Forming Acrylic Mirror

I think all this information exists in previous entries, but it's nice to have all my knowledge, however little that is, dumped into one spot.

Heat forming acrylic (aka plexiglass) is pretty simple. Just get the temperature up around 220°F and it's pliable. I put it into the kitchen oven. Don't rely on the oven temperature, though, use something with a probe to tell if you've got the real temp. For one thing, you can put the probe right down where the mirror is, not just floating around in the air inside the oven. Also, when I was researching this I found some warnings about fumes. I think that's only if you overheat because I never smelled or sensed anything. Maybe I've silently shortened my life by 20 years.

I tried a couple different methods of using a form. The first couple tries were "open-faced," meaning I laid the acrylic on top of something and counted on the weight of the material itself to cause it to sag into the form. It isn't really heavy enough for that, so I moved to a two-part form. It takes longer to heat that way because of the mass of the form itself. Another disadvantage of this method is that you have to be able to force the mirror into shape before you heat it, which means no 2D curves (like a bowl, if you see what I mean). Actually, you probably could do that by putting the acrylic sheet between the halves of the form and then weighing the top part down. Like the open-faced method, but with extra weight. Drill a hole in the form so the temp probe can sit right on the acrylic.

The above should give pretty nicely formed acrylic sheet even in complex shapes. Unfortunately it's totally unworkable for acrylic mirror. I'm not sure if it's the acrylic or the mirror backing, but the heat stresses the material such that you don't get a smooth reflection anymore. For a while I thought it was imperfections in the form, but various experiments ruled that out. (You can get bumps and ridges from a rough form, though, which can be eliminated by loosening the form a little or by lining the form with some kind of bumper material. I used a sliced up silicone baking sheet.)

Also: I never got far enough to have this problem, but eventually you'll run out of space in the oven. Multi-part forms? Some other heating method?

The curve I wanted wasn't 2D, so how about cold forming (aka "bending")? I have not yet determined the point at which the acrylic cracks or deforms. I haven't even determined if there's some point short of complete destruction where the mirror breaks down, a la the heat deformation. It seems to be pretty sturdy and stable, but then I'm not making tiny radius curves. If you imagine bending a yard stick, that's about what it seems like, or maybe a little stiffer. Put another way: I have a square of acrylic mirror that's 2'x2' bent into a curve about the same as the side of a 55 gallon drum and I haven't see any problems with it. (This is the 1/8" thick stuff. There's also a 1/4" thick stuff that I've never tried.)

The trick with cold forming is that you have to hold it in place somehow. Even drilling a hole through the acrylic will deform it, although only in the immediate area surrounding the hole. I started with a sandwich method--cutting two parts and then cramming the mirror between. That works, but you have to have some method of securing the bread of the sandwich that doesn't involve drilling through the mirror. Also, you cover up some portion of the mirror surface.

Of course, you can drill through the mirror, but in that case why even have the top piece of bread? That's what I switched to: Put the mirror on an open-faced form and drill a few well-placed holes to secure it down. Don't overtighten, because the force isn't distributed over a wide area like with the sandwich, so the local area can get quite deformed. Maybe a reason to have the top piece of bread after all. Or maybe just some rubber washers or silicone baking sheet for padding.

You can't get too fancy with this method, but for a single, relatively gentle curve it works great.

Massively Unparalyzed

• I think I'm out of the summer diet doldrums. I suddenly realized that if I stayed focused, I could meet my original goal by Christmas. I have to lose 1 lb every 10 days, which seems pretty reasonable. That's just ~350 calories/day. This is the same rate my overall history has been up til now, so as long as I keep, or *cough* get back on, that average I should be good. (I say "original goal" because after I meet that goal I think I'm going to go an additional 10 lbs. But I'll have to see how things stand when I get there.)


• I needed to sense sunlight vs shadow and I'd read somewhere you can do that using an LED somehow. I got that working but they aren't made for that and so it didn't meet even the low standards of this application. Some photoresistors, even at RadioShack's redonk prices, were only $3 and work so much better.


• Also needed, or at least wanted to evaluate, pulleys. McMaster-Carr's site is so awesome, why can't my local hardware store (or even Lowe's) have a website like that? Or McMaster-Carr have an outlet near me.

  That said, my local hardware store fits an amazing depth and breadth of stuff into their store. I don't think there's been a single thing I couldn't find there that I was able to find at Lowe's, in the fields the LHS covers. (i.e. they don't sell appliances at all, so obviously they have a smaller selection of dishwashers.) And the reverse is not true--I've found things at the LHS that Lowe's didn't have. And they manage to fit all this stuff into an area that's probably literally no larger than the area devoted to just cash registers at Lowe's. Like, how is it possible that my LHS has a 9 ft² caribiners + pulleys display right next to the pumps I need (though are still more than I want to pay)?

  Oh and the staff actually know where things are and how to use them, unlike at Lowe's.

  I guess what I'm saying is, local hardware store:Lowe's::small European country:US. It may be a little more expensive, but the service and selection are great.



