Vegetable oil can be extracted from almost any plant, but most oil is extracted from seeds. Obtainment of oil comes through pressing, in ram presses or screw presses. Building a screw press is a bit hard but it will last for a long time and will be able to provide you with oil in the long term. Modern techniques use chemical extraction with hexane. Palm oil is the most consumed oil, followed by soybean and rapeseed oil.
Cut all the parts according to the measurements. Sizes and amount of pieces are well detailed in the "Things You'll Need" section. To cut steel and wood, use the saw with its correspondent blade. For plastic, simply use scissors. Make sure you use sandpaper to smooth off the edges on every cut.
Drill two holes in the two angle iron bases. Use the drill with the 9/32-inch bit. Make sure each hole is 1/2-inch away from the outer edge on its side.
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Weld the two 24-inch tubes, the 6 1/2-inch tube, the 5 1/2-inch flat bar, and angle iron bases together. The two 24-inch tubes are the uprights, the 6 1/2-inch tube is the center tube, the flat bar works as a cross member and the angle iron bases compose the base. Consult the reference at http://journeytoforever.org/biofuel_library/oilpress.html for a diagram of how it should look. This is the frame.
Center the frame on the white pine wood base. Signal the location for the four mounting holes that will hold the frame and the 1 1/4-inch tube. Drill the holes with the 7/8-inch bit to a depth of 1/2-inch on the frame only.
Drill four holes through the mounting board. Use the 5/16-inch bit. Make sure the holes fit with the centers from step 4. Use sandpaper to round all the edges and uneven surfaces. Install the T-nuts. Paint if desired. Let the paint set and add clear lacquer.
Weld a 3 1/2-inch steel disk to one end of the 3 1/4-inch steel tube. Drill many holes on the side of the tube. They should be done with the 3/32-inch bit, and be separated by 1/2 inch. Use sandpaper to remove any burrs on the inside and the outside of the steel tube. This is the cylinder.
Weld the 1-inch steel tube to the 3 3/8-inch steel disk. Make sure the welding is done in the inside of the tube and never on the outside. This is the piston. Use a lathe to fit the diameter of the piston to the cylinder (3 15/64-inches). Smooth the edges.
Create a 1/8-inch by 1-inch slot at one edge of the bottom of the plastic bottle. Attach a receiving cup to this slot. Oil will pour from the press into the collector and out to the receiving cup.
Only build a homemade oil press if you plan on pressing a lot of oil. Otherwise, buying oil from the store is cheaper.
Wear gloves and a mask when using a saw. Small wood or steel particles may go into the air and breathing them could be harmful.