Wednesday, February 26, 2014

Away From The Heat Gun and Into The Oven - The PVC Bow Hot Box Story

My favorite thing about selfbowyery has always been the tillering process. This is where the spirit of a bow lies. It's a vital process that in the end determines what a bow will be like, how it will handle, how it will shoot, and ultimately how long its life will be. It's easily the most important part of building a bow.

The problem with tapering PVC pipe is that this process pretty much takes care of itself. Because of this, a PVC bow's spirit lies in the details, the degree of taper, the reflex and deflex, the recurves and siyahs, and all the other things that can be done to a PVC bow once tillered. As a result, my favorite part of PVC bowyery has become what happens after the taper, not tapering itself.

While there are ways to taper a bow completely by hand, using a jig to flatten my bows has become standard for me. It's the one process I care the least for and one I've tried to speed up or automate for some time. A heat gun, my tool of choice, is quick and fairly efficient, but it requires your undivided attention to get good results. The goal for me was to build something or find a technique that heated a pipe to the perfect temperature for flattening without risk of overheating and without active participation.

I tried many things. Of them, heating through another medium was always the easiest but always frustrated me. Boiling water is only hot enough to bring PVC pipe to the lower range of its ideal temp making getting the pipe to jig frustrating with little working time. Sand and other particulate mediums worked but dealing with hot sand in a quick, low-involvement way always seemed too involved. Steam worked, but the quick cooling time always became more detrimental than helpful.

So I turned to dry heat. I built various hot-boxes and ovens, even a dedicated kiln. The ones that worked worked amazingly well, heating a pipe quickly and without a need to actually do anything aside from putting the pipe in and taking it out. They all worked, though realistically they were way too expensive and some of them were hardly safe. And while I used the best ones for a while, replacing coils and refilling tanks sort of just killed the fun of building bows. I scrapped the idea, going back to the heat gun after all.

My first successful foam board hot box that didn't melt.
A few months ago I had the idea sparked again. After following a lead that ultimately became a dead-end, I remembered something. A few years ago I went into a laminated bow kick and had built a simple hot-box. It was crude but it worked and it was cheap. It used light bulbs for the heat source, and while I never got it hotter than 200 degrees, it made sense. It was a start.

I started out building a simple box made of insulating foam board based on Sam Harper's hot box from After playing around with a couple of them, I halved the design making it cheaper and more efficient for what I needed. After literally melting a couple boxes, I finally came up with one that worked and cost less than a good heat gun. The final touch came from my dad and my friend Ken, a new PVC bowyer. After replacing the $15 foam board with a $10 drywall board I had a solid prototype that worked well, was inexpensive, and wouldn't melt. It was also sturdy enough to be used as a table for flattening jigs.

The final PVC bow hot box in all its glory!
After refining the design, I came up with the box you see above. It's very simple, yet works really well for heating multiple pipes at once evenly and quickly. It takes less than 6 minutes to heat most PVC pipes till they can be flattened yet it holds the pipe below its melting point, almost eliminating the chance of overheating. It's simple and can be built with minimal tools and minimal experience. With a few simple additions like a dimmer switch and thermostat, the box's temperature would be controllable and exact. And another plus is that the box is silent, unlike the heat gun or other forced-air hot boxes.

Thanks for reading!

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