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 Post subject: Radius on top ?
PostPosted: Thu Oct 23, 2014 8:33 am 
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Joined: Sat May 26, 2012 12:35 pm
Posts: 301
Location: Ireland
Hi, I'm building a stewmac dreadnought kit and I'm about to glue on the top braces. I used a 15 ft radius dish for the back braces but thats the only dish I own and I have built a go bar deck to use with that so it worked out really well.
My question is --would it be ok to use that same dish--15ft--for the top as well ? I am assuming that the top braces that came from stewmac are pre radiused but I'm not sure.
Its just that the go bar deck is so handy for gluing and would save all that clamping. Its probably not a good idea to use the same radius back and front but just thought I'd ask anyway to see if anyone has ever tried it here?
many thanks
Rusty

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 Post subject: Re: Radius on top ?
PostPosted: Fri Oct 24, 2014 8:35 pm 
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Joined: Wed Nov 02, 2011 9:13 pm
Posts: 162
Zen wrote:
My question is --would it be ok to use that same dish--15ft--for the top as well ? I am assuming that the top braces that came from stewmac are pre radiused but I'm not sure.



It's easy to figure out if they are. Just hold a straight edge along them.

I think that 15-foot radius for the top is overdoing it by quite a bit, but I'll let the more experienced luthiers on the forum weigh in on this.

Quote:
Its just that the go bar deck is so handy for gluing and would save all that clamping.


There's no reason you can't use the go-bar deck for gluing the top braces. I do it all the time. What I do is cut a caul (basically a piece of pine 2" x 2" and long enough to match your longest brace) with one side band-sawed to the radius I want. Then I glue some cork (available in sheets from your friendly office supply store) onto the arced side, where it contacts the actual guitar plate. I put the caul on the floor of the go-bar deck and then put the guitar plate in it. When I install the brace, the go-bar struts push the top into the desired radius. (I actually have two of these cauls for the guitar top, one cut in half in the middle, so that I can glue X-braces that way in one go.) It's true that you can't install all the braces at once, but I'm in no hurry.

I've made a bunch of these cauls with different radius arcs for use with guitar tops and backs, mandolin tops and backs, and whatever else I want to make. It's a lot cheaper than buying a dish for each contour you wish to use, as long as you're not in production.


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 Post subject: Re: Radius on top ?
PostPosted: Sat Oct 25, 2014 6:07 am 
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Joined: Wed Aug 13, 2014 12:14 pm
Posts: 940
What does it mean:

If you have a one foot span of top (i.e. a chord one foot long on the 15' radius circle), the center of that span will be elevated 0.1" compared to the ends.

As the span increases beyond a foot though, it starts to get more significant. With a two foot span (i.e. a chord 2' long), the center rise is 0.4".

That's considerably less than an archtop guitar, but whether a "flattop" guitar can work with a 15' radius or not....I dunno. I suspect it's more than you'd really want. It would be enough, probably, to seriously affect the relationship of the fingerboard with the body. You would definitely need a wedge between the underside of the fingerboard and the top. I suppose you need a wedge even with a more typical larger top radius, but this would quite significant.

But that is all inexperienced speculation. I hope one or more of the experienced builders will reply.

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 Post subject: Re: Radius on top ?
PostPosted: Sat Oct 25, 2014 6:32 am 
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Joined: Fri Mar 03, 2006 7:09 pm
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Location: Hegins, Pa
the top is about 28 but here is what you need to understand the most . It is about the neck angle . Here is a past post that will cover this very well
viewtopic.php?f=5&t=6493&hilit=getting+the+neck+right

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 Post subject: Re: Radius on top ?
PostPosted: Sat Oct 25, 2014 12:56 pm 
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Joined: Sat May 26, 2012 12:35 pm
Posts: 301
Location: Ireland
Thanks to all of you who replied--I get it now--actually should have got it long ago but then these crazy ideas pop into my old head from time to time ha ha.

John is also correct when he says 28ft for the top to match the neck angle later on.

This forum rocks !!


Rusty

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