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Body Construction


The Unpacked Kit

Martin provide the sides bent (well sort of - see later), a top that has been joined, the soundhole cut and the soundhole rosette already inlaid. The fingerboard has been radiused and the fret slots cut (as well as the dots inlaid). The neck has been roughly shaped and the dovetail joint has been cut. Also, the peghead has already been veneered. The end blocks need trimming, but have been shaped. Everything else has been provided apart from the braces to place around the rim to impart compression strength and to prevent longitudinal cracks spreading around the sides. It seems a strange omission, given that everything else (including strings is provided).

The instructions with the kit are, to put it mildly, terse. You definitely need to have done plenty of reading as you could not rely on the instructions to build the guitar.

The worst thing about the kit is the lack of plans. Martin have drawn on the bracing positions (but not the bridge plate). The position of the braces is suspect to say the least and leave a lot open to interpretation. However, without a plan, I was left in the dark somewhat and end up making guesses based on bracing patterns on other OMs that I had seen - not very good really.

Note: I didn't take a picture of the whole kit to start with, so this image is the earliest I have of the whole kit. Obiously, I have placed the sides in a mould and glued in the end blocks here.


Rim Construction

Martin define the rim as the kerfed sides with the end blocks glued in. Ideally you need some form of mould for this step in order to keep everything aligned. There are many ways of doing this, including using cardboard formers (as recommended by Stewart Macdonald). I decided to use a plywood mould which I bought from John Hall of Blues Creek Guitars.

The Sides

Unfortunately, when the sides arrived they were very straight ! I contacted Martin (with pictures) and they told me to just force them into the mould and glue the end blocks on. There was very considerable tension in the sides and I was afraid of breaking them or, if they didn't break, introducing unnecessary stress into the guitar. (From the image below, it is clear how far they have become unbent - look carefully at the outline on the cardboard for where they should be - it's faint in the photograph, but the ends of the sides should be perpendicular to the line of the sides !).

I decided that they needed rebending. This was my first attempt at guitar construction so I did not have a bending iron (and due to time constraints did not have time to make one). I approached a friendly luthier in Bristol who very kindly spent time with me and rebent the sides.

Gluing The End Blocks

I spent a fair amount of time setting everything up for gluing the end blocks - getting the neck block absolutely true is very important. The mould from John Hall was not flat at the neck block so I ended up making a rounded former to fit in the mould and create a flat gluing surface for the neck block. (Don't forget the greaseproof paper to prevent the sides being glued to the mould).

Kerfing

I made my first real mistake here - I did not cut the kerfing into small enough sections. This meant that, when following the curvature of the the sides, the kerfing pulled out slightly on the bottom of the upper bout. This is because the height of the sides is not constant so when the curfing is bent to follow the curvature of the body it will pull away at the bottom. It can be hidden with glue and saw dust, but it's irritating to make such a silly mistake. Apart from that it was pretty straight forward.

Extra Bracing (Vertical Struts)

Martin recommend gluing vertical sticks (about the size of lolly pop sticks) every 3-4 inches around the sides to prevent cracks propagating and to provide compression strength. I made these from a block of mahogany to try to match the colour of the kerfing, sides and back.

Completed Rim


Top Construction

Using some cardboard, I made a dish for the top to sit on to lend it a slight camber when the braces were glued.

In the completed bracing picture, note the cap on the X-braces. I decided to put this on as I did not like how far apart the X-braces were at the join (due to the profiling at the top of the members, which I also filled with tiny slivers of spruce).

The finger braces have not been profiled yet and the braces have not been scalloped. This was done just before the top was glued on to the sides to reduce the risk of warping. Also, I did not like the position (angle) of the tone bars as they did not sit against the angle of the bridge (nor look like other OM bracing patterns I had seen), so I changed the angle slightly.

The following two images show the scalloped braces once the top has been tuned by tapping.

I was not too sure how to do this, but I could hear the tone changing as the braces changed shape. I scalloped the braces quite conservatively on the basis that I can do this later from through the sound hole (albeit with some difficulty).


Back Construction

Joining The Back

Shooting Board

The centre line join in the back needs to be precisely cut. The two book matched halves from Martin are not straight and so need to be jointed accurately. I used a number 5 plane on a cobbled together shooting board to get an exact join on the book matched halves.

Also, after looking closely at the grain pattern of the mahogany back, I decided not to put in the marquetry strip - I really like backs without the contrasting stripe for some reason. Hey ho...

