heat blanket/dimmer results

[Don Harris]

I just hooked up a 1500 watt light dimmer to a 10" x 18"- 900 watt silicone heat blanket and it seemed to work fine. I started with the dimmer at a little less that half way, and the blanket heated to 180 degrees F. and stayed right there. I bumped it up a bit and it climbed to 248 and stayed within a couple degrees for several minutes. I did have a light bulb plugged into the dimmed socket as well, not sure if that affects anything. So now I need to wire it up for real to make it safe and actually bend some wood.

I found the dimmer on ebay for $30. I'd seen similar ones on websites for $160! -don

[Bob Branstetter]

Originally Posted by Don Harris

I just hooked up a 1500 watt light dimmer to a 10" x 18"- 900 watt silicone heat blanket and it seemed to work fine. I started with the dimmer at a little less that half way, and the blanket heated to 180 degrees F. and stayed right there. I bumped it up a bit and it climbed to 248 and stayed within a couple degrees for several minutes. I did have a light bulb plugged into the dimmed socket as well, not sure if that affects anything. So now I need to wire it up for real to make it safe and actually bend some wood.

I found the dimmer on ebay for $30. I'd seen similar ones on websites for $160! -don

Thanks for the report Don. I'll have to see if I can find one of those cheap 1500 dimmers myself.

[Don Harris]

So everything is wired safetly in switch boxes with a timer so I don't burn down the house if I pass out. The bending straps are grounded and I'm ready to bend. I spritzed instead of soaked. Wrapped the wood in packing paper and put together my bending "sandwich" (aluminum, paper, wood, paper, heat blanket, aluminum). Fired up the blanket. Steam was coming out. I pushed with my clamping cauls and waited for the wood turn to plastic and bend beautifully over my form as I gracefully bungee-ed the cauls in place for a perfectly bent C-bout. Didn't happen. Not even close. The wood never really gave very much and it took a lot of pressure to get it to bend at all. As I pushed I heard a sickening cracking sound. Not good. I peeked under the aluminum bending strap and saw that it wasn't just a little "it will be ok" crack. It was a "I ruined this piece of wood" crack.

The next try I soaked the wood in the tub for about 5 minutes and put the blanket on the bottom with no paper (the wood stained last time). The wood did seem to give more and I got one side not so gracefully clamped. The other side is a tighter bend and I never got it close enough to clamp. I now realize that I need plenty of extra wood on the ends to bend and clamp. I was trying to get by with just a smidge more than the final length which was a definite mistake. The good news is that I've still got another piece of test wood before I get into the real ribs. The bad news is I don't have a lot of confidence about it being much better next time. The wood was a bit rippley/wavy afterwards too, even though I have some cross supports. I was hoping my bending "sandwich" would keep things pretty smooth. I'm sure glad I didn't start with my curly maple rib stock. I'll figure it out, but my fantasy of having the ribs glued up before my next luthier's group meeting next week went down in flames. Not literally. I'll keep trying.... -don

Now I've un-clamped the second try. I scorched it a bit and every ripple and texture of the heat blanket surface is molded into the wood. I think I left the heat on for too long waiting for the wood to get "plastic". Getting the corner parts of the "C" is going to be a challenge I think.

I've learned that the bungee idea for clamping while bending is not a good idea. Not enough pressure into the tight bends. I still think it might work for glue up. I'm going to use eye-bolts and wing nuts in the cauls on the next try.

