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Rule

Looks like I'm no oven designer. :-) Oh, it looked good, and dry runs (no cane, just checking that the built-in temp probe read as it should, and that the thermostat cycled according to expectation) indicated everything was performing as expected.

Sooooo... Since it was past time to heat treat the roughed out sections of #2 that had been air drying in the closet for a while, they were all duly loaded in the oven, the door closed, and voltage applied...

First stop was a drying cycle, at ~210 degrees F for an hour. As it came up to indicated temp, I noted a little "steam" escaping around the hole the temp. probe wires came out of, but that was more or less expected, given I was out in a cold garage. Didn't smell like I was toasting anything.

After 60 minutes of baby-sitting the oven, monitoring temperature all the while,  it was time to run it up to ~325F for a slow heat treat. About the time it hit an indicated 240F, I could smell cooking cane. That's odd... When the temp probe read 250F, it was smelling pretty acrid, and I had a wave of impending disaster swept over me...

Popped the end cap off, and the smoke billowed out... :-(

All four sections of the 3/2 rod were in there. :-( If I planed the strips out and impregnated the blank, would it be a carbon fiber rod? :-) Not sure whether to laugh or cry.  (Todd Enders)

    Sorry to hear of your problem.  What were you using for a temperature sensor?  I know that there is an LM34 and an LM35 and one is Celsius and the other is Fahrenheit.  Maybe you just used the wrong temperature scale?  (George Bourke)

    I think I have what went wrong pretty well figured out.  Temp probe mounted too far from the heat source, as was the thermostat probe.  Classic hot spot/cold spot situation.  Used a fiberglass/silicone reinforced flexible heater, 6"x48" as the heating element, figuring the broad heat source would reduce hot spots... :-/

    The temp. probe was an industrial type "K" thermocouple, hooked to a Fluke DVM.  The probe is accurate to within a couple degrees F, and had been checked for accuracy beforehand (read 212F on the nuts in boiling water).

    Of course, the smart thing, the careful thing, would have been to run a few scrap strips through ahead of the actual rod sections, but when one has a notion, and thinks they have it all figured out, caution disappears on the wind as thistle down.  (Todd Enders)

      Go to an Industrial Electronics store and get some more fiberglass coated Type "K" thermocouple wire and another plug of the style used for your meter. You can also try Omega.com for these items.

      Make up a half a dozen thermocouple sensor ends, (Just twist the ends of the wire together. Weld them if you have access to an Oxy/Acet torch.)  Now twist together the like wires of the ends of these sensors and twist in another lead of this wire. It is good to use wire nuts or some mechanical means to keep the wires tight. The final end of the last lead connect to the plug.  You can insert the sensor ends into however many areas you have ends and you now have an averaging sensor that will give the average temperature of the oven.

      I worked the mechanical lab of a heating and cooling manufacturer for over 30 years and we used to do this to get the average temperature of the air into and out of a unit.  (Dick Fuhrman)

    I know the feeling - commiseration. It happened to me on my first set of rough strips (of which I was very proud) in my heat gun oven. I blame my father-in-law who was challenging me with vexing engineering questions such as "shouldn't you try one set of strips first?" whilst I should have been monitoring more carefully what was going on.

    It looks like you could still make a 3 or 4-piece-whatever-the-length rod out of the not so charred end bits. I did this with mine and now have a delightful 5’ 3" 2 wt 2 piece Sir D with a longer tip than butt and with 'slight' discoloring at the ferrule end of the tip section! It is quite amazing what can be hidden under a grip. (BTW this rod is surprisingly strong given its heat treatment. It successfully fought and landed a brace of strong 5 lb trout on its first outing, both of which were released in good form.)  (Stephen Dugmore)

    Looks like one end was fired and the other justly barely cooked. If you are using a strip type of oven, this is about par for the course. Some work OK if you are into flipping end for end.

