Bending Sides With Silicon Blankets

By Michael Keller

From his 1990 GAL Convention Workshop

used with permission

mossman_benderThough 1 attended the 1977 Guild convention in Tacoma, I exhibited my instruments for the first time at the following year’s convention in Winfield, Kansas. I visited Stuart Mossman’s shop while I was there, and I saw the side-bending mold that he had. It must have cost a fortune. It was about the size of a Volkswagen van standing on end, and it had all sorts of hydraulic pumps and pistons. In a production shop that kind of tooling might make sense, but for a small shop like mine, making twenty to thirty instruments a year and bending wood for repairs. I don’t need that kind of investment.

I bent sides far years over a hot pipe I bought at Lewis Music in Vancouver, B.C. I had to work at a regular job and save money for quite a while before I could afford to buy two Overholtzer side-bending molds. A friend of mine had a custom mold made by the Overholtzer company, and it cost $1,000, I believe. That’s a lot of money. I can bend guitar sides with either a hot pipe or a cast mold quickly and accurately, but I am in this to make a living, and if I can save time and money I will do it. That’s why I prefer my new method. (By the way. . .Would anybody like to buy two nice Overholtzer molds?)

Silicone heater blankets are a spin-off from NASA research. They were originally built into the walls of satellites to keep them warm, being run by solar panels. They have been adapted throughout the world to all sorts of uses, like thermoplastics manufacturers that need to form flat sheets of plastic. You can bend something to any shape of mold that you can lay this thing on. This technology is available cheap and easy.

The blanket is made of silicone rubber .055. thick, with a wire grid inside it. When you plug it in. the grid heats up. If you bold the blanket you can feel it hum and start to get hot right away. In two minutes it can hit and maintain a maximum temperature of 500"F. with a uniform density of heat. Once you lay it on something the temperature is affected by the heat sink properties of the material. When I bought this blanket the company supplied me with two sheets of heat curves and graphs showing the drop in temperature as related to the density and thickness of the material being heated, so that when you are building jigs you cou1d calculate what the temperature of the blanket would be. To get ahold of that information, contact Ash Equipment of Batavia, Illinois (630-406-0300; www.ashequipment.com ). They can put you in touch with the Watlow company which makes these blankets. Watlow has thirty or forty offices worldwide.

When I ordered a heater it was at my door C.O.D. in three days. These are stock items, not specially made. The catalog lists sizes starting at 1/1 square, then 1×2, lx3, 1×4,1×5.1×6. And then it jumps to 2" square. then 2X3, 2×4. 2×5. and so on. They come in hundreds of different sizes, all square or rectangular, and they can also be ordered in custom sizes. The one 1 am using here is 5" wide and 35" long, and it cost about $90. Watlow can supply a heat-resistant glue with which silicone blankets can be glued to almost anything. An article in the March-April 1988 Fine Woodworking showed The Blue Lion Dulcimer Co. using these blankets glued into their molds. In my mind you lose one of the big advantages by doing that I build nine different body shapes. If I glued all the blankets down I would have to buy nine blankets, and there goes your cost advantage. I just use one blanket with all of my molds.

The molds are cheap and easy to make. 1 recently made three of them in a weekend for about $25. The first time I tried this I glued up a plywood blank to the full finished thickness which is about 1′. Then I drew the guitar shape on it and cut it out. Bandsawing that thick blank was quite a chore. I made a few mistakes and it look me a few extra hours to go back and fix them. It is quite a bit of work to get a nice, fair curve. My first mold took me a whole day to make. Now I only glue up half the thickness of the mold. For my equipment and abilities it is much easier to cut a nice uniform curve in that thickness of stock. I use sanding blocks and a square to fair out the curves. Then I will glue two more sheets of plywood to the outsides of that mold, and trim the overhang off with a flush-cutting router. A little sanding and you’re done. These molds are pretty much the actual outline of the instrument, rather than having a Jot of overbend built into them.

The working surface of the mold is then covered with standard .020” sheet aluminum. A heavier piece of metal might be more difficult to get around the mold and still have a nice uniform curve. You could probably go thinner. The function of the aluminum is just to keep the mold from charring. Steve Grimes says he uses stainless steel for his facings, which I imagine would cost a little more. 1 start from the waist and work in each direction. 1 clamp the aluminum into the waist with two damps. and I’ll drill a hole right in the lowest point of the waist curve, and then put a screw on each side. 1 bevel the holes a little bit so that the screws are countersunk. Then 1 work in one direction putting in pairs of screws, one on each side. The reason for making the mold considerably wider than the side is to allow the blanket to lie between the rows of screws without touching them. The first molds I made I stopped the aluminum right at the end, but it left a sharp edge that caught on the table. Now I fold the aluminum about 1” underneath and screw it in place.

