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Radius above the Soundhole
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10101&t=56954		 Page 1 of 2
Author:  guitarmaker78 [ Mon Jan 20, 2025 1:14 pm ]
Post subject:  Radius above the Soundhole
I’ve noticed i get a gap under my fretboard right where the neck attaches to the body that I have to sand down.

I don’t like sanding the top thinner here to get rid of the gap. And then feathering it in to the sides of the top to even out the low spot.

I glue up the top bracing there above the soundhole flat, but the radiused sides pulls it back down when I glue the top on.

I’m thinking of sanding that area of the rims above the soundhole flat on my next guitar. I’m wondering if you guys do that, how do you go about it and how do you keep the geometry correct?

My initial thoughts are to put a straightedge on the fretboard of a finished guitar and get a measurement of the gap at the tailblock for reference. Then use the (flat) back of a radius dish to sand a flat spot on the rims above the soundhole while maintaining the proper gap at the tailblock so I don’t mess up the fretboard angle where it falls off at the body.
Author:  Hesh [ Mon Jan 20, 2025 1:41 pm ]
Post subject:  Re: Radius above the Soundhole
Here you go: http://www.luthiersforum.com/forum/viewtopic.php?f=10117&t=25931
Author:  Terence Kennedy [ Mon Jan 20, 2025 4:33 pm ]
Post subject:  Re: Radius above the Soundhole
I clamp a board across the lower bout the height of the bridge and use a flat sanding board that has a piece of 1/4” plexiglass attached to represent the fretboard so sanding produces an upper bout pretty close to the desired neck angle. (Old picture, I have an extension on my headblock now and rout the mortise after the box is closed.)

ImageIMG_2428 by Terence Kennedy, on Flickr

ImageIMG_2429 by Terence Kennedy, on Flickr

ImageIMG_2430 by Terence Kennedy, on Flickr

This is followed after the top is on with a sanding jig I saw in the Fox course to fine tune the upper bout.

ImageIMG_2431 by Terence Kennedy, on Flickr

Jim Olson and Charlie Hoffman had router jigs to angle the upper bout of the rimset but I think they both built with an unradiused flat top. I use a 30’ radius. I am thinking about making one.
Author:  guitarmaker78 [ Tue Jan 21, 2025 6:01 am ]
Post subject:  Re: Radius above the Soundhole
Hesh wrote:
Here you go: http://www.luthiersforum.com/forum/viewtopic.php?f=10117&t=25931


Thanks, Hesh. That’s pretty much what I was thinking.
Author:  Hesh [ Tue Jan 21, 2025 12:22 pm ]
Post subject:  Re: Radius above the Soundhole
You are very welcome Thomas. The method was originally a Martin Method and then some of us learned it and I documented it here writing the tutorial. It works very well.
Author:  Tim McKnight [ Wed Jan 22, 2025 1:48 pm ]
Post subject:  Re: Radius above the Soundhole
Hesh wrote:
You are very welcome Thomas. The method was originally a Martin Method and then some of us learned it and I documented it here writing the tutorial. It works very well.

Hi Hesh,
I use a similar method but on my powered dish sander. After the rim has been chalked and sanded. I elevate the bottom end of the rim about .080”, resting it on a shim and sand the neck and side areas up to the top of the sound hole. I was taught this method by a veteran luthier who told me that this method was developed at Gibson and Martin later copied it. I guess it’s semantics but thought I’d throw that out there.
Author:  bcombs510 [ Wed Jan 22, 2025 2:32 pm ]
Post subject:  Re: Radius above the Soundhole
Tim McKnight wrote:
…up to the top of the sound hole….


Thats a great tip, Tim. I have been flattening almost to the waist and thought it was imparting too much fall away on the extension. I was already planning to try shortening the flatted area, this confirms it will be a good idea. ;)

Brad


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Author:  meddlingfool [ Wed Jan 22, 2025 3:27 pm ]
Post subject:  Re: Radius above the Soundhole
Just needs to go to the UTB…
Author:  doncaparker [ Wed Jan 22, 2025 6:22 pm ]
Post subject:  Re: Radius above the Soundhole
Just an observation that is relevant for those who are doing a fully bolt on neck, like the Gore/Gilet bolt on, bolt off:

When you rout out the pocket for the neck extension, and glue on the neck extension, those things are flat. The pocket is flat because you used a flat template to rout it. The extension itself is flat, because you started with a flat blank and glued it to the flat bottom of a fingerboard extension. The only non-flat bits are the edges of the pocket. Using the Charles Fox paddle shown by Terence, you can flatten those edges with hardly any work at all, and hardly any wood being removed. My overall point is that, if you are building in this way, and using something like a 28 foot top radius, there is no problem with just building with the radius for the whole top (including the upper bout) and then just sanding the edges of the neck extension pocket a little bit to flatten it to match the neck extension.

