Compressed air is delivered from the measuring chamber of a back-pressure gauging system and passes along channel a of gauging plug 1 to radial orifices d and b. These orifices taper outward at their outer ends. Placed into these taper holes are steel balls 4 of a diameter close or equal to the most expedient diameter of the wires used in the three-wire method of measuring thread pitch diameters for the given pitch. The diameter of the cylindrical part of the orifices should be less than the diameter of the steel balls to prevent their dropping into channel a. The balls are kept from dropping out when the gauge is not in operation by sleeve 2, which is pushed over the gauge by spring 3. During the inspection of a threaded hole, the gauge is screwed into the hole in the work and the sleeve is held by the spring against the end face of the threaded hole. The pressure of the compressed air pushes balls 4 into the internal thread being checked. The clearance between the balls and the tapered holes depends on the actual size of the pitch diameter being checked and this determines the air flow which is indicated by the pneumatic back-pressure gauging system.
$3704$SHP,M$