news3.htm
This page will include progress reports, notes, random brain farts, and other ramblings.
My friend Alan recently bought a 240V spot welder for his projects. Since he has little room at his place I have it at mine. The welder is a nice one, costing about $450.
When I first tried it on my 0.008 inch tinplate pieces the welder immediately blew a hole in the steel. I read the manual and ground down the tips to 1/4 inch wide. This did not help. After checking the circuit diagram (extremely simple) I reconnected the 240V machine to 120V. This worked very well. About 2-4 seconds is all it takes and it barely dims the lights.
This is great for my work here but eventually I need to find a cheaper way to join metal. Few people can justify $450 for one step in fabricating a hobby engine. And eventually Alan will want his welder back.
Getting sheet metal thin enough to be practical for a model airplane has been difficult. Mostly I've been cutting up one-gallon coleman fuel cans. Coleman fuel, however, costs about $8/gallon. The other day at Home Depot I found Mineral Spirits in one-gallon cans for $2.10. Each can gives me enough metal for a flame holder.
What it doesn't give me is metal for the engine body. I found a website for a company called Vulcan in Canada which makes ornamental cans for Christmas popcorn, wine bottles, and etc. They told me that they buy their metal from Dofasco in Canada, whom I haven't contacted yet. This metal is called "tinplate" which is, apparently, where the term "tin can" comes from. When I heated tin cans to red hot there is a thin layer of stuff that flakes off. I assume that it's tin. This needs to be cleaned off to get to the real metal for welding and such.
I also talked with my local steel supply house. I was told that there is a company called "Thin Metal Sales" (why didn't I think of looking for that name?) in the LA area (actually Chino). I was told that TMS sells 0.010 steel in 24 inch wide rolls, 10 lbm (about 10 ft long) for $72. I haven't contacted them yet either.
I also got curious about what "Mineral Spirits" is. I tried burning it and it seemed to be between gasoline and kerosene. A little more checking (http://hazard.com/msds/) and I found out that, besides being paint thinner, it is one of the dry cleaning fluids and has properties that are between kerosene and gasoline. Not only do I get the metal, I also get a fuel to use.
Last night I spot welded up a cone of tinplate about 10 inches long and 4 inches at the base. The top fits snugly to a 1 3/4 inch tin can which will be the pilot. I put one spot weld to hold the two together but then had to quit for the night. Next I need to cut the air holes. At first I'll run it in a 4 inch stove pipe, like before, to verify that it works right. I'll try running the fuel in from the back trying different configurations of fuel line to preheat the fuel. First I'll try the 1/8 inch copper because it is light weight and thermal conductivity. After that I'll try the 1/4 inch copper then the 1/8 inch steel brake line, if needed. The fuels will be gasoline, mineral spirits, and kerosene.
I want the pilot to run nearly stoichiometric at all conditions to ensure good flame holding. I think I can do this with a k-type thermocouple, a simple amplifier circuit, and a small DC motor. The thermocouples are only a few dollars each, are light weight, and fairly rugged. The amplifier circuit should be less than $10 for parts from Radio Shack.
Possible motor configurations are the ones from a small cassette player. The motors are small and have a high rubber-band driven gear ratio. I also found on the web some screw-drive DC motors of the right size.
I'm also considering a model airplane servo. The FPS3003 sells for $12 from Central Hobbies (http://www.centralhobbies.com/Servos/FPS3003.html). These are normally controlled by a pulsed DC signal. The relative widths of the off and on pulses controls the position. After talking with a friend I realized that, rather than construct a complex controller to run it, I could remove the decoder in the servo and run the DC motor inside directly. It's lightweight, rugged, and already has a gear reducer. This would make it easy to drive a small fuel control valve.
I'm also considering different ways of fabricating a small, lightweight fuel control valve. The ones in the catalogs all seem to be either too large or too expensive.
This Page created March 26, 2000
Last Updated Sept 4, 2000