A different way of thinking about this is that, by sanding the upper bout flat before putting on the top, you are trying to avoid a hump under the fingerboard extension that needs moderate sanding to eliminate. But if you are installing a neck extension, most of the parts are already flat. You just need to flatten the edges of the neck extension pocket. There is not really a hump to be sanded out; just the edges around a pocket that would lead to a hump if there weren't a pocket routed in the top.

I hope that makes sense.
Author:  pkdz [ Wed Jan 22, 2025 10:56 pm ]
Post subject:  Re: Radius above the Soundhole
When I needed a flat plate to sand the upper bout I attached a 24" piece of sand paper from the rental department at Home Depot on the backside of my radius dish. I forget where I got the Idea but it's always handy and doesn't take up extra room in a small shop.
Author:  guitarjtb [ Fri Jan 24, 2025 7:31 pm ]
Post subject:  Re: Radius above the Soundhole
All of my radius dishes are glued to a 24" piece of 3/4" plywood, to keep the dish from warping. I have sticky back 80 grit on all the flat surfaces and they are very handy for any flat sanding, and they take up no additional room in my shop. I only use the radius side as a backer for gluing braces. I sand the top of my rims flat, and then create the fall away in front of the sound hole using pretty much the method Hesh described.
A small tip that works:
I sometimes tape a piece of poster board on half of the flat sanding board. This way, the shim that is taped on the back end of the guitar rim can be slid in any direction, and the shim is not sliding on the sand paper.
Author:  guitarmaker78 [ Sat Jan 25, 2025 1:22 pm ]
Post subject:  Re: Radius above the Soundhole
doncaparker wrote:
Just an observation that is relevant for those who are doing a fully bolt on neck, like the Gore/Gilet bolt on, bolt off:

When you rout out the pocket for the neck extension, and glue on the neck extension, those things are flat. The pocket is flat because you used a flat template to rout it. The extension itself is flat, because you started with a flat blank and glued it to the flat bottom of a fingerboard extension. The only non-flat bits are the edges of the pocket. Using the Charles Fox paddle shown by Terence, you can flatten those edges with hardly any work at all, and hardly any wood being removed. My overall point is that, if you are building in this way, and using something like a 28 foot top radius, there is no problem with just building with the radius for the whole top (including the upper bout) and then just sanding the edges of the neck extension pocket a little bit to flatten it to match the neck extension.

A different way of thinking about this is that, by sanding the upper bout flat before putting on the top, you are trying to avoid a hump under the fingerboard extension that needs moderate sanding to eliminate. But if you are installing a neck extension, most of the parts are already flat. You just need to flatten the edges of the neck extension pocket. There is not really a hump to be sanded out; just the edges around a pocket that would lead to a hump if there weren't a pocket routed in the top.

I hope that makes sense.


I was thinking a fully bolt-on neck would solve the problem. I’d like to get there one day. I’d actually like to do a fully bolt-on dovetail neck. Alas, I am about to start guitar number 5 and wish to gain more experience before undertaking such a challenge.
Author:  Woodie G [ Wed Jan 29, 2025 8:27 am ]
Post subject:  Re: Radius above the Soundhole
We had a slightly different approach that provided correct geometry without much in the way of jigs. I suppose it may have been the 90 years of woodworking experience and 80 of engineering expertise in the shop that yielded the idea that - to riff on Mr. Musk - the best jig or fixture is no jig or fixture at all (as in not needed due to engineering done elsewhere). Like stuff in the Apple ecosystem, it just worked... for Size 5 to Large Jumbo and 12 fret d-bodies. The most significant advantage is that the process avoids an eventual concave top above the soundhole by generating a preload using a 60' radius UTB and a flat caul for glue up of UTG, UTB, and soundhole bracing (following X, fingers, tone bars, bridge plate glueup in 28' dish).

