[Jeff] recently bought an SX2 mini milling machine with plans to eventually automate it for use as a CNC mill. After paying nearly $700 for the mill, he decided there was no way he was willing to pay for the $125 tachometer add on as well. Instead, he reverse-engineered the mill and constructed a tachometer of his own.

He opened the control box and started looking around. After identifying most of the components, he got sidetracked by a 3-pin header that didn’t seem to have any particular function. That is, until he realized that a lathe by the same manufacturer uses the same components, and figured that the header might be used for reversing the motor. Sure enough he was right, and after adding a reverse switch, he got back to business.

He probed the 7-pin socket with his logic analyzer and quickly picked out the mill’s data line. He hooked the line up to an Arduino and in no time had the RPM displayed on an LCD screen.

[Jeff] says that this little experiment is the first of many, since the mill is so hacker friendly. We definitely look forward to seeing a CNC conversion tutorial in the near future.