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Large Solar Forge nearly complete from 1977

The Solar Forge

and General Purpose, All Around

Light Bucket

  1. Introduction to the Solar Forge

    The Solar Forge is a large device to dramatically demonstrate the power of solar energy. It is hot, hot, hot. It makes things first smoke and then burst into flames. It has enough power to really be useful for small commercial processes in very sunny climes.

    Small Solar Forge burns wood from 1976
    1. Uses

      People being people, this forge is most likely to be used just to burn stuff up. A 2-by-4 will burst into flames in a few seconds. You can even make offering to the old sun gods by tying old cell phones to the target with iron wire and sending them off in a blaze of glory (take care, some devices explode).

      1. General Description

        The Solar Forge consists of a large parabola section, a carriage, and a target. The parabola concentrates sunlight, the carriage allows the parabola to track the sun, and the target allows the resulting heat to be put to a practical purpose.

        The parabola is the size of the floor of a garage and is attached to the carriage by a hinge. To track the sun, the parabola section is rolled in a circle around the target and tipped over on the hinge. The target stays in one place making it much easier to use the heat, but it also features a wind shield that turns so that its opening always faces the parabola. All the sun light falling on the 74 square foot collection area is focused to less than one square foot in the target. On a sunny day this size forge will continuously put about 7.0 kilowatts of heat on the target.

        The collection area of the forge is a pie-shaped section of a much larger parabola that has been turned around a central axis to make a surface. This structure is made like an airplane wing of either wood, or sheet metal. The front surface is either covered with aluminum foil, made of polished metal, or tiled with 4" by 6" pieces of glass mirror to reflect sun light.

        The parabola section is carried around a central target on a carriage made from an old pickup truck frame. This frame can be made to double as a trailer when moving the forge from place to place.

        The target is in center of the turning circle on a simple steel tower and is shielded from the wind by a metal cover to form a small kiln. The target remains in one place as the parabola moves around it making it far easier to may use of the heat. It is set intentionally set high to insure that the dangerous focal point is well above everybody's head.

      2. Developing Countries

        In developing countries the Solar Forge can be used as a small industrial power source to forge recycled metal and to do arts and crafts work. As it happens, it is impossible to make the springs or drive shaft of any car out of bad steel. Most of this steel is better than that used in the famous Japanese swords. It is a shame to let such good material be mixed in with scrap iron and not to forge it into tools people really need. A wide variety of tools and decorations can be made from this good steel. Also, crafts items made with the Solar Forge way can even be sold with the claim, "forged by the sun," which should give them a good market in developed countries.

      3. Developed Countries

        In developed counties, the Solar Forge could be towed to county fairs and alternative energy demonstrations. However, it will be much easier and safer to simply make videos for show and sale. It would make a flashy project for a cable TV do-it-yourself show.

      4. Space Applications

        People interested in space exploration will like this design because it would be very easy to use on the Moon. On the Moon, this type of tracking collector could be used both for power and for receiving interplanetary communications by laser.

        Most large sun tracking power sources need a complex and massive foundation. Foundations are hard to install in the Moon's pulverized rock surface. The Solar Forge needs only a flat space and the same rolling technology proven by the Apollo Lunar Rovers.

      Small Solar Forge from 1976
    2. Past Constructions of the Solar Forge

      I built a small version of the Solar Forge in 1976 with about 12 square feet of collection area. I would set a 2-by-4 in flames in less than a minute. I then built a larger parabola in 1977 on a truck frame, but was not able to get a shinny surface mounted on it before money ran out. Neither of these projects has ever appeared on the Web before.

      Also, in 2004 a team of my students built a Solar Forge for a science fair project.

      With the current need to move to renewable energy, I think the time is now right to build a new, big, powerful solar forge.

    3. Why do it, or Hubbert's Peak and Global Warming

      We are now at a historic watershed with respect to energy. Late in 2005, the human race used up exactly one half of all the oil that the human race will ever use. The Earth's gas tank was exactly at the 1/2 mark.

      This historic event is called Hubbert's Peak. It will be a defining event of the first decade of the 21st century. Its social, economic, and lifestyle effects will be comparable with those of the industrial revolution.

      There will be winners, and there will be losers. If we want to be one of the winners, all we have to do is redesign the entire world energy infrastructure. Nothing less will do. It is important that we get this process started immediately with a major effort that includes educational projects like the Solar Forge.

      The only good technical (and very readable) references on Hubbert's Peak are:

      1. Kenneth Deffeyes, Hubbert's Peak, (Princeton University Press, 2001)
      2. Kenneth Deffeyes, Beyond Oil, The View from Hubbert's Peak, (Princeton University Press, 2005)

      Also, in developing countries the most widely used fuel is fire wood. Over use of this fuel has resulted in deforestation in many areas which is adding to global warming. Alternative power sources, like solar, are sorely needed there.

