My Latest and greatest grinding machine is a Draper style machine. Draper machines push the lap back and forth over a mirror using long stroke and their action is more reported to be more like handwork. I made the Draper machine when I had trouble controlling the surface of a 12.5" flat with my large Waineo machine. At this point I know it gives me better control, but I don't have enough experience with it have all the bugs worked out.
The design of this machine was inspired by John Nichol's Draper machine that can be found on his web page.
The machine features a welded steel frame for stiffness, mounted bearings and a ball joint for the drive arm. The drive arm is adjusted by sliding a 1" aluminum square tube through a slot and then clamping it against the rotating piece with two 1/4-20 bolts and square nuts that slip into the tube. One advantage of the Draper design is that the length of the stroke is set independent of its centering and offset. (This approach is very different from my Waineo setup where the stroke length and the stroke centering need to be adjusted together.) The mounted bearings were purcahsed on e-Bay, and the 1" shafts and many other pieces came from the McMaster-Carr catalogue. The top arm is 1-1/4" Al square tubing. The white table is just some bathroom wall laminate that was cheap. The white bead is my silicon caulk dam that was a quick and easy kluge.
The motor is a 28 RPM 1/6 HP continuous duty one that was purchased on e-Bay. It is rated at 3.6 AMP (often a better guide than rated HP) and 270 lb-in. It will move a 12" lap easily--I don't know its limits yet. I have run the machine at 29 RPM but I have the pulleys to adjust it to 45 RPM and or 12 RPM. The turntable goes at 5.36 RPM when the arm is at 29 RPM and it has a fixed ratio to the arm speed.
The frame was welded together from 1.5" X 16 gauge steel tubing. Rust may be a problem that I will watch for. It is a much stiffer frame than anything I could have build out of wood for a comparable weight. I put wheels on everything these days.
The drive train is shown below. The pulley at the motor is switchable to adjust the speed of the whole setup. The main drive shaft and the turntable drive shaft has a fixed ratio that could be adjusted by changing pulleys and belts. The wood shims under the mounted bearings on the main shaft are used to adjust the tension of the turntable belt. The motor belt tension is adjusted by sliding and clamping the whole plywood motor mount.
The turntable is 3 plys of 3/4" Baltic birch. There are 3/4" square brass rods mounted on top, and the mirror clamps have slots that fit into these slots. The 1/4-20 SS bolts secure the wooden part of the mirror clamp. The brass rods are set into dadoed slots for alignment. It sounds complicated but it is simple to set up and is the best method I have come up with for holding a variety of mirror sizes. The turntable rests on a 12" pulley (a feature borrowed from the mirror-o-matic.) It isn't all that solid and I may machine a replacement piece if it gets to be a problem.
The turntable rides on the turntable shaft. That shaft is secured on the bottom with a mounted bearing and at the top with a 8" X 8" by 1.5" plywood board with a bronze bearing fixed in it. I adjusted the alignment by assembling everything so that the shaft rotated easily and only then drilling holes to secure the plywood board to the steel bars. You can see the bottom of the bearing board in the picture above.
The drive shaft is shown here. The square 1" Al tube rests in a slot and is held in place by two 1/4-20 bolts coming up through the bottom. They go into square nuts in the tube. The bottom of the tube is slotted for most of its length. The ball joint is fixed into place and secured against a square nut that slides into the tube. There is 6" of adjustment in the slide for a 0" to 12" stroke length. I put rulers everywhere to facilitate the recording and resetting of strokes. I would recommend this setup if one can make the metal pieces. It is simple to adjust and has very little slop.
More details can be found in the accompanying pdf document that gives a partial parts lists, diagrams with measurements, and machining plans for the drive pieces shown above.