This is a description of a heat tempering oven I have recently 
         built. The germ for this idea came from the Planing Form (Sept/Oct 97) 
         which described the bamboo bazooka driven by a heat gun. This seemed 
         like an economical approach since you have to get a heat gun anyway 
         and thus can avoid the cost of the electrical hardware needed for 
         convection ovens. However, the design of the b-bazooka struck me as 
         being overlong for my workshop area and intuitively seemed likely to 
         have a temperature gradient from one end to the other since it has a 
         large exposed surface area relative to its internal volume (This may 
         or may not be true; I have not built or tested that design and is only 
         my rule-of-thumb impression). However, these factors ruled out this 
         design for me. The oven described below was built for $45 (not 
         including the heat gun) since I had some scrap lumber and fiberglass 
         insulation laying around. From the ground up it would probably cost 
         about $65-70 (sans gun). 
     
                Temperature testing of the oven that I built showed a zero 
         temperature gradient from one end of the oven to the other (yes, zero; 
         same exact temp at both ends and middle tested at 364 degrees F) so I 
         expect little trouble with hot spots. Since it is a forced air design 
         you may have to modify tempering times since the humidity inside will 
         not increase as the cane vaporizes its moisture. The primary caveat 
         with this design is that there is no R and D.; It was conceptualized, 
	    built and worked well as a one-shot build. It very likely could be improved 
         upon. Its main advantages are its relatively short length, low surface 
         area for heat loss compared to the distance traveled by the airflow, 
         efficient performance, and minimal cost over and above the heat gun. 
         In addition, it should be easy to lengthen with minimal investment if 
         you wanted to try some one piece rods. The disadvanges I see are its 
         bulk and weight. It is easy to roll around on casters but it is about 
         15.5 inches square by 6.5 feet long and is heavy because of the wood 
         construction. On the good side of this, the surface of the wood hardly 
         warms up at all and it is sturdy enough to use to stack tool boxes, 
         materials, etc. on top of it. 
     
                Design concepts: The basic construction concept is of two steel 
        duct pipes (one inside the other) which are capped so that air flow 
        provided by the heat gun flows in one direction through the outer tube 
        and flows back through the inside of the inner tube to be vented via a 
        chimney. Thus, the inner tube gets heat via air flow and from heating of 
        its walls. Countercurrent heat exchange minimizes the potential thermal 
        gradients that may occur (in theory). There should be no radiant heat 
        affecting the cane.
          The inner duct pipe tube is supported by galvanized steel discs cut to 
        the size of the I.D. of the larger outer duct pipe. The center of these 
        discs are cut out to the size of the inner tube so they may slide down 
        over this inner tube. These discs (support baffles) are cut to create 
        flanges on the outer edge which are bent to create fins (like a fan 
        blade) to create turbulent spin to the air flow and prevent streaming of 
        the forced air along the inner tube which might create hot spots. A 
        conical deflector is constructed of galvanized steel to prevent direct 
        heating of the end of the inner tube and to start channeling the airflow 
        to the surface of the outer tube.
          This outfit is nested in a wood box packed with fiberglass insulation 
        to prevent heat loss. The box is lined with heavy duty aluminum foil to 
        act as a heat reflector back into the system. A removable wood cap-box 
        with insulation is also made to prevent the protruding end, where the 
        rods are inserted, from acting as a source of heat loss and possibly 
        creating a temperature gradient.
           The end of the outer box where the pipe cap for the heat gun is 
        mounted was originally made of wood. This is probably OK. However, in a 
        first run I didn't realize how efficient this unit would be and had the 
        heat gun turned up way too high (max temp.- this was dumb) and ended up 
        with scorching around the protruding end cap (but no where else) so I 
        replaced it with a sheet of steel. 
                The heat gun I am using is the Sears industrial strength heat 
        gun (as suggested by Darryl Hayashida in the Rodmakers archives). It is 
        infinitely adjustable for temperature and has two air flow rates. I use 
        the high flow rate (17 cfm). Set at about "6" I get a temp of 364 
        degrees F in the oven. A continuously adjustable heat gun such as this 
        one is desirable as it will allow you to fine tune the oven 
        temperature. Temp is monitored with a good candy thermometer.
     
