Thursday, December 28, 2006


Mothra CnC

It's been a while, so I thought I'd give an update on my incantation of this project, and sum up all that has been done to date.

The original Mothra concept was to be a simple 3 axis router that I could use for both CNC (computer numeric control) and for Reprap (polymer extrusion). I'm happy to say that it works as a router, with average play less than 1mm and enough torque to cut 1/4" chunks of wood with a router bit. As a reprap, not so much, though I am starting to look at carving parts to improve this one.

Setup + Notes

The frame is roughly 24x12x18in and has a working area of 18x9x8. It is made from 1x2 to 1x6 lumber cut to length and bored with a drill press. Motors are unipolar NEMA 17's ((ELECTRONICS LINK)) with the Z axis driven through a 4to1 gear train, to lift heavy tools. The gears were taken from a thrift-shop dot matrix printer.
The router is a dremel tool purchased at a hardware store, with a variety of bits I have been experimenting with.
The X platform is screw-driven 1/4"/20 rod with 2 nuts epoxied to a wooden guide. This guide pulls the platform, which is supported on one side by a drawer slide. The other side rests on a piece of wood, but the wood-to-wood friction is minimal and there is little to no deviation.
(Note, X axis steel guide rods were removed after taking picture. Were replaced fully with drawer slide)

(Also note, mothra only has one X axis drawer slide. 2 would generate unneeded friction and the 1 has no play to speak of.)
(Also also note, CnCZone is a great source for designs if you need more inspiration)

Controller is 2 PIC chips that drive 2 ULN2803 darlington driver chips. This lets me drive any small unipolar stepper without much trouble. I did have an early issue with power, as my workbench variable power supply was getting maxed out and causing the early versions to skip with the tiniest resistance.

I'll post schematics and the .hex program when I have some more free time.

The controller is driven by 6 paralell port bits in the following manner:
1 x step
2 x dir
3 y step
4 y dir
5 z step
6 z dir

every time the STEP bit shifts, the motor is stepped. If dir is high, it steps clockwise, if low it steps ccw.
This is a standard format used by KCam4, Turbocnc, and Mach2/3.

In my practices I have been using KCam4 since I find it was the first one I actually got working. It's a steep learning curve, but once you get the concepts behind it,

For the moment, I have been producing gcode with meshcam and A9CAD. Also have been finding some code on CnCZone.

Problems encountered:
The biggest single problem that I had was alignment. It just seemed too hard to have a machine tight enough to provide .1mm accuracy yet be smooth enough to be driven by relatively small steppers. This problem was essentially fixed by converting the X and Z axes to drawer-slide bearings rather than 1/4 steel rod. When I decide to rebuild this completely, I'll do the Y axis as well.

Future plans:
I want to finish an extruder for reprap, starting with a modified hot-glue gun. Also considering making an aluminum version, for a high-torque metal cutting capability.
Designing an interface for the current serial reprap would be useful too, so it can run regular reprap software in addition to Kcam.
One day, I'll also finish my autocad diagram for mothra2.5, publish the full plans, and (re)build it.


an hour or so later:

Total cost has been around 200-300$ . But much of that was extra wood and bolts and parts used to hone in on a good design. And a Melabs pic programmer. I'd guess at there being maybe 50-85$ worth of parts in the final project.
If anyone has any questions on details I may have left out, please ask. The more people we have making and completing repsraps, the faster this project will start to take off.

Looking good! I don't believe that $55-80 parts thing for a minute, but never mind. It looks really nice!
Hey, why don't you just move this post up to the head of the list instead of linking back to it. That's not at all hard to do and people can find it easier. :-)
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