The Build: Martin Mahogany Dreadnought Kit (Part 2)
In part two of his quest to build a classic Martin acoustic, editor Jamie has a bracing experience – and gets thoroughly trussed up
As regular readers may know, I recently embarked on a mission to build the nearest thing I could get to a Martin D-18, using the company’s official ‘Mahogany Dreadnought’ kit. As this kit is not quite a bolt-together affair but requires some moderately serious luthiery skills to assemble properly, I enlisted the aid of Bristol guitar maker Alex Bishop, whose tools and seasoned judgement would help my greenhorn guitar-making skills meet the mark.
In the first instalment of this feature, published a few issues back, Alex and I laid the foundations of the instrument, joining the two halves of the back together after learning how to sharpen our tools properly – an important aspect of building guitars correctly. With the back set aside to dry, we turned our attention to the braces.
As mentioned, Martin supplies the kit with all the braces for the top pre-cut roughly to shape. For the uninitiated, braces are reinforcing struts of spruce that prevent the wood of the guitar top being ripped apart by the tension exerted by the strings when tuned to pitch, which equates to 75kg of pull for a standard set of 0.012s – quite a load for a flimsy sheet of spruce to stand up to for years on end. For reference, a hefty Fender Twin only weighs 29kg. So it’s important for those reinforcing braces to impart sufficient strength to the top to prevent it from warping or breaking apart. However, the devil is in the detail (as always), because the braces must also be slender enough to permit the top to resonate freely. It’s this balance between strength and resonance that defines the voice of the guitar. What’s more, the kind of playing you do will also influence how robust the bracing needs to be. It’s a good job Alex is an old hand at this, because his judgment will help me judge how to tailor the bracing for the best balance of sound and strength.
Most of the parts supplied in the kit require further fettling before they are ready to be fitted. Straight out of the box, the braces supplied with the kit are rather chunky. But that’s not a problem, because, as mentioned, there’s plenty of scope to shape and refine them into a more svelte form later on – as Martin fully expects kit builders to do and as its builders would make sure to do in its own factory. First, however, we’ll need to glue each brace into its correct position on the underside of the top. To help us do this, Martin has pencilled lines onto the underside of the top marking where the 11 braces of the X-brace configuration should be placed.
Brace Yourself
Most important are the two main braces of the central ‘X’, followed by two long diagonal braces that sit inside the legs of the ‘X’ in the area behind the bridge, plus four small braces that are positioned diagonally, two a side, on either leg of the ‘X’. Additionally, there’s a heavy bar-like brace near the neck joint that runs horizontally from side to side of the body. This chunky brace reinforces the vulnerable part of the top that sits beneath the end of the fingerboard and helps the top to withstand the impact of frets being tapped in during the build. There’s also a lollipop stick-like flat brace that sits behind that, which is designed to prevent the top splitting due to fingerboard shrinkage adjacent to it – a problem that guitars with ebony ’boards are particularly vulnerable to.
“The fingerboard can shrink much faster than the surrounding materials and that puts a great amount of tension in the top,” Alex explains, “which is why that brace is there – but even then it doesn’t always prevent splitting due to shrinkage.”
“There’s scope to refine the braces into a more svelte form later on – as Martin fully expects kit builders to do”
Finally, there’s a rhombus-shaped bridge plate that sits under the bridge and provides reinforcement in that stressed area, acting as an anchor point for the ball-ends of the strings.
“The central X-brace is the biggie,” Alex says of this design, “because the big stress is in the bridge. The tension of the strings is trying to pull that bridge forward towards the centre of the X-brace, where reinforcement is at its strongest – which is why it’s a genius bit of design. It’s the focal point of a lot of stresses, so the X-brace is really structurally key.
“The two braces inside the lower legs of the ‘X’ are interesting – just how necessary they are is still up for debate,” he continues. “You can sculpt until they’re quite light and resonant, so long as the main X-brace is strong enough, which is why they’re referred to as the ‘tone bars’. There are also four smaller braces that sit outside the legs of the ‘X’ that you can also make quite fine. In fact, as soon as I started doing really fine braces in these areas, all my guitars started sounding better.”
That process of fine-tuning the braces comes a little further down the line, however. First, we must fit them to the top. After some basic prep, Alex introduces me to a contraption known as a ‘go-bar deck’. Available from parts retailers such as StewMac, but also possible to make yourself, the deck provides a way to apply even, firm pressure to multiple braces being glued down to the top. It’s basically a cube with only two solid sides, opposite one another. The way it works is simple. You place the workpiece – in this case, a guitar top – onto a disc-shaped jig on the lower surface of the go-bar deck. This disc is slightly dished, by about 3mm to 6mm of deflection, in order to form a mild convex arch in the top.
“We call them flat-top acoustics but… they’re not,” Alex says. “The top is actually slightly arched.”
Once the top is in place on the concave jig, you put your braces in position, as marked by Martin. You then fit a selection of oak and willow sticks, each bent slightly for tension, between the braces and the top of the go-bar deck. Yes, it’s that simple: bendy sticks push the braces down tightly in place while the glue dries. The oak sticks are relatively stiff and are used to press down the centre of the biggest braces: maximum force is applied here to ensure those points are stuck solidly.
