A backhoe subframe is the assembly skeleton for building a stand-alone backhoe or one that attaches to an existing tractor. The primary structural element of the subframe, to which all other elements of the backhoe are mounted or connected, is typically a heavy steel channel, tubular or I-beam. The axle and feet assemblies are welded or bolted below the beam and the engine, seat, control and power assemblies are bolted or welded to its upper surface. The control plate -- the forward-most control surface -- is typically bolted or welded vertically to the forward vertical surface of the subframe beam.
Place a piece of 5-inch tubular steel on a pair of saw horses. Measure and cut a length of 6 feet, using a power hacksaw. Use a power grinder to smooth off the edges.
Measure, mark and cut an 18-inch length of 1-by-5-inch steel channel beam (the control plate). Use a steel file to clean up and smooth the edges.
Mount the control plate in a heavy steel vise and measure and mark two drill holes. Make the locations 2 inches in from the flange edges and at 4 and 14 inches from one end.
Use a power drill to drill a 3/4-inch hole at both locations.
Use a 140-amp stick welder to weld the control plate to the forward vertical surface of the 6-foot tubular steel beam. Mount the plate equally centered, laterally and vertically. Double-weld the welds. This completes the beam skeleton of the subframe.
Mount the chosen wheel assembly or rear stationary feet of the backhoe to the underside of the rear of the tubular steel beam. Mount the chosen front wheel or stationary feet of the backhoe to the underside of the front of the beam. Cut and weld an engine support frame for the engine -- this will vary by engine choice -- onto the upper rear surface of the subframe. Mount both the seat assembly and the control assembly, respectively, to the middle and forward positions on the subframe.
Bolt the hydraulic control arm and the structural control arm through the upper and lower drilled locations, respectively, on the control plate.