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Top/side geometry issue

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Top/side geometry issue

Postby Gerry Thompson » Tue Aug 08, 2017 8:49 am

Hi all

I wonder if I could trouble you all for some advice.

I have built an acoustic guitar body using a 28' radius dish for the top and a 20' dish for the back. I sanded the side set using the "driving the bus" method. I arched the X brace and the tone bars using the 28' dish and left the other braces flat. Once I had sanded the 28' radius into the side set I then switched to a flat board and sanded the area around the upper braces and neck block flat but attempted to maintain the angle established by the 28' dish.

On gluing the top on I seem to have lost a fair bit of the angle in the upper bout such that a straightedge laid along the centreline gives a gap of about 3/4mm or 1/32". A fair bit lower than I expected/need.

So...by my calculation I can save the situation by using a 7mm thick fretboard and a 9mm thick bridge. My fret crown height is 1.3mm. So....7mm plus 1.3mm plus 0.75mm is roughly 9mm.

Are my calculations ok do you think and the save plan itself? Also can anyone give me tips for getting the proper side to top angle using a 28' dish for future builds?

Many thanks.
Gerry Thompson
 
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Joined: Thu Mar 09, 2017 7:20 pm
Location: Speyside, Scotland

Re: Top/side geometry issue

Postby Randy Roberts » Tue Aug 08, 2017 8:47 pm

Any chance of a picture?
Randy Roberts
 
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Re: Top/side geometry issue

Postby Gerry Thompson » Wed Aug 09, 2017 6:29 am

Randy there's not really that much to see so I'm not sure a picture would be that helpful. It's just a straightedge on the top of the soundboard (on a closed box) at the centreline and a gap of about 1/32" above where the saddle will be.
Gerry Thompson
 
Posts: 4
Joined: Thu Mar 09, 2017 7:20 pm
Location: Speyside, Scotland

Re: Top/side geometry issue

Postby Randy Roberts » Wed Aug 09, 2017 9:54 pm

Gerry,
I may be totally misunderstanding the problem but…
Is the neck already on this thing?
If not, I don’t know that you have a problem to solve . The angle that the neck joins the body will take care that. As long as you decide the height you want the strings over the soundboard at the saddle, and the thickness of the fretboard, frets, and nominal action, this will tell you the angle you need the neck to join the body . Draw it all out full scale in a side view and I think it will make sense.

If the neck is already on and not easily (bolt on) removed, then again it will be worth drawing out what you currently have side view full scale. That will let you see where your solutions might come from.

I’m not sure what you mean with the last question “getting the proper side to top angle using a 28' dish for future builds?” unless you are talking about the angle of the top to the sides at the neck block. Generally the neck joins the body at about a 1 ½ degree angle, but that can vary with the architecture of everything else and is usually most easily dealt with with the neck's heel rather than the body. Generally the side's plane is 90 degrees to the top, ignoring whatever doming is or isn't going on .

Again I may be totally misunderstanding the question. But one thing I can guarantee you is that it almost always takes less time to tear up a sheet of paper than to redo a body or neck. So if in doubt, draw it out.
Randy Roberts
 
Posts: 379
Joined: Sat Jan 07, 2012 9:11 pm
Location: Omaha, NE (a suburb of Iowa)

Re: Top/side geometry issue

Postby Gerry Thompson » Thu Aug 10, 2017 12:38 am

Thanks for that Randy. Nope not built the neck yet so it's purely a question about the geometry of the box. Specifically that created by the side to top at the neck block area. In order for the fingerboard extension to lie flat on the soundboard and for good string action the angle of side to top in that area has to be greater than 90 degrees surely - the top getting a slight downwards slope to the side from the radius dish. I have an angle of about 90.5 degrees whereas 91 or 91.5 might have been optimal. Like you say and I suggested I can vary other parameters to sort this out but I think you're absolutely right that drawing it out to scale would be a good move.

Thanks again much appreciated
Gerry Thompson
 
Posts: 4
Joined: Thu Mar 09, 2017 7:20 pm
Location: Speyside, Scotland

Re: Top/side geometry issue

Postby Stephen Neal Saqui » Thu Aug 10, 2017 10:09 am

Keep the dovetail jigs simple to use so that you can adapt to any angle that you have. It's good to have your dimensions set, but it's better that you have flexibility in the jigs. You may want to do experiments in future guitars regarding the top arching.

Making the neck should solve your understanding now.
Stephen Neal Saqui
 
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Location: Challis, Idaho

Re: Top/side geometry issue

Postby Randy Roberts » Fri Aug 11, 2017 7:22 pm

Gerry,
It might be more useful to you, in visualizing what's going on, to picture that the dome of the top is gradually dropping away from the plane of the strings.

Again, what's needed to make it all fall together right usually all takes place at the heel of the neck.
Randy Roberts
 
Posts: 379
Joined: Sat Jan 07, 2012 9:11 pm
Location: Omaha, NE (a suburb of Iowa)


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