    4. Elegance of the Design

      The Solar Forge is designed using only a compass and straightedge. The process can be taught easily and does not even require you to do so much mathematics as to multiple two numbers together. This makes the forge very easy to adapt to local conditions, uses, and materials.

      This layout process is called lofting and was used to design the great sailing ships of old. You simply layout the design on a large flat surface, such as sheets of plywood, and swiftly layout the curves for the parabola in only a few minutes. You then use this full-sized curve to make patterns for all the other parts.

      This design process is easy to do, easy to teach, and is great fun.

    5. Limitation on the Forge's Use

      There are however limitations on the Solar Forge's use. Like all solar concentrating devices, the forge can only be used in locations with a large number of cloudless days each year. Unlike solar hot water heats and photovoltaic panels that produce reduced output on cloudy days, all solar concentrators are useless with anything more than a few scattered clouds. This limits the use of solar concentrators to areas like the American Southwest, but these areas do include regions like North Africa where millions of people live.

      The Solar Forge takes a lot of room and can only operate on a flat surface of either dirt, grass, or parking lot. The space must be large enough for a compact car to turn a complete circle. It is best suited for a rural location like a farm. The frame will last many more seasons if it is stored under a roof such as a farm equipment shed in off seasons.

      The focal length of the forge is fixed in the design. The hot spot cannot be moved around or extended out to a distance. Also the quality of the optics is too poor for the light to be farther concentrated, reflected, relayed, or formed into a beam. This is why the reflector can be fairly described as only a light bucket.

    6. Temperature Requirements

      Just as one would expect, low temperatures are easy to achieve with solar devices and higher ones much more difficult.

      If all you need is hot water for domestic use (45 C or 110 F), then a flat solar panel that does not track the sun will do well and be the cheapest and most easy to build. These work fairly well even on cloudy days.

      If you need hot water for light industry or heat to run a Sterling engine (95 C or 200 F), you can make a trough type collector that concentrates solar energy in one axis only. These are moderately easy to build and straight forward to make automatically track the sun because they need move only in the one axis. The Solar Forge process can be used to design the curve of the collect trough for this type of collector.

      If you need higher temperatures (above 100 C or 212 F), then you must concentrate the sun light in two axis with a reflector that is a section of a rotated parabola. These are much harder to build than the simpler types and must track the sun in two axis. The Solar Forge is of this type.

      The most likely use of a Solar Forge is as heat source for small businesses. The following table shows the temperatures that are needed to work some common recyclable materials into new products:

      Material F C
      Steel, work 800-1000 430-540
      Steel, anneal 1350-1500 730-820
      Steel, melt 2500-2700 1350-1460
      Aluminum, melt 1218 660
      Brass, melt 1652-1724 900-940
      Bronze, melt 1562-1832 850-1000
      Copper, melt 1981 1083
      Lead, melt 621 327
      Zinc, melt 786 419
      Silver, melt 1762 961
      Gold, melt 1946 1063
      Glass, work 1200-1800 650-980
      Glass, melt 2000-2400 1100-1320

      From this table we can see that the Solar Forge is just a toy unless it can raise a bar of steel with a cross-section larger than your thumb (20 mm) to a good hammering cherry red (800 F, 430 C) in 20 minutes. Furthermore, to be of commercial value, it must be able melt a crucible with 2.2 pounds (1 kilogram) of aluminum (1218 F, 659 C) in one hour. These will make good test criteria for Solar Forge prototypes. No Solar Forge prototype has yet passed these tests.

    7. Choices

      Before you begin building your solar forge, you need to think through exactly what you are trying to do and to make several key decisions:

      • Product -- Are you just doing a solar energy demonstration or are you going to make a produce? Do you want to do black smithing? Do you want to make jewelry? To you want to make stained glass?
      • Size -- Some tasks, like solar power demonstration for a science fair, may need only a small unit. Most applications like metal work need a really big unit.
      • Weather protection -- If you want your forge to last a long time you will have to carefully seal the edges of the plywood and paint the backs of the mirrors.
      • Height of target -- The higher the target the more effective the mirror and the safer the hot spot. However working with hot materials over you head can in itself be dangerous.

      The example described below is a large unit with 74 sq. ft.(6.6 m2) of effective area . It is intended for light manufacture in a developing nation.

      Rockler order link to first page
    8. Construction

      The construction of the solar forge requires a moderate level of construction skills and commonly available tools. It does require work in a number of different areas that include automobile mechanics, woodworking or sheet metal work, and metal fabrication. Some welding will be needed on the frame. This is not a lone beginner's project. The work will best be done as a team effort.

