Single Phase Speed Controllers

The angle grinder will use a universal motor, so you can control its speed with a triac circuit like those found in domestic lamp dimmers.

If you know what you're doing with a soldering iron: get a cheap dimmer switch rated past 400W to cope with the surge current at startup. Open the case and replace the large wound inductor (should only be one) with a short circuit; the motor has it's own inductance, and you want to minimise the total. Connect up the dimmer in series with the motor, and off you go - I wouldn't stand too close the first time, and check for hot smells afterwards :) You may find adjustment jumpy at minimum speed, or if you change speed fast - inherent limitations of this simple circuit.

HTH Guy

If you havent already got the grinder what about a variable speed router?

just an idea

Dave