls all the stepping patterns, temperatures, etc., and has a parallel port interface (for interfacing to various pieces of milling software). We've just recently ordered a full set of Generation 3 electronics from Makerbot so that we can easily interface with standard reprap software, but if I have time some day I'd like to consider porting the software to the dsPIC.

All in all, we figure that we have invested maybe $100 in the machine (aside from the Gen3 electronics we just ordered), and the rest was from bits laying around. The plastic was salvaged, the steppers were both from old printers and from a local electronics store (we use two different kinds), the belts, cogs, and table were from old plotters, the darlington arrays were from old printers, and so on. We did pick up some drawer slides from Lowes that had ball bearings in them, and run very smoothly for the length of the machine.

So far we've just printed out a couple of boxes -- essentially a few simple patterns that I could easily express in some dsPIC code. We were awe struck when it actually started printing things out, and have watched both times in amazement as it creates things.

Thanks for reading. :)

Hi Peter,
Nice machine.

Re-threaded rod: I find the stuff sold in the local DIY shops is poor quality as you describe, but I can buy stainless steel studding online for about £2 per meter. I don't think that is any more expensive, but it is much straighter and has a much better thread.
Nice work. I like the moving gantry design.

It looks like your extruder works really well! Which motor are you using? is that a DC with gearbox?
Funny, I have actually been oogling car jacks for using the screws in them ;)

This would be for a normal CNC mill though, not a reprap machine.
I used drawer slides for my positioning system too, I also bought them at lowes LOL.
How fast is /pours/? Faster than 3 mm^3/s?

Looking good!

Hi all, thanks for your comments!

Nophead: thanks! I will definitely keep that in mind the next time we need some good threaded rod. that sounds both good and inexpensive!

Renoir: thanks! I really enjoy the gantry style system as well, it just seems kind of neat. :) the motor for the pinch-wheel extruder we're currently using is something salvaged from (I think) a plotter. It just looks to be a standard 12V DC motor with a gearbox to slow it down and increase the torque.

Sandos and Ping: Isn't it funny when you find common parts that work really well? Remember ours started out as a CNC mill, then moved to something with an extruder head on it. :)

Wade: We haven't measured the extrusion rate yet. Part of the issue is that the DC motor and toothed gear we're using aren't ideal -- the gear's teeth are shallow (but numerous), and it tends to slip at many speeds. Strangely enough, at some higher speeds it doesn't seem to slip at all -- so we really have some issues to figure out with our specific setup.

If the 3mm diameter filament contains about 3.14*(1.5^2)= ~7.1mm^3 per 1mm length, I'd say (from memory) at best we're likely feeding the filament in at /at least/ that rate. When we get these good feed rates, we've tended to just set the heater to constantly on, and fed it with a little more than 12V. Otherwise the heater barrel tends to cool down much too quickly.

Keep in mind that we haven't actually built anything at that feed rate (we've only built two boxes total, and at much slower speeds/feedrates), and that at the higher feeds the filament often seems to almost blob and become much larger than the 0.5mm nozel aperture (maybe double the size?).

hopefully I'll have some better ideas of numbers once we have a good set of control electronics, and have tinkered with constructing real models. :)

PS -- We're the folks from just outside Toronto, who came to see your machine after you had made the first set of replicated parts!
Hi Peter

Looks like great initial results!

It would be interesting to see more detail about the pinch wheel layout portion.
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