      This is a large construction and you must take safety seriously. Be sure to understand the proper use of all your tools before you use them. Do not work off the ground without proper scaffolds and ladders. Many of the operations require moving and particularly turning over heavy structures, be sure you have enough people to do the job safely. This is not a one-man project.
      1. Large Solar Forge truck frame from 1977
      2. Building the Carriage

        You need to secure the carriage before you can customize the light collection parabola to your needs. The carriage can be an old light truck frame for large forges or a hand build go-cart like frame for smaller ones. Using a small car with unibody construction but real no frame, can be done but would be more difficult and requires a cutting touch. The example below uses and old truck frame and which resulted in a rather larger than desired turning radius.

        To build the carriage, think Junk Yard Wars on cable TV. You need an old truck or SUV frame. You will have to remove the body and most of the engine. The best frame would:

        1. Be free or at least very cheap
        2. Be small with a tight turning radius
        3. Have good running gear and suspension
        4. Have a standard transmission
        5. Have four wheels and tires that hold air
        6. Have a working rear axial and differential
        7. Did I mention: be free

        You need to remove and hopefully recycle:

        1. The gas tank and contents
        2. The oil and antifreeze
        3. The interior
        4. The entire body
        5. The steering column
        6. The alternator, corroborator, radiator, fuel pump, fan, and hoses
        7. The break pads or disks with the calipers
        8. The heads, pistons, connecting rods, and timing belt

        You need to keep (even if you do not end up using them all):

        1. The frame
        2. The axles, wheels, and tires
        3. The tie rods, steering box, and other steering parts
        4. The block, transmission, and drive shaft
        5. The bumpers
        6. The battery and starter motor

        When you are through with your stripping, you should really be down to the bare bones, the frame should roll easily around in the tightest circle possible.

        Solar Forge Planning Top View showing pickup frame and turning radius

        Securing this frame is the first job needed to start this project. You must then determine the smallest turning radius possible for your frame, so that you can customize the solar forge design to this specific carriage and also the desired target height. You will need the following measurements from your stripped frame before you can design the parabola section:

        1. Mid-radius -- The turning radius measured at the inside center of the frame
        2. Mid carriage height -- The height of the frame above the ground at its middle with some weight on the frame to approximate the weight of the parabola structure.

        If you are going to trail the forge on the highway you will have to make a proper towing bar that steers the front wheels. You will need decent tires. This will cost work several hundred dollars. You will also have to limit the width to under 8 feet. This can be done by making the parabola in three pieces. The main body will be just under eight feet wide. Top and bottom sections will then be bolted on and are about two feet wide each. The forge will always be clumsy to haul as the wind effects it strongly and this will put a limit on your speed. I would not enjoy hauling it for long distances, for example going on a circuit of county fairs.

        Solar Forge Layout Tools, compuses and straightedges

      3. Special Tools

        You do need a few special tools to layout your solar forge. None of them are expensive, none of them take power, and some can be home made.

        1. Three compasses -- You could use an adjustable compass, but it is much faster just to make three compasses to suit your specific work. These can be easily made from a stick of wood, a masonry nail, a pencil, and a bolt. The short one should be the height of your mirror segment plus the distance between the rows of mirrors, 6.25 inches in the example. The mid-sized one should be one to two feet and the long one about a meter.
        2. Big T square -- You will need a big T square like the ones to work dry wall with a length of 48 inches.
        3. Several long straight edges -- You need one moderate straight edge about a yard long. You will need a very long straightedge or two that can reach all the way across your lofting. You can make them from three pieces of molding from the home supply store. You use one piece by itself and joint the other two end to end with a shorter plywood plate on top.
        4. Trammel points -- You will also need a set of trammel points which are a widely used tool that lets you make a very long compass with any small dimension wooden beam. These let you turn you long straight edge into a really wide compass.

        The wooden parts can be cut out with a simple jigsaw. A cut-off or radial arm saw are a big help in making the ribs. You will also need to drill holes and install screws.

      4. Making your custom design by lofting

        You can do the detailed design of your solar forge all by yourself without so much as adding two numbers together. The process is called lofting and is the traditional design technique for laying out the ribs of wooden boat hulls. However, you should contact me for a more detailed procedure before starting work, or if you need a metric version.

