DIY hot knife

Old soldering pencil modified to accept X-acto blade holder.

For years I tried soldering with the old, cheap, 20- or 25W soldering pencil shown below. I was never very good at it and was continually frustrated, imagining there must be something wrong with my technique. I read tutorial after tutorial claiming that good soldering required heating the work rather than the solder directly, but that trick never seemed to work for me. If I wanted to make a joint I had to melt solder onto the iron tip and then try to smear it onto the work, which, as everyone knows, is no way to make a solder joint.

Old, nearly worthless, low-wattage soldering pencil.

Finally I figured out the problem and bought a more powerful 40W iron, which instantly started producing good joints for me with proper technique. The upgrade left me with a surplus soldering iron and the opportunity to try a modification which first occurred to me when I was 10 or 11 and still playing around with Dad's tools in our old garage, which was to try to adapt a soldering pencil barrel to accept the threads of an X-acto Type A blade holder. The idea was to be able to conveniently adapt my trusty hobby knife for hot-blade work, useful for instance in the cutting of plastics.

The classic, type A Xacto hobby knife with No. 11 blade, shown with blade-holder assembly demounted.

In short, it works. The soldering pencil was clamped in a vice with the tip up. The original tip was removed and discarded, and the threaded hole in the barrel was lubricated with 3-in-1 oil and drilled out with a #21 drill bit to a depth of approximately 5/8". This was a straightforward operation because the #21 made a hole not much bigger than the one already there. After drilling, lube was reapplied and the hole was tapped at 10NF32, using a plug tap with generous lubrication, completely backing out the tap after each cutting half-turn. With the hole rebored and -tapped, the blade holder assembly from a Type A Xacto hobby knife was removed and smoothly threaded into it. Upon first heating, the tool was observed to smoke lightly for several minutes as the excess cutting oil burned away. After about seven minutes, a test cut was made in a polyethylene terephthalate (PETE) beverage bottle, as shown below. The knife cut well with light forward pressure and did not cause undue melting or warping of cut edges.

A PETE (#1) beverage bottle cut easily around its long axis using the DIY hot knife.