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-   -   vintage flattop radius question (https://www.acousticguitarforum.com/forums/showthread.php?t=468345)

nick1018 04-29-2017 06:32 PM

vintage flattop radius question
 
hey, first time writer long time reader here. So I am doing some research on a build i want to do. I know a lot of prewar guitars were built with a radiused top and back but had flat sides. I am mostly interested in gibson. Meaning the side edges were not radiused to match the top or back, the kerfing is sanded to a 90 degree angle so when the top or back is glued & clamped into place, the outer rim of the sound board is flat, though the very center of the X brace or back bracing retains some radius. My question is, how is there enough gluing surface on the sides to glue down the top or back? Or is it just forced down by clamps? Obviously it works since so many old guitars are still together. any info would be appreciated. thanks! nick

Truckjohn 04-29-2017 08:37 PM

Linings are glued to the rims to give more glue area.

In terms of the radius on the sides - the amount of radius on a 0.200" wide side + lining is pretty small. Clamps do just fine gluing the top to the sides either way.

But.. Different makers did different things for the shape of the rims. Gibson left the neck heel end and butt on many models the same body depth. So their rims are flat flat. Martin tended to make the neck heel end shallower than the butt by 1/2" or 3/4". To accomplish this - the body gets a slope. Cumpiano's book details how to make the shape... Its not particularly hard to do...

In terms of "radius". Thats really a misnomer on old guitars... Curve is a better description. Very few guitar makers used an actual "cylindrical" or "spherical" radius.... Most of those old guitars used braces that were curved using a simple fair curve. You can make a pretty good curve template by flexing a metal or wood yardstick between 3 pins... 2 outside and 1 in the middle.

ricklt 04-29-2017 10:49 PM

Look up kerfing on eBay. That's the lining he Truckjohn was talking about. I think it's pretty self explanatory once you see it.

Sent from my XT1650 using Tapatalk

mirwa 04-29-2017 11:05 PM

I can only speak on how I do it.

When building an acoustic guitar, an example here is a guitar with a back of 20ft radius, I sand the sides of the guitar and heel block and neck block so they conform to that 20ft, then I apply my kerfing.

Once the kerfing is dry and glued in placeI then resand my sides,neck block, heel block and now kerfing to conform to that radius.

Then it is ready to receive the back and be glued into place.

When I pull the back on most old guitars, I do not think I have ever seen true 90 degree kerfing remaining, but more pre-shaped kerfing as explained by how I do it. So I would assume that's how they did it as well.

Steve

nick1018 04-29-2017 11:38 PM

thanks for the info. I know what kerfing is and how it works. I was just talking about on some prewar guitars the sides and kerfing were 90 degrees, not matching the radius of the back. So if the back and sides didnt meet at the same radius it wouldnt be a perfect joint. So was there just more clamping pressure to make them meet?

murrmac123 04-30-2017 11:41 AM

What is this "kerfing" whereof you all speak ?

(With the honourable exception of Truckjohn ...:up:)

Do you mean the "lining" ?


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