Some people use two sheets of plywood, one on each side, with spacers in between, rather than solid molds. The man who turned me on to this method used to make his molds that way. Then he found that the heat caused the metal to distort a little bit. So he fit blocks of wood in between the outer parts to create a continuous surface under the aluminum. "

The temperature of the blanket can be controlled in one of three ways. I just unplug it when it gets too hot, which is the low-tech way. Or, you can get a light dimmer with a high enough power rating; that would let you run the blanket at a constant lower heat. Then for $80-$100. Watlow will sell you a little box that you plug the blanket into; you can set it to a temperature and the blanket will come up to that temperature and then shut off. Then when it cools to a certain point, it will come back on and heat up to that temperature again. I don’t use either of the controls and never have. Jim Olson, who has a shop in my part of the country. started out using the controls, but after a few years he quit using them. He has a box full of these very expensive thermal controls collecting dust.

Jim has about fifteen blankets of every conceivable size hanging around his shop, and every place he needs to heat wood he has a blanket to do it. I once watched him take a bridge off of a 12-string in under two minutes. I take bridges off all the time with an alcohol lamp and a pallet knife, and it takes me a long time to get one off cleanly. He takes the pins, strings and saddle off, then he takes his little 2×5 blanket and lays it on the bridge, puts little round brass weights on each end of the bridge to hold it in place, plugs it in, and a minute later unplugs it and just pulls the bridge off. You can straighten necks with these things, you can take fingerboards off. They can be used anyplace you use conventional heating. It is a very versatile tool: I bend all my purflings, bindings, and cutaways this way, too.

I have tried bending wood dry, but even a wood that bends easily, like mahogany, will fight you a little bit. If you soak the wood for five to ten minutes, it bends just like pasta. I throw my guitar sides in my bathtub, run some hot tap water in there and set a shampoo bottle on top of them. I have been doing that for ten years. My wife says it’s time I figured out a new method.

You don’t need to clamp the waist too tightly. I have scorched wood a few times at the waist, but I have never burnt it. The blanket is not fragile. I have cranked this down fairly tight. I’ve been using this blanket for three years and it is still in good condition.

I feel it is important to err on the side of safety. so I’ll use this device only with a special ground-fault interrupter receptacle that I installed for this purpose. I have never had a shock from a heater blanket.

Without further ado I will bend these sides.

michael_bending I have just laid the blanket on the surface of the mold. I don’t treat the surface or do anything to it, I just lay the blanket on there. I have never done so, but the blanket could be laid on the outside, the upper surface of the side.’ I have marked the center block with a dot because the curve is not uniform, so it is critical which direction this block is turned.

I’ll just set the block in there and plug the blanket in. After a few seconds it will start to bend right down. I am just bearing down very lightly on this wood block. I can hear it starting to sizzle. I can feel it getting springy now. It is starting to loosen up. Sometimes after it has been on here a little while it will dry out, and I will just dampen it from the top a little bit.

It’s almost all the way down. Right about this point I just flip up the clamp bolts for the waist block. Now the waist is down. While I have been talking I have been screwing both of these bolts down. It moves around a little bit, but that is no problem. I can seat it.

If I push gently on the end of the side it will eventually bend around the mold, but to speed things up I’ll reach down with the end clamping block and just bring the.blanket up in contact with the side. So now it’s touching the whole piece and is heating it all the way to the end. It is really starting to get this wood hot and flexible.

The wood is getting too hot for me to hold on this side. This end is just about ready to clamp. It is almost down. Don’t force the wood. It will bend all by itself.

The side has gotten quite dry from about 5” from the center of the waist, so I usually just dampen it a little. When wood bends like this, the cells on the top of the bend have to stretch, so I just wet the top a little bit. The cells on the bottom have to compress, so I usually slide a wet rag between the side and the blanket to get a little water back in there. This is going really fast now. It just wraps right around there.

Bending the side and clamping it to the form takes about five minutes, and I usually let it heat for another five minutes, so the blanket is hot for only ten minutes. I never walk away from this while I am using it like I did with the Overholtzer mold. If I were to leave the blanket plugged in for fifteen minutes it would cook the wood – just bake it. I am done bending it, by the way.

The blanket and wood will be cold in maybe thirty minutes, where my Overholtzer mold would stay hot for a very long time. I set it on a little bench outside my shop and let the cool air blow the heat off. If I am in a hurry, I will pop the side into the outside mold that I use to assemble instruments and clamp it well with little wood cross bars before it is even cool. If I am not in a hurry I’ll just let it sit overnight, and the next day plug it in for maybe five minutes and let it get hot. When I do reheat it the next day, those sides just suck right down to the mold. They hold the shape perfectly.

I sometimes rebend sides. If I get busy with another project after I have taken a set of sides off the mold they may have opened out a little by the time I get back to them. So I will just pop them back on the mold for five minutes and let it cool down. and they will be bent perfectly again.

For a cutaway I bend the side over a full shape mold. and then remove the cutaway area. The cutaway piece is bent over a separate mold, a wood block that is shaped like the cutaway. the same width as the side and faced with aluminum just like this mold. I use a smaller heater blanket for this. The big one would work fine, but it would be less convenient.