Convection Oven

It's Thanksgiving evening and I'm a little lethargic. It's been a beautiful day here in Arkansas. The family meal was outstanding as always. Family, food, shelter and good jobs are tremendous blessings compared to the political, economic and criminal chaos around the world. It's nice to stop and count blessings now and then. This seems like a good time to post something on the blog.

Someone asked about my oven recently. I don't think I've talked much about it here. There are pictures on Todd's tips site and I've mentioned it a time or two on the rodmakers list. My oven is the most overbuilt piece of equipment I own. It started out as a simple thing built to the exact specifications in Wayne Cattanach's book (Handcrafting Bamboo Fly Rods) beginning on page 66. That oven is sufficient for any hobby shop, but I just couldn't leave it alone.

The basic oven includes a 650 watt mica heat strip in a 6 inch by 10 inch by 60 inch insulated steel box. Add a simple oven thermostat and you're good to go (see Wayne’s book for more detail). I drilled holes at each end and in the middle of the oven to check the temperature from end to end and found that there was a hot spot near the middle of the oven. Apparently, that's characteristic of a mica heat strip. Wayne suggested flipping the strips half way through the heat treating cycle. That eliminates the adverse effects of the hot spot for the most part. But I'd read about using convection heat to stabilize the temperature in the oven. I had to have a convection oven.

I looked at all the stuff on Todd's site and read about other makers that had built convection ovens. I remember one in particular that was referred to as the "mother of all ovens". That was MD's oven. It was a masterpiece and monster piece of equipment with multiple circulating chambers inside a really big steel box. There are probably still pictures floating around out there on the internet. That design was way over my head. Besides, I had already built Wayne's oven. Actually, a friend of mine in the HVAC business had the sheet metal formed for me and I just put it together.

I also read about ovens with a circulating chamber (stove pipe) run outside the oven. Those ovens seemed to loose a lot of heat in the stove pipe requiring a much more robust heat source than my little 650 watt heat strip. I decided to try to circulate the heat inside the oven I had already built. After several pencil drawings, I finally came up with an idea. Turn the box horizontal, divide it into two small 4 inch by 4 inch internal chambers with a metal divider and mount a circulating fan in the door. Cool...except where do you get the right size fan without spending a bunch of money?

I searched flea markets for months for the right motor and fan. I love flea markets. Doing business at a flea market is a respectable way to help folks out that may be struggling to make ends meet or just need a little extra money. I'd rather help the guy out at the flea market and buy a used gadget for more than it's probably worth than to buy a new one at the local hardware store. Support your local flea markets! So anyway, back to the motor and fan. It took a while but I finally found the perfect fan and motor combination at the flea market. It's the blower motor and fan for a camper furnace. With some minor modifications, the squirrel cage blower fit perfectly and the heat resistant motor fit nicely on the door. I had to engineer a door that would hold it all together. That took some trial and error, but it didn't take too long to get it all lined up.

I had a metal divider bent at a local steel shop to go down the middle of the oven. After a few modifications, it has a perfect friction fit. The divider is flush with the front of the oven and a there's a hole cut in the far end of the divider to allow circulation between the chambers. So...when the door is mounted, the fan is drawing air across the mica heat strip on one side and pushing it back down the other side over the oven rack that holds the bamboo in the middle of the chamber. That solved the hot spot problem.

But that wasn’t good enough. I had also read about PID controllers. The “mother of all ovens” had one. So my modified Cattanach oven should have one too. A PID Controller is a fancy thermostat that replaces the oven thermostat in Wayne’s oven. It “learns” the temperature variations in the heat cycle of the oven and adjusts the heat strip to stabilize the temperature to within one or two degrees. A regular oven thermostat could vary as much as 20 or 30 degrees as it tries to maintain a target temperature.

I found a used PID controller on E-bay for about $75 including shipping and found a steel box to mount it at the equipment bone yard at work. My HVAC friend found a sensor that we could use in his equipment bone yard. All that was left to do was to wire it up...power to controller, switch for fan, switch for controller, power to fan, power to heat strip, sensor to controller. I found the manual for the controller on the internet. The wiring diagrams were way over my head, so I dropped the oven and all the parts off in my HVAC friend’s garage and about two weeks later we had a fully functional miniature version of the “mother of all ovens”. How cool is that!

I’ve been using the oven now for about three years with no problems. The only thing I would change would be the heat strip. It takes an hour for the oven to pre-heat to 390 degrees. When I open the door and insert cool strips, the temperature drops about 20 degrees and takes a while to heat back up. My 650 watt mica heat strip is just too weak. It should be about double, like maybe 1200 or 1400 watts. I’ll replace it some day, but it’s not a priority at this point.

The oven was a fun project, but all the extra stuff was unnecessary. The only advantage I may have is heat treating at the upper limit. Much more than 390 degrees and the bamboo would be destroyed. I couldn’t do that with the standard Cattanach oven. But the benefits of heat treating at such a high temperature may not be worth all the extra time and effort. If you want to learn more about the effects of heat treating at various temperatures, download Wolfram Schott’s “Bamboo in the Laboratory” from the Power Fibers web site.

The basic oven in Wayne’s book is all anyone will ever need for a hobby shop. But if you like making tools, a computer controlled convection oven is a fun project. I have about $100 invested in my oven including all the parts and assembly. Well, not really. I also made my HVAC friend a rod. That didn’t come close to the value he added to the project.