The Bipolar ORD Bot



It’s ORD Camp time again this weekend.  Every year I have done a gonzo build of some type of CNC machine.  This year I only had a few hours to spare, so I wanted something simple.  These are never meant to be practical machines, just conversation starters.

This was hacked together and programmed in about two evenings with stuff I had laying around, but working at Inventables means there is a lot of cool stuff “laying around”.  It was inspired by the RepRap Wally 3D printer, but vastly simpler in construction.  This only uses a couple of fabricated parts.  There are (2) sets of indentical actuator arms.  The inner arms are hard mounted to small NEMA 14 stepper motors.  The other end is attached  to a wood base, but free to rotate on a bearing.  The outer arms are mounted to the stepper motor shafts using Actobotics hubs.  The other ends have 1/4″ I.D. flange bearings.  These are bolted together, but free to rotate using a screw with a holed drilled for the pen.  That is basically it for the mechanics.

The stepper motors are driven with some high resolution stepper drivers.  These are driven by stock grbl 0.9 firmware running on an Arduino UNO.  The UNO does not know what the heck it is driving though.  The resolution is done in degrees.  I wrote a quick conversion tool that converts Cartesian gcode to bipolar gcode using these formula.

  • L = 150mm
  • A = 90mm


I have my CAM software output circles as multiple lines, so circles don’t need to be dealt with.  It has an odd, shield, shaped work area that you need to stay within.  Before powering on the steppers, you place the pen at the top middle of the work area.  You then tell grbl that both angles are at 51 degrees with G92 X51 Y51.



Here are a few more pictures taken at this weeks Beer and Making session at Inventables.

0121151912 0121151912b 0121151912c


The shield has a solenoid driver that I was going to use for pen up, but I never got around to that.  I kind of like how it runs so silently.

Here is a video of it running.  It is rerunning over an old plot to show the repeatability.   I think if I used true inverse kinematics the plots would look even better.  Maybe Machine Kit on a Beagle Bone is in its future.


A few people have asked if the motors could be moved to different locations.  Yes, I think you could put the (2) motors on any (2) joints and still have a controllable machine.  Not all work areas would be the same size and some might have issues with much higher torque requirements.  I believe separating the the motors by one linkage, like this one, yields the best results.

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3 Responses to “The Bipolar ORD Bot”

  1. Bipolar bot for drawing spirals | Hackaday

    […] Inventables for the Carvey CNC machine. They’re all popular projects and all very useful. This one, not so much. It’s a bipolar bot that doesn’t take itself too seriously, and this year’s build […]

  2. The Arduino-based Bipolar Bot can draw impressive spirals | Bits & Pieces from the Embedded Design World

    […] Dring has created what he calls the Bipolar Bot just in time for ORD Camp 2015. The spiral-drawing contraption was inspired by the RepRap […]

  3. Bipolar bot for drawing spirals | Hack The Planet

    […] Inventables for the Carvey CNC machine. They’re all popular projects and all very useful. This one, not so much. It’s a bipolar bot that doesn’t take itself too seriously, and this year’s build […]