referring now to the drawings in greater detail , fig1 through 7 illustrate a hard rock trench cutting machine 10 cutting a trench 11 in a floor 12 of a mine . the cutting machine 10 includes a main body assembly 13 , a cutter wheel assembly 14 , and a longitudinal thrust assembly 15 . the main body assembly includes a main body 16 which has a pair of rearward projecting slide tubes 17 and 18 fixed thereto . the slide tubes terminate in a rear support assembly 19 . the rear support assembly includes a pair of cylinders 20 and 21 which are tied together by a rear cross beam 22 ( fig6 ) and which are fixed to the slide tubes 17 and 18 . a pair of support feet 23 and 24 are adjustably positioned under the rear of the machine by hydraulic cylinders 25 and 26 which are pivoted to the cylinders 20 and 21 and to the support feet 25 and 24 . the support feet are also connected to the cross beam 22 by pivoted links 27 and 28 . the rear support assembly further includes an operator &# 39 ; s station 29 which includes an operator seat 30 , a control panel 31 , and an electrical cabinet 32 . the cutter wheel assembly 14 consists of an octagonal frame 33 having a number of cutting spokes 34 mounted thereon . the wheel is mounted by bearings ( not shown ) on a hydraulic cylinder 35 which adjustably supports the cutting wheel and the front of the machine during a cutting operation . the cutting wheel is driven about its central bearing by motors 36 which are mounted in the main frame , as is indicated in fig1 . the motors 36 are coupled to drive shafts 37 which constitutes the input to a pair of transmissions 38 . the output of each transmission is coupled to a pinion 39 and the pinion 39 drives a spur gear 40 . the spur gear 40 is keyed to a pinion gear 41 which drives a ring gear 42 mounted within the cutter assembly . since the motors 36 and the transmissions 38 are identical , only one is shown in detail . the cutter wheel is advanced into the face of the trench in a manner which will hereinafter be explained . as the cutters grind the rock at the trench face , the rock is scraped by scrapers 43 onto a muck ramp 44 . the crushed rock is then pushed onto a conveyor 45 which conveys the rock to a shuttle car s which trails behind the cutter . as was previously indicated , the cutting head is incrementally advanced to perform its cutting operation . to this end , there is provided a clamp carrier assembly 46 which is slidably mounted on the slide tubes 17 and 18 . the clamp carrier assembly includes a pair of hollow tubes 47 which carry a hydraulic piston housing 48 . the piston housing 48 carries a cylinder 49 which has a bore 50 therethrough and pressure legs 51 and 52 project from the bore 50 as pistons . the legs 51 and 52 , together with the bore 50 , constitute a common pressure chamber 53 . with the pressure chamber 53 pressurized and with clamping feet 54 and 55 firmly locked in place by pressurization of the hydraulic chamber 53 , the machine is driven forwardly by thrust cylinders 55 and 56 , which extend from the feet 54 to the main frame 13 and which are pivotally connected to the feet and to the main frame . extension of the thrust cylinders 55 and 56 forces the cutting head forwardly in the trench , since the entire main frame is extended and the slide tubes 17 and 18 slide in the tubes 47 . steering is accomplished by a pair of steering feet 57 and 58 which are connected to the main frame assembly by steering cylinders 59 and 60 . steering links 61 and 62 are pivotally attached to the main frame assembly and to each steering foot 57 and 58 . thus , with the chamber 53 pressurized , the machine may be steered to the right or left by extending one of the steering feet 57 or 58 and by retracting the other foot a like extent and concomitantly extending one of the cylinders 55 or 56 and retracting the other . this action causes the main frame assembly to pivot about the axis of rotation of the cutter wheel while the legs 51 and 52 shift their angular relationship to the trench walls . when the desired angle of attack is achieved , the cylinders 55 and 56 are pressurized to begin a cutting stroke . during the cutting stroke , the steering arms are dragged along the walls while the rear reset legs are retracted . after the cylinders 55 and 56 have reached the end of their stroke , the rear reset legs are extended to support the rear of the machine , the chamber 53 is depressurized , the legs 51 and 52 are retracted , and the cylinders 55 and 56 are retracted to draw the clamp assembly to the position illustrated in the drawings . to further understand the operation of the machine , reference may be had to fig7 which illustrates the hydraulic circuitry involved in the previously described arrangement . in