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 Post subject: Top/side geometry issue
PostPosted: Tue Aug 08, 2017 1:12 pm 
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Joined: Mon Jun 12, 2017 3:23 pm
Posts: 3
Location: Scotland
Hi all

I wonder if I could trouble you 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 0.75mm 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.


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PostPosted: Tue Aug 08, 2017 7:53 pm 
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Some people build their guitars with a radius in the upper bout. I use the method used by Martin, which is that nothing is radiused above the soundhole, except for the x-brace, which is what you've done, if I'm reading your question right. Since the transverse brace is dead flat, none of my guitars have a radius on top bout, and neither will yours. As far as I can see, you've made no errors and you're good to go.

The typical Martin style belly bridge is ~ 3/8" tall, or .375". Your calculations look right and I'm sure you're fine to go with the typical 1/4" thick fretboard. Remember that, although you're using 1.3mm tall fretwire, you will have to level the fretwire, then recrown it after you install it. Also, your neck will have an angle such that, when you lay a flatedge on top of the frets, the bottom of the flatedge will just grace the top front edge of the bridge. I can't remember the exact angle, I'm sure others can give you an idea.

I hope this helps you. We love questions, so please don't be shy about asking them.


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PostPosted: Tue Aug 08, 2017 10:28 pm 
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Joined: Sat Apr 10, 2010 10:50 pm
Posts: 2281
Location: Seattle
Instead of a 7 mm fretboard you can slope the fret board a bit. Taking the nut side down 1 mm gives you 1 mm of additional height at the saddle. I think a fret board sloping from 5 mm to 6 mm nut to body would look better than a 7 mm fretboard.

Depending where you are there are other ways to fix this,

I check the angle before gluing on the top by clamping the top on and running a straight edge along the fretboard extension path and check the height at the saddle. I correct the angle on the rins if necessary.

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PostPosted: Wed Aug 09, 2017 6:36 am 
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Joined: Mon Jun 12, 2017 3:23 pm
Posts: 3
Location: Scotland
Thanks for the replies guys. Much appreciated

I don't think I'm miles off Diane but I would have liked a gap of 2+mm at the saddle with a straightedge sitting on the soundboard of the completed box.

That's something I didn't think about doing John. I might make a fingerboard with a taper as you suggest and one of 7mm and see which I prefer. The guitar's just for me and the first acoustic I've built for ages so it's a bit of a practice run. I'll be checking the angle on the sides/top before closing the box and adjusting as necessary on the next two guitars I'm about to start.

On the 28' radius dish question I see you keep your upper bout flat Diane. John do you do likewise or leave the radius in the sides and then flatten the soundboard for the fingerboard to sit on? I've seen a sanding paddle anchored at the bridge on Kathy Matsushita's site which looks like it might be a good idea.

Ps - I'm in Scotland. I'll need to update my profile with a location asap


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PostPosted: Wed Aug 09, 2017 3:43 pm 
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Joined: Sat Apr 10, 2010 10:50 pm
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Location: Seattle
drumgerry wrote:
...
John do you do likewise or leave the radius in the sides and then flatten the soundboard for the fingerboard to sit on? I've seen a sanding paddle anchored at the bridge on Kathy Matsushita's site which looks like it might be a good idea.

Ps - I'm in Scotland. I'll need to update my profile with a location asap


I do it a little different than John and Diane. Although I am going to try their way next build. I radius the rims with a 32' radius but then flatten the upper bout a bit taking mostly off of the waist to reduce the angle to where I want it. My transverse brace is flat in the center with a bit of radius on the ends. I have no magic formula so my method gets close and I test and adjust before gluing on the top.

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PostPosted: Thu Aug 10, 2017 6:57 pm 
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Joined: Thu May 24, 2012 8:03 am
Posts: 637
Location: Chestertown Maryland
Tell me if it is wrong to cite another forum, please. Here is a method I use and it seems to work regardless of the size of the guitar body. It is from Hesh Breakstone, a well know builder and repair guy:

http://www.luthiersforum.com/forum/view ... 17&t=25931

Ed


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PostPosted: Fri Aug 11, 2017 8:25 am 
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Joined: Mon Jun 12, 2017 3:23 pm
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Location: Scotland
That looks a decent method to try Ed. Thanks for the link.


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