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Top Radius
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Author:  MikeyV [ Fri Mar 25, 2011 12:24 pm ]
Post subject:  Top Radius
I'm coming up on the part of my build where I need to figure out what radius I will use for the top of the box.

I hear 28' is common, and quickly drawing this up in CAD, I see this gives the favored 1.5 degree neck back angle.
I was planning to use this radius for the build. Do alot of small builders use this number?
I also hear Martin uses a 52' radius. Do thay build the top to this radius braces etc.., the just sand a 1.5 degree angle at the neckblock/upper bout area to create the neck back angle?

Thanks for the help guys, always helpful.
Mike
Author:  jfmckenna [ Fri Mar 25, 2011 12:35 pm ]
Post subject:  Re: Top Radius
Mike,

For what it's worth that's what I do is plane out the upper bout. I don't work to tight CAD specs but I eyeball it as close as possible then just set the rest of the neck angle by planing the heal to get it right. I think it's best to do it this way so that you don't have an excessive hump at the body joint.
Author:  meddlingfool [ Fri Mar 25, 2011 2:06 pm ]
Post subject:  Re: Top Radius
When I was doing dovetails I found that gluing a top with a 28' radius onto rims that were perfectly flat worked very well. But I aslo profile the fingerboard radius after the neck is attached.
Author:  MikeyV [ Fri Mar 25, 2011 3:42 pm ]
Post subject:  Re: Top Radius
Med, With the rim perfectly flat, wouldn't that cause a dip where the top is glued down to the top of the neck block, or more precicely, anrise right after the top leaves the neck block heading for the tail?

Or do you find that that rise is really small, and just having the soundboard rise at the 1.5 degrees given by the 28' radius makes the fingerboard angle match up?

Thanks for your opinions, guys.
Author:  bluescreek [ Fri Mar 25, 2011 4:15 pm ]
Post subject:  Re: Top Radius
http://www.youtube.com/watch?v=QYHPCeVRUA4 this may help . The top can be flat or radiused . I use a partial radius . The key here it so aim for the perfect set up . That is with the string height at 1/2 off the top at the front of the bridge . This vid will show how I do it.
CF Martin uses an angle of 1 1/2 degrees worked into the sides that would equate from the top of the sound hole to the neck block .
there are many ways to do this so find the technique that works for you and give your process repeatability .
Author:  cwood8656 [ Fri Mar 25, 2011 4:17 pm ]
Post subject:  Re: Top Radius
I build my tops on a 28 foot dish and profile the rim that way. I do build the upper bout area flat, and sand the rim flat in that area. Makes fitting the neck pretty easy.

Chris.
Author:  gozierdt [ Fri Mar 25, 2011 4:21 pm ]
Post subject:  Re: Top Radius
A lot of us use 25' to 28' for the top radius for the 1.5 degree angle you mentioned. Some,
I'm one, also plane the upper bout linings on the sides so the upper bout is flat from the
center of the soundhole to the upper edge. The angle on the heel of the neck and/or the upper
bout can be fine tuned during final assembly to hit the saddle at the height you are aiming
for, typically about 3/8" bridge height and about .125-.150" saddle height above the bridge.
Search on Neck Setting and related topics, there are lots of relevant threads.
Author:  meddlingfool [ Fri Mar 25, 2011 5:34 pm ]
Post subject:  Re: Top Radius
I also put the 28' radius into the UTB. Found this adequate to get thing very close. It never bothered me too much whether the tongue went up or down a few thou as I'd just flatten it if it rose and let it be if it dropped as long as it wasn't extreme.
That being said my tolerances are a lot tighter these days.
One thing that can help is to tape a precisely 1/8 shim exactly where the saddle will be. Then take a flat board with fresh 100 grit on it and gently sand the upper bout between the soundhole and the dovetail. The board should ride constantly on the shim. Mine is about 18"x4" 5/8" MDF. Make witness lines where the tongue falls and sand till they're gone. Don't push hard or you'll compress the top and throw off the angle. This will give you your 1/2" saddle height. Maybe not the most graceful but certainly effective and served me well for 45 guitars till I switched to double mortise and tenon, a whole new game.
Hope that helps...
Oh, this also assumes you are using a 1/4" fingerboard.
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