        There are four stages to the lofting process:

        1. Laying out Mirrors -- Laying out the positions for the mirrors along each rib.
        2. Designing the ribs and stringers -- The mirror backing forms the front of the fibs, the backs are laid out in this step. The stringers, or semicircular pieces that run across the ribs, are also drawn directly on plywood sheets.
        3. Marking the stringers -- The positions of the ribs are developed and marked directly on the stringers.
        4. Designing the Surface -- The conic strips on which the mirrors are mounted are laid out directly on plywood.
      5. Preparing the Floor

        The design is worked out full scale on a large drawing area which could be a smooth floor, but is more often sheets of plywood or pink resin paper. This example uses plywood sheets and requires a flat work area 20 feet by 27 feet. The top section of the lofting with the side view of a rib is kept for the whole process. The bottom section is first added to lay out the rib spacing and then replaced with the construction plywood to lay out the surface curves directly.

        This example starts on a drawing area equal to two and one half pieces of thin plywood. This area will be used to make a paper pattern that is the transferred the construction materials. Another half piece of plywood is cut into strips and used to provide a place to draw the axis line. You may want to tape the plywood pieces together and tape the pieces to the floor. The top edge of the plywood sheets defines the direction of the sun and the factory edges help to keep everything square.

        Solar Forge first mirror layout showing arcs

      6. Starting the Lofting and the Theory

        As shown above, the first thing you do is lay out the top section of the lofting. It has two key lines the axis of rotation and the ground line. Draw a line to be the axis of rotation for the parabola parallel to the factory edge of the plywood. Perpendicular to that line, you lay out the ground line. You then choose the height you want for the target and draw a four inch circle on the axis line there.

        The only other measured point needed is a reference point for the bottom of the mirror closest to the carriage frame. Mark the location of the carriage frame closest the axis on the lofting using the turning radius and carriage height you took earlier. Then make your first mirror reference point about 10 inches in higher and about 10 inches closer to the axis line to give yourself room to build the parabola structure.

        Using the top edge of the plywood for reference, draw a line straight down to the first mirror reference point with your T square. This represents a ray of sun light striking the bottom of the mirror. Now using your long straightedge draw a line from the reference point to the bottom of the target circle. This line represents the ray of light being reflected to the target.

        We now need draw the first mirror so that it will reflect the incoming ray along the focus ray to the target. We know that the angle of incidence is equal to the angle of reflectance. So we need to bisect an angle. The question is which angle?

        The angle of incidence and the angle of refection are measured from a line perpendicular to the mirror. We could use our compass to bisect the top angle formed by the sun ray and the focus ray and then draw a second line perpendicular to the bisection. There is a faster way. Simply extend the focus ray past the sun ray and bisect the larger angle formed by the sun ray and the extension of the focus ray. This process first adds 180 degrees to the original angle and then bisects the total angle which produces a result identical to the two step process we first suggested.

        To do the bisection, place the medium compass on the reference point and draw two arcs, (#1 arc pair in the drawing). Then place the long compass on each of these marks and swing crossing arcs above and below the focus ray (arcs #2). You can then connect these arc crossings with a straightedge and have the correct angle for the mirror. Use the short compass to define the length of the mirror plus gap (arc #3) and thereby find the reference mark for the bottom of the next mirror up the line.

        The next step is to use the long straightedge to draw the mirror line all the way to the axis line. Be as accurate as you can in hitting the marks. This is easiest if these crossed arcs are far apart. This is why it is best to always use the largest compass possible that still leaves marks on the writing surface.

        This surface radius line gives you the information you need to cut the mirror row backing material so that it will bend to hold the mirror at the correct angle and fit in the curved parabola structure. Why this is so can be seen with a simple experiment.

        Take a piece of scrap typing paper. Roll it into a cone shape and tape the overlapping edge. Put a mark on the tip of the cone. Now cut the open top of the cone off flat with a pair of scissors. The result should now look like the container for a snow cone. Mark the two edges where they over lap. Now cut the tape and unroll the cone. The result is not obvious at first but it is clear if you think about it. The cone unfolds into a section of a perfect circle. The radius of the circle is the distance from the cone point to the top edge.

        The surfaces we that mount the mirrors on are sections of a cone. Extending the mirror line all the way to the axis gives us the radius of the needed cone section. Each row of mirrors has a very slightly different radius that get longer from front to back and are always longer than the top view radius.

        This process qualifies as a cute trick even if I do say so myself.

        Solar Forge first stage in lofting where the mirrors are laid out

      7. Lofting the Mirrors

        In this next step the location and angle of each mirror row is worked out. This process also defines the front edge of the ribs.

        Drawing in the rest of the mirrors simply follows the same procedure as the first mirror. When you work up the parabola, you define the top point of the last mirror (with the space) to be the bottom of the next mirror you are working on. When you are working up the parabola in this way, be sure to run the focus ray to the bottom of the 4" target circle. You can stop when you get as high as you think you want your parabola to reach.