        Tools: The tubes were assembled, where necessary, using steel pop 
        rivets. To do the metal work you will need 1) tin snips; 2) drill with 
        appropriate bits (one sized to the pop rivets, one large bit to start 
        the hole cuts for the tin snips in the support baffle discs and tubes, 
        and one sized to your wood screws for the outer box construction); 3) 
        pop rivet gun and steel rivets; 4) compass to scribe circles to be cut 
        in the metal; 5) basic woodworking tools to build the box. 
     
        Materials: For the inner tube construction you need 1) one 6 inch diam. 
        galvanized steel duct pipe 6 ft. long; 2) two 6 inch steel duct end-caps 
        (these have a flange on the end); 3) a smaller steel duct pipe 6 ft. 
        long; I used a 3 inch diam. pipe. If you want a larger internal chamber 
        you could probably use a 4 inch pipe although I have not tested the 
        performance of this combination; 4) one 1.5 inch I.D. x 12" long pipe 
        nipple for the chimney; 5) one 6 inch diam. duct pipe connector piece 
        (crimped on both ends- about 6 inches long); 6) one end cap for the 
        inner tube (size matched to tube); 7) a couple of pieces of square duct 
        work walls (these are supplied so as to be assembled to make a square 
        duct; I just chose pieces with the largest surface area/$ to cut out the 
        support baffle discs and conical deflector as shown below); 8) heavy 
        duty aluminum foil; 9) steel grating for rod rack; 10) aluminum or steel 
        screen door handle to mount on the removable end-cap; 11) file to smooth 
        off the holes cut with tin snips; 12) staple gun; 13) glass of single 
        malt scotch. Everything except the aluminum foil and the single malt I 
        obtained at the local hardware store. 
     
