Screw-Piston Box Churn: The Case Turns and a Piston Runs End to End Along a Five-Start Screw

"The operator now takes hold of the crank handle (h) and gives rotary motion to the case (A), thus causing the diaphragm or piston (C) to be forced, by the screw shaft (D), toward, and eventually against, one of the ends (e or e') of the said case (A), when he then gives an opposite rotary motion to the said case (A), and thus forces the said diaphragm (C) to the other end of the same".

From 20 mm hardwood cut four sides 412 mm long, two 200 mm and two 240 mm wide, and two end pieces 280 x 280 x 40 mm. Rout a groove 6 mm deep and 20.5 mm wide in each end's inner face to take the sides' edges as a square 200 mm inside. Cut a 180 x 120 mm opening in one 200 mm side for the lid. Bore the ends 11 mm at the four corners, 128 mm from the middle each way, and draw the case together with four M10 rods, washers and nuts. Bore a blind 20.6 mm hole 30 mm deep in the middle of one end for journal i, and a 20.6 mm hole through the other for journal k. Soak the case in water overnight so it swells tight.

Thread the four screw sections on a 580 mm M12 stainless rod so their threads run on from one to the next, with a 20 mm stainless tube on each side as journals i and k. Clamp the stack with nuts and washers so the screw turns with the rod. Thread the piston on before the last section goes on. Put journal i in the blind hole, pass journal k through the other end, and draw the case up round them.

Glue the middle plate to the plain outer plate, drop the disk into its hole, peg up, and glue the slotted plate on top so the peg comes out through the slot and the disk turns freely. Line up the nuts' threads before the glue sets by running the piston onto a screw section.

Make the lid from hardwood to fit the opening, with a 6 mm lip all round, and two hardwood buttons r screwed to the case to turn over it. Fix a hardwood block on the outside of end e and pin a 20 mm steel stub in it as journal g, with a crank h. Fix a crank l on the rod's end outside journal k. Build a frame of two uprights on a base, 580 mm apart, with open slots for the journals. Bore 10 mm holes through each upright and its end piece, and through the crank l, for the pin v.

Turn the disk's peg to open or close the holes for the cream's thickness. Pour about 6 litres of heavy cream at about 15 °C, checked with the thermometer, through the lid opening, button the lid down and pin crank l to its upright. Turn crank h one way about seven and a half turns until the piston stops at one end, then back the other way, timing it. When the butter comes, pin the case instead and turn crank l to drive the piston against each end and press the butter there. Open the lid, lift the butter out, drain the buttermilk, wash the butter in cold water until it runs clear, work in a pinch of salt and weigh it.

**E. L. Pratt** of Philadelphia, assignor to himself and R. B. Fitts, US 25,306, 30 August 1859, "Churn". He claimed the rotary case with a piston that both moves along and turns on a screw, with grooves across its edges, and the piston of three pieces with a disk to open, adjust or close its holes. He gives no sizes, so every size here is ours; he made the piston of wood, and ours is printed, with a peg through a slot to turn the disk.