If you want to change the rotational speed of all the drills, you can disconnect the motor from the power head and change the number of teeth of the motor gears and the gears connected to the motor and the power head to change the rotational speed of all the drills in the power head. (But to meet the three conditions, 1, the center distance of the two shafts can not be changed, that is to say, the sum of the number of teeth of the two gears can not be changed, a gear plus a tooth, the other gear must be reduced by one tooth. 2, the gears are best not to root cut. 3, the change must not be interfered with other gears or shafts.)
If you want to change the rotational speed of a certain drill, you have to familiarize yourself with the drawings, take out the gears on the main shaft connected to this drill and the gears connected to this gear, and then install a pair of gears from a new machining according to your requirements. (But also to meet the three conditions, 1, the center distance between the two shafts can not change, that is to say, the sum of the number of teeth of the two gears can not be changed, a gear plus a tooth, the other gear must be reduced by one tooth. 2, the gears are best not to root cut. 3, the change must not be interfered with other gears or shafts.)