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CM6 - Compliant 3D printed robotic arm

Cheap, safe, and compliant 6 - Axis 3d printed robotic arm based on Quasi direct drive BLDC drives.

Petar CrnjakPetar Crnjak
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The goal of CM6 is to be a go-to robotic arm framework for people interested in robotics. CM6 uses 6 gimbal BLDC motors paired with small gear ratio gearboxes ( from 1:5 to 1:9), by doing that it is passively compliant and safe. Each Joint is using an S-Drive BLDC driver that is mounted on modular actuator designs for specific gimbal motors. Design can be changed easily by using different size aluminum extrusions or changing the gear ratio of modular gearboxes.

The total price of this first version is around 1000 dollars in raw materials. But I believe that the future version could be pushed to around 700-800 dollars.

Today every engineer, maker, and tinkerer can get access to a decent 3D printer for less than 300$ and start creating. While industrial robotic arms are too expensive only alternative are DIY robotic arms that are in most cases not really easy to program, they are not safe, or capable of doing useful tasks.


CM6 is a robotic arm that could fill that gap. It is safe, compliant, and most of all cheap. CM6 is a 3D printable COBOT robotic arm that uses QDD (Quasi direct drive actuators). QDD gives CM6 passive compliance which makes it behave like a human arm.

  • 7 × Radial Ball bearing 35x47x7
  • 3 × Radial Ball bearing 50x67x7
  • 4 × Radial Ball bearing 25x32x4
  • 4 × Radial Ball bearing 3x8x4
  • 1 × Thrust Ball Bearing 50x70x14

View all 29 components

  • Load test video!

    Petar Crnjak • 04/17/2021 at 15:52 • 0 comments

  • HIGH speed movement

    Petar Crnjak • 04/10/2021 at 15:03 • 0 comments

    In this video robot is moving from point to point with acceleration and deceleration. This makes it start and stop smoothly without jerky motion.

  • Video of motor jogging!

    Petar Crnjak • 03/25/2021 at 16:01 • 0 comments



  • Kinematic diagram of the robot and Denavit-Hartenberg parameters

    Petar Crnjak • 03/12/2021 at 18:01 • 0 comments

    So this is a kinematic diagram of the CM6 robotic arm. you can see that robot has a spherical wrist since axes Z5, Z4, and Z3 always intersect. 

    And here is DH table!

    I also always like to have written what does every parameter mean since I forget every time so here they are! :D

    Link offset = distance Zi-1 from Oi-1 to the intersection with Xi.

    Link Length = Distance from Oi-1 and Oi measured along Xi

    θ THETA = Rotation around Zi-1 to get Xi-1 to match Xi

    α ALPHA = Rotation around Xi to get Zi-1 to match Zi (We are rotating frame Oi-1 around Xi)

  • First video of CM6 in action!

    Petar Crnjak • 03/07/2021 at 12:06 • 5 comments

    First video of CM6 in action! Here you can see joint 1 being used as master and joint 2 as a slave (joint 1 is disabled, so it is really easy to move it). Atm, I only wired up the first 3 joints, and the software I wrote is configured for 3 DOF arm so there is a lot of work there. I tested multiple control techniques and one in this video works best and smoothest. In the future, I will show you other techniques I used but they did not perform as well as this one.

  • Total assembly!

    Petar Crnjak • 02/26/2021 at 19:05 • 0 comments

    I finally assembled the arm! It is looking really promising but everything will be revealed when I start some hardcore testing! Here are some pictures :D

  • Assembled base (Joint 1 and 2)

    Petar Crnjak • 02/25/2021 at 16:25 • 0 comments

    Assembled the last parts of the robot. Now the only thing left is to put it all together and start testing and playing around with it. Joint 1 these pictures is using 8:1 planetary gear reduction and joint 2 is also using 8:1 planetary reduction.

  • Upper arm + forearm

    Petar Crnjak • 02/24/2021 at 11:18 • 0 comments

    So this is assembled forearm and upper arm. So upper arm is using aluminum extrusion to connect joint 3 with joint 4. By using aluminum extrusion we can lengthen and shorten the arm and change the reduction ratio of joint 3 by changing a few parts. The length of extrusion in this picture is 200mm. Gear reduction of joint3 is 8:1 planetary gear ratio and then 32:28 belt reduction. The total reduction of joint3 is 9.14285:1.

  • Forearm assembled

    Petar Crnjak • 02/23/2021 at 16:36 • 0 comments

    So this is a complete forearm assembly. It is still looking pretty clean because there are no wires but once it is wired up wires will be visible. 

    Joint 6 is 5:1 planetary reducer. Joint 5 is plantery 6:1 and join 4 is 6.5: 1 bevel gear + HTD M5 belt. 

    as you can see here joint 5 is using a 2:1 bevel gear reduction and after that 3:1 belt reduction. Total reduction is then 6:1.

  • Gallery of robot renders

    Petar Crnjak • 02/22/2021 at 15:54 • 0 comments

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Petar Crnjak wrote 04/10/2021 at 20:11 • point

It could lift 1.5Kg but its precision suffers then. Since it is using BLDC gimbals and low gear ratio gearboxes it acts differently when under load. I will make a more detailed video about load capability and repetability.

  Are you sure? yes | no

ParadoxRobotics wrote 04/10/2021 at 16:39 • point

What is the rated torque and the max payload of the arm ?

  Are you sure? yes | no

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