The willow sticks, meanwhile, are bendier and provide gentler downforce, and these are used to hold down the delicate scalloped tips of the braces while the glue dries. Large or small, it’s vital to make sure the braces are glued securely. The tips of braces, in particular, are apt to
“It’s vital to make sure the braces are glued securely. The tips of the braces are apt to lift if not glued properly”
lift eventually if they are not glued down properly to begin with – and then you end up with a buzzing loose component deep inside the bowels of your guitar.
Sticky Business
Once we’ve done the dry run to figure out how best to place the sticks, it’s glue time again – this time applying the Titebond more generously than when we were joining the two halves of the back, because, as mentioned, we don’t want any braces to lift or rattle during the lifespan of the guitar. As we apply the glue, Alex explains that we want to see plenty of ‘squeeze out’ – a continuous line of beads of glue pressed out all along the joint between each brace and the top. Alex cautions, however, that we’ll need to chip off any dried-up globs of glue later on with a chisel – it looks unprofessional to leave them in place – so you don’t want to go too crazy with the glue as you’d be making a lot of extra work for yourself.
“If there are pools of glue all the way along the bottom of the brace, you’ve got to chisel it all out later – and that risks damage to the top,” he says.
He also advises that we go no further than glueing down the main X-brace to begin with. Because it’s so structurally important, we’ll let that central X-brace dry properly before fitting the other braces so as not to disturb the setting glue.
As you can see, even this one operation of glueing down the top bracing involves quite a lot of procedural forethought, and that really says a lot about what guitar making is about. The result you achieve at the end of a build will ultimately depend on how consistent your planning and quality control has been at every stage. Taking a meticulous approach to everything from tool sharpness to applying glue adds up, little by little, to the quality of the final instrument. Like golf, it’s about making as few errors as possible over the whole duration of the game…
Trussed Up
With the glue drying on the first of the bracing, it’s time to turn our attention to the neck. Alex thinks we could usefully install the reinforcing truss rod in the neck while we wait for the bracing glue to dry. As we’ve got all the parts ready to hand, he takes the opportunity to demonstrate how it works.
“Basically, this is a double-action truss rod and it’s simple,” Alex explains. “What it is is a threaded rod, with an Allen key socket at one end and a [rotating] barrel at either end. The two ends of the truss rod are also connected by a metal bar that runs parallel to the threaded rod. This bar will flex into either a convex or concave shape when you adjust the truss rod with an Allen key. So if I tighten it, the bar will flex upwards – but if I turn it the other way, it will flex downwards.
“The reason that it took a while for people to come up with this so-called ‘double-action’ truss rod design is that the main thing a truss rod is designed to do is oppose the tension of the strings. If you start with a really straight neck but then put some 13-gauge strings on it, they’ll pull the neck into a bow shape. So what
you want is to be able to control that bend, which the truss rod allows you to do in a finely adjustable way.
“Different woods or even different pieces of wood from the same tree will all bow by different amounts under strain,” Alex continues. “You generally want a tiny amount of bow in the neck, so the strings can vibrate freely. So, in my head, the perfect neck and truss rod combo is one where I tension up the guitar and the neck gives me a tiny bit of bow naturally – and I don’t have to actually touch the truss rod, in terms of adjustment.
“If, however, the neck is made out of a piece of low-grade mahogany or cedar or something, it will probably bow quite a lot – and in those instances you will need to tighten the truss rod clockwise so it bends downwards at either end, counteracting that bow in the neck. You have to beware of over-tightening, though, because the truss rod may snap or even break through the fingerboard.
“Double-action truss rods can go the other way, of course – unlike single-action truss rods – and that’s useful if your neck warps or something like that. For example, I did a Lowden refret once that was supposed to have a double-action truss rod, because that’s what was spec’d for that guitar. Anyway, I refretted the whole thing with the fret slots cut so it would be quite a snug fit for the frets I was using. And, in fact, the compression from the frets [being inserted into the narrow slots] caused the neck to bow backwards very slightly. I did anticipate that, however, but thought, ‘That’s fine, because it’s got a double-action truss rod and it’ll go back the other way.’ But with this particular Lowden they’d decided to fit a singleaction truss-rod in! And so I had to refret that whole thing just because of that, which was a pain in the backside – and a lesson learned.”
With that explained, it’s now time for us to secure the truss rod in place in its channel in the neck with the everdependable Titebond glue. Alex tells us that the last thing you want is a loose truss rod rattling around inside the neck once the fingerboard is fitted on top – and as the truss rod is both under stress and entirely hidden from sight, we can be pretty generous with the glue holding it in position. With that job completed, our next task on the build will be bending the sides of the guitar into the exact shape they’ll need to be in order to be fitted to the top and back of the guitar – but that’s a story for next time.
In the meantime, it’s time to turn the kettle on, put a bit of Django Reinhardt on the workshop stereo and wait patiently for a whole lot of glue to dry…
“Be beware of overtightening, because the truss rod may snap or even break through the fingerboard”