• How can I completely waterproof a wooden structure? How about sunproofing? Like, is it going to get baked and shrunken on one side? Probably somebody already knows all this stuff. No wonder people just build with metal. Oh wait, how about PVC? Does that degrade in the sun? Oh and it's hollow, so you can put wires in there....HMMMMM


• I was going to have an entry in here talking about all the things I suddenly (as of this morning!) have to do, but I can't even spend time enumerating them!

Paralyzed by Obstacles (i.e. whining)

(throughout this entry substitute "can't find at a reasonable price" for "can't find" and "free or under $40" for "reasonable price")

The next stage in the parabolic solar collector thing is to have the fluid circulating. I want to have it run through the collector, then down a hose into a tub of water and then back. Measuring the heating rate of the water will give me a better value for the power.

For this, I clearly need a pump. And can I find a pump? Clearly not. For simplicity, I think I want one that has garden hose fittings on both ends. Sump pumps usually have that on one end, but the other end doesn't have any fitting at all. (Maybe I could disassemble one to find out, but I can't find any to do that with either.) I bought a drill pump for like $6 but it was too wimpy to be useful.

Not that it has to be that powerful. According to the back of my envelope, I only need about 60 gph in the worst case. Typical sump pump numbers are 10x that. I could probably do it myself by putting a squeezable diaphragm inline and stomping on it. What a lot of work, though.

But I have feelers out on craigslist and freecycle, so in the meantime how about something else? I know, I can try to make a much larger stirling. What'll I use for the piston? Oooh, brill idea: a cut off bike pump!

Take 2: And can I find a pump? Clearly not.

Maybe I'm just too cheap. Or maybe I haven't figured out the right workaround. Or maybe I'm asking for too much. All of these questions (plus a couple other things I'm stopped on but aren't worth describing) has my brain in vapor lock. I am unable to move on anything.

Stirling Walking Beam

I made a Stirling engine before, but it wasn't "real" in the sense of "having traditional engine parts, like a flywheel and crankshaft". Also, it would be pretty difficult to scale that one up or extract power from it as is. So this is my first regular ENGINE engine. (This is called a "walking beam" engine--many other configurations are possible.)

One reason I built this one was to prove to myself that I really understood how they worked. For that reason, I designed this all myself. Not that there's SO MUCH there. Also, there are tons of videos on YouTube that are identical to mine.

It's be really awesome to scale this up. Like with an oil drum for the displacer cylinder. I'd start that right away, except not only do I not have an oil drum, I'd need access to machine tools to make the power cylinder.

Tips

• With the power piston shaft and displacer piston shaft mounted on the same point, getting distances right is a little tricky. It's a big parallelogram this way. Make them separate next time. Maybe even on a rotating collar so the phase angle between can be modified.
• If the shaft tiepoints have a lot of play in them, the engine works jerkily if at all (because all the motion is taken up in using the play).
• I think I overdid it on the height and underdid it on the width. Could have used a little angle-reducing distance on the piston shaft. Alternatively, shorten the stroke.
• The flywheel is a little heavy. The momentum should carry it through the compression stroke, it shouldn't have to be barely sucked in.

New Results

Latest solar trough configuration:

A few details:

1. Those are just simple, band-sawn parabolic arches. The mirror is attached with velcro. Works awesome. Ly.
2. I suspended the mirror from the pipe thinking that would keep the focal length constant while allowing simples changes of elevation. Why do I always go straight for the complicatedest solution in the universe?

And data from same:

The highest temp on there is almost 140°C. That's over 280°F. According to my rough calculations, the four steepest upward slopes indicate powers in the range of 20-25 watts. (Some of those dropouts are me messing around with the setup, some are cloud cover. Also, I later discovered that true solar south is like 15 or 20° east of where I've been pointing.)

Two big changes from the last run, other than the already-mentioned one of shortening the excess pipe.

1. Painted the pipe flat black.
2. Used a laser pointer to adjust the focus. I mounted a frosted glass square (I happen to have a bunch I bought for just such a use as this) at what I thought would be the focal length. Then I stood back and aimed the laser pointer in a roughly perpendicular way and looked where the point fell. This really needs a System to keep it perpendicular, but anyway I was able to determine that my focus was off by over an inch.

I'm thinking the Mark III will be the last iteration. Pointing in the right direction, and simplifying the pointing a little, may let me add another 10-20 degrees to the peak but that will be about as far as I can go with these simple materials and using an open-air design.

Belated Mirror Squeezer Results

I did one "live" run of data collection from the parabolic mirror squeezer, but after I did it I realized there was a major problem. The pipe was much longer than the mirror width, so it overhung. Too late, I realized that this meant there was unheated oil in there, making the temperature measurement invalid. And I don't even know what direction the error was worse in, since the overhang could also act as cooling fins. Anyway, here's the graph of the results:

It looks very similar to the original run in a hotbox:

One major difference: Check the x-axis. The parabola run is at least 4x steeper.

Anyway, if I get some sun this weekend I should be able to re-run with more accurate results.

