X bracing postion ?

[Max Gisko]

Is there a formulae for finding the placement of the X brace. I'm working on a 12fret to the body, 24.9" scale length OM guitar

[Mark McLean]

Max

Are you working off an OM plan?  OM guitars are usually built with the neck joining at the 14th fret and you will be making something more like a 12-fret OOO.  You might be interested in Paul Hostetter's nice essay on the evolution of 12-fret to 14-fret designs here.  The Martin 12-fret guitar actually has a longer upper bout, reaching up towards the 12-th fret but leaving the anatomy of the bottom end largely unchanged.  If you put a 12-fret neck on an OM body you will need to shift the bracing and the bridge towards the end-block, by a distance equivalent to the space between 12-14 frets.  I can be done, and check out the Martin Normal Blake model, as well as the various 12, 13 or 14 fret versions of the Gibson L-00 if you want to get the idea.   But be careful to keep the bridge and the bracing in relationship to each other as you slide things up and down within the shape of the box. 

[Max Gisko]

Thanks for the reply. I am working from an OM plan and I want it to be a 12 fret to the body neck. I know the bridge will move further down into the lower bout, so I asumed that the intersection of the X brace would move as well. But does the angle of the X change as well, or am I over thinking this and should I just go with my gut and do what feels right? I will definitely check out the essay on 12 fret to 14 fret design.

[Mark McLean]

If the bridge moves south it is important that the ends of the bridge still overlap the main X-braces.  You don't want to have your bridge sitting entirely inside that triangle of the bracing in the lower bout.  The easiest way to achieve that is to move the bracing down as well.  Or you can close down the angle of the X to bring the arms a little closer together.  Or do a little of both.  If the whole X-bracing moves south it will be further away from the soundhole.  That area of soundboard between the X and the soundhole bears quite a lot of pressure so you will want to keep your bracing fairly tall in that area, and maybe use a fairly large soundhole doubling patch to reinforce the top around that region.  Essentially, you will be making an equivalent of the Martin 000NB (Norman Blake signature model).  See if you can find any info on how Martin do the bracing placement for that model.  And this build thread on the ANZLF is about a similar project and has a good discussion of the bracing adaptations. I am sure you would find some useful stuff on the Unofficial Martin Guitar Forum if you look up the 000NB.  It is all manageable.  Good luck with it. 

[Max Gisko]

Thanks again for all your help, The ANZLF build thread is great info, this makes a lot of sense and was the direction i was looking for.