Concerned about attaching 30ft radiuses top with flat upper braces to inside heel block

[Jim Martinez]

I am building my 2nd guitar, a OM model steel string acoustic with 15ft radius back and 30ft radius top. My 1st was a Hauser Classical with a 15ft radius back and a flat top. There lies my concern, I am worried about the geometry of the neck and top coming together so that the fret board sits flat on the top after the rims and heel block get radiused. I am using a mortise and tenon neck with barrel bolts to attach. I radius the X brace but glued the two upper braces above the sound hole FLAT. If I place a straight edge over the top in the area where the fretboard would get attached before gluing it to the radiuses sides and heel block - it is flat. My Question - Since I have radius the sides and heel block already should I leave it that way or should I make the top of the heel block flat before attaching the top Any advise would help, it seems none of the books address this matter other than talking about adjusting the neck angle at the tenon of the neck. Thanks, Jim Martinez

[Todd Stock]

I don't, and no issues. 28' radius on X, tone bars, fingers and bridge plate, then UTB radiused to 60' glued on top on flat surface...perfect extension geometry from Size 5 to 12 fret dreads or grand auds. Absolute worst case for you is having to flatten the top where the extension will and, and that is easily accomplished, either with a jig per kennedy or others (check archives), or just a flat block and some witness lines to check progress.

[Jim Martinez]

It sounds like your saying I should be ok leaving the sides and heel block at the 30ft radius and attaching the top without issues of the fretboard lying flat above the sound hole.

[Barry Daniels]

I don't think that is what Todd is saying. I happen to use the same approach that he does whereby the upper bout (including the head block get flattened a bit, down to a 60' radius. Some folks flatten the upper bout completely flat. Leaving the head block at a 30' radius tends to make the fretboard flare up a bit, which you can deal with by taking some material off the end of the fretboard or by planning some material from the top. So there are various ways to deal with the issue but it does require some method of fitting.

[Jim Martinez]

Thanks I understand now leaving the upper bout area at the 30ft radius would be a problem. I am following the Robert O'Brien DVD on building a steel string guitar. He radius the X brace but the upper two braces are glued flat in the video. But he does not talk about how to deal with the upper bout of the sides and heel block. He shows putting a 30ft radius on the entire sides which confused me. My guess it is impossible to show and explain all of it in a video. But thanks for MIMF, I see there is help out there. I will figure this out, which is the challenge I look forward to. But I"d rather not make expensive fire wood as Robbie would say. I still am very new to building guitars and even thou I have built one classical guitar and am proud of the finial result and sound I have a lot to learn - But look forward to building. I am a 30 year veteren of Miami Dade Fire Rescue Department now retired and have found the love of building guitars as a hobby. Again thanks for the help.

[Todd Stock]

I don't flatten the rim, although my students have tried it and got largely the same result as I do. Upper bout rim stays at 28'...only thing that gets 60' radius is the UTB, but it is glued to top on a flat caul. The objective is a near-flat upper bout under string load...combination of the 60' UTB and whatever small contribution the neck block makes (mine is 1-3/4" x 3") ends up just fine.

[Michael Lazar]

It's hard for me to imagine how one could radius the sides and attach them to a flat upper bout. It would make more sense to me if the sides were made perfectly flat This would require the top to be "forced " a little. Alternately one could make a fully domed top and sides. When I do this, I bring the plane of the neck up about 2 mm and put a shim made from wood matching the neck beneath the fret board. Here is an exaggerated illustration. In reality the shim is much smaller and quite unobtrusive but the result can be quite elegant.

[Barry Daniels]

I couldn't quite see what was going on with the drawing until I clicked on it and saw the closeup.

[Todd Stock]

Crazy...I know...but I never have to resort to shimming or other workarounds to get a perfect extension angle. Unless the shim is intentional, as in a negative angle neck, it always seems to me to be a fix, rather than a feature.