Potpourri

• The diet hit a plateau. I'm down by 43 lbs from when I started, but in the last 30 days it's only changed by .5 lb. Looking at the chart from a year ago, I think I see a pattern. It was going down very slowly in May/June 2007, too. Looks like maybe 3 lbs in 60 days.

  There's probably two things going on. First, every year I imagine early summer weight loss should be easy. "Winter hibernation pounds just melt away naturally!" Second, a lot of good, fresh food shows up at the store.



• I used to listen to Pandora at work. The advertised feature, discovery and laser(ish) targetting of your tastes, worked great. The problem was that the selection wasn't so great. I was down to about 30 songs that they just endlessly looped for me. Then I switched to Last.fm. The targetting isn't very good, but the selection is pretty huge. Exhibit A probably says more about how out of the loop I am for just now finding this, but still: Pretty hilarious (Lyrics)


• And speaking of nerds: Do you like math? Do you also like mechanical devices? Then you will probably love How Round Is Your Circle. It's filled with mechanical ways to make, or approximate, mathematical functions, such as for linkages. The main problem with the book is that it's too short. It should really be a set of volumes so he can better explain each item. (Video teasers).


• I have dismantled and cannibalized the Squeezer for a much simpler version of a parabolic trough now in production. Stay tuned!

Speaking Of Things That Don't Work

My house is crammed with computers, but only two of them (mine and Mrs Username's) are connected to the internet. Which means the kids always want to use them to play games.

The basement is wired, so last night I finally got around to hooking one up. It's got some version of Windows on there and already has a network card, so it should be easy, right? Drivers?? I don't even know what the card is! Not to mention the fact that I'm not too comfortable putting a Windows machine on the internet.

Then I get a crazy idea: I'll make it dual boot Ubuntu¹! Takes me 45 minutes to download the latest disc, i.e. less time than it would have to identify, locate, download and install a NIC driver. Completely easy to install: Other than the fact that I had to adjust a slider to say how much room to leave for the existing Windows partition, I didn't have to tell it anything at all. It even got on the internet all by itself to download updates.

The question is: Is making my kids dual boot into Linux to play games child abuse or child undangerment? Either way, it certainly builds character.

¹Actually, Xubuntu, because this is an older machine. I considered Edubuntu, but I think I'll go with the small, fast one and then add kid-oriented packages.

Things That Don't Work vs Things That Do

I've never been completely happy with my temperature logger. It's a bit fragile in the sense that if anything at all goes wrong, and there's no way to tell that at the time, I lose the entire run. I lost a run Memorial Day weekend.

Instead of storing the measurements on the Arduino, I'd like to instantly beam them onto my computer far inside the safety of the house. That way I can track things realtime as well as be assured that I have them. Coincidentally, for my birthday, I got both another Arduino¹ and Making Things Talk.

The book describes a great number of schemes to make microcontrollers talk to each other and to computers. You can use wireless networking, bluetooth, XBee, etc (I have only the vaguest notion what some of these are). Naturally the easiest protocols require the most expensive hardware. I only need one way, slow communication, so I got a simple RF module.

Claim:

It works just like a serial port! Just connect the transmitter to the TX pin and the receiver to the RX pin! It Just Works(tm)!

Reality:

No.

Perhaps my unit was faulty. I found many tutorials and guides across the internets and while results varies, I can't ever really say it worked. I did see data appear for a short time, but mainly what I saw was noise. Or nothing at all, which is even less explicable.

Last night I had a brainwave. Or brainstorm. Something happened to my brain and it resulted in an idea. Why not use a wireless laptop as the go between? The kids have these OLPC dealies. The laptop has a USB port and does WiFi. I have a WiFi router (specifically purchased, used, to work with these laptops). About 30 minutes and 10 lines of Python later, I was reading values from /dev/ttyUSB0 and sending them out over a socket to my desktop to another 15 minutes and 20 lines of Python.

The guts of the entire scheme are already there. But with so much success so fast, I'd like to add features. For instance, instant graphing of values on both ends. A protocol so that the laptop knows if there's been an error and can tell me, out in the field. When I have more than one sensor, I'll need a way to indicate which sensor had what value. It'll be like a complete Science Sensing Station!

¹If you are at all interested in robots, sensors, controlling stuff with computers, electronics or just plain messing around, I highly recommend the Arduino. That SparkFun item is all you need, assuming you have a USB port (and possibly a cable). Well....you may also need some external electronics, depending on what you want to do. LEDs, resistors, motors, etc.

Ta-Da!

Once I abandoned the dead end of heat-formed acrylic, the ideas for cold-forming have been coming thick and fast. Here's the result of the squeezer configuration.

First, the squeezer itself:

Closeup of squeezing mechanism:

Result:

Closeup of accuracy:

Soo....yeah. That works. The squeezer is a bit overbuilt. I could go with 2x2s or even smaller next time. And one bolt every couple feet instead of every 6 inches. Other minor improvements. Those may never be made, though, since I had a real brainstorm on an extremely simple way to accomplish the same thing. More on that another time, though. I'm going to use this for my next experiment.