A tool change system was part of the requirements from the start of this project. Being able to use multiple tools like brushes, palette knifes, pens, markers, and pencils unlocks so many creative possibilities.
I tried to take inspiration from other cnc automated tool change mechanisms but most are overkill for this application. Most cnc machines that use tool changes need an end effector connection to withstand the high torques and forces experienced during milling. In contrast, end effector connections for cnc painting only need a to withstand the small torques and forces between paint filled brush bristles and a canvas. Ultimately though, it was the cost and large space requirements of existing tool change systems that sent me down the path of developing something new.
I settled on a simple screw-in design. Drill chucks are used to accommodate tool handle diameters up to 0.375in. A 3D printed part is fastened to the top of each drill chuck to assist with locating the tool into the tool docks on the workspace. Four tool docks are available on the workspace next to the canvas. Each drill chuck assembly has a 3/8-24 threaded hole on the top. A 3/8-24 bolt is secured to the C axis (aka sixth axis). To pick up a tool, the machine simply screws the C axis bolt into the threaded hole on one of the drill chuck assemblies. The C axis is commanded to turn and screw into the drill chuck assembly until the head of the screw contacts the drill chuck assembly and the C axis stepper motor overtorques. At which point, the C axis is set to zero in software. This same process is reversed for returning a tool to a dock.
On the topic of tool holders, I open sourced two laser cut artist tool chuck designs that can be used to retrofit a three axis cnc mill for cnc art: https://mega.nz/#F!Tc9w1DCI!gCbwh5lii0nDh_tOyw0b2g