        You can then finish by working down from the bottom mirror by using the bottom of the first mirror as the top of the one below it and working down. When you work down, run the focus ray to the top of the 4" target circle. Stop working down when you get about 10 inches from the ground line. The example worked out to 22 mirror rows with 6.0 inch mirrors and 0.25 inch spaces.

        Check the line of points where the mirror extensions cross the axis. These points should make a nice progress and not be out of order or have two points close together and then a large space. In our example, the spacing started out at about 2 inches for mirrors near the bottom and grew to almost 5 inches near the top. If the progress does not look smooth, try redrawing some of the mirrors using bigger compasses. Clearly number each of the crossing points to match the mirror number.

        Solar Forge second stage in lofting where The ribs and stringers are laid out
      8. Lofting the Ribs

        You can now lay out the rib pattern on the same flat surface. It is easiest to make a pattern in paper or masonite from the lofting and then use the pattern to cut out the pieces for the nine ribs. In the example we are making the ribs from 3/4 inch plywood. In the 1970 example pictured at the start, I made them from 2 x 6 lumber. The full line of mirrors make up the front section of the ribs.

        Divide the length of the complete rib up into easier to work pieces. In the example, we start with the mirror reference point and go up just under eight feet. We then lay out a short section of rib at the top and one at the bottom. The rib sections can be later attached with single plywood plates on one side. Two plates would be much stronger but you could not then remove the sections. Where the plates are permanent, use screws and glue. Where the plates are to be removed use bolts, nuts, and washers. Always place a flat washer between wood and a bolt head or nut.

        Solar Forge rib patterns

        Draw a line from the bottom of the first mirror in each section to the top of the last mirror. Measure back and draw a parallel line the width of your rib material back. This width is 6.0 inches in the example. Draw lines straight down, like the sun rays, from the mirror points that define the ends of the rib sections.

        Since a number of ribs must be made it is best to make a pattern out of heavy paper or masonite. The problem is to transfer the points from the lofting to the pattern. One way to do this is to drive a very small nail into each point on the lofting of a rib and then cut the nails off diagonally to form very short sharp points close to the surface. You can then press the pattern material on to the cutoff nails using a cloth or cardboard pad. You can then connect the dots to transfer the information from the lofting to the pattern. When not in use the points will have to be protected with a piece of corrugated cardboard.

        The design needs stringers to run at right angles to the ribs and space them out properly. The stringers in our example are semicircles that are flat with respect to the ground line. This makes them easy to layout and build, but does require you to cut notches in the ribs. The ribs can later be reinforced at these notches with 3/8" plywood blocks.

      9. Lofting the Stringers

        The example shows six stringers that run from side to side of the parabola section. One at the top, one at the bottom, one at the top of rib mid-section, one at the bottom of the rib mid-section, and two in the middle of the rib mid-section. Most are 5.0 inches wide but the top one is only 4.0 inches. The radius for each of these stringers is taken from a line drawn from the stringer to the axis line.

        You lay out two sheets of 3/4" plywood below the first lofting. These sheets can be used to make stringers directly. Also add a strip of plywood against the axis strip down the middle of these pieces and draw a centerline.

        Start with the back rib and use the trammel points and long straightedge to draw to sections for the top stringer. Since the parabola section will be more than 8.0 feet wide, two sections are needed (except for the shortest one). Continue the layout of the other stringers but skip the area with the seam between the two sheets of plywood. The center points for the curves can fall anywhere along the center line strip.

        When you are happy with the stringer layout, you can take up the plywood and cut out these pieces. Be sure to mark them clearly.

        You then need to join the two pieces of the stringers. This can be done by overlapping the two pieces by about a foot and carefully aligning the outer curve. You tack the two pieces together with nails and cut straight through both pieces. You then joint the two pieces by nailing a 3/8" plywood block on the bottom side of the stringer being careful to retain the curve.

        Solar Forge thrid stage in lofting were the stringers are Markd for the ribs.

      10. Lofting the rib locations on the stringers

        The next lofting requires four pieces of 3/8" plywood that replace the pieces used to make the stringers. On these you will lay out the ribs and define the size of the parabola section. Only benchmarks and measurements will be taken from this lofting so it can be done on pink paper. Layout the plywood as shown above and draw the centerline.

        In our example the width of the parabola is set at 12 feet by this layout. Draw side lines from the far corners to the center line and axis line crossing. Use the trammel points to bisect the angle beaten the side lines and center line. We now have the position of five ribs. Bisect each angle again and we have the positions of nine ribs with a spacing of less that two feet at their widest part.

        Mark out the width of the rib material on this lofting. Center all but the outer two which go just inside the lines.

        Draw the curves for the stringers on this top view, this time all from the center point on the axis line. Lay each of the stringers semicircles in its position. Adjust the stringers so that the joints do not fall between the same ribs. Mark the stringer ends where they cross the side lines and cut them off with about two extra inches to spare each way. The ends will be trimmed off later.

