the housing may be obtained as by instrumentation at 37 a fig3 also shows interengaged cam surfaces 125 and 126 and 127 and 128 on the piston driven actuators 129 and 130 and on the pads to effect outward driving of the pads piston cylinders appear at 39 a and at 40 a fig4 shows a schematic diagram of one version of the hydraulic control system a source of filtered fluid at internal drill string pressure is shown at 58 this internal pressure is designated p 1 a source of filtered fluid at the borehole annulus pressure outside of the housing 30 is shown at 63 this external annulus pressure is designate pa when the source of drilling fluid pressure generally mud pumps is not operating the internal pressure p 1 and the external annulus pressure pa will be equal when such pumps are operating there will be a substantial pressure drop across the bit resulting from the mud flow through the bit thus the internal pressure p 1 may typically be on the order of 300 to 600 p s i higher than the external annulus pressure the charge discharge valve 50 is spring loaded to expose channels 53 54 note high pressure from filtered source 58 is provided each channel and the upper piston 51 from internal pressure p 1 to each of the pistons 51 and 51 a note channel 53 is connected to port 57 as is channel 54 to port 56 other pairs of pistons not shown are similarly connected and nominally equally spaced to the pair shown when the mud pumps are operated the pressure p 1 at 58 increases and is applied directly to.