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Did the bending on my first set of sides. I am bending some sapele sides that are .080 thick. Actually it went pretty well, but when I took them off the mold, they had some spring back. I can fit them into the outside building mold but not without springing them into the mold.

Question is should I put them back on the form with the heating blanket and bend again or is there a better approach?

Thoughts on what I should do to avoid the spring back. Did I not leave them on the mold long enough, etc?

Help would be appreciated.

Thanks
Dave
djmrozinski@chartermi.net

Hi Dave and welcome to the OLF - I see that you are from Michigan too - cool!

The black topped dread that I just finished has sapele sides and the sides that I bent seemed to be a bit tougher than average to bend and hold the bend. Like Todd said I bent with less water, more heat and faster and that helped.

What seems to make the difference is that about 1.5 hours later after the first bend I simply turn the bender on again having left the side in the bender and bring it up to bending temp again and then shut it all down and leave it overnight. So I am cooking the side twice but it only goes in the bender once and it stays there overnight or 12 hours or so.

Sorry to be a nag but I have to say this too - when ever my bender is plugged in I stand there with it until I am done and unplug the thing - they can start a fire if left unattended long enough to heat up too much.

My sapele sides had some spring back too which is normal and as long as the spring back is not substantial and the side fits the mold with minimal assistance it's OK to proceed. Once the linings and blocks are glued-up the "rim" can be removed from the mold and holds it's shape. I use reversed kerfed linings which I believe make for a stiffer rim than triangular linings.

I had a discussion about a week about about how much spring back is acceptable and will the wood relax over time. The general opinion was that some spring back is OK and wood does relax over time to some degree.

Also there are a couple of good bending videos that you can find in the tutorial section of the OLF.

Sounds like 1 of 2 things , too much water or not enough heat. Try taking them to 375 for 2 minutes then letting the set for 15 at 225. allow them to totally cool before pulling out of the bend.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Thanks for all the comment. I bent the second side and let it sit overnight before I released it from the mold. Spring back was only about 1/4" on the second side. Much better than the first side which I only left on the mold for about 2 hours.

I am bending with a silicone blanket and I made a heat controller that has a K-thermocouple input to read temp and control the blanket (I am a EE so that part was easy). I followed John's approach with the kraft paper and three slats, so I think I have the process down. I guess you just need to do this a few time to get the hang of it.

Seems like leaving it in the mold longer was the trick.

Thanks again for all the replies.

Dave
djmrozinski@chartermi.net

Hi Dave,

As Hesh mentioned, a little springback is acceptable, provided that the sides don't go near breaking point when inserting them in their outside mold.

I find that leaving the side overnight in the bender is overkill. A loss of time basically. What I do is simply recook the side twice for 5 minutes once the side is bent. It does the trick. In more details, once the side is bent I turn off the blanket for 5 minutes. Then restart it for another five minutes and repeat the process once again. When I can touch the slats with my bare hands without burning myself (about 20 minutes later...), I then take the side out of the mold. I rarely get any springback with this method.

I do leave the sides and bindings in their outside mold overnight before I start working on them, just to make sure they're dry.

Hope this helps!

_________________
Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

If you are comfortable working with simple electrical components, it's a useful thing to wire up a timer (bathroom wind-up style) and a dimmer switch in a controller box for your heating blanket.
Then if the phone rings or the dog needs 'out' or .... , you won't burn down the shop or incinerate that guitar side!

John, is this a standard dimmer? When shoping around looking for parts to build my bender, the only dimmer I could find would only handle 600W max. Shoping around on the internet I could find some 1200W dimmer but they were way too expensive. Stopped my choice on a router speed control, like most of us here I believe.

All this to say, if your dimmer is 600W max and you are using a heating blanket with it, than you're not avoiding fire hazards as much as you think....

_________________
Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Unless he's using a 2.5 W/sq.in. blanket instead of 5, then standard dimmer and timer work great.

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Jonathan Kendall, Siloam Springs AR

So nice to see you guys are worried about my safety!

But not to worry...
1-It's a 1000W dimmer, which is readily available at HomeDepot in my part of the world. And the timer is rated 15A @120V.

2- Most electrical devices that carry the UL/CSA certification are designed to fail by not catching fire- they are somewhat 'idiot proof', so that if somebody puts higher wattage bulbs in the dining room lighting fixture, the house won't burn down.
Dimmer switches usually will fail by blowing the triac and cutting off the flow of current.

Cheers
John

Thanks for the precision John.

I hope you didn't take my comment the wrong way. You obviously know your way around electrical wires and stuff. But since many would read this thread without knowing The details you just supplied, I felt it was needed to point out that we can't just use any dimmer on a heating blanket.

Although I didn't know the dimmer would just fail without hazard if it can't handle the power going through it. We learn something everyday I guess!

_________________
Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Not a problem at all, Alain.

I wouldn't want somebody inexperienced with electricity to make a bad mistake because of some detail I forgot to mention.
Thanks for bringing it up.

Cheers
John