|Venerable old ways to straighten a neck with
First, a caveat: I don't employ this technique much any more except in certain circumstances. In the absence of an adjustable trussrod, I prefer to plane and refret or simply replace a board to true up an errant neck. Working with wood as it is yields much better results than forcing it where it doesn't want to be.Heat-straightening is predicated on the notion that the glue joint between the board and the neck is able to be softened by heat, enabling you to realign the components which share the glue joint slightly. In very rare circumstances this heating technique will make the wood itself go limp (as occurs when bending sides, for example) but this is very uncommon. While it's possible to heat wood to the point of limpness, it will likely ruin not only the fingerboard but the finish on the back of the neck. I've seen it, it's not pretty. And it doesn't really soften the neck enough to take a twist or a bow out of it. This technique is primarily about softening glue and realigning glued surfaces.
Before electric heating blankets and pads, there was this tool:
It's a very rigid steel box with a reliable true
electric heating elements inside, and an on/off switch or a rheostat
Through that little hole to the right of the rheostat is a small bulb
tells you if it's on or not. Mine came with a pair of heavy padded
like the one above in the center, and another one for clamping from
through the soundhole onto the fingerboard extension.
This photo is purloined from
somewhere in cyberspace—it's not mine, but
it's just like mine was when it was new. My rheostat died almost as
soon as I got it, and ever
since, it's only had an on/off switch.
If this is of use, here's the inside of mine:
If you open this image by itself ("view image" or "open in
separate tab"), it'll be quite large.
Here's the skinny on the voltage and watts and so on:
120V, 100 watt element times two = 200 watt item. My unit is
The idea was to heat and soften the glue joint between the board and the neck, force the neck into the desired shape under that heat and pressure while the glue is soft, then turn it off and let it cool, still clamped. The glue would reset, the two surfaces that the glue bonded would be in a slightly different new relationship, and the neck would stay that way. That was the theory. In practice it took some farting around to get correct results.
In many instances, this works like a charm. Martins usually
work well, as do
many Asian instruments glued with epoxy. Gibsons and Guilds were iffy.
Older guitars of any make don't usually respond well because their glue
is no longer softenable. Hide glue changes over time. Water-based glues
in general do best if they still
contain a bit of water, or if the wood around them does.
These devices will
make crispy critters out of any plastic fingerboard inlays, too. Very
fragrant. And in efforts to get old glue joints to give, I have left
this 200W device on long enough to really cook the fingerboard—to the
limits of actual damage—without ever having the glue budge.
Here's a crude sketch of the heater (light blue thing) in full idealized contact.
Closer up, here:
The heater lays right on the frets, which keep the wood from
In fact, the real thing is shorter than most boards, but long
The heating box is dead true, so that when you rest it on the frets, it serves as a straightedge. Thus you often see the expected gap, which if all was well, you would believe it could fix. But simply clamping the neck flat to the heater alone is usually not sufficient.
Instead, I put a small shim (such as a piece of cardboard) between the heater and the first fret, to overbend the neck a bit. After the whole thing cools and the clamps come off, there will be a bit of springback, and the desired relief will be there.
This takes care of the most common problems. A twist can be
by strategic use of shims in various places. A lot about this tool
trial-and-error. And a willingness to punt and use other methods.
The shape of the wedge is, of course, exaggerated for this illustration. The actual one I've used for years is made of pine. In this procedure, the fingerboard extension moves ever so slightly back from the soundhole. No touchup is required, but it moves enough to re-angle the neck a bit.
Sometimes, when dealing with a typical bowed neck, I'll put a shim under the fingerboard extension to push the neck back as it heats the misshapen part of the neck (below). This is a lighter "reset" yet, but it can really save the day with guitars with borderline reset issues. The heating device is extremely strong and rigid, so you can clamp like mad and it will stay straight. This gives it a distinct advantage over silicone blankets and so on.
Here's a tool that I have never used:
However, I have a similar thing I made for this task from a
huge oilite bronze
roller bearing many years ago.The bronze bearing I use was sliced down
the center and
has a porcelain handle on it. I heat it on a hotplate, and use it to
the glue under the end of the f'bd when I'm preparing to remove a neck
for a reset. Again, I prefer to keep the heater off raw wood, as it
will scorch it; the frets are a good spacer. So I'm not a sales
prospect for the thing
above. But it's potentially a good idea for a specific task.
One thing I still routinely use my big neck heater for is removing fingerboards from necks. Again, there are old glues that are simply not affected by heat (anything by Lyon and Healy comes to mind). But when all you want to do is get the old board off, even ancient glue joints can be coaxed into yielding with a little help from the heat.
I hear of other people getting amazing results on older
I tend to see their amazing results showing up in my shop a few years
later after the
neck remembered where it had been before. I don't think they're a Big
Answer tool, and most people I know who used them starting back in the
70s have moved on to other methods of straightening necks, as I have.
But they're an interesting tool and serve an occasional purpose. If I
lost mine I'd make something similar using heating blankets.
a line or something:
* Back around 1972, a local music distributor had a batch of about 500 copies of a Martin 000-42 made in Japan. All had the same cluster of production flaws which rendered each and every one of them utterly unplayable and unsalable: bowed and unadjustable necks, and ill-considered neck angles. I got the gig to straighten all 500 necks, shave the bridges, and set them up to play well. I don’t remember the brand, but I think it was either Coast or Sahlein that imported them and dumped the gig on me. In any case it was a very long time ago. They either had that tool made for the job, or somehow landed one, and gave it to me to do the job. Actually, I had two, the other one stayed with the shop I was working with back then that had connected me with the weird “summer job.”