Bending Koa

[KLW]

So I'm making a tenor guitar my 3 guitar and just bent the sides, Koa, it bent fine BUT scorched quite a bit, luckily mostly on the inside! I also don't pre-taper my sides so a lot of the scorching is in the area that will be removed when tapered.. I'm pretty sure I can sand the outside to look god but I want to understand why this happened.

I use the Blues Creek bender and LMI controller and blanket with the sandwich being steel slat-->blanket-->wood wetted lightly and wrapped in foil-->steel slat.. For the first side I bent at 295 deg cooled then ran back up to 295 again and cooled when I saw the first side was scorched I reduced the bending temp to 285 and followed the same basic process but bent quicker.. neither time took longer than 5 min to bend..

Am I doing something wrong here? Is this just a characteristic of Koa?? Am I Just plain bending too hot?? Both side where completely bent by the time the temp reached the set point I started at about 230 and slowly bent as the temp rose...

KLW

--

Kerry-Lee Werry, http://klwstrings.blogspot.ca/

[Kevin Sjostrand]

Hey Kerry,
I have never bent Koa, but I can tell you that when I bent Santos Mahogany, It scortched a little bit on a practice side at about 350 degrees, so I reduced the heat some and then it was okay. I had soaked the sides for about 10 minutes in the bathtub, so I don't think "not enough moisture" was an issue. I now bend at 300 degrees. My EIR sides bend really well at that temp with no scortching. I would guess too much heat. You are using a blanket so the heat should be about the same top and bottom, but you are containing the heat more underneath with the mold. If you are using a blanket, I would not think you need to cover your mold. Slats top and bottom, and you have the blanket right against the wood on the bottom, but not on the top, so you are obviously going to get more heat to the underside. Try the blanket on top of the side with a very thin slat under it. This might distribute the heat more evenly.
Personally I use light bulbs for a heart source and I am getting good results.

Kevin

[KLW]

Yes the stack does have the heat pad just under the wood, I'm just surprised that at 295 (or 285) things scorched as much as they did... I'm thinking given how things felt this time I can bend Koa at least at 250..

KLW

[tippie53]

how long did you heat it? I use wet paper between the slat and the wood and I do not put the blanket on the bottom
Pattern- slat - wet paper - wood - wet paper - Blanket - therm - top slat.
start bending when you see 220 to 225 degree F then start at the lower bout , then the upper bout then the waist. allow the heat to hit 325F then turn controller to 220 for about 10 min to dry things off and allow to cool. See if that don't help

[KLW]

The whole bend time was about 7 min but left it at temp for another good 7-8 min.. Thinking I should have lowered temp and left it at a lower temp not the full 295..

Thanks for the help

KLW

[tippie53]

yes
you let it too hot
once you get it hot say to 325 you turn the heat down to 220 for 10 min to dry it off