    Recirculation ovens make the problem go away.  (Don Anderson)

    I recommend switching to a heat gun oven.  It is so much simpler and cheaper to build than a conventional oven or a conventional oven with a recirculating fan.  The only problems I've encountered with uneven heat were because the blast from the heat gun in my oven is aimed directly at the wall of the inner piece of duct.  This caused the wall to heat up and radiate heat into the top of the oven.  Simply placing a extra piece of sheet metal as a heat shield fixed that problem.

    My oven consists of an outer wall of 8" duct 5' long, 1" of insulation, a middle wall of 6" duct, and an inner tube of 4" duct separated from the 6" by 1" of air space.  I use it vertically with the heat gun at the top.  The gun blasts hot air  through holes in the 8" and 6" tubes.  The hot air goes down in the 1" air space and then comes up through the 4" tube and out the top.  I hang the bound sections from a wire across the top of the oven and measure the temperature with a candy/deep frying thermometer with the probe placed in the oven's exhaust.  The whole thing cost about $50 without the heat gun.  I already had a heat gun anyway for straightening.  The flow of air through the oven works like a convection oven, and assures that the heat is uniform.  (Robert Kope)

      Just a question, what kind of insulation did you use? I would appreciate specifics so I can buy some and finish my heat gun oven.  (Randy Tuttle)

        You could also use the stainless steel, double walled, insulated chimney liner/duct. Lowes has 5' sections for what seemed a decent price which of course I can't remember now.  (Larry Puckett)

          The outer pipe can be made reasonably with a 6" stove pipe inside an 8 " stove pipe. get a steel plate and self-tapping screw an 8" end cap and a 6" end cap to the plate. Install the pipes and sheet metal screw them to the caps. The steel plate makes a nice base so the oven does not tip over.  (Dave Norling)

        I used fiberglass insulation.  I don't recall what it was, but it may have been 1' insulation for furnace ducts.  This was not a good idea. I've heard that fiberglass can't take the heat, and mine was paper backed. After a couple of years, I stopped smelling the paper and adhesive burning.   The double walled chimney pipe is a MUCH better idea.  Barring that use some kind of high temp insulation.  I know that Don Schneider found some real good ceramic insulation for his oven.  (Robert Kope)

          William Servey and I built two ovens. The insulation we used is called Ceramic Wool rated to 2200°, 1400° direct flame. Had to buy a minimum amount, 2"x24"x25', enough for 4+ ovens. I'll have to search for the supplier name if anyone is interested.

          Because of the thickness, had to place the insulation over the 6" pipe and placed the open 8" over everything & used hose clamps to compress the 8" pipe to latch the seam. You can't slide a latched 8" over everything.

          Used 9 1" long steel standoffs, (turned on lathe with hole for steel pop rivets in each end) pop riveted in place, 3 on each end & 3 in the middle of the 4" pipe to hold it in place. Used 8" caps with a layer on insulation inside on the ends.

          The tube for the heat gun comes in from the side near the top and is pointing down at about a 45° angle. Idea here was the air would spiral around the 4" on its way down the outside of the pipe. Mounted a piece of sheet metal to deflect the direct blast of the heat gun from the 4" to eliminate a hot spot.

          Once up to temp, which doesn't take long, 425° at the exit for 15 minutes seems to work well. We decided we didn't need a higher temp because it is a convection oven. Air volume will make a difference in the operation, use a damper on the exit. To much volume will scorch the bamboo near the exit. When set right you will get an even tone. Why? We have no idea.  (Don Schneider)

    I gotta agree with Robert on the heat gun oven. After spending over $1,000 getting a circulation type of oven up and running, the heat gun oven works as well and is well within the home builders abilities.

    I won't build another circulation oven if I had to do it again, I wished the Frank had come up with his idea sooner.

    Still, what seems really important is air flow around the cane thereby getting a homogenous air temp. This is one thing that strip type of ovens do very poorly.  (Don Anderson)

    I have a vertical oven that only uses two vent ducts (a 6" and a 4" duct).  I wrap the outside of the oven with a foil backed insulation and duct tape.

