Design a working mechanism with 3D model, 2D layout DXF, bill of materials and engineering checks. machine='gearbox' {input_rpm, input_torque_nm, ratio, stages, safety_factor}: picks tooth counts, module (Lewis bending check), shaft diameters, bearings, meshed gear layout and per-gear cut files. machine='four_bar' {ground, crank, coupler, rocker, coupler_point_distance, coupler_point_angle_deg}: Grashof type, transmission angle, rocker swing and coupler curve. machine='robot_arm' {link1, link2, payload_kg, tube_width, tube_wall, shoulder_deg, elbow_deg}: reach, workspace, joint torques, motor sizing, tip deflection and stress. Cost: 1 credit per call.