shopbuilt handheld hot caul, aka 'the hotwand'

[Bob Hammond]

Hello,

The other day I started a post about releasing a misplaced fingerboard. I didn't have a storebought hot caul to do that, so I improvised a gadget from something left behind a few years ago by a daughter, from her high school era. It's a cosmetic accessory known as a 'hair straightener'. I was never sure about the utility of the object for that purpose, but hey I'm a Dad, not a Fashionista.

The object has the power slideswitch on one paddle (nearer), and the pilot light and tiny thermostatic control knob on the other paddle (farther away).

[Bob Hammond]

After removing the screws and twisting it apart, we see the two heating elements and the control circuit, connected by (hay) wires. Aside from the heating elements and circuit board, I see that there are two rectangular silvery 2x3" wafers behind the heating elements. After tracing the wires and clipping away some of the mechanical wire restraints, I made some sense of the scheme. The first objective is to establish which wire supplies electrical power (the 'hot' wire), and the wire ('neutral' ) wire that safely returns the electricity to ground. By convention in the US, the hot wire should go to the switch; I verified that was so and flagged the wire with black tape.

To be continued...

[Bob Hammond]

An inspection of the circuit board shows that the heating elements are connected in parallel , by four wires to each element. Two have braided insulation (power), and the other two are red and yellow (likely for a themocouple or thermistor sensor). It was easy to cleanly clip one element away rather than desolder it.

Incidentally, I did not notice a fuse on the board. The labeling on the outer shell states that the power consumption is 170 watts, so it's reasonable to assume that each element is rated ~85 watts.

[Bob Hammond]

Now, for reassembly, I noted that as produced by the manufacturer, that the line cord with the strain relief and power switch was fitted into the discarded outer shell. But since the outer shells are bilaterally symmetrical, I could see that the line cord could be fitted into the board's shell, but that the power switch could not easily be fitted. So I cut a notch in the outer shell, cut away the switch, and soldered the line cord, with the splices offset and insulated with shrinkwrap tubing. Then I embedded the line cord in hotmelt glue as added reinforcement.

[Bob Hammond]

The silvery plate heat shield was replaced, and the outer shell reassembled. In the picture, you can see the separated parts, the removed element, heat shield, and shell.

I've never been sure if the thermostatic control is very effective; the elements were always too hot to touch. I may make a second caul with the leftover element, using a lamp dimmer or router controller, but probably not. I will definitely keep the silver heat shield. I think it's a mica base material, and will be an effective shield for delicate surfaces. When the element was hot enough to sizzle water instantly, I was able to place the shield in contact for about 5-10 seconds before it got too hot for my finger.

[Peter Wilcox]

Hi Bob

I was at the thrift store the other day and saw one of these. I remembered your thread, and since it was only $1.50, I bought it. But I have a couple of questions:

1) What do you use it for, and how do you use it?

2) Why break it into separate pieces, instead of just using it as is and keeping the unused half out of the way (or being able to switch it off), or applying it to both sides of whatever you're heating up?

[Bob Hammond]

I used it to slowly free up a fingerboard, but it could also be used to lift a bridge. But I would be careful to shield the top around the bridge, because this thing gets pretty hot - hot enough to make a drop of water sizzle instantly. Hmm, it might make a nice mini-hotplate for warming things, such as pre-warming a bridge for hot-hide gluing, or for keeping a cup of coffee warm (off the bench, of course).

As for separating the halves, that's necessary because as manufactured in the 'salad tongs' design, I found that it was not possible to apply heat to a surface.

[Peter Wilcox]

Thanks Bob. I guess some day if I am ever at loose ends I'll take it apart and separate the halves.