        Solar Forge fourth stage in lofting were parabola surface is laid out

      11. Lofting the parabola surface

        If you have the money you can layout the mirror surface on four and a half new sheets of 3/8" plywood. Otherwise, sand the curved lines until they are dim. Keep the two side lines.

        On the mirror lofting make a mark on each mirror line to divide the mirror from the gap. The gaps are important because the let each surface strip curve independent of its neighbors.

        Using the trammel points and your longest straight edge, draw a circle for the top mirror surface section. This distance runs from the top of the top mirror (not the gap) to the point where its extension line crosses the axis line. You will have to use a strip of plywood to extend the center line out past the axis line to get enough space. Now shorten the radius by the width of the mirror ( 6.00 inch in this example), using the same center point, and draw the inter edge of this section. Only one section 12 feed wide section is needed for each surface section. Mark the strip and center point with the mirror number.

        Continue this process for each mirror. Each strip will have a different center point. Be sure to skip sections parts of the plywood joints where a section would cross. Mark the side lines on the sections.

        Cut out the surface strips. The back sections are made from two pieces. These need to jointed like the stringers were with blocks of 3/8" plywood. Where possible, use the scrap materials at the ends and slide the strips around so that the joints will not fall on the same space between the ribs. Trim the ends of the surface strips with a couple inches to spare on each end. Number the sections carefully.

      Large Solar Forge parabola started from 1977

    9. The Build

      The construction of the parabola is very like building a wooden sailing boat hull. The construction material in our example are plywood, exterior glue, and screws. The construction is in eight steps:

      1. Preparation
      2. Building the parabola structure
      3. Attaching the surface
      4. Install the mast
      5. Building the hinge
      6. Building the tracker
      7. Installing the mirrors
      8. Building the target
      1. Preparation

        Plywood does not last a long time left out in the rain. The Solar Forge is used only in sunny climates but still we need to protect it against moisture as best we can. This requires filling and sealing the edge of all the plywood parts before assembly.

        The first step is to fill all the voids in the edge of the plywood with splinters of wood. The bamboo slivers used for oriental cooking work well for long voids and tooth picks for short one. The wood splinters should slide easily into the holes and not to be too tight as this plies the plywood layers open. Install the splinters with lots of water resistant glue and leave some extra sticking out until all the glue is dry. This step is particularly important for the surface strips as it helps them bend uniformly.

        Cut off the splinters ends and sand all the plywood edges. Seal the edges with wood sealer or thinned shellac. Seal only the edges not the flat parts.

        Prime all the surfaces that will not to be glued with water based wood primer. You may want to mask off the area to be glued and use a roller of spray gun. You may have to wait on painting the backs of the surface strips as you do not yet know the exact location of the ribs, but paint the edges of the plywood well.

        Large Solar Forge parabola attached from 1977

      2. Building the parabola structure

        Build up a full set of ribs. Install the plywood blocks in the weak areas. If you have bolt on sections, install their ribs but do not glue the areas that have bolts.

        The ribs and stringers are most easily assembled upside down. A simple support line the one in the picture above is a great help. This is at least a two person job. Be careful when working above the ground.

        The stringers are attached to the notches in the underside of the ribs. Start with the top and bottom stringers. Screw and glue all the joints. Be careful to get all the ribs in exactly the right place as marked in the lofting.

      3. Attaching the surface

        The frame now needs to be turned over. Use a number of people with push sticks and ropes. Be sure that all the people are so placed that they are not hit if the frame moves unexpectedly. Prop up the top end of the ribs so that the frame is steady and top and bottom are fairly easy to reach.

        Install the top surface row by bending it to fit the frame. Look carefully at the mirror marks on the front of the ribs. Attach it with glue and screws.

        Repeat the process for the bottom surface strip. Then work you way from the top and bottom to the middle.

        After the glue is set, trip off the waste wood that is outside the sidelines.

        Solar Forge Mast with cables, turnbuckles, and anchors

      4. The Mast

        The mast counter acts the forces in the surface sections that otherwise would try to flatten out the parabola section. It is a piece of thin metal tubing with wooden dowels fitted into both ends and is about one inch in diameter. The base is bolted to the central rib in about the middle of the parabola section. It is best to cut half circles in two surface strips rather than cut away most one. The top should have two through holes about an inch apart that point toward the four corners of the parabola sections. The holes must be well rounded so the cable is not cut.

        The cable must be anchored to the ends of the outer ribs. If the ribs are in three sections, anchor the cables to only the mid section. Place four turnbuckles in the lines so their tightness can be adjusted.

