1. Field of the Invention
The invention relates generally to electric power tools, and more particularly to power tools equipped with variable speed switches for varying tool speed.
2. Reference to Prior Art
In some power tools, tool speed can be varied by controlling the pressure applied to the trigger. However, varying trigger pressure may not be an acceptable way to vary tool speed since holding a trigger in a less than fully depressed position causes fatigue and accurate trigger control is difficult.
It is known to provide electric tools with slide switches to vary tool speed when the trigger of the tool is fully depressed. However, a slide switch only provides an operator with a choice of high and low speeds.
It is also known to provide an electric tool with a variable speed switch which is useful regardless of power line voltage and which employs a potentiometer that is adjustable with a thumb wheel to control tool speed. Thumb wheel adjustment permits an operator to simply dial in a desired tool speed when the trigger is fully depressed. Prior art variable speed switch assemblies include multiple components and considerable care must be exercised to insure that the components are properly assembled to form the switch unit. Some of these components, such as the thumb wheel for example, must be made of costly heat resistant materials to withstand the temperatures generated when the potentiometer is soldered to a circuit board. Furthermore, care must be taken to avoid damaging prior art switch assemblies through normal use. For example, mechanical stops within the potentiometer absorb energy exerted by an operator when the thumb wheel reaches the limits of potentiometer adjustment range.