[arnoldschnitzer]

Sorry but I don't understand the fascination with heat blankets. Why not buy or make a traditional bender and learn how to do it the old-fashioned way? With your hands on the wood you are more intimately involved in the bending process and you are better able to judge when to push and when to back off. And there is a certain beauty to forming the ribs with your own hands. Not to mention you have the ability to touch-up small areas where the wood may be a little harder or just didn't take the bend the way you wanted. BTW, any time you soak rib wood you will end up with cross-grain waves. Use as little water as possible. Just my $.02. Call me a cremudgeon

[RCWilliams]

stove top controlls work well for heat units. I'm sure the bending forms I built require are a bit more effort than you would want to go to, but I used oven elemets and stove top controlls for the heat. if you have a way to caprure the thermostat rig in your rib bending aparatus an electric oven controll unit would allow temperature controll. Our bending jigs are aluminum, and we spritz the water onto the wood, the contact with the alu flashes the water to steam and plasticises the wood. we use an outside skin of aircraft alu to pull the wood to the form and clamp the rib to the bending form. we bake the ribs dry and then go straight into the rib form and clamp them into position. this reduces the ripples.

it is just a conjecture, but I wonder if you ceated a wooden form of the final shape of your c or, slightly overexagerated by about 10% for spring back, and used a skin of air craft alu between the heat blanket and the rib to pull tension on the rib stock arround the form and kept the heat blanket in contact with the alu, but not where it could modify the surface textuer of the wood, if that would work for you.

because the grain is not straight flamed wood is more suceptable to compression and tension failure in the bending process

[Don Harris]

<<Call me a cremudgeon>>

I'll call you incredibly helpful. I'm grateful to anyone who would take the time to help point me in the right direction. I was thinking a lot about doing the bends the traditional way after yesterday, but I bristle at being beat by this thing. If you've got to soak the ribs to make the heat blanket work, and soaking causes cross grain ripples, I'm not sure I'll be happy with it.

<<it is just a conjecture, but I wonder if you ceated a wooden form of the final shape of your c or, slightly overexagerated by about 10% for spring back, and used a skin of air craft alu between the heat blanket>>

I did build in some overbend on the c-bout form. The heat blanket was right on the wood with the alum over the heat blanket. I'll try the alum on either side of the wood this time. Not sure what I was thinking there.

I added a couple more cross supports and I'm soaking the wood (some that I partially bent yesterday) in the tub. It went in before I learned that soaking can cause ripples. I'm still on my test wood. I got some cello reject wood from Orcas Island Tonewoods that I'm using for these ribs and have two extra pieces. It's beautiful wood, just not highly figured. This piece was $25 because of some end checking, but the usable part is still 32" x 9-1/2 x 2". It's nice to not freak out about ruining expensive wood. I had eight pieces of cut rib stock. Now I'm down to seven. I'll need six to build the bass unless I can get two c-bouts out of one piece. Technically it could work, but I think I need more length to clamp successfully. -don

[Don Harris]

Just did another test bend and it went much better but I'm not happy about the ripple and sag effect going on. I've got aluminum tubes for cross supports every 3" or so, but it still doesn't look good to me.

The light dimmer is doing what I wanted it to do. It may not be super accurate, but it keeps the heat within a 10 to 20 degree window. But unless I can get the wood to look more uniform, I'll be using the dimmer to control a charcoal grill lighting element inside a pipe. My luthier/teacher guy warned me that even with forms, you will still have to do touch up on a pipe, so why not just use the pipe? We'll see.

[RCWilliams]

if I might make one more suggestion, it sounds as if the form you are bending to has spaces in it, basically a series of bulkheads if you will, if you add a layer of alu thick enough to carry the load between the spaces, then the rib, then the next layer of thinner alu then the blanket, then clamp the ends of the outer layer of alu with enough tension to pull it hard against the inside alu, it might help. we have plywood handles screwed to the ends of the alu outer layer the length is short enough that when pulled tight it does not quite close on the other side of the form. then we can use a hand screw or other clamp to tension the outer layer and leave it untill the wood is baked dry uner tension.

if you go to our web sight, kcstrings.com, and look at the makers shop photos under the "about us" button, second page,first pic it shows our bending forms for cello and bass. the alu form first on the right is the cello c form same principal as the bass c the other forms are for the other boughts. with a properly built aparatus no touch up is required. the question is, is the cost of building the jigs appropriate for the number of basses you are going to build?