        The mast stays also will support a tarpaulins away from the glass for weather protection when the forge is not in use.

        Solar Forge hinge structure.

      5. Building the hinge Structure

        The hinge line must point directly at the center of the target for the tracking to work properly. It is drawn in first lofting. The back end of the hinge line should be about 10 inches above the frame to give you room to work.

        The hinge structure is in two sections. Note that the hinge structure only tips over in one direction. The bottom section must be custom designed to fit your specific carriage. Draw it out on the first lofting. It may include double plywood sections, a heavy beam or angle iron across the bottom, and bolts to the frame. Again, always place flat washers between wood and a bolt head or nut. You may also wish to add diagonal braces of metal or wood down to the carriage frame.

        The top section is made of heavy plywood and overlaps a rib for several feet. It should not actuarially touch front surface and may have notches for various blocks. It does not attach to the center rib, but rather one closer to the low edge of the parabola when tracking the morning sun. The fourth rib was used in the 1970 example. You may wish to add diagonal braces to other ribs. Do not glue this section to a rib until you have fitted the entire hinge structure to the carriage.

        The hinges are heavy agricultural gate hinges with fixed pins. They are held on with bolts. Three of them will be twice as strong as two.

        When you fit the entire parabola structure to the carriage, you will probably find that you have to cut away a few sections of the lower edge to make them miss the carriage wheels. Simply the structure cut back a whole surface pieces to a rib end. The resulting outline has a bit of a rectangular look.

        To keep the sun focus on the target, the entire parabola structure will have to be tipped over as the carriage rolls around in a circle. The carriage can be rolled by pushing it or by rigging up a crack to the deferential. Simply having a place to sit so you can push with your feet works well.

        A means is needed to tip the parabola over. This tipping can be done with a small pulley and rope, a small hand winch, or with push poles. This structure is heavy so you may need more than one method working together, such as pulleys and push poles. You will also need to add reinforcing elements where the tipping structure attach to the ribs.

        Small Solar Forge Sun Tracker Solar Forge sun tracker drawing

      6. The Tracker

        It is not obvious at first that this design can track the sun at all. You can prove to yourself that it can with a simple experiment with an umbrella. Open an umbrella and place a 8.5" x 11" piece of white paper inside it by working the paper under the ribs (cut the paper if needed). Hold the umbrella upside down. Pretend the umbrella is a complete solar parabola, and the piece of paper is the section you are going to build. The sunlight is coming from the direction of the umbrella shaft.

        Grab the shaft about 6" up from the rib hub so your fist represents the target. Hold the umbrella above the ground and move it around so that the shaft points at any place in the sky. As you move the shaft, also rotate the umbrella so that the white piece of paper remains about the same distance above the ground but tilts to different angles. Satisfy yourself that you can point the shaft to any point in the sky and keep the parabola section the same distance from the ground.

        You would have to tip the parabola all the way over to the vertical to point at the sun on the horizon. It is all right if it only reaches far enough to see the sun two hours after dawn as there is no real power available until the sun is at least that high in the sky.

        The aiming device, or Sun Tracker, shows an image of the sun on a frosted screen. The operator never has to look directly at the sun. The operator must simply keep the image in the middle of the screen by moving the carriage and adjusting the tilt of the frame. This device is simple to construct from a inexpensive lens, piece of PVC pipe, and a piece of frosted plastic or glass. It is mechanically aligned with the axis of the parabola. The lens can come from inexpensive reading glasses.

        The tracking is physically done by hand and is a bit awkward, so it takes practice. You must keep the image of the sun in the middle of the Sun Tracker window by moving the carriage around in a circle. You may push the carriage or rig a crank on the differential. With some additional work, you can move the carriage using pulses of electricity to the starter motor. You also have to adjust the tilt of the parabola with either a rope and pulley, or a screw jack. It takes a while to learn to track the sun as the amount of adjustment needed varies with the time of day. It is best to have an assistant do the tracking while the master works with the heated material, just as the blacksmith's apprentice worked the bellows in olden days.

        The tracker may be attached to the back of the parabola structure but this may be too high. If you let it through the surface you will have to build a small box against a rib to support the weaken surface material.

      7. Attaching the Mirrors

        Paint the front surface with one or two coats of water based house paint. Be sure to cover all the plywood edges.

        The cheapest available mirrors I found were thin glass in one foot square sections. In the example these were cut into 4 inch by 6.0 inch rectangles with a hand held glass cutter. Wear gloves. A few pieces will be broken in the process.

        I chose to soften the edges by hand rubbing them against a course carbide stone. This made later handling much safer.

        To extend the life of the mirrors, I also painted the back sides and edges with water based house primer. I do not know if this is worth the effort.

