Can someone please explain neck-body alignment to a newby?

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Richardl
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Can someone please explain neck-body alignment to a newby?

Post by Richardl » Sun Sep 10, 2017 1:19 pm

Hi Folks...sorry this might be a bit lengthy...

I haven’t posted anything here for a while! Some years ago I made a guitar using the Kinkead book and got lots of helpful advice on this forum and from local luthiers viewtopic.php?t=5517.

The guitar I made had a flat top to keep it simple. It turned out, hmmm, ok I suppose, sounds OK, but I can see plenty of room for improvement - one aspect being that the string height is a bit much at the 12th fret (and the whole thing is slightly wonky), so I thought I'd try another, with a domed soundboard.

The one thing that stumped me last time and still stumps me is the soundboard curvature and the way the fret board aligns. Some folk wrote comments on why the Kinkead (and Cumpiano) approach was not correct but although several explanations were made, I'm not sure I entirely followed them.

One of the more detailed replies was this one..."Whilst many people seem to have made respectable guitars following either the Cumpiano or the Kinkead method, many haven't and there's reasons for that which you can read about here and here, which are just a couple of references for these issues that continue to arise. Basically, if you follow the instructions closely as written in those books, the neck angle will likely come out wrong.

To get it right, without having to wedge the fretboard extension, the top of the neck blank has to be coplanar with the upper bout AND the extension of this plane over the saddle position should clear the top of the guitar by 2 to 2.5 mm. This means that the neck angle is essentially set by the longitudinal curvature of the upper bout and that the angle that the heel makes with the top of the guitar is largely irrelevant, provided the gap between the heel and the sides is closed. If you use a radius dish for the top in the range 25' to 33' (8 to 10m) you can get the right geometry by flattening the UTB slightly from the initial dish radius (varies by dish radius, scale length and frets clear of the body, so draw it out), but ~1mm flattening is in the ball park, then glue the UTB in separately (i.e. not in the dish). You then need to maintain this geometry as you close the box. Any minor residual error that you have once the box is closed you can take out by skimming the top wood in the upper bout until the projection over the saddle position is correct and the neck shaft is coplanar with the upper bout and the heel is fitted to the sides."


So...from this I gather that the braces are curved to accommodate the radius of the soundboard except the main brace across the upper bout which is curved less (and therefore is glued in separately). This gives the soundboard, I think, quite a complicated geometry where it meets the sides but only by a smidgen.

Finally, my question, if most folk curve the soundboard to a 25 -33 ' radius (and 'drive the bus' with the radius dish), there must be a straightforward way of achieving a good string height along the fretboard and at the bridge. Is the above method the one that is typically used (flattening the upper bout slightly) or is this a remedy to the apparent deficiency of the Kinkead approach? I understand that the Kinkead approach results in a slightly bent fretboard at the upper bout which is the main problem) so how does his general approach differ from the correct way...or indeed, what is the correct way?

Blimey, sorry that was an essay!

I'd appreciate any advice or am I roughly on the right track?

Thanks
Richard

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Trevor Gore
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Re: Can someone please explain neck-body alignment to a newby?

Post by Trevor Gore » Sun Sep 10, 2017 11:03 pm

Richardl wrote:
Sun Sep 10, 2017 1:19 pm
One of the more detailed replies was this one..."Whilst many people seem to have made respectable guitars...
Sounds remarkably like something I wrote a few years ago!

There's more stuff here.

There are numerous ways of getting this right, and many more ways of getting it wrong. Unfortunately, most of the well known texts (with one notable exception :wink: ) get it wrong, as you have found.

One way of getting it right, for any build method, is to fully understand the geometry, draw it out full size, and measure off all the offsets you need. However, if you're not confident with the geometry, follow the methodology outlined in your quote, which is also fully detailed in the book for the dishes/gobars/outside mould/bolt on neck build method. If you apply that technique to a different build method, unless you really know what you're doing and can make the appropriate compensations, you will have problems.

