rheostat with router/ effective?

Don't want a resistive, rather a "chopper." It'll slow a universal motor down all right, and you could get a tach setup off of the number of commutator bars and a brush. But why bother?

because the router runs too fast for the larger bits.

A resistive type of voltage reducer (rheostat) is the wrong thing to buy because it will not only reduce the speed of the router but the power too. A speed controller that is designed for controlling the speed of routers and other brush type motors sends full voltage to the motor to maintain full power, but it does so in short pulses that vary in width as the speed setting is changed. These controllers will allow you to set a lower speed, but still have the power that you need. The knob is usually marked 0-100 meaning % of full speed. This is more than close enough to get the speed that you need for running panel raising bits. These controllers work great from about 25% to 100% but are significantly less effective at controlling speed between 0% and about 25%.

thanks you, that's very helpfull, if that's a percentage thats roughly accurate it should be all i'll need. thank again

You'll put 1/3 the cost of a new variable speed, soft-start, constant-speed feedback router into gizmos, but won't consider replacement?

The gain is great.

Not sure about this, but I remember reading somewhere that an aftermarket variable-speed controller should not be used if you have a soft-start router.

I don't know if your 3 1/4 hp Bosch is (my 2 1/4 hp Bosch is), and it may depend on the _type_ of variable-speed controller, but something you might look into.