[Don Harris]

I built a bending pipe using 2" electrical conduit with a charcoal grill starter element inside. For a test I bent the wood I had already used in the heat blanket form which was pretty badly scorched and bent up. I liked the process. Got an upper bout bent and clamped and got the dimmer dialed in for the right temp. I was going to give the heat blanket another chance, but I'll try a c-bout on the pipe tomorrow and if it goes well, I probably won't look back. I'll still use the form to clamp the c-bout after bending.

[Don Harris]

Thanks RC. I have looked at your website and found it very interesting. In fact, my dad lives in KC, and I would love to see the shop next time I'm in town if that's OK. Like you said, I'm not sure the time spent tooling up for bending forms is worth it as a hobbyist, especially when pipe bending gives good results. As you'll see, I did some of what you suggested:

I changed my mind and decided to modify my c-bout form and try that again. I bought a small sheet of heating/cooling sheet metal and bent it to fit perfecty on the form and screwed it down. I then put a piece of damp cloth on the wood (didn't soak the wood) and put aluminum on either side of that. Put the heat blanket on top and heated it to about 260 F. Didn't see steam coming out, but as I pushed on the wood I could feel it start to give. I started tightening the wingnuts and very quickly had the whole thing done. Let it cool and then took it up to 260 F. again. Once it cooled down I HAD to see what was happening, so I pulled the wood out. Apart from some staining on the damp cloth side and a minor (I hope) blow out from the side I started on (didn't let it get hot enough first I guess), it looks good. It has a lot of spring back, but I'll leave it clamped overnight and see where it ends up. I'll do the other "C" tomorrow and then decide if I'll do more "C's" on the hot pipe. I'm definitely doing the upper and lower bouts on the pipe, as I'm afraid that clamping on the fragile corner blocks with the heat blanket routine could damage them, and with any spring back, the corners won't be a tight enough curve. So I'd end up on the pipe anyway. -don

[arnoldschnitzer]

Bending manually is one of the skill sets that is important if you are going to be an instrument maker (whether amateur or pro). You can always work out a more appropriate method for commercial work later, but it's a good idea to study the basics first. Like in music, we learn major scales before we get into diminished, altered dominant, etc.

[Don Harris]

I think I started with the idea of using forms thinking it would be easier and more uniform. But now I see that it takes some tweaking just to get the forms right (at least for me) and to get the method to work. I don't think I'd enjoy bass building as a hobby if it were TOO mechanized. Like buying a CNC machine to make electric basses for myself. What would be the point? That's not a good comparison...but... just thinking out loud. I do think that pipe bending is more romantic. More tactile feedback. Plus I like the thought that pipe bending is a fundamental skill in instrument building that I need to master, like Arnold said.

I went to get a bite to eat and go see Die Hard then took the "C" out of the form. It held its shape beautifully, but the little blowout left a slight crease on the opposite side. I won't be using this one on the bass. Too bad, because it looks really good in every other respect. I'll do another "C" on the pipe and see if I can make it better than the form. I'm enjoying this process immensly. I've been dreaming about bending guitar sides since I was a kid, so this is a kick.

[RCWilliams]

Don,
All are welcom at K.C. Strings... even Arnold. believe it or not we average of over hundred people a month who tour the shop. if you get the chance please do come by.

[mojoluthier]

I have hand bent perhaps 100 guitars, and find that using the biggest diameter tube practical is advantageous for getting smooth curves. I have built twice as many using blankets and forms, then corrected with the pipe. I bend much hotter than 250 f, perhaps 400. I do not linger or smoke appears. I use a layer of SS inside and out, and put my blanket between the wood and the outside, though other prefer the other option. I spritz. I preheat the bender, turn it off, put in the wood, reheat for 5 minutes, bend over a 2 minute period, and turn off the heat. There is about a ten percent spring back, but I add it to my forms. There is nothing wrong with hand bending, but it is not easier. It is a very satisfying skill, however. I break perhaps 1 piece in 100.