Clamping

Provided the joint is accurate, there is no need for a huge amount of clamping pressure. There are many ways to clamp the halves whilst gluing. I used some blocks of wood and small wedges to create the pressure and some weights to prevent the boards buckling (probably overkill but who knows ?). I also used newspaper rather than greaseproof paper under the back, so that the glue can cure better as I really want this joint to be strong !

Bracing

The reinforcing bar down the centre was supplied in two pieces. I jointed the strip and then glued it down. Mortices were chiselled for the braces later - I felt that this would give closer joins and be better aligned than gluing individual strips. Prior to gluing the braces the top was scraped with a cabinet scraper and prepared to the final finish. The reinforcing strip was also sanded to a smooth arch.

The braces are ready shaped, just requiring finishing and fitting. They are also over length and will be trimmed later.

I used a hardwood strip (planed down to a couple of millimetres so that it would bend) to support each brace as it was gluing, as suggested by Cumpiano. The second image shows a close up of the bar. (The clamps on the small brace in the photo are not doing anything other then holding the back off the work board !)

After trimming the braced back looks like this:


Fitting The Top And The Back

Fitting the Back

In order to get the right angle for the kerfing that will join with the back, I made up a sanding board with an angle of about 4.5 degrees. By moving this around the rim the correct angle could be set.

The back braces needed to be trimmed to length. Mortices for the back braces (which are visible in the first picture) also needed to be cut into the kerfing to allow the back to sit flush with the sides and to allow the braces to add structural strength to the sides.

Fitting the Top

No photos as fitting the top is a lot more straight forward than the back - the angle of the kerfing is pretty flat (about one degree by my estimate). I also sanded in a drop off from the sound hole to the neck block to allow for fingerboard to sit on the sound board due to the neck angle.


Gluing The Top And The Back

How to Clamp the Top and the Back ?

There are many different ways to clamp the top and the back to the sides: spool clamps, cam clamps, "roping" with elastic, clamping with a custom made former, etc. I spent ages trying to decide which approach to use. I was not too happy with trimming the top and bottom exactly to the sides prior to gluing (as you would need to do for roping and spool clamps). Also, with roping you need to squash the top flat in order to glue the back on (and the Martin kit has a domed top). I did not like the idea of using former as I was not sure that you would get even clamping pressure. So I decided to use cam clamps, which meant making a lot of them, as I could not afford to buy the commercial ones.

Should You Glue the Top or the Back First ?

I really don't know. I thought about this for ages. If you glue the top first, it is much harded to clean glue squeeze out from the back (which will be visible through the sound hole). However, if you glue the back first, it will be more difficult to shape the braces in the top (if you want to). In the end, I decided to glue the top first...

Gluing the Top

Not too much to say here really - just that you need to work fast and put lots of clamps on quickly ! This is also why I spent so long making lots of cam clamps (it probably took almost as long to build the clamps as to make the rim !)

Gluing the Back

Very similar to the front. (I protected the top with a thick layer of cardboard).

Completed Soundbox

Once the top and back was glued the dovetail joint was opened up. The overhang on the top and back was trimmed with a flush cutting router bit and the soundbox was sanded to 80 grit in preparation for cutting the binding channels. The following two images show the soundbox after these steps.


Binding

Router Attachment

I used a twin flute router cutter and made an attachment for my router out of MDF. As can be seen in the two photos below, the attachment can slide forward and back to vary the depth of cut. There is a flat sole that is perpendicular to the handle. There is also a chamfer on part of the sole to accomodate the curve of the back. It is used by keeping the handle part tight against the sides (it has an "easy slip" piece of tape attached to it). The second picture shows it coming up against the side, but not yet tight against it.

Routed Channels

Following routing, the channels were cleaned up with a file:

Finished Routing

The back, completely routed:

I decided not to install a purfling strip as I want to keep it simple and minimalist.

Tail Wedge

The tail wedge needed to be chiselled out and then glued. The first image has had the first coat of glue applied.

Gluing Bindings

Before gluing I chamfered the inside edge of the binding to make it fit tighter and cleaned up the binding channels. I also did a dry run of the fitting. You need lots and lots of strips of masking tape already laid out as you do not have much time before the Weld-On glue sets !

Bindings Scraped Flush

After gluing, the bindings were scraped flush with the plates and the sides. Any imperfections were filled with super glue.