From the Build Guide Concise Order of Build (with what was referred to as the secret sauce in italics... Boomers!!!):

Top and back prep:
- Thin top to joining thickness (we use a minimum of .140", but usually as thick as we can manage; most tops will be joined at 3/16”/0.193”, then thinned, but minimum thickness to allow leveling on one side after joining is about 0.140”)
- Lay out top, joint, dog-ear center joint on one of the sides, then join
- Thin back to joining thickness (we use a minimum of .125", but usually as thick as we can manage)
- Lay out back
- Mill and fit back trim (if used...we usually sandwich trim and mahogany filler to get to back thickness)
- Joint back, dog ear, and join back (with back trim if used)
- Sand top to rosetteing thickness (usually 0.005-0.010" thinner than joining thickness - just enough to smooth up the show face)
- Install the rosette
- Sand the rosette flush and then reduce top thickness to target thickness (this is the starting thickness for top tuning...for us, this is a flex and feel thing, but other approaches use deflection testing, nodal pattern, etc.)
- Mill the sound hole (we save the offcuts and label for student, guitar, etc.)
- Smooth and radius the edge of the sound hole
- Sand the back to final thickness
- Make up the bracing kit:
--Rip out all top and back braces and sand or plane to dimension
--Thin the top offcuts being used for the back center graft to 0.100"
--Mill the bridge plate to thickness and width
--Radius the back braces to 15', the X, fingers, and tone bars to 28', and the UTB to 60'
--Lay out and drill drill truss rod access hole in UTB [we use 6mm for our 5mm wrench; for smaller wrench sizes, use smaller holes and space the end of the hole 1/16" in from the soundboard edge)
- Lay out back bracing on interior of back
- Install center graft on back (use a straight edge offset by ½ the width of the graft to line up multiple sections)
- Profile center graft
- Notch center graft for braces
- Install back braces
- Lay out the top bracing
- Mill the X joint, dry-install X and UTB, trim brace ends to meeting angles (ends of fingers, tone bars, and sound hole braces), then fit bridge plate
- Radius glued face of bridge plate to 28'
- Install X, tone bars, fingers, and bridge plate on top in 28' dish
- Install UTG, UTB, and sound hole braces on top using a flat caul (this preloads the 60' radius UTB to avoid a concave upper bout)
- Shape back bracing/voice back
- Shape top bracing/voice top

Assembly of Rim and Plates:
- Fit back to rim (notch for brace ends and neck/tail block)
- Fit top to rim (notch for UTB and upper arms of X only)
- Glue back to rim (we let this dry 20 minutes, then pull out of go-bar deck to clean up any hide glue runs or squeeze-out)
- Glue top to rim
- Trim overhang
- Scrape and block sand sides to flat and true
- Sand and final voice top and back
- Bind (worth another thread for order and fine points)
- Mill neck mortise or dovetail socket
- Mill truss rod access in top and neck block
- Sand sound hole edges and radius
Author:  guitarjtb [ Thu Jan 30, 2025 10:27 am ]
Post subject:  Re: Radius above the Soundhole
Woodie,
Are you saying that the top of your rim is flat, from rear to front, when you glue the top on?

James
Author:  doncaparker [ Thu Jan 30, 2025 10:55 am ]
Post subject:  Re: Radius above the Soundhole
I'm not answering for Woodie, but reporting my own experience with the plan she outlines. Essentially, as I understood it from her prior posts about this a few years ago, you sand a 28 foot radius in the rims and leave it that way. The top braces below the upper transverse brace are all shaped to a 28 foot radius, but the upper transverse brace is shaped to a 60 foot radius. When you glue the top to the rims, this causes a bit of flattening in the upper bout, with no need for any special flattening. It is enough to make everything work out exactly the way it should. I have built two guitars using that plan, and it works great. This assumes a glued down fingerboard extension, not a completely glue-less, bolt on/off system.
Author:  bcombs510 [ Thu Jan 30, 2025 1:07 pm ]
Post subject:  Re: Radius above the Soundhole
Are the UTB and UTG glued to the soundboard against a flat caul as well? I took it that is what pre-loads the tension.


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Author:  SteveSmith [ Thu Jan 30, 2025 2:24 pm ]
Post subject:  Re: Radius above the Soundhole
I’ve been doing it the way Woodie described for years and it works great. I use a clamping caul with 60’ radius but that’s not far from flat and probably makes no difference.
Author:  doncaparker [ Thu Jan 30, 2025 5:29 pm ]
Post subject:  Re: Radius above the Soundhole
I’m with Steve. At that short length, the difference between using a flat caul and a 60 foot radius caul for the brace is practically nothing. And for the graft(s), having them just be flat is fine. I’m not too worried about springing or not springing the top and braces at curves that shallow.
Author:  Woodie G [ Thu Jan 30, 2025 5:56 pm ]
Post subject:  Re: Radius above the Soundhole
Thanks all for stepping in... I have the flu and am vegetating versus properly monitoring my media accounts. I was able to dig through the mounds of discarded tissues accumulating on all of the available horizontal surfaces within reach and - after disinfecting my computer- log in here to find I was remiss in paying prompt attention to your queries. I pleaded sickness and the fact that I ran out of raspberry Jello last night.

