the pressure differential causes the carriage to return to the loading position upon removal of the higher pressure air on the back side of the piston when valve pcv closes an important feature of the machine includes a soft touch feature generally shown by the numeral 88 in fig4 and provides a third regulator 90 and solenoid operated valve stv referred to as a soft touch solenoid operated valve soft touch regulator 90 is set to reduce the supply air pressure to about 5 to 15 psi below that of the pressure supplied to the backside of the piston through flow control valve 82 when energized soft touch solenoid valve stv allows the regulated air to pass through to shuttle valve 86 whereat it overrides the lower pressure air from regulator 84 and passes into cylinder 66 on the front of the piston because the soft touch air supply is less than that supplied to the rear side of the piston through flow valve 82 by about 5 to 15 psi the differential causes the piston speed to slow considerably thereby causing the motion of the carriage and tool toward the grinding wheel to slow the slowed motion continues as the tool contacts the wheel and provides a reduced pressure between the wheel and tool it can be appreciated that wheel life is prolonged less heat is generated due to the reduced pressure between the wheel and tool and a high quality surface is ground on the tool the soft touch.