cutaway at neck body join

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Mark Parker
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cutaway at neck body join

Post by Mark Parker »

This may have been discussed, but I couldn't figure out a way to search for it successfully. If previous discussions exist, please accept my apologies and point me to them.
I am making my first archtop and following Benedetto's book. He says the neck block (and therefor the cutaway) should be a perfect 90*. BUT the neck/fretboard has a taper (3/8" increase from nut to 12th). If the edge of the heel exactly matches (covers) the edge of the cutaway, won't the fretboard overhang the cutaway for an inch or two before it starts to curve? I would think the angle of the cutaway side of the neck block should equal the angle of the fretboard taper. I realize we are talking a very tiny variation from 90, but it seems like it matters, no?
Thanks for your input. This is a great forum that I am just beginning to explore.
Fair Winds and Fair Tunes,
Mark (a veteran sailor and a VERY newbie luthier)

Brian Evans
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Re: cutaway at neck body join

Post by Brian Evans »

Making the neck block and cutaway side match the exact width of the neck AND it's taper is part of making a great guitar, in my opinion. Neck width at the 14th fret body joint is just about the first thing you need to establish as you build your body mold. I also establish the nut width and string spacing at the bridge at that point - to establish the neck taper, and I do build that into the mold as well. Others may not, I never thought to ask the question to be honest. If you do make it 90 degrees, which does seem simpler, simply tapering the fretboard extension down from the fretboard to the 1 1/2" of so of side where the two touch in the cutaway section will mask it completely. Regardless, you need to decide the neck width at the body joint any time you have a cutaway to allow you to fully design that part of the body, no matter how you do it.

Mark Parker
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Re: cutaway at neck body join

Post by Mark Parker »

Brian Evans wrote: simply tapering the fretboard extension down from the fretboard to the 1 1/2" of so of side where the two touch in the cutaway section will mask it completely.
Thanks for a) validating my concern and b) the simple solution
Fair Winds and Fair Tunes,
Mark (a veteran sailor and a VERY newbie luthier)

John Clifford
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Re: cutaway at neck body join

Post by John Clifford »

Yeah, my neck blocks have a 90 degree angle, and I resolve it the same way, by slightly tapering the fingerboard extension to where it meets the top. A tiny bit of extra room for your hand in the cutaway area is not, in my opinion, a bad thing.

John Clifford
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Re: cutaway at neck body join

Post by John Clifford »

Oops, I meant the neck extension, not the fretboard or fingerboard or whatever that thing is called. I haven’t had my coffee yet this morning!

Brian Evans
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Re: cutaway at neck body join

Post by Brian Evans »

When I made this guitar, I blithely set up my sides in a mold that I'd had made for a guitar with a classical width neck. I was all glued up and almost ready to close the box when I realized I had the cutaway close to a quarter inch in the wrong place. Because this design is weird, I was able to simply trim the neck block and re-veneer with birdseye maple from the same side piece. Taught me in no uncertain terms that neck width is very important in getting the end result to come out right.
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Mark Parker
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Re: cutaway at neck body join

Post by Mark Parker »

Off topic a bit, but is that a bolt-on neck? I would be interested in details - either here, new post, or PM whichever is more appropriate.
Fair Winds and Fair Tunes,
Mark (a veteran sailor and a VERY newbie luthier)

Brian Evans
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Re: cutaway at neck body join

Post by Brian Evans »

It's a simple bolt on neck. The esthetic of that guitar is modern, I have a few bits of exposed bolts and joinery, so the neck joint is a plain 1" wide 1/2" deep tenon to locate the neck, and a single 1/4" furniture bolt from the outside heel, with a threaded insert in the neck block. The tenon is fitted quite snug in the mortise so that the single bolt simply reacts against the 7 or 8 lbs of "lift" at the heel from string tension. After a year it's completely stable.

Slightly different view. The neck is birdseye, same board as the back and sides, three piece. It's interesting to cross over engineering work that I did when racing cars with lutherie. The clamping force of that 1/4" bolt done up to 20 lb-inch of torque is close to 500 lbs. 20 lb-inch is just finger tight. The ultimate tensile strength of that 1/4" bolt is probably around 3,000 lbs.
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Last edited by Brian Evans on Sun Aug 04, 2019 7:22 am, edited 1 time in total.

Mark Parker
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Re: cutaway at neck body join

Post by Mark Parker »

Nice!
Fair Winds and Fair Tunes,
Mark (a veteran sailor and a VERY newbie luthier)

John Clifford
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Re: cutaway at neck body join

Post by John Clifford »

Brian, you're making me wonder about the wisdom of struggling with dovetail joints. If I ever make a perfect one, then maybe I can abandon the practice.

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Randolph Rhett
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Re: cutaway at neck body join

Post by Randolph Rhett »

I do actually contour my neck block to match the taper and even swoop a little into the cutaway (by sanding it a little against a form). But it never seems to line up perfectly anyway :( so I live with a little mismatch and finesse the fingerboard and extension a little. The results are usually fine and cause me less grief than other spots in the build. I find that the final alignment of the parts on an arch top is more challenging than on a flat top, so in some ways this is a low priority concern for me. Getting that dovetail right, the neck alignment right, the cheeks sitting flush on the sides, the extension mated to the top over a few inches, the neck angle correct, the heel still square, the... aaaahhhh! Why do I build these infernal things?!!! ;)

Brian Evans
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Re: cutaway at neck body join

Post by Brian Evans »

I did a dovetail neck once, so that I could learn how. Given the issues and requirements pointed out by Randolph, it wasn't any harder or easier than doing my style of bolt-on after I got the router jig fine-tuned with test pieces. I do a different version where the head of the bolt is inside and I use a long tee handle hex head driver. In both cases the wood is the part that will fail first, the forces are steady but not really all that great. I did one with two bolts once (since converted to one bolt) and tightened the bolts too tight. Over a few months I literally crushed the sides of the guitar under the cheeks of the neck, and needed to do a reset, build the sides backup to flush, re-do how the cheeks fit on the sides, re-establish neck angle. That's when I refreshed my memory about bolt clamping forces.

Christ Kacoyannakis
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Re: cutaway at neck body join

Post by Christ Kacoyannakis »

The comment about the 90 degree angle didn't really sound like something Bob Benedetto would make, because he is very sensitive to what players feel, and how the player interacts with the instrument, and I think that change in angle (although blended with the neck extension) would be noticeable. However, I did check Bob's book, and he does say that. I know I have his guitar plans and his video. Maybe he makes a comment about it in the accompanying video, but when I designed my guitar (I used Bob's book as a guide, but I designed my own body shape) I did make the cutaway angle match the taper of the neck. It is a very smooth and even cutaway, and no blending of the neck extension was involved. I guess you could do it either way. I know I didn't figure that out on my own, so I must have heard about that detail from somewhere.

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