Building the marble pump body { font-family: sans-serif; font-size: 100%;} img { margin-right: 12px; margin-top:0px; margin-bottom:4px; } Building the marble pump The heart of the marble machine is the marble pump. It's a deceptively simple device, with just three moving parts - a crank, a piston, and a slider. Its action is elegant and smooth, but it needs to be constructed with precision. This is how the pump parts fit together. The outside pieces are 10 by 6 centimeters and 1.8 centimeter thick. The thick slider in the middle is 10 by 6 centimeters and 3.4 centimeters thick. The slider The slider is made by drilling an 11/16 (or 17 mm) hole through the middle of the block. Then, two 1" (25 mm) holes are drilled exactly from the middle from either side, to a depth of 5 mm. After that, a 5 mm wide slot is cut into each of the holes on either side. It's best to use a 3/16" brad-point drill to minimize chipout on the inside of the hole. Drill a series of holes to open up the slot, then use a carving knife to smooth out the edges of this slot. When I built the machine, I cut this slot before drilling the main hole for the piston. It might be more work, but easier to cut a shallow slot from either side, and drill the hole for the piston afterward. Crankshaft The crankshaft is just a bent piece of coat hanger wire. Use a relatively heavy coat hanger, with a wire diameter of about 2.5 millimeters (.100 inches). If you don't have any wire coat hangers handy, you can buy one of those really big "jumbo" paperclips at an office supply store. Those are made of the same size wire. Extract a straight piece of wire 26 cm long. Make two marks on the wire, one 3 cm from the end, the other 6.7 cm from the same end. Use a wide pair of pliers and grab the wire so that the mark nearest the end is at the edge of the pliers. The 3 cm section should go through your pliers. While firmly holding the wire, twist the pliers by 90 degrees to make a 90 degree bend. Next you need small short and sturdy needlenose pliers. Long needlenose pliers don't have the strength needed to bend coat hanger wire. Find the spot at which the nose of the pliers is exactly 6 mm wide, and mark that spot on the pliers. Grab the long straight section of the wire from the previous step with the pliers at the point where the jaws are 6 mm wide as close as you can to your previous bend. Now bend the long straight section of the wire 90 degrees to form a 90 degree 'S' curve. This forms one side of the crank. Next, clamp your square nose pliers at the second mark (the you made 6.7 cm from the end of the wire), with the side that already has bends on the side of the pliers. Bend 90 degrees to form the third bend in your crank. The 'throw' of your crank should be about 10.5 millimeters. With a wire thickness of about 2.5 millimeters, the width of the whole crank should come to about 13 millimeters. The straight section between the two bends should be no less than 22 millimeters in length and the length of the crank part of your wire should be no more than 34 millimeters. It's better to be closer to 34 millimeters than too short. With a narrower crank, it may slip side to side and potentially jam itself in the slot in the slider. It might take more than one try to get it right. The piston The piston is a cylindrical piece of wood sized to slide easily in the 11/16" (17 mm) hole. The slot cut into the bottom of the piston slides over the crankshaft when the pump is assembled. Next we need to make a short dowel that slides easily in the 11/16" hole. 11/16" dowels are unfortunately not readily available, so it's best to whittle one out of a larger piece of wood using a spokeshave. You could also make it on the lathe, but whittling it doesn't take much time either. Drill a 11/16" (or 17 mm) hole in a scrap of hardwood and use that to check the roundness of your dowel. As soon as you can force it onto the end, push it on as far as it will go and turn it back and forth. Any part of the dowel that is rubbing against the hole will become slightly indented and shiny from this. This will tell you where you need to carve away more material. The goal is to make a dowel that slides very loosely in the hole. There must not be any binding at all. Next drill a 1/8" (3 mm) hole in the piston, centered 2.4 cm from the end of the dowel. Then cut a slot in the dowel. I'm resting the dowel on a V-block to make sure I don't accidentally rotate it side to side as I'm making this cut. This slot needs to be just wide enough for coat hanger wire to slide freely in the slot. The slot would ideally be just slightly narrower than your hole, so that the crank can help pull the piston down. When the crank faces down, the piston actually bottoms out against the base of the pump, not the crank. So the crank needs to be able to slide down this slot. put the piston back in the machine and turn the shaft until the piston is at its highest point in the middle. Mark the edge of the piston Next cut the piston off so that it is one millimeter longer than the mark. Taper the top of the piston so that it is roughly dome shaped, with the edges coming down to the pencil mark in the previous step. Fitting the crank and piston Try out the crankshaft, piston, and sides of the pump. Check that the slider does in fact slide back and forth by 2 centimeters. If the travel is within the range of 19 to 21 millimeters, you should be ok. If it's outside that range, try making another crank. The piston should protrude about a millimeter above the top of the slider. We will make allowances for that in the shape of the top of the pump. On the down stroke, make sure that a marble will just barely fit into the hole. The piston would bottom out against the surface that the slider rests on, not against the crank. Getting the vertical dimensions just right will make for a pump that operates smoothly and quietly. If the piston has too much vertical travel, the marble that is inserted under the stack of output marbles will be too far down, and the stack of marbles will lower down onto the new marble before being pushed back up. The pump would still work, but the action would be less smooth. Pump top Drill two angled holes into the marble pump top. Drill these holes from the bottom side of the top, and space them to be exactly equal to the amount that your piston slider moves back and forth by (in the range of 19 to 21 millimeters). The holes should diverge at about a 10 degree angle from square (20 degrees between the two of them). Use a 11/16" or 17 mm forstner bit. You should use high quality birch plywood for this. If you don't have any high quality birch plywood, make the top out of solid hardwood instead. Use a chisel to carve out an indentation between the holes on the bottom of the top. This indentation gives room for the top of the marble to pass forward and the top of the piston to pass back. Clamp the sides, slider and piston together. Use two sheets of paper between the slider and the sides on each side. This will allow for some slack when the pump is finally screwed together. Clamp the top onto the marble pump and drill the holes for the screws. Use drill size that allows the screws to engage, and drill through both parts. After drilling both parts, remove the top, and countersink the holes. Sand the tops and bottom of the slider so that it will have some slack to slide freely inside the pump. Before adding the bottom side of the pump, we can test it by resting everything on a smooth surface. It's easier to tweak the pump before the bottom cover is added. Use the same procedure to drill the screw holes for the bottom of the marble pump. The bottom side is larger than the pump to leave a flange around the edges for screwing the pump into the base of the machine. The pins sticking out from the top of the pump are to ensure that the elevator column that sits on top of the pump is aligned properly. We will add these once we build the elevator column. Dimensioned drawings for the marble pump Next: Building the elevator column