                Construction: Scribe a hole in the smaller duct pipe about 2-3" 
        from the end to just fit the 1.5 inch pipe nipple. Drill a starter hole 
        and cut out the scribed hole with tin snips. This should be a tight fit 
        so the threads of the pipe nipple engage the hole and the nipple can be 
        screwed into place when ready. Test the fit and leave disassembled for 
        now. Rivet the end cap onto the small duct pipe. 
                Scribe circles on the sheets of duct work wall pieces so that 
        the cut out disc will just fit inside the 6" diam. duct pipe. Scribe a 
        smaller circle on each disc using the same center to create a center 
        hole that will just fit the O.D. of the smaller duct pipe. Cut these 
        out (5 discs will do). Make angled cuts from the outside edge part way 
        (2/3) toward the center hole (see schematic) and bend these tabs in to 
        create fan blade-like structures. Slide the baffle/support discs over 
        the small duct pipe leaving them spaced at even intervals with the 
        blades all going in the same direction. You will cut off a few inches 
        off the small inner duct pipe later so the last disc can be placed so 
        it can be moved back close to the end after this has been done. Set 
        aside. 
                Scribe a circle and cut out a disc larger than the outside duct 
        pipe diameter (maybe 8" diam.; its not critical as you will cut it 
        again). Cut a straight cut from the edge to the center hole. Overlap 
        the cut metal edges to form a cone and rivet the overlapping edges to 
        hold it in this shape. Place the cone in the end of the large duct pipe 
        and mark the pipe size. Cut the cone down so it is about 0.5-1" smaller 
        in diameter than the large duct pipe. DON'T FORGET TO LEAVE ON 3-4 
        PROTRUDING SQUARE TABS spaced around the cone WHICH WILL BE USED TO 
        RIVET IT IN PLACE. Rivet the cone into the 6" diam. duct pipe connector 
        piece by folding the tabs up along the pipe wall and drilling through 
        the two layers. 
                Scribe and cut a circle to fit the 1.5" pipe nipple through the 
        sidewall near the end of the 6 foot long 6" duct pipe. This hole can be 
        a few inches from the end which is not crimped. Make sure it is not so 
        close to the end that it will obstruct the insertion of the duct pipe 
        connector piece into the end of the larger duct pipe. This should be a 
        tight fit on the pipe nipple to minimize air escape. 
                Do a test fit of the inner duct pipe with support baffle discs 
        in place into the larger duct pipe and line up the two pipe nipple 
        holes. Check the end where the bamboo is to be inserted and make sure 
        the small pipe is a few inches shorter than the end of the larger one. 
        Cut off the small pipe to make this fit so as to not constrict air flow 
        from the larger to the smaller pipe at this point when the end cap is 
        in place on the larger duct pipe. Smooth this cut end off well as your 
        fingers will be in this area quite a bit when inserting the rod racks, 
        etc.
                Cut a sheet of 1/4" plywood or metal sheet to size to fit on the 
        end of the outer housing box. This end-plate was 15.5" square on my 
        version. If you use metal cut it a little larger and you can pound the 
        edges around the wood with a hammer when you are all done which gives a 
        nice tight fit. Cut a hole in the center of this sheet which just allows 
        insertion of the 6" duct end cap. Cut a centered hole in this cap which 
        just tightly fits the nozzle of your heat gun.
                Put the small inner duct pipe with support baffled discs in 
        place into the larger 6" duct pipe and align the pipe nipple holes. 
        Screw the pipe nipple through the outer pipe and into the inner pipe. 
        It should protrude about a 1/2" into the inner tube for a secure fit. 
                Insert the 6" duct pipe connector piece with deflector cone into 
        the 6 ft. duct pipe and rivet them together. Slide the outer box 
        end-plate you have made over the 6" connector piece and larger duct 
        pipe. Slide the 6" duct end cap with the hole for the heat gun on to 
        the connector piece and rivet into place.
                Construct a wood outer box but leave the ends open. I used 1x10" 
        and 1x6" boards to get a 15.5" square box. These were internally framed 
        (minimal) with 2x2". I made the box this size to get several good inches 
        of insulation around the pipe in all directions. The length of the box 
        should be to cover the duct pipe from its end-cap where the heat gun 
        inserts to the swelling where the crimped end starts (this is where the 
        end-cap fits on). This was about 5.5 ft. in my version. The tube 
        protrudes out a plywood end plate with a center cut hole similar to the 
        other end. Mount the pipe in the box using these end-plates to suspend 
        it by screwing them on to the end of the box. Fill the box with 
        insulation material. I used liberal amounts of fiberglass insulation.
                Mount the handle on the remaining end-cap with rivets. Cut the 
        grating to make a rod rack and place inside small inner tube. 
                If you get 6 or 7 ft boards, the scrap ends can be used to make 
        the cover box for the end of the oven pipe. Make this box similar to 
        the larger box. Fill with insulation with space for the protruding pipe 
        end and handle. Take several layers of heavy duty aluminum foil or an 
        aluminum lasagna pan and mold it carefully around the end of the long 
        box and pipe and handle. This will make a form-fitting metal skin which 
        will be used to cover the fiberglass in the end-cap box. Be sure it 
        reaches all the way out to where the end-cap box will contact the oven 
        box. Put the insulation filled end box over the foil in the position 
        you ultimately want it. Pull off together and fasten on with staples. 
        This end box can be held on by hinges or any other system. Mine is held 
        on by a bungee cord attached to two eye bolts. It works.
                Next to last step- insert the heat gun in the end and fire up 
        the oven. Do the first take outside or in a well ventilated area as oils 
        on the pipes will burn off with a light smoke. Don't overdo the heat 
        like I did- this thing will cook. When set at "6" on the gun my oven 
        stabilizes at 364 F in about 40 minutes.
                Last step. Drink the scotch and contemplate how many hours of 
        draw filing you have left on your steel forms.