To your question:
Richardl wrote:
Sun Sep 10, 2017 1:19 pm
Is the above method the one that is typically used (flattening the upper bout slightly) or is this a remedy to the apparent deficiency of the Kinkead approach?
If you're building with radius dishes, it is a standard approach. There are others, but if you are also using a bolt on neck and don't want to wedge the fretboard (up or down) it is arguably the most straight forward. There are a couple of guys whose surnames begin with G who have tried a lot of different methods but this method is the one they use.

Richardl
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Re: Can someone please explain neck-body alignment to a newby?

Post by Richardl » Mon Sep 11, 2017 3:50 pm

Thanks Trevor; yes your post from 3 years ago :) . I read it a few times before it really started to sink in so I'm glad I have more-or-less understood. The Kinkead soundboard curvature is less than seems to be typical (25'). Not sure if that has further disadvantages in strength and tone but wouldn't it lessen the complex geometry at the wall in the upper bout? What is the advantage of the more domed soundboard? Yes, maybe I should buy the book :wink: . Being a beginner guitar player as well as builder, I guess the subtleties are a bit lost on me so I hadn't realised that the slightly high string position might be a consequence of the design, rather than my build or the trussrod setup (which I havent touched). Anyway, I have yet to make a start so thought I'd just confirm a few things, if possible, to begin with. So thanks again for your help.

Cheers
Richard

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Re: Can someone please explain neck-body alignment to a newby?

Post by jeffhigh » Tue Sep 12, 2017 11:45 am

Hi Richard,
I would encourage you to develop your understanding of the geometry involved.
Firstly note that a 25' radius top will have greater curvature than a 33' not less

It is worth looking beyond the rules of thumb and developing an understanding of the effects of neck angle. fretboard thickness and fret size, soundboard curvature, upper bout slope and desired action and bridge height etc.
Often there is only an emphasis on matching the neck angle and the upper bout slope.
Your aim should also be to get the desired bridge saddle height off the soundboard at the required string action.

Drawing up crossections and rotating around the neck body joint may help you with this.

Richardl
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Re: Can someone please explain neck-body alignment to a newby?

Post by Richardl » Tue Sep 12, 2017 8:08 pm

jeffhigh wrote:
Tue Sep 12, 2017 11:45 am
Firstly note that a 25' radius top will have greater curvature than a 33' not less
Hi Jeff, thanks for your comments, yes I think I had understood that far. What I meant to say (I see it's a bit ambiguous) was the Kinkead method, I gather, used a flatter top, (I haven’t calculated it but is it a 50 ' curvature or something like that?), however, the more domed ~25' radius seems to be a more standard curvature used by many (I stand to be corrected on that). I was just interested in the disadvantages of Kinkead's lower curvature. Anyway, I get the drift that making a decent drawing might help me with planning. I suppose I had thought that drawings might be done for the beginner in a book (a paint by numbers approach of a standard design) with that all accounted for and demystified. Understanding would come with following a precise method. Maybe there is a book - or two with the method I should follow :wink:

Cheers
Richard

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kiwigeo
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Re: Can someone please explain neck-body alignment to a newby?

Post by kiwigeo » Tue Sep 12, 2017 9:29 pm

Richardl wrote:
Tue Sep 12, 2017 8:08 pm
Maybe there is a book - or two with the method I should follow :wink:

Cheers
Richard
Two books..one black one white :mrgreen:
Martin

Richardl
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Re: Can someone please explain neck-body alignment to a newby?

Post by Richardl » Wed Sep 13, 2017 4:58 am

kiwigeo wrote:
Tue Sep 12, 2017 9:29 pm
Richardl wrote:
Tue Sep 12, 2017 8:08 pm
Maybe there is a book - or two with the method I should follow :wink:

Cheers
Richard
Two books..one black one white :mrgreen:
Yes, Ive contacted the bank to see if I can raise the mortgage :wink:

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