Automated 5-Axis Calibration

Calibrating* 5-axis robotic mills is particularly challenging because the cutting head is at the end of a robotic arm, so each axis’s alignment depends on that of the previous axis such that slight perturbations in position - either from regular wear or sudden events such as tool crashes - can compound through the length of the arm.

After visiting 5-axis tools in production facilities and watching ShopBot customers struggle with calibrating them, I decided to write a program that would automate the process. It works as follows: the user first attaches a precision machined aluminum block to the x-axis, inserts a metal rod into the cutting head, and attaches an alligator clip to the cutting head. The block is connected by a wire to a low voltage sensor which can detect when the metal rod makes contact. To calibrate, the user runs a software program that causes the machine to touch off on the block in varying positions, registering its position and using some math to calibrate each of its axes in turn. I coded the routine in Visual Basic and G-code.

*Note: 5-axis calibration is different than 3-axis “zeroing” (as discussed in Automated 3-Axis Zeroing Device) in that calibration is agnostic to the material being machined; the goal of calibration is ensuring the robot knows at what motor positions its cutting head is aligned with its other axes, a challenge 3-axis machines do not have post assembly.