This is Mechpen, my newest drawbot.
The idea was to have a robot arm that could sketch on a rather large surface.

It is a SCARA (Selective Compliance Assembly Robot Arm) robot arm, meaning the robot has a shoulder and an elbow joint and a hand. Mechpen has a reach of 140 cm which means it could sketch up to A0 format.

Hardware

Most self-built SCARA robots you find online are used for 3D-printing and have a rather limited range but good accuracy. Mechpen in contrast was designed to have a long range while still have ok-ish precision. So the arm and everything that reaches out from the base should be as light weight as possible. It don’t has to carry more than a pen. Both motors and all the electronics have to sit in the base structure. The base structure can and should be heavy, so it doesn’t tip over if the arm is fully extended. All in all Mechpen weighs around 15 kg.

I bought the pulleys, bearings, belts and that kind of stuff. The rest of the hardware was custom machined. I even drilled a lots of holes in almost every part of the arm to save some weight.

Each part of the arm has a length of around 700 mm and is built from 6 mm carbon tubes. Each tube is slightly angled inwards in the forward direction. This alone was not enough, the upper arm twisted quite a bit when the forearm was at 90°, so I added two more diagonal tubes to the upper arm. That helped a lot, but even that did not solve the problem completely. More on that later.

The motors have a 0.9° step, which means 400 steps for a full circle. Each motor is geared down with belts by factor 5. With the drivers set to 1/32th stepping that makes 177.778 steps per degree.

Initially I wanted to have the possibility to rotate the hand with attached tools or pens. That plan was dropped because too complicated (and not needed for pens). The easier version was a servo that could lift the pen. As I wanted to lift the pen more than just a few mm, I designed a linear rail and a wheel that is attached to the servo. Then there is a nylon wire attached to both ends of the rail and with a 180° servo I could get a travel of around 70mm max.
At the bottom end of the rail I attached a small piece of Delrin in which I inserted two small magnets. For the pen I machined a spring loaded mechanism, which also has two small magnets and snaps perfectly into place when it comes close to the rail. This way I can swap out pens easily or even replace it with a sensor.”

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