        Attach the mirror sections to the surface with general propose construction adhesive. Five quarter sized blobs should be enough. Press the glass plates uniformly onto the surface strips. Avoid pointing the parabola at the sun during this process.

        Solar Forge targe drawing

      8. Building the Target

        The design of the target is completely dependent on the work you plan to do with your forge. The example target is made from a 55 gallon drum, the front barring and wheel from a junk car, rebar, and fire bricks. It is intended for metal work.

        For the target to get very hot, it must be protected from the wind. The size of the required opening can be measure on The first lofting by drawing a line from the top of the top mirror to the target and a line from the bottom of the bottom mirror to the target. You can then sketch out the size of your target structure and measure the needed opening. As the parabola tips over on its side, the opening has to be about as wide as it is tall.

        The target mounted on a pivot made from an old car front wheel assembly and loosely attached to the carriage with a piece of pipe just off the ground. The opening of the target then follows the carriage automatically.

        The back and bottom of the target are lined with one inch thick fire bricks and mortar to hold heat. The opening incorporated two hooks that let a straight piece of steel stick straight through the hottest point.

    10. Cost

      Don't start this project without a $1000 US budget in hand. You will probably need to raise another $2000 US in money and donated materials before you are through. The safety equipment alone will cost several hundred, do not skimp there. Please email me if you need to work out a detailed cost estimate.

      How much good stuff and labor can you get for free?

    11. Special Features

      Some people will want to add special features. For example you can retain the starter motor and battery, then move the carriage by pulsing the starter in first and reverse gear. Adding a small photovoltaic panel to charge the battery will also let you play a radio. The radio can provide critical information on weather, security, and markets as well as entertainment.

      You can also add a seat in the shade and good places to fit your body as you to push on the carriage around with your legs. You may like a tool shed and shade for the work area.

      In many developing countries, the user will certainly want to paint the whole structure bright colors and give the forge a name like a boat.

    12. Safety Concerns

      This forge has the power of the propane burner used for hot air balloons. It produces light intensity heat comparable to that of a welding arc, only larger in area. It can do damage within its turning circle, fortunately when not tracking the sun it tends to spread the light falling on it over a large area of the sky. Care is needed to safely use this device. This is an industrial device, not just a toy.

      The Solar Forge has a number of important built-in safety features most of which are not present in most solar concentrator designs. These include:

      1. The Solar Forge cannot start fires at a distance. At twice its focal distance the light cone has expanded to the same size as the original mirror surface. It is then no more dangerous than a large number of shinny objects laying in the sun.
      2. The focus is at a fixed location.
      3. The workers do not look at the sun. They need only look at a safe tracker.
      4. The workers can wait in the shade when not needed for specific actions.

      However, safety equipment is still required for safe operation. This equipment includes:

      1. Fire Extinguisher
      2. Welder's helmet
      3. Welder's gloves
      4. Welder's tongs
      5. Welder's apron
      6. Welder's goggles for all viewers
      7. Bucket of water
      8. Warning signs
      9. Tarpaulin to cover collecting surface when not in use

      Do not operate this piece of equipment without all the safety equipment on hand. Do not even think of building this device if you are not willing to use it responsibly, with power comes responsibility.

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    13. Myth Busting

      The Myth Busters television show on the cable Discovery Channel has built solar concentrators several times. They have made at least three attempts to test the Archimedes myth that a Roman invasion fleet was set on fire with mirrors, but they have never got a fire started by sun power alone. The Solar Forge shows both that why this process is difficult and how a practical machine to do real work can be built.

      To work a practical large solar concentrator must:

      • Hold the mirrors at precise angles in two degrees of freedom
      • Track the sun
      • Have a fixed target

      The Myth Busters trials so far have not met these requirements, but the Solar Forge does.

      I would be happy to help the Myth Busters build a proper Solar Forge, but first we must have a new myth. I would like to suggest:

      • Home-build solar energy devices are nothing but hippy-dippy toys that can ever-so-slowly cook a meal or make a gallon of sun tea, but they are simply more trouble than the are worth.

      Any member of the Myth Busters crew would be competent to build a Solar Forge and its size and power put the design right down their ally.

    14. The Next Step

      Please email me if you are interested in building the Solar Forge or would like to see the sketches in more detail. I am looking for a team of students to take on this project and a sponsor for the modest amount of money needed to build it. I will help you detail the design to your specific needs, tools, and scavenged parts. You will only need to convince me that you will handle the Solar Forge's power responsibly.

      Email me and I will send you a full size rib pattern and a set of the sketches from this article.

      I am particularly interested in finding someone in a developing country who would be interested in building a Solar Forge. I am sure it could be of practical use.

      Thanks again for visiting the Woodware Designs Web site.

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