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PostPosted: Thu Sep 06, 2018 8:47 am 
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Koa
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Woodie G wrote:
15'/28' for us, but those radii are based on what has worked well acoustically, gives the correct fretboard extension fit with the building system, and provides adequate range on handling RH changes. Back radius does have some impact on acoustics, but there's no real coupling between back radius and the neck/body geometry we need to establish...in other words, you might just pick something that has worked for others and go with it.

For top radius, it's really a question of the building system. If you'll be wedging the upper bout to establish the fretboard extension fit, those builders using that approach seem to build with anything between 20 and 30 foot radius. For those using a flat or nearly flat 60' UTB glued on a flat caul, 28' radius provides good extension geometry for 12 and 14 fret bodies from Size 5 to 12 fret dreadnaught.

Having both dishes allows the body to be braced up and closed in the go-bar deck, which eliminates a lot of clamps. With a deck and dishes, 48 bars (24" x 3/16" with vinyl caps from Goodwinds) will run $80 delivered...or about the cost of 4-5 good cam clamps, and will let you get by with far fewer cam and F-style clamps. Also an option to rip up some straight-grained hickory or ash to make up wood spring bars...$15 worth of 5/4 stock gets out a lot of wooden bars.


I have very low floor joists in my basement, my head just clears them so I can screw a piece plywood to them and have a perfect go bar deck using my bench and some strips of wood.

I don't even know what a UTB is so that's out for me :-) I may buy the 28' for #2. Too many things to buy when your starting. For this one I'll follow John Halls method. I'm not sure but I think he used a flat dish and did some rocking and pressure to get the right angle for the neck alignment. Don't quote me I need to watch the video a couple more times.

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PostPosted: Thu Sep 06, 2018 9:08 am 
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In my opinion the important issue is to understand the top/neck/bridge geometry. One thing that helped me was to tape some freezer paper to the bench and draw the neck, top and bridge at full scale exactly as I planned to make them. I start with a line representing the strings and go from there. Once you understand that then the rest of it is just using tools that you have available to shape everything correctly. Obviously the radius dish makes things a lot easier.

UTB = upper transverse brace. The large brace that supports the top between the soundhole and the neck block.

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PostPosted: Thu Sep 06, 2018 10:51 am 
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banjopicks wrote:
I notice you don't have a large piece of sandpaper stuck on. Do you remove it after sanding the sides?


I hadn't bought a big piece of sand paper at the time that was taken, was trying to stick 8x10 sheets to the dish with double stick tape. Didn't work very well, the rim grabs the edge of the sandpaper and rips it. Have bought a big piece of 120 since then.

My go bar is very simple as can be seen in the photo - I take it apart and put it away between builds. For bars I bought a bunch of fiberglass wands at Lowes and cut them into some different lengths. Sometimes if I don't have enough of the correct length I put little blocks of wood between the bar and whatever I'm clamping. Again, my first couple were built without a go bar, I just used my 15 and 25 foot radius sanding beams as backing cauls when I glued braces - It worked but was tedious and at about #3 I put the go bar together.

And you are very correct that tooling up is a big problem in building your first guitar - I tell people not to build because they want a cheap guitar. I started with a pretty minimal shop and with each one I bought the tool or whatever would have made the last one easier. GAS gets replaced by TAS (tool acquisition syndrome)


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PostPosted: Thu Sep 06, 2018 1:34 pm 
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OK, don't everybody jump on me.

The old techniques for classical guitar construction included steps that deliberately did not devote effort to get the edge seam perfect. Tops were attached with tentalons (dentalons) and it did not matter if the the sides even touched the top. Then the edge was cut out for the binding, in which case even if you did match the sides to the top, you cut out the entire seam that you so carefully constructed.

The back is a little more difficult. Try to get it close, but do not run a series of clamps to hold the pieces tight. Instead use a backing strip to distribute the clamping force to bridge any local areas that do not exactly close. After you cut the binding channel, the occasional slight gaps are irrelevant. But, if you had run a whole bunch of clamps without the backer, you certainly would deform the back to close the gaps, which would only ripple the back and maybe set you up for future cracks.

This is not just a lazy expediency. The old masters, and many of the current ones, often developed the top with a very complex shape prior to assembly. The tentalon method is pretty much mandatory to get a fit.


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PostPosted: Thu Sep 06, 2018 1:51 pm 
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wbergman wrote:
OK, don't everybody jump on me.