    The oven is a common design and with a heat gun of at least 1800 Watts, it cranks up to a 400 degree temperature quickly and holds nice at 350 degrees (or whatever temp you require).  (Scott Turner)

      I built my oven years ago with basically the same design (I don't have foil backed insulation, but matte faced HVAC insulation that I got from work).  I have temp probes at the base, middle, and upper sections.   There's typically  not 5 degree variation from top to bottom.  BTW, I used a 6" end cap on the top with holes cut for the heat gun nozzle and one for the exhaust from the 4" duct.  I use a removable 4" elbow to slow down the air flow and place a hook for the sections to hang from.   The air flows from the gun into the 6" pipe, down the pipe, and is forced up into the 4" pipe by the gun's blower pressure.  My 6" pipe and 4" pipe are not offset.  I used a 6" flange (commercially available right by the pipe, elbows, and end caps) to lift the sections up and create the space needed for the air to flow into the 4" pipe.  I put a circular cut piece of sheet metal on a 1"x12" section of board and screwed the flange onto that.  Makes it own stand.  (Rick Crenshaw)

        I initially had problems with uneven heat in my vertical 'Frank Neunemann' heat gun oven but have solved that with advice from the list. It now works really well and I would recommend it as a very cheap and effective oven. I would advise 4 things from my experience:

        1.. Insert a short length of pipe into the heat gun hole directed away from the inner pipe. This pushes the hot air further down into the oven and I think assists with getting circulation going. This certainly stopped over heating at the top of my oven.

        2.. Insulate the base of the oven as well as the side (or as per Frank's original design use a thicker plate as the base to retain heat). I simply stand the oven on a piece of wood rather than on the cold floor and this definitely helps with getting to temperature quicker.

        3.. I wrapped my oven in 2 layers of fiberglass woven tape, which is effective on its own but much more so if you can create some sort of an air gap around the pipe as well - at the moment I just have some cardboard sheet wrapped around, which works fine. A double pipe as suggested by other would be the best solution but more complicated to make.

        4.. I use a piece of loose metal plate to reduce the size of the outlet until the oven gets close to the right temperature. I think this also helps in getting there quicker  (Stephen Dugmore)

      I basically used the same ducts and foil backed insulation held in place by duct tape.  The other thing that I did was use an elbow duct for the exhaust to keep the hot air away from the heat gun.  I also have a thermometer at the top and bottom to check the temp and it usually only varies about 1 or 2 degrees difference.  It works very well at 370 degrees for 15 to 30 minutes depending on how dark you want the sections.  (Tom Peters)

Rule

I searched the archives and couldn't find the answer so... I built a mica strip oven per Wayne C's design. 54 inch mica strip and a Robert Shaw thermostat. When I set the thermostat, the temp goes up for a while even after the thermostat clicks off.  And when the thermostat clicks on the temp drops for a while before climbing back up. This happens even if it has cycled a couple of times. The question... Is this what most of you find? And, since there are such swings beyond the 350 desired temp, do you usually resort to manually switching the thermostat  on and off to keep the temps within reasonable range? I have the probe bare wire attached to the rat screen. I am wondering if that is the reason for the problem.  (Ron Revelle)

    That isn't a problem, it's a feature. There might be an adjustment for hysteresis, but for the most part it just comes with the territory.  (Larry Blan)

    You will always have that problem with an “ON/OFF” controller.  The heating element has a lot of energy in it so when the thermostat clicks off that energy continues to be given up to the air, causing it to go over set point. The inverse is true on the down side.  The element cools off and when the thermostat clicks on it take a while to bring the temperature of the element up to where it can start heating the air again, hence undershoot.  The cure is to use a PID controller which attempts to anticipate the energy shifts in the element as well as the surrounding metal enclosure.  It will hold the temperature much more consistent assuming you have adequate air flow.  What’ s enough?  My experience is about 700 CFM per 1000 watts to keep the temperature within about 5 degrees.  I don’t have a recommendation on a PID controller since I got mine through an industrial supplier.  (Al Baldauski)

      ...and lots of high temp insulation is not a bad idea either.  (Timothy Troester)

Rule

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