that figure , the cylinder 49 has its pressure chamber 53 connected to a line 65 . pressure chambers 66 and 67 outboard of the pressure chamber 53 are intended as an aid to the steering function and are connected to lines 68 and 69 , respectively . in order to accommodate the steering function , a valve 70 is provided . the valve 70 , when shifted to the right , connects the line 68 to the pump and connects the line 69 to drain so that the feet are shifted to the right as a unit , since fluid in the pressure chamber 53 is locked therein by a pilot - operated check valve 71 . if the valve 70 is shifted to the left , the feet 51 and 52 would be shifted to the left . to provide for clamp pressure , a valve 72 is provided which controls fluid flow through the line 65 and through a line 73 . to apply clamp pressure , the valve 72 is shifted to the left to connect the line 65 to the pump ; and to connect line 73 to the sump , fluid flows past the one - way check valve and into the pressure chamber 53 . at the same time , the chambers 66 and 67 are exhausted in the following manner . pressure in the line 65 is applied to pilot - operated valves 74 and 74a through pilot lines 75 and 76 , which opens the lines 68 and 69 to the line 73 which is connected to the sump . to retract the legs 51 and 52 , the valve 72 is shifted to the right to connect the line 73 to the pump and to connect the line 65 to drain . pressurizing the line opens the pilot - operated check valve through a pilot line 77 , while the lines 68 and 69 are pressurized past check valves 78 and 79 . the thrust cylinders are extended by supplying fluid pressure to a line 80 and exhausting fluid through a line 81 . this procedure is controlled by a valve 82 . if the valve 82 is shifted to the left , fluid pressure is applied to the line 80 only if a pressure - responsive valve 83 is open . opening of the valve may only occur if the pressure in the chamber 53 is about 3 , 000 psi , since this pressure is sensed by the valve through a pilot line 84 . this feature is to ensure that the legs 51 and 52 are securely clamped prior to a thrust operation . as the thrust cylinders are extended , fluid is drained from the chambers ahead of the advancing pistons through the line 81 , which is connected to the sump . to reset the thrust cylinders , the valve 82 is shifted to the right to connect the line 80 to the sump and to connect the line 81 to the pump . the steering control for the trenching machine is controlled by a valve 100 which , when shifted to the left , connects to the cylinder 59 to the pump through a line 101 . the cylinder 60 is exhausted to the sump through a line 102 . when the desired steering has been accomplished , the valve 100 is centered and the cylinders are locked in place at a pressure of about 500 psi by a valve 103 . this is accomplished by shifting the valve 103 to the left to connect a line 104 to the lines 101 and 102 through a line 105 and past pilot - operated one - way check valves 106 and 107 . fluid ahead of the pistons in the cylinders 59 and 60 is exhausted through a line 108 . a pressure of 500 psi is maintained by a valve 109 . to retract the steering cylinders , the valve 103 is shifted to the right to connect the line 108 to the pump and to open the pilot - operated check valve 106 and 107 so that the cylinders 59 and 60 may be exhausted through the check valve , the line 105 , and the line 104 to the drain . the cutting wheel support cylinder 35 is operated by a valve 90 which , when shifted to the left , connects a line 91 to the top of the cylinder to raise the cutting wheel , since a line 92 on the other side of the piston is connected to drain . the cutting wheel is lowered by shifting the valve 90 to the right to connect the lower portion of the cylinder to the pump and to connect the upper portion of the cylinder to drain through a pilot - operated , one - way check valve 93 which is opened by the pressure in the line 92 . the rear leg lift cylinders 23 and 24 are independently operated by valves 94 and 95 , and since the operation of the cylinders is identical , only the operation of the cylinder 23 will be discussed . the cylinder is extended by shifting the valve 94 to the left to connect a line 96 to the pump to pressurize the upper half of the cylinder and to connect the lower half of the cylinder to the drain through a line 97 . the piston is retracted by shifting the valve 94 to the right to connect the line 97 to the pump and to connect the line 96 to the drain . it should be evident that this disclosure is by way of example and that various changes may be made by adding , modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure . the invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited .