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I am considering building my first OM style 14 fret cutaway and I have some questions before moving forward. I have attached a couple of pictures showing the cutaway design, similar to the Martin style. The dashed lines are the neck block and material to be removed. Do I need to consider reinforcing the cutaway portion of the upper bout? Do I need to reinforce the neck area to prevent possible twisting?

I read here that the cutaway portion of the upper bout can be sanded a little thinner to help with the bending. Do I need to do anything to the rim, cutaway side, to compensate for the cutaway?

Anyway, I appreciate any and all feedback on this.

Thanks,
Patrick

"Do I need to consider reinforcing the cutaway portion of the upper bout?"

You still need to have the upper face brace run across the top below the cutaway (like in pic below), but otherwise, no.


"Do I need to reinforce the neck area to prevent possible twisting?"

No.


"Do I need to do anything to the rim, cutaway side, to compensate for the cutaway?"

Yes. You need to add a bulge to the treble side rim profile in the area of the cutaway to allow for the radius of the back plate (as on the upper of the two sides in the 2nd pic below).


When laying out your neck block to cut the width on the cutaway side, remember to factor the thickness of the treble side into your calculation so that the surface of the side at the end of the cutaway ends up flush with the edge of the fretboard at the 14th fret.

1/2 the fretboard width at the 14th fret minus the thickness of the side = the width of the treble side of the neck block from its center line.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Thank you Jay. How did you calculate the bulge to compensate for the cutaway?

I don't calculate it, actually. The luthier who taught me did it by eye. The bulge is about 1/2" wide at its widest relative to the non-cutaway side profile. That's enough for a back with a 15' radius. I added some markings to the photo to give you some landmarks for where it goes. You can make it a bit wider if you want to play it safe the first time. It would just make a bit more work when profiling the rims in the sanding dish (if that's what you will be using) if there's a bunch of excess width there.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I am using 15’ radius for my backs and a radius dish to sand the rims. How many inches is it between the waist centerline and upper bout tip? Interesting enough, I was reading your thread last night as I was figuring out what I needed to build a cutaway. Thanks Jay!

For the sides in the photo, the distance between the yellow lines is 3 5/8".

And just to make sure my labeling in the photo isn't ambiguous, "upper bout tip" means the widest point of the upper bout. Just want to avoid any potential confusion on that since "upper bout tip" isn't a standard term.

One other thing. I don't know if it matters in your thinking about the layout, but the bend for the cutaway on the guitar in the first photo starts about 1 1/2" past the widest point of the upper bout. The cutaway on your plan is quite a bit deeper and it looks like the bend for the horn of the cutaway starts pretty much at the widest point of the upper bout.

Here are a couple examples of how some builders adjust the upper face brace to accommodate a deep cutaway in case you might need to do a similar adjustment:

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I kind of designed the horn placement based on photos of Martin guitars today. Based on what I have laid out are there potential pitfalls bending a curvy cutaway versus the image in your first post?

I haven't made a cutaway as curvy as the one in your plan so I can't add anything based on experience. I'm sure someone on the OLF has done cutaways like that and maybe they can comment.

Btw, what wood are you planning to use for the sides?

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Found this photo of the bracing on a Martin with a cutaway.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Heh, I actually own that model. It's an all Ovangkol guitar. Quite enjoyable!


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I see from the image that I should lessen the depth of the cutout, making it a little straighter. As far as woods go, something that is not figured, maybe EIR or Mahogany. Do you have some of your build pictures on the forum?

Patrick

Remember the linings and thickness of the side., a bit of space to fit them. Nearly missed that on my first.
Fortunately I drew out a full sized plan and added the bracing on it, that's when I noticed.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I have bent cutaways with that tight a radius and in my experience- well I’ve cracked some sides. More than one. Definitely use an easy to bend wood like Indian rosewood or cocobolo.

Straight grain EIR or mahogany would be good choices. Walnut is another very nice wood that bends easily.


"o you have some of your build pictures on the forum?"

