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Hi guys,

Noob question alert;

The Cumpiano book suggests that you angle the neck such that a straight edge sits something like 1/8-1/16” (from memory) above where the top of the bridge at the suggested thickness will be.

In doing so the neck will be slightly angled back which means that the fretboard past the 14th fret hovers slightly above the soundboard, unsupported.

On build #2 I glued in a wedge of hardwood to close the gap and support the extension, and on #3 I had the neck parallel to the fretboard but this meant the bridge had to be quite thin - basically the thickness of the fretboard plus frets.

What is the usual practice here?

Thanks in advance.

Tom


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I don’t think there really is a “usual” practice. There are lots of different ways of dealing with the issue.

One way is to just glue the extension down and live with fall away over the body. Many acoustic players don’t use those frets that much, anyway.

Another way is to install a wedge.

Another way is to use a spherically domed top of a radius that makes it all come out right, with very minor flattening under the extension.

Another way is to spherically dome the top, and spherically dome the top-facing edge of the sides, but sand an angled flat spot into the part of the edge that is north of the upper face brace, and use a flat upper face brace. I’m liking this approach at the moment.

I’m sure there are others I have not mentioned. The point is that there is not a single standard way.

As Don says, modify the upper bout geometry in whatever way you prefer. Another option is to increase the arch height of the upper transverse brace according to the size of gap you're getting right now. I also flatten the middle part of it that goes under the fingerboard extension, so the arching is not spherical anyway. Draw a straight line on the brace to use as a visual reference while you plane/sand the arch.

The radius in the upper transverse brace (I use approximately 50') should get you pretty close without the need for a shim. A little falloff of the fretboard over the body is acceptable.

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Good judgement comes from experience. Experience comes from poor judgement.

lately i've been building true flat tops and i'll mill relief angle into the neck block, UTG, and rim area and glue the top and the fb extension onto that. for my 24" scale instruments this basically amounts to a 1/2 degree of taper which the top easily conforms to.

radiusing the top can get you close as well and all that you need to do is provide a small flat area for the extension to glue down to.

i'm not a fan of just forcing the extension down and going for it. (imo) this induces some stress that I don't care for and has the potential for loosening the fret at the body joint.

imo the neck angle should be designed in.

Thanks everyone, that’s really helpful. It pretty much confirmed what I suspected but I just wanted to be sure I wasn’t being dumb or missing anything obvious!

The Cumpiano book is great save for a few glaring omissions like this


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Cumpiano has lots of company when it comes to avoiding the topic of establishing neck angle.

Ken Cierpolowski covered this topic extensively in his builder's manual. Short version, my paraphrasing from memory, to copy what Martin did at the factory, the whole top of the body was sanded level with a great big disk sander, the top being square to the sides, then the body was elevated at the front by 1.3 degrees, and the sander brought back to sand the region north of the soundhole flat, leaving the top sloped down at the front, north of the soundhole and the neck was trimmed at a complementary angle so that a straight line from one end of the fretboard to the other was established.

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Peter Havriluk

As noted, the challenge is to get the correct clearance of the string plane (or surrogate) over the bridge location, while also getting the fretboard extension to lay flat on the upper bout without excessive distortion. Martin seems to do a fairly poor job of this, given they have to resort to several different bridge heights (with more than 0.100" of slop) to address issues in their production methodology, so I am not certain that their model is one to follow for a small shop builder. As mentioned, wedged upper bouts get things closer, but given the issues the Martin factory has, it seems the wedge angle needs to be fine-tuned for actual geometry versus rote application of a set angle (if Martin's 1/10" or more of slop is to be reduced to the 0.020" variance that can be easily accommodated with minor saddle height changes).

As we don't use the wedge method in this shop for new guitars, I'd be interested in any keys to consistency by those using upper bout wedges - how do you correct your setting angle to dial in the desired 0.350" bridge thickness, 0.150" saddle height, and 1/2" string height at saddle on a consistent basis?

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A constellation only takes shape when one maps the whole.
- Beth Brower

Woodie, we're finding me drifting into an area where I'm really ignorant. Ken's discussion of what Martin did was offered as his take on injecting some precision into the process, I think. But Ken's choosing an angle which he claimed was Martin's production standard works for the home builder only if his soundboard has the same dome as Martin's. Wonder how that's done. But ya gotta start somewhere, and that looks like a workable start.

Should be interesting to see how somebody could apply Ken's process to a 12-fret neck married to a 19-fret fingerboard, created so that the soundhole turns up about where it would for a 14-fret neck on that same body, but the bridge is farther south on the 12-fretter.

The home builder also has an opportunity (or a need) to tune his bridge height to complement the neck position that the body preparation gives him, within reason. Beats incantations and incense.

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Peter Havriluk

From what I've seen, Martin can throw some fat, ugly ebony bridges on their guitars to compensate for lack of neck angle accuracy in their construction. Just saw a 000 today where I'd hazard the bridge wings were maybe 3/16" thick at the ends and who knows how fat the center of the bridge was - looked like a woodshop worker's first-time attempt at a scratch build bridge. By the way, this was no cheap Martin ... an Eric Clapton model. I'm sure '39 Martin from which this guitar was patterned had the same height bridge ..............

Andy

From what I have seen here, body geometry is used to avoid having to adjust bridge thickness, assuring that eventual string height at comfortable action is within a 1/64" or so of desired (we usually see 0.520"-0.550" at initial string-up, and a fairly rapid progression to about 0.490"-0.510" after a few weeks. While I have a good understanding of the method in use here in this shop, I'd really like to know if fine tuning of upper bout wedge angle can be optimized to the same degree that other methods are, or must we settle for what seems like a wholly unacceptable degree of 'kludgeiness' in final assembly. What I am really after is an expert's methods to address what appear to be weaknesses in this particular method for controlling neck geometry.

More methods = more tools to address unique problems down the road.

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A constellation only takes shape when one maps the whole.
- Beth Brower

You did the right thing by gluing in a hardwood wedge. A better method would be as others have explainded, to work out the geometry in the upper bout. Since you build on an open workboard as I do you may benefit from the picture below. I sat down one day and really worked this out. I use balsa wood in various thicknesses and an arched sanding template and from a 2x4 and dish out the balsa shims such that the UTB is arched and also there is a ramp created from about the top of the soundhole to the head block that makes the angle perfect. BTW I think in C&N's book the height of the ruler over the bridge should be only about 1/32nd inch. After I get the dish just right I then glaze the balsa with epoxy to harden it up nice.

This is the workboard with dished out balsa shims.



This is the angle that it creates on the top of the guitar:

I like my fat Martin bridges like I like my ...

Andy