Body neck geometry

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Dave M
Blackwood
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Joined: Tue Jul 15, 2014 6:44 am
Location: Somerset UK

Body neck geometry

Post by Dave M » Thu Jun 30, 2016 7:43 am

I have struggled with earlier builds to get the right angle between the neck and the upper bout right, leading to the correct height at the saddle.

I am now working on a cutaway 12 fret join and am trying to get it right this time. I have too great an angle at the moment with the position at the saddle too high - about 4 mm. This is a simple bolt on neck.

So am I aiming to just flatten the head block, allowing the flexibility of the top to follow the existing contours of the sides, given that the head block is currently higher than the sides? And what shape am I aiming for on the top of the head block? Do I still need to maintain some of the curve or is flat good?

Puzzled in Somerset.
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Dave

johnparchem
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Re: Body neck geometry

Post by johnparchem » Thu Jun 30, 2016 8:06 am

Flat is good as the the fret board is and wants to remain flat.

Dave M
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Joined: Tue Jul 15, 2014 6:44 am
Location: Somerset UK

Re: Body neck geometry

Post by Dave M » Fri Jul 01, 2016 6:15 am

John thanks, as soon as you say it it is obvious that's the way to go.

Do you in your falcate builds (and I often seem to be following along behind you on these) do much flattening of the top of the sides as well?
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Dave

johnparchem
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Re: Body neck geometry

Post by johnparchem » Fri Jul 01, 2016 6:39 am

I have not found any magic yet to get the rims with the proper angle without a bit of trial and error.

I radius the transfer brace but I flatten the center of the brace to accommodate the fret board and a bit.

I do radius the entire top with a 10 M radius board and plane the heel block to remove the radius, still I find when I am done I have a bit too much of an angle. I am not sure if that is my sanding technique with the radius dish. I am horrible sanding and always seem to add a radius even when try to sand flat.

I have found that building the top in the 10 m radius dish and the above described transverse brace that a straight edge on the unclamped top does give me a good angle. When I fit the top on the rims I usually find that the rims have a bit more angle\radius than the top. Before gluing on the top I clamp my top on the rims and check the angle. When I find that I have a bit too much angle I protect the heel block and sand the upper bout a bit (mostly toward the waist) with a flat dish to reduce the angle.

Dave M
Blackwood
Posts: 268
Joined: Tue Jul 15, 2014 6:44 am
Location: Somerset UK

Re: Body neck geometry

Post by Dave M » Fri Jul 01, 2016 6:57 am

John that is really useful. We obviously go through similar issues. In the last (SS) build I ended up having to taper the fretboard like a classical having had to sort the angle out with the tenon.

I have been reluctant to do too much to the top of sides/linings after having sanded the body in the radiused workboard as you describe.

Well back to it tomorrow. I will start with the head block then check how the top is sitting on the sides. I have to say the feeler gauges have never seen so much use!
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Dave

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Trevor Gore
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Re: Body neck geometry

Post by Trevor Gore » Fri Jul 01, 2016 9:06 am

Dave, Check out page 4-93 et seq of the Design volume, which goes into the detail of how it is all supposed to work. Basically, radius the top rims with a 10m dish and use a flattened upper transverse brace (and head block if using the BOBO joint), as it is the projection of a tangent to the upper bout over the saddle position that sets the neck angle. Using the book techniques you can check before, during and after the top is being put on that the neck angle is correct. The things that send it wrong (and there are many possibilities) include not setting up to the reference lines, winding up the stretchers too tight and tilting the head block (and distorting the rest of the geometry) and not checking that all is right just before you glue the top on. After gluing the back on, then removing the shell from the mould to trim the overhang, be sure you have all the geometry right before you glue the top down. If you do all the check steps you should never have more than ~0.25mm (if any) sanding to do on the upper bout to get the angle spot on.

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