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Building In Dishes - Some Questions


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I'm just on my way through my first guitar built in radiused dishes, and there's a couple of questions I've got for guys who've done it before.

Prior to using dishes I used to add arching to the top by measuring the curvature on braces. My previous guitars have got 3mm curvatures (the mid point of the brace is 3mm higher than the ends) meaning the tops are pretty flat. The top I've just braced and started to voice was done in a 25ft radius dish - and I'm finding it pretty stiff (I expect due to the extra arching). For you people who've experimented with top arching - is this normal?

I'm also having the normal fretboard extension queries. Prior to dishes my tops would go on after planing the neck block to an angle where, if a straight edge was put on it along the centre line of the guitar there would be about a 1.5mm gap between the straight edge and the top at the point where the bridge lies. The increased arch on this instrument means the gap is nearer 1cm, not a problem - as I've decided to do an archtop style fretboard extension with a wedge. But for future instruments I'd like to know what to look out for and a way to stop this happening.

Sorry if that's not worded well - it was a late night last night, and it's not the easiest of stuff to explain! :D

Cheers for any help

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Arch adds stiffness, and raises the reasonant peak somewhat, yes. If you prefer it looser/flatter, you could always build with a 30' arch or so (25' is, what, 5mm deflection over a 50cm span or so?). I found that on my first (25' dish), the fingerboard sat almost flat with the straighedge on the fingerboard, hitting the top of the bridge, give or take. You can either leave the upper transverse brace flat (some do), sand the rim flat after dishing past the soundhole so it angles down a little, and either brace the upper bout flat or leave it arched, or 'simply' flatten the area under the fingerboard by hand - there's a little jig described in the tips and tricks section at luthiersforum.com. Takes off maybe half a mm on the upper bout area, which I left/leave a little thicker on purpose, also because I don't tune there anyway.

Alternately, flatter dish :D

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So, -Im doing carpathian at 1.10, should I thin that a little more to compensate for the extra stiffness??, (I also have a 25' dish)

:D

You could do, but you can always adjust it after you brace by fiddling with either brace carving or by sanding/planing the top then.

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So, -Im doing carpathian at 1.10, should I thin that a little more to compensate for the extra stiffness??, (I also have a 25' dish)

:D

GW- That depends on the Spruce. I am used to 25' and 30' radius, and just adjust the stiffness/thickness of the soundboard itself based on what I am used to. Every piece of wood is a little different. This is just something you get a feel for as you go along building. Probably a safer method if you are not sure(and this is what I did-**and am still doing as we will always be learning how to make these adjustments) is to lean a little thicker on your first. Then study your results and adjust(conservatively) on your next, and just keep on refining. My method generally starts by thicknessing to a given number as you have done, and then I send it through the thickness sander taking very small amounts until it feels right(I forget calipers during the adjusting, and then for my notes check it when it feels like what I wants). I have started with Sitka @.110" and after I felt it was right I was @.092". I have done Redwood starting @.135" and finished @.120". I could go on, but like I said it depends a lot on each piece of wood. It sounds like you have a good starting number(not likely to fail because you are too thin), and you could fine tune the edges of the lower bout when the box is assembled(that is a nice way to fine tune, and can also clue you in as to how much thinner you may be able to go based on your notes).

Honestly I have always adjusted for arching with bracing adjustments. I would not really adjust my soundboard thickness itself much because of the radius I choose.(FWIW)

Peace,Rich

P.S. If you want to speed up the learning curve on these things. You could consider taking a class with a master builder. They are spendy, but you will be recieving the insights from years worth of their study and notes :D .

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