The old techniques for classical guitar construction included steps that deliberately did not devote effort to get the edge seam perfect. Tops were attached with tentalons (dentalons) and it did not matter if the the sides even touched the top. Then the edge was cut out for the binding, in which case even if you did match the sides to the top, you cut out the entire seam that you so carefully constructed.

The back is a little more difficult. Try to get it close, but do not run a series of clamps to hold the pieces tight. Instead use a backing strip to distribute the clamping force to bridge any local areas that do not exactly close. After you cut the binding channel, the occasional slight gaps are irrelevant. But, if you had run a whole bunch of clamps without the backer, you certainly would deform the back to close the gaps, which would only ripple the back and maybe set you up for future cracks.

This is not just a lazy expediency. The old masters, and many of the current ones, often developed the top with a very complex shape prior to assembly. The tentalon method is pretty much mandatory to get a fit.


No argument there. It's the linings and the blocks that need to fit and I think most folks here would agree. I've used sides that weren't quite tall enough several times by letting the linings sit proud of the edge of the side. However, I would not accept gaps between the linings and the top or bottom since a good fit is important to the integrity of the final joint.

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PostPosted: Thu Sep 06, 2018 2:14 pm 
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Brazilian Rosewood
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Freeman wrote:
"was trying to stick 8x10 sheets to the dish with double stick tape. Didn't work very well, the rim grabs the edge of the sandpaper and rips it. Have bought a big piece of 120 since then."


If you use contact cement (as used for gluing formica to particle board) it will work a bit better - at least for several years, and then you might want to change it anyway.
I made a radius dish by gluing small wedges around the edge of a circle of 3/4 inch plywood and then placing a circle of 1/4 inch plywood on top, gluing it to the wedges and screwing it in the center. It's probably more of a spline curve but it is close enough for radiusing the top. It's virtue was it was cheap and I didn't have to swallow a lot of sawdust to make it.


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PostPosted: Thu Sep 06, 2018 2:24 pm 
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Clay S. wrote:
I made a radius dish by gluing small wedges around the edge of a circle of 3/4 inch plywood and then placing a circle of 1/4 inch plywood on top, gluing it to the wedges and screwing it in the center. It's probably more of a spline curve but it is close enough for radiusing the top. It's virtue was it was cheap and I didn't have to swallow a lot of sawdust to make it.


That seems like a great idea to me. Do you still use it/like it?

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PostPosted: Thu Sep 06, 2018 2:41 pm 
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banjopicks wrote:
Clay S. wrote:
I made a radius dish by gluing small wedges around the edge of a circle of 3/4 inch plywood and then placing a circle of 1/4 inch plywood on top, gluing it to the wedges and screwing it in the center. It's probably more of a spline curve but it is close enough for radiusing the top. It's virtue was it was cheap and I didn't have to swallow a lot of sawdust to make it.


That seems like a great idea to me. Do you still use it/like it?


My "radius" dish is similar to Clay's. I have a frame with an open circle in the top (I think 20" in diameter). On top of that, I have a doubled up piece of 1/8" masonite glued together with a bolt buried between them so it protrudes through the bottom and not the top. A wing nut at the bottom allows me to pull it into what ever deflection I want. In practice, I never adjust it and it is set at the equivalent to a 28' radius.

Clay is right, it is not a radius but a spline curve. The difference is negligible in this application. My only complaint is that my dish is bulky and heavy; at almost 4" high it is a bit of a pain to store and pull out to use. I keep telling myself I am going to make a smaller/lighter one but I never get around to it. I guess that means I'm happy with what I am using.

As Steve says, the lining joint is what counts. I have fixed lots of problems with the edges of the sides knowing I was going to rout it away for binding anyway. Once I went so far as to include side purflings to save even more side width ;)

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PostPosted: Thu Sep 06, 2018 2:49 pm 
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Koa
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I'm going to purchase a 20' for the back and make one like this for the top.

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PostPosted: Thu Sep 06, 2018 4:19 pm 
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Sounds like you have reached a resolution. Using a radius dish for the back does seem to be the most popular method. I believe most factories do it that way, or a facsimile there of.

I still do mine following the Cumpiano method of keeping the sides parallel nearly to the waist. I cut the sides about a 1/4" shy of the finished profile using a side profile template, bend the sides, glue in the neck and end blocks then finish the transition to the neck block with a plane and a radiused sanding stick like Freeman showed. Its easy and I like the results

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