spindle speed controller

sherline mill, but i think the question might still apply here...

how do you set up a speed control interface for your spindle??

is there a commercial product or a free schematic??

looking thru the archived messages someone mentions some i2c
(whatever that is...noobe here) but i cant find any details...

i would rather breadboard a simple control if possible instead of
spending hundreds of dollars...

thanks,
any input apprecitated,
fdk