Abstract: A grinder for removing a predetermined amount of rubber at a predetermined angle from a raised or recessed area on the sidewall of an inflated tire as the tire rotates. A pair of spaced sensors engage the sidewall area of the tire at predetermined radial spaced locations on opposite sides of the area to be worked. The sensors generate a pair of electrical signals in relationship to the profiles of the sidewall areas engaged thereby. The signals are fed to a microprocessor which calculates the average angle of the raised or recessed area by the difference between the outputs from the two sensors along with the known spacing therebetween and generates a third signal. This third signal is supplied to a servo mechanism which automatically adjusts the grind angle of the grinding wheel to match the average angle of the area to be ground.