MIMF

Hi all.

I'm very new to the whole forum so please bear with me.
Plus, I hope I'm posting in the right place!

So, bending acoustic guitar sides....
I'm cracking my way through far too many sets and really need some help.

My first build wen't fairly well (maple) after using a traditional bending iron.
However, I went on to build a Fox Box style bender, very solid and made from MDF, and now I'm having problems.
I'm also using spring steel slats and a silicone heat blanket.

My first sapele sides were more scorched than cracked, so not too bad.
Then my walnut sides cracked, in the same place (waist and upper bout/shoulder).
After ordering more sapele (to practice with) and walnut I hit the same problem.
I only got as far as cracking the sapele and I'm now to scared to even try the walnut.

I'm trying everything.
More water - less water, more heat - less heat, more time- less time, and still things are cracking.
I'm going really easy when screwing in the waist caul and letting gravity do most of the work on the bouts.
I was going to use my traditional bending iron to bend the sides into a ball park area, then tweak them on the Fox Box. Good idea?

Any suggestions?

Sapele can be very difficult to bend. Sometimes, no problem, sometimes, one guitar takes three sides. How thick are the sides you're trying to bend. Most woods like to be somewhere between .075" and .083", or so, to bend. I've always found walnut easy to bend at those thicknesses. I've never used a bender but always a pipe, so I can't speak to the bender issues.

Walnut should be very easy to bend. The fact that you are having trouble with it makes me think you are trying to bend too thick. Walnut should be pretty easy under 0.09".

Also, from what you desribed I think you may want to tweak your process. If you are letting gravity do most of the work for the bouts you may be going too slow. If you are doing the waist all at once try taking the waist down most of the way with the bout pulls holding it in tension. When you get close to bottoming out the waist, pull the bouts down fairly quickly (but not wrecklessly so). The waist caul will hold them in tension. Once they are bent, go ahead and bottom out the waist.

Going slow allows the wood to dry out which leads to breaks. Check out some of the videos on YouTube to see how fast you need to go.

Here's a video I made a few years ago. Notice how quickly I bend...

https://youtu.be/sUJbB0pa7Zs

Also, are you placing the blanket above or below? If you're placing it above, that is a big part of your problem. Place it under the rib you're bending. To get heat on the top surface, you close-up the bender for a few minutes to pre-heat the upper slat. I bend directly on the blanket.

If all of that fails, you're simply trying to bend it too thick. Walnut can almost be bent at room temperature, it bends so readily...

How are you controlling the heat from your blanket?

I use white or brown kraft paper as a water reservoir for bending, which removes the variable re: how much water is in the bending package if just the wood is wetted out. The stack from bottom to top is:

- Spring steel slat
- Sheet of standard household aluminum foil, cut to size of slat
- Sheet of kraft paper, cut to size of slat and wetted with warm water
- Side with tape tabs marking waist
- Paper
- Foil
- Slat
- Blanket
- Slat (this protects the blanket)

After the stack is in the bender, I hook up the blanket to a router speed control, set that to 'full', plug it into the timer/shutoff/outlet on the bender and run the timer to 45 minutes. On a 5 watt/in^2 blanket, you should be able to run it full out until the bend is completed and still have some water in the reservoir, so as soon as the power is applied, note temp increase on slats (they get hot quickly) and wait for steam to become visible. Once visible, run the waist down to 3/8" off the form, immediately move to the lower bout and bend, then bend the upper...all this should take about 90 seconds. Run the waist down the rest of the way, check the ends to make sure the cauls are in the right location, then wait for the steam to exhaust itself (no visible steam = no more liquid water in package). Switch the speed control to 'Variable' with the range knob set for the low red, or about 3/4 rotation. After 10 minutes on 'Variable', dribble a drop or two on the slat at the 'top' of the lower bout...at correct drying temp, you should see the water quickly evaporate...if it immediately dances off the metal, turn the speed control down a bit more...this is a drying cycle, which drives any moisture out of the stack and sets the bend. After 45 minutes, the timer will click off...let the stack cool to shop temperature (overnight for acacias and mahogany where minimal spring-back is desired...rosewoods and a few other woods like zebrawood can come out once temp is back to shop temp.

The process is shown in a series of three videos I did a couple years back. Between Mario's approach and mine, you should be able to get the sides bent with minimal issues.

A few other considerations:

- Thicker sides are tougher to bend...the side is just a beam in bending, so a .090" thick side will be close to half again as stiff as a .080" thick side.
- Treating mahogany and other woods with SuperSoft 2 will allow them to take tighter bends without either compressive or tensile failures in waist or cutaway. I've found SS2 to be next to useless on resin-laden woods like most rosewoods and some ebonies...just not needed for wood that just needs a bit of heat to bend.
- Some rosewoods - cocobolo in particular - will change color in bending due to oxidation of resins...best to bend coco at lower temp (just 250 deg F seems to be enough to bend) and to consider heating the other pieces (back, or for coco binding, the tail graft, heel cap, etc.) to same temp for same period of time if you want a match.
- Do not wrap or 'envelop' the side in foil or paper...paper will wrinkle and mark relatively soft woods like mahogany, and foil does not allow steam to freely leave the package. As Mario said, bend quickly once the temp is high enough...excessive spring back is almost always the result of dawdling.

Here are the links to the vids:

Part 1: https://youtu.be/Q7vd9wGG4LM
Part 2: https://youtu.be/qhrHvfzfDLo
Part 3: https://youtu.be/ZjRDv9e6MFM

I really appreciate all the comments guys! Very useful stuff.
I'm moving onto my next built very soon so will take everything on board and see how I go.
Thanks again.