Well, after almost 20 days I've finally received the parts I ordered from Ponoko. They look to be good quality parts, but I'm anything but pleased with the turnaround time. Honestly, I could have machined my own (except for the gears) in at most a 5th of the time and for much lower prices... Next time, that's the route I'm going to take. In any case, these parts are for an automated pH adjustment apparatus that I'm going to attach to my hydroponic system. The design calls for a set of gears which are driven by two stepper motors. These gears, in turn, drive two screws which will depress the plungers on a pair of syringes filled with acidic and basic pH adjustment solutions. The entire device will be controlled by an Arduino board, and bolt on to the lip of the nutrient container.
The parts look pretty nice! I'm definitely pleased with the black acrylic. Unfortunately, some of the holes of the retaining rings are a bit off; but it's only a couple of thousands of an inch. Ideally, the large gears will be attached to the retaining rings with some epoxy. These assemblies will fit into the large holes in the top plates, with the gears on the bottom; and a captive nut will fit into the hex hole in them. A piece of 1/2" threaded rod will be threaded through that nut, and epoxied to the bottom piece of the plunger on the syringes after it's cut off using a band saw. The syringes are held in the bottom plate, and this is attached to the top plate with a pair of 1/4" threaded rods and associated nuts. The stepper motors will bolt onto the top plate through the small holes, and protrude through the two medium-sized holes. Their shafts will be epoxied onto the smaller gears, and they will in turn drive the large gears. The whole assembly will cost around $25 for the acrylic parts, $30 for the stepper motors and h-bridges to drive them, and possibly an additional $10 for a power supply.
Of course, the beauty of using the stepper motors is that they can be run open-loop; this means that the adjuster can be run without a feedback mechanism. On the other hand, I'm somewhat concerned about the amount of torque required to drive the screw. Hopefully the gearing will bring the torque requirements down sufficiently for a pair of Nema-17 motors to drive this. Unfortunately, I can't test it today; I'm still waiting on the electronic parts from SparkFun. But I expect that they'll be here shortly! (I've never had a bad experience buying from SparkFun, and I've even been able to email them and alter my order before it's shipped. Always a good experience with them.)
Here it is, with the mechanical parts assembled. Notice the lack of stepper motors.