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So here's the thing good people. I'm building my first post-retirement acoustic Grand Auditorium here in Ohoka New Zealand, and going along pretty well. I'm completely absorbed by the thrill of it.

But I'm having a lot of trouble with the side bending. I built an LMI style bender heated by 2x150 watt and 1x100 watt globes. And just to be sure it would get hot enough, I arranged two x 200 watt infrareds to shine on the outside of the upper steel sheet as well. The sides are Sapele at about 2.8mm. I spritzed them and wrapped in brown paper, between two spring steel sheets.

The outside top and inside-the-jig temperature was between 220 and 230 fahrenheit according to my wife's meat-cooking probe (just don't tell her please!).The sides bent well enough - yes you can feel when they are ready to co-operate can't you. But I have a lot of springback. While I have managed to wrestle them into the outside mould, I'm feeling that there's going to be a lot of tension if I glue it all up. I imagine the heel and butt blocks will cope with much of that but am haunted by the thought that tension is a tone killer.

So help please!

Am I jumping at shadows?

Is there something I'm doing wrong?

Is there restorative action I should take?

MAC

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I think you are jumping at shadows .All you have to do is clamp the sides to the outside form and put your two end block in and the linenings .Then glue the top on before you take it out of the form Bill............

Hi Mac.  Let's take this one as two issues with the issues being bending methodologies and spring back respectively.

There are lots of ways to bend guitar sides and what ever works well in all respects is likely a decent way to go.

What I do is use a heat blanket instead of bulbs.  Forgive me please but I have to cover one base just to make sure that we have our terms defined in a manner that we are both speaking about the same thing.  When you say "globes" you are not referring to those fat-arse, Al Gore CFL light bulbs are ya?  ;)  If we are speaking of conventional incandescent bulbs I can't tell you how many folks that I know from another forum who initially went this route and had lots of issues.  Mind you it can be done, lots of folks still use bulbs instead of blankets but the results take much longer to obtain decent results AND many folks have cracked sides too struggling with bulbs.

From a safety stand point a bender should never be left unattended if plugged in be it bulbs of blanket(s).  With blankets those of us with CRS and ADHD, like me... don't have to stay with the darn thing for 1/2 and hour waiting for it to heat up so I could make the case that blankets may be safer too at least from user error.

There is also the issue of the wood having a window of opportunity where bending well is possible but if the temp and time goes beyond that point and for lack of a better term some woods, in my experience, will "case harden" from excessive heat and time and then become more brittle and prone to breaking.

Mahoganies are on the tougher side to bend and require high heat and fast action so as to not case harden.  The more figured the wood often times the more problematic it can be.

Side thickness is an issue IMHO and your 2.8mm is about .030" thicker than I would attempt to bend any wood requiring higher heat and avoiding over heating or prolonged heating.  I bend my sides, including sapele at around .080 with good results and no breakage or facets.

So my read of your post indicates that your sides were too thick to begin with, the wood type is known to be temperamental at times, and I'm not a fan of bulbs because of the inconsistent heating with hot spots in some places, cool spots in others, and the overall idea that bending with bulbs can take 1/2 and hour exposing the wood to the possibility of case hardening where with my blanket I can bend my sides in 5 - 7 minutes.

Now for the spring back issue.  Again lots of ways to do this.  What I do is bend as per my method and then turn off the blanket and go have lunch.  I wait for the bender and sides to cool completely to room temp and then about 2 hours later, I like long lunches..., I turn on the bender again having never loosened the cauls and clamps and again cook the side, now bent, for about 5 - 7 minutes longer.  I used to use a thermometer but I found that your results will vary widely as to where you stick it and these thermometers are not all that accurate anyway.  So I wanted to develop a "feel" for this process and not rely on readings and settings as much as I rely on what I believe the wood wants and needs based on experience, observation, and the feel of it too with leather gloves on of course.

So I cook it twice and on the second bake I leave it in the bender, unplugged and off, overnight.  The next day my spring back is minimal to nonexistent and this seems to work well for me.

By the way for the price of a heating blanket you can save yourself the price of one set of cracked sides if the wood is something more exotic and expensive so I also see the blanket as inexpensive insurance.

Now back to what to worry about since you have a lot of spring back.  I have some guitars here from my early days as a builder in a condo spare bedroom with white carpet in my shop.... that had a lot of spring back too.  When coaxing them into the mold I was worried that they might crack now or some day.  So far so good and all is well so I believe that at least to some degree the wood will relax a bit and the tension on it will to some degree lesson.  Do I recommend this - no.  It's always better to have the sides fit the mold well with minimal pressure from the spreaders/clamps.

If the spring back bothers you to the point of action you can always touch up the sides on a bending pipe.  Use a backer slat to avoid blow-out.

I hope this helps.

Right on the money as always Hesh. I used those confounded bulbs for several years and the one Brazilian Rosewood side I broke at the waist was the end for me. Ordered the blanket that day and haven't had a problem since. I've found that by adding a slight taper in my mold at the head and tailblock to pull in a bit extra has corrected all of my springback issues. Side thickness between .070 and .085 depending on the stiffness of the wood I'm using. And lastly, Mac use the sides the way they are, hand bending a little to improve the fit, and then take off .8 millimeters in sanding the sides true after you assemble the rim. Post some pis as you progress and I hope you have a successful and fun build.

Lets not forget this is his first build. And i think we all have been there before.The first one I built I bent the sides over a two inch pipe.Bill..............

Me too Bill, with a propane torch stuck in the end. Odd thing was the brown scorch mark on the front of my shirt.

Lots of folks on another forum used to know that I liked building in my boxers because it got hot in there with the heat of the machines, etc.  Boxers and electric bending pipes clamped to the edge of a bench are NOT a good idea....  I still have a scar to prove it.....  And no I am not posting any pics either....

Thanks everyone for your input. What a great resource frets.net and you guys are.

I have five things to take forward to next time:

- get thinner. 2.0mm from 2.4mm presently

- get hotter. From bulbs to blanket. Heat twice.

- get wetter. More water. 

- take longer. Cool overnight

- persevere!

Watch this space,

MAC

You are very welcome Mac and thanks to You too!  Why?  Because your very good questions, like many good questions gets us thinking about what we do.  Then when we attempt to write up our thoughts and vet it for clarity, soundness of methods, safety, etc. at least in my case I think that I benefit from having to express my thoughts in an hopefully understandable manner.  Kind of like taking a test back in school only a heck of a lot more fun.  And then in time we get to see your progress and what you create too and that makes us all proud of what we are doing on this very fine forum.

Regarding the water, water is a dicey thing in so much as a bit too much and other issues may happen such as the sides cupping.  Figured wood such as African rope mahogany may cup in the figure only, etc.  So what I do in respect to water is lightly spritz the side and then wrap it in brown paper like you do.  Then I lightly spritz the paper too.  This lets me have some control over uniformly applying some moisture but not soaking anything and thusly avoiding the cupping.  I also like distilled water but I'm not sure that there is much benefit in this beyond a bit less mineral deposit stains on the wood, slats, etc.

Then like most things in Lutherie there are folks such as Wayne Henderson who loves pie and soaks his sides (BRW) in a tub of water for very long periods of time....  Wayne did a commission for Clapton and is the Luthier in the very good book "Clapton's Guitar."  So my point is that water usage can vary widely according to what we are bending, who is doing it, and what works for them.  There is no right or wrong here, just different ways to get decent results.

Good luck and above all this DOES get way easier after one does it a few times.

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