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Heating Pad (or other) Recommendation for Backbow?
P
Private member
Have an electric archtop that has a .012" backbow strung with 12-48s with truss rod completely loosened. 3 piece maple neck with a Ziricote fingerboard...
Any tips or recommendations to straighten this neck would be appreciated... Currently trying humidity (in the case).
What makes the neck change shape under heat? ..softening glue?, fundamental change in the wood?, something else? I don't know the answer but Scott Freilich, who ran Top Shelf Music in Buffalo, was a pioneer of neck heaters and claimed that the heater did the same thing as a hair curler did with hair.
I bought one of his last aluminum heaters a few years before he retired and have used it successfully for a number of years. Since I work on a lot of 'catalog' guitars from pre WWII with poplar or basswood necks, the heater works well. Mahogany doesn't seem to bend as well, and often needs a few 'heat sessions'.
Almost always, my necks are waaay bowed. I don't measure, but while on the heater, I clamp until just convex, then let it heat for an hour or so, then unplug and let it set overnight. One thing I found is that you need to assess exactly where the 'bend' begins and ends, so you can place the shims at just the right spot along the fb, and yes, leather is a good shim choice. With really spongy necks I just pull the board and install carbon fiber rods.
Tom
PPrivate member
Hi Tom and John.
We're trying to slip the glue joint and heat the lignin in the fingerboard. Lignin is a natural glue in wood that holds the cell structure together and acts much like glue we use and when heated, softens, and when cooled, hardens. So if we apply science and thermodynamics, we are using heat to soften the lignin, glue and the cell structure, and then let the wood and glue cool and harden to hold the new shape of the cell structure and the glue joint to hold the new shape. And we try to compensate for recidivism of the glue joint and fingerboard shape, overcompensating by bending beyond where we want the end result. The key temperatures to remember are: Lignin softens at around 266 to 320 degrees. Most glues soften at around 150. Most finishes start to deteriorate at around 140 but I've heated everything from shellac to poly to about 200 without noticeable damage. Wood starts to scorch at around 356. So the goal is to not only slip the joint (current methods seem to treat this as the end goal) but to change the shape of the cell configuration of the fingerboard wood as well. To do this, we need to heat the fingerboard to a temperature that is hot enough to soften the lignin and also heat the glue hot enough to soften. And not heat the finish on the neck enough to damage it. So with all of this in mind, this is how I do this:
I bend the neck plus 1/8" past the shape of the neck/fingerboard you wish to achieve with whatever method you choose. I then heat fingerboard to about 290 degrees, a middle ground between the 266-320 needed to soften the lignin. Now the fingerboard acts as a highly efficient heat sink insulator, so heating the fingerboard to 150 will not heat the glue joint to 150. So it takes at least (based on calculations using a temp gun) 30 to 40 degrees more to heat the glue joint to 150. By heating the fingerboard to the 290 mark, we achieve the temp necessary to soften the lignin, overcome the insulating of the fingerboard thickness, and soften the glue joint without damaging the finish or scorching the wood. If the fingerboard is finished, then we can only heat the fingerboard to about 200 so as not to damage the finish, but this will only soften the glue joint (current goal thinking), and not have the added help of reshaping the fingerboard wood. It is this component of having both the re-shaping of the fingerboard coupled with the slipping and resetting of the glue joint that makes this method more effective then just slipping the glue joint because we now have two elements helping to hold the neck in the re-shaped position instead of one.
What if the fingerboard re-shaping and glue slip aren't strong enough to hold position?
If we want to bend the neck itself because it is bent or twisted beyond what a glue joint and fingerboard re-shape may be able to hold over time, we need to re-shape the neck itself. So to that, I had a discussion with my long time builder friend about wood;
Heat, cell structure, and lignin (a natural glue like matter holding the cellulose, fibrous structure together. Some believe that as wood ages, the lignin starts to harden and cells start to harden and are not prone to heat changes but he believes the cell structure and lignin is still changeable and so do I after heating older wood to change multiple times. After the wood cools, the lignin hardens and the cell structure hardens, making this basically a one shot deal. Some manufactures are starting to preheat wood before building (torrefaction, caramelization etc.) to lock the lignin and cell structure ahead of time so that the instruments are less prone to humidity changes. Some report that the wood after this process is harder to work because the cells and lignin have crystalized.
Based on all of this, I applied a method to a bass neck that was over bent 1/4". I tried slipping the glue joint and re-shaping the fingerboard but it didn't overcome the shape of the neck. So I removed the fingerboard, exposing the wood in that area and removing it as an insulator. I then used two heat lamps from above the fingerboard area and used sheetmetal under the neck to reflect and heat the neck. I was unconcerned about the finish because the customer wanted to keep the neck and was fine with refinishing. The finish was poly. I bent the neck to the desired position and heated to 290 degrees, a middle ground between 266 and 320. I also applied water to the raw wood area of the fingerboard (I scraped the glue off) because water softens wood, easing fibers and we can use less heat. I let cool in clamped position for a week then released clamps. The finish was fine and no recidivism after a year and a half.
PPrivate member
How durable is this approach in your collective experiences? Does the heat actually alter the wood fiber arrangement in the neck? If there was fingerboard creep will it gradually return?
PPrivate member
Hi David,
I finally got around to making my own heat press device very cheaply, with: a) a small length of box aluminium and b) a heating blanket that's suitably proportioned...
It's totally possible to do heat pressing without it, but having a tool as above will be definitely worth it.
Having said all that, what I've learned so far is:
1. Put truss rod in neutral;
2. Tension with clamps and a known good straight edge to adjust neck to desired geometry (Tony's reply talked about how to calculate this - agree with the "double it" rule of thumb)....use leather cauls to pull the neck to the target...
3. Apply a consistent heat along the neck. I've seen people doing this apparently successfully with sports socks filled with (dry) rice hot from the microwave....
4. Once the neck is warm and relaxed and in the right geometry.... If you're looking for more forward bow, only tighten the truss rod until it bites. If you're looking to correct excessive bow then adjust a bit more, so it's tight, but still has room to travel.
5. Leave to cool (clamped up) for at least 24hrs, 36 preferable.
cheers
Steve
PPrivate member
Doug. Please see “Washburn Neck Reset”. You should be able to straighten .012 out. I forgot to mention that you tighten the truss rod after letting fingerboard cool. The formula I use is bending the neck/fingerboard is twice what you want plus .015. I imagine you’re shooting for around .006 to 007? So .012 to flat + .007 = .019 x 2 = .038 + .015 = .053. I’ve done 1/8” of back bow and worked great. You should be OK.
PPrivate member
I've had minor success with a heat lamp on one while it's left under tension, but .012 is quite a lot.