Saturday, July 26, 2008

Hard Playdough

This is a glob of hard "Playdough" painted black. It's 1 part cornstarch, and two parts baking soda, 1.5 parts water. Bring to a boil and it polymerizes. I did it in the microwave. At that point it's very rubbery, but you can roll it and treat it like clay, with some difficulty. From there it's just a mater of curing it to be hard. I microwaved it to get it hot about half a dozen times then let it cool in-between. The finished bit is very hard, pie dough colored, with a coat of paint for looks and water protection. It's probably a bit more brittle than wood, but you can form it. This is probably good enough to RepStrap many of the Darwin brackets, though it'd be tough to get enough detail for the gears, and it might not be strong enough for the corner brackets.

Friday, July 25, 2008

Stepper circa 1983 details

Unfortunately it looks like the stepper I found can't be wired as a bipolar, it has five wires, four of them go to one end of the four coils, and the other one ties all the coils together. It was being driven from the transformer, then controlled through a Darlington array. This is a simple way to drive a stepper as long as you don't want to half-step, or micro-step. The Darlington was a ULN 2003A, which is rated to 500 mA, so the motor must be 500 mA or less per coil. Since you can't wire the motor bipolar, the Stepper Motor Driver 1.1 won't work to drive this stepper. Need to find or design a unipolar stepper driver I suppose. There's newer quad Darlington arrays available that can put out 1.5 A per channel for about $2, or you could run the common wire to ground I suppose. Maybe there's a way to get the motor casing open and re-wire it, though it looks spot welded.

Thursday, July 24, 2008

Stepper circa 1983

I dismantled two Commodore dot matrix printers that were in the trailer, circa 1983, and one of them had an 18 volt stepper motor, there were some fuses labeled 1.6 amps, so I'm guessing that this stepper should handle 1.5 amps or so. There were also servo motors complete with attached encoder wheels, though there is a gap in the holes, I think that the paper advance would spin the motors one revolution and that was geared down to match the right distance. The other printer used a veritable tangle of wires, springs and gears to position the carriage with a solenoid instead of a stepper.

Saturday, July 19, 2008

Backyard Achaeology

This week I've been cleaning out this cargo trailer. There's stuff in there that has been in there since the 80's, but remarkably, there are things in the stuff that date to the 40's. My grandfather worked at a failing hardware store before moving to Alaska to work for the Air Force as a civilian junk reclaimer. There's a lot of real junk too though, and the whole thing was flood damaged in '96, the paper boxes were falling apart, and there was dried silt dust enough to choke on. Odd stuff though, for example there's a hand powered hydraulic pump in perfect working order, but unattached to anything, just with empty fittings for hoses. Then there's a weather balloon, in a can, with a balloon valve, in another can, and hydrogen to fill the balloon, in another can, just add water. And propane, that's right, underneath all the boxes was a 5 gallon tank full of propane, rusting away, waiting to explode some day! Fortunately it seems that it's undamaged, and not really very rusty, but it's been in there for 30 years. There was one bag, inside it was a bunch of little pouch plastic bags with a metal tab for a sealer, inside the pouch bags was another plastic bag with writing on it, "while wearing, put mask inlet in bag and seal with rubber band, provided, don't breath into bag for too long, try not to store mask in bag, don't store food in bag due to toxic properties." In that there's a cardboard-foil pouch containing three rubber bands, on the pouch it says that the cardboard foil pouch is good to 500 F operating temperature. The rubber bands themselves are nice and fresh, strapped to piece of cardboard. This seems to be a lesson in bureaucracy.

Wednesday, July 9, 2008


The workbench is done, it took much more time than I had hoped, but it has a very sturdy surface, which is mostly level. Soon I'll be on to other more interesting projects, like RepRap. This was built entirely from materials on hand, some 4x4 fence posts, and some salvaged 4x6 joists, a sheet of siding, some short 2x6 scraps, and a pile of screws and nails.

Wednesday, July 2, 2008


I melted some HDPE (2) from a laundry softener bottle into an aluminum star shaped mold meant for foodstuffs. It has a low glass transition, it was rubbery feeling when hot, even when very hot. (500 F) When I kicked it up to 500 it started to brown a bit, but it never flowed freely. From browsing the web I found that HDPE doesn't have benzine attached to the polymers like in PS, that is a big positive, the HDPE didn't smell at all, unlike PS. The result seems very strong and not at all brittle like PS as well. It never smoked. To mold this by hand you'd need to place the plastic under some amount of pressure, it seems to have a bit of memory to overcome. Pieces added to melt would stick, and then shrink and get thicker. It stuck well to the aluminum.