Yes - the rim is radiused at 28', and the caul used for the UTB, UTG, and soundhole braces is flat - as much for generating that preload as to eliminate the need for an expensive, space-consuming 60' radius dish (re: the note above on "...the best jig or fixture is the one no longer needed, blah, blah..."

I am NOT an engineer, so if you need the why in addition to the how, I am not your best source. As Mr. Parker and Mr. Smith mentioned, it just works, which is more than i can say for TheraFlu or those tissues which are supposed to be gentle on my poor, abused, very congested nose. And no - this is not me wallowing... that is even less attractive than this endless complaining.
Author:  Hesh [ Thu Jan 30, 2025 6:16 pm ]
Post subject:  Re: Radius above the Soundhole
Feel better Woodie.
Author:  guitarjtb [ Thu Jan 30, 2025 7:33 pm ]
Post subject:  Re: Radius above the Soundhole
Woodie G wrote:
Yes - the rim is radiused at 28', and the caul used for the UTB, UTG, and soundhole braces is flat - as much for generating that preload as to eliminate the need for an expensive, space-consuming 60' radius dish (re: the note above on "...the best jig or fixture is the one no longer needed, blah, blah..."


Sorry that you have been fighting the flue.

So to clarify, the rim is radiused to 28' using the dish and the "drive the bus" method?
This would take the neck area and the heel area a little lower than the bridge area. That will help create the angle needed in front of the sound hole. Since I don't use a dish to radius my top rims, I need to drop the front end of the rim a little to get the same angle that you would with a dish.
I like a flat rim on top, with radiused top braces.
Author:  James Orr [ Fri Jan 31, 2025 12:34 am ]
Post subject:  Re: Radius above the Soundhole
It’s simple enough to glue the brace against a flat surface, but won’t the beefy UTB pull that thin little top right back to 60’?


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Author:  doncaparker [ Fri Jan 31, 2025 8:12 am ]
Post subject:  Re: Radius above the Soundhole
James Orr wrote:
It’s simple enough to glue the brace against a flat surface, but won’t the beefy UTB pull that thin little top right back to 60’?


Sent from my iPhone using Tapatalk


Sure, I think it is safe to assume that, or at least something between flat and the 60 foot radius. But the goal isn’t for it to be completely flat; the goal is for it to be curved (a convex curve) such a tiny bit that the difference between flat and curved essentially doesn’t matter.
Author:  Woodie G [ Fri Jan 31, 2025 8:16 am ]
Post subject:  Re: Radius above the Soundhole
Thanks for the kind thoughts, gentlemen. Type A atypical flu should be on your list of things to avoid... I am going to give it a single star when I identify the website for viral reviews.

On radiusing the rim - 'driving the bus' is certainly one way to accomplish the task, but we used a straight linear motion as much because our radius dishes were rather generously sized as because it allowed some adjustment in cutting action using a piece or two of butcher paper. A powered radius dish as well as radius sticks are other methods I have seen. The linear method also has the slight advantage of appearing a bit more dignified than 'driving the bus,' although likely not an issue for shops lacking distaff members of the enterprise.

Yes, Mr. Orr - the UTB definitely forces the top to a radius which is essentially 60' absent other loading from the rim, etc., but that slight preload is still there, as is the rest of the top at 28' radius... while a wooden spring may see some relaxation and change in spring constant over time, it is still a spring . What we found is that - over time - the area above the soundhole might flatten, but it did not move beyond flat into concavity as what seemed to me to be too many larger-bodied Larrivees did after too few years (as well as the occasional late-1960 and 1970's Gibson).
Author:  guitarjtb [ Fri Jan 31, 2025 10:37 am ]
Post subject:  Re: Radius above the Soundhole
Woodie G wrote:
On radiusing the rim - 'driving the bus' is certainly one way to accomplish the task, but we used a straight linear motion as much because our radius dishes were rather generously sized as because it allowed some adjustment in cutting action using a piece or two of butcher paper. A powered radius dish as well as radius sticks are other methods I have seen. The linear method also has the slight advantage of appearing a bit more dignified than 'driving the bus,' although likely not an issue for shops lacking distaff members of the enterprise.


No matter how large the dish is, and whether you "drive the bus", use a powered radius dish, or move it in a more dignified linear motion, if it is a dish, it will still create a rim that is lower at the heel and neck block than the rim is near the bridge area. Most guitar bodies that I am familiar with, are longer than the width across the bridge area. This is why the method works. Those that prefer a flat top rim have to create this situation by other means, described by others in earlier posts.
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