Here's a build thread from the 2016 Challenge. https://www.luthiersforum.com/forum/viewtopic.php?f=10145&t=47888
Another parlor guitar: https://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=49296&hilit=ziricote
A dreadnought. https://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=51842&hilit=sipo
The guitar I play most. https://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=51883&hilit=ambrosia+maple
A build thread of an octave mandolin. https://www.luthiersforum.com/forum/viewtopic.php?f=10128&t=53129

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

The top in the upper bout is not a big contributor to the sound, but is structurally important. I would certainly not make the cutaway area of the top any thinner. There should be no need to make it thicker, either.

Here is what I came up with for a revision. The horn is forward and the cutaway is not as aggressive. I drew in the upper face brace to see if that would be affected. The top image shows the revised design and the second image I overlaid the first drawing over the revision to better see the changes.

What challenges are there routing the binding/purfling in the cutaway, particularly where the side meets the neck block?

I made a rookie mistake with my first cutaway. It happened at the time of gluing in the neckblock. You need to be careful that the block is at the correct angle relative to the plane of the soundboard. In other words this is the moment, surprisingly early in the build, when you are locking in the neck-to-body angle. It is locked in now because you are gluing sides to two faces of the neck block. In a non-cutaway build you only glue onto one face and make a ring which is still a bit flexible, so the angle of the block is not actually fixed until the back or top get glued on. In your cutaway build the block will be unable to rotate at all once you have made the ring. You can be distracted with gluing and clamping two sides of the block simultaneously and not notice that the block has rotated a bit forward or back. Next thing you know you have locked in the wrong neck angle and it is complicated to fix it later (especially if, like me, you don't notice the mistake until after the box is closed and the bindings are on).

Thanks for the heads up Mark. Would it make sense to glue the bass side rim first to the neck block, then screw the neck block against the mold to insure the block cannot move, something I learned from John Hall, then glue the treble side rim last?

Yep, doing it sequentially like that would be a way to avoid the trap that I fell into.

Maybe I'm not understanding this correctly, or maybe it's down to a difference in methods, but for me the neck-to-body angle isn't determined by the angle of the head block relative to the soundboard. It's determined by what I do to the body end of the neck when fitting it to the body. If the head block ended up angled slightly back or forward after gluing it to the rims, it would have no effect on the neck-to-body angle, at least in my case.

I've glued the bass side to the block first followed by the treble side, and I've glued the treble side to the block first followed by the bass side. Both worked equally well with regard to alignment. I slightly favor gluing the treble side first because I butt the end of the bass side up to the treble side which extends past the end of the head block to cover the end of the bass side.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Yes Jay, it is a difference in method, and it wouldn't happen by the (better) method that you illustrate. You have the block clamped to the table and presumable have squared up the top side (the face for the soundboard), so it should all stay square. I did it in an external mold and evidently did not secure the block in a vertical manner. You are right that the final angle is determined when you mate the heel of the neck to the block. But when my block went in off square I ended up having to do a substantial "reset" of the neck. Maybe nobody else would be stupid enough to make the mistake that I did? Your pic shows Patrick very nicely what he should do. You really don't need to build in a mold - sometimes it gets in the way.

To your point Mark I did have a similar issue happen to me requiring a bit of work getting the fretboard to body angle right with a dovetail. I appreciate everyone’s suggestions and tips as there are other factors to consider building a cutaway style guitar.

Jay makes a really good point about gluing the treble (cutaway) side first and a bit over-length - and then butting the bass side one against it to hide the end.

Here is another point which could qualify as too dumb to mention, but I will put it out there anyway. The first straight bit of the cutaway side (where it is glued to the neck block) does not run absolutely parallel to the mid-line of the guitar. I am talking about that part of the body where the fretboard from 14th to about 16th fret needs to align with the cutaway side. Remember that the fretboard is a bit wider at 16th than at 14th.

Mark you are correct and I took measurements off one of my guitars to account for the fretboard flair towards the sound hole. I did ask a question regarding cutting the binding/purfling channels on the cutaway side that got missed. What do I need to pay attention to when cutting them. I am using a cutter similar to the S-M unit.

No special challenges that I can think of. This is what it ends up looking like.



What is the heel going to look like? Will the treble side of the heel be flush with the end of the cutaway side all the way down to the heel cap, or will the side of the heel taper from the fretboard down to the heel cap?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter