Build a steel frame to support your treadmill deck and belt. The size will depend on the width and length of the deck you choose. Measure the length of your deck and the width of your front and rear rollers. Add 3 inches of clearance to each section. Cut the front, back and two sides of your frame from a 3-by-1-inch steel channel to these dimensions with a miter saw.
Mark the location of the front of the back rollers on you deck and drill holes on the sides of your frame accordingly with a 1/2-inch drill to accommodate the roller shafts.
Weld the front and back sections to one of the side steel channels. Make sure the channel of the steel sections are facing the center of the frame. Insert the treadmill deck into the open frame by matching the rollers to the holes you drilled in Step 2. Weld the other side section to the frame. Secure the roller shafts to the frame with the bolts and nuts. This is the frame of your treadmill.
Bend the rigid conduit into a "U-shape" to build your treadmill's handrail and uprights. Determine the dimensions of the handrail and uprights by the height of the treadmill's users. You should be able to comfortably hold onto the handrail while running. Adapt the dimensions of the handrail's uprights to the height of your choice. Drill holes through the lower sections of your uprights. The height at which you drill the holes will determine the incline of the treadmill.
Drill holes on the front corners of the treadmill's frame. Align these holes with those on the uprights and secure them together with carriage bolts.