MIMF

I have built a few instruments before at a luthiers workshop so this is not my first instrument, but this time i I wanted to build a guitar for a friend, so as I don't have a workshop I bought one of those Stewmac dreadnought kits. Looks good but checking the instructions (https://www.stewmac.com/freeinfo/i-5295/i-5295.pdf) I have a doubt.
At the part where the kerfing is sanded, I can see that the back radius is built into the kerfing, head and tail blocks. But when the guitar top is done, everything is sanded down flat and squared to the guitar sides. Is it ok to do it like this? The guitar top also has a radius, and I know some builders also build this radius on the kerfing and end blocks, so I wondered if sanding them flat could cause any problem when gluing down the top.

Thanks a lot in advance.

Usually the top and back edges of the rim are profiled to the radius of the desired dish. Making them flat will simply mean that you'll have to pull them in a little to get the plates into contact, but should not cause any structural issues I can think of. As for the angle around the edge; while its probably better to have the liners rise a bit into the guitar to match the dome, the actual difference in height across a quarter inch or so for a 15 or 25 foot radius is probably not enough to seriously weaken the glue joint.

I'll note that classical guitars built in the traditional style on a solera have rims that are level on the top and back, but the pates are arched at least somewhat. The back arch varies in radius from place to place, and the top is only arched in the lower bout, with different makers using somewhat different arch shapes there. They do usually try to match the angle around the edge, at least on the back, but since it varies from place to place, and the fitting is done by hand and eye, it may not always be super precise.

Jorge, as Alan said, the back (and tops) are usually more or less spherically domed which is kind of tricky to achieve without a radius dish. The method described in the StewMac instructions sort of works - I used it on my first couple of guitars. I just made a couple of sanding sticks long enough to span the guitar with 15 and 24 foot radii. There are some tricky ways to draw that curve - you can calculate it mathematically, tie a pencil to a 15 foot piece of rope and make a giant compass, or draw it with some sort of CAD or drafting program (which is what I did). Anyway here is a guitar in its mold getting the kerfing sanded.
That will put the correct angle on the sides but it doesn't take into account that at the waist the sides are actually taller because they extend more into the sphere. I broke down and bought a couple of radius dishes and I now sand the rims to fit the dish
You can see that the waist does not (yet) touch the sand paper, that means the more needs to be taken off the upper and lower bouts. (Many people leave the rim in the mold while they do this sanding - I find that hard to hold and so I just try to be gentle)

I always felt that while the first method worked, it did introduce a lot of stress into the back. The top has a much flatter dome so its probably not as critical, but I feel that I get a much better fit with the dish. I think if you try to visualize what is going on you'll have better luck fitting the back.

Good luck, hope this helps

Thank you both! That is some interesting information.

Investigating a bit more I think that sanding the linings flat on the top must have been how Martin used to build their guitars back in the 30s, so that's why Stewmac is suggesting this method (?). Of course that does not mean it is the best way, and I'm sure that the method Freeman is suggesting is probably better as it makes sense geometrically.

I guess that the classical guitar building ways have something to do with it also. Actually checking Cumpiano's famous book, that's how he builds both the steel string and classical guitar. Again, that does not mean it is the best way to do it, but as Alan says it will work for sure.

Not sure what I will do yet, but now I have some useful information to make a decision.

Jorge

Huss and Dalton build some of their guitars with a domed top pulled down to a flat set of rims. They say this adds a bit of tension which helps tone.

Jorge, I think flat on the top is OK but remember that the back is not only domed but it tapers from tail block to the head, Normally the lower bout is about the same thickness but the sides taper significantly from the waist to the neck. I rough cut that taper into my sides before I bend them but I rely on the sanding to actually give them the compound curve shape. It also means that the back is in a compound shape - the back braces will give the side to side dome but it is the glue joint to the sides that makes the lengthwise arch. The wood wants to bend from side to side but will fight the lengthwise bend and it is possible to have some real stress points along that glue seam. I just wasn't happy trying to make that joint so I bought the radius dish.

You can do it for the first guitar or two but at some point you'll want to move to a dish.

Unless your head and tail blocks match the back and top curvature, they will telegraph through. It’s not pretty.

Barry Daniels wrote:Huss and Dalton build some of their guitars with a domed top pulled down to a flat set of rims. They say this adds a bit of tension which helps tone.

Yes, I read that somewhere as well. They call that line "Traditional", so I guess that's the "vintage way". Nowadays I think most people build a radius on the top.

Freeman Keller wrote:Jorge, I think flat on the top is OK but remember that the back is not only domed but it tapers from tail block to the head, Normally the lower bout is about the same thickness but the sides taper significantly from the waist to the neck. I rough cut that taper into my sides before I bend them but I rely on the sanding to actually give them the compound curve shape. It also means that the back is in a compound shape - the back braces will give the side to side dome but it is the glue joint to the sides that makes the lengthwise arch. The wood wants to bend from side to side but will fight the lengthwise bend and it is possible to have some real stress points along that glue seam. I just wasn't happy trying to make that joint so I bought the radius dish.

You can do it for the first guitar or two but at some point you'll want to move to a dish.

Thanks Freeman. Actually this kit comes with the sides tapered, so that's one thing less I need to do

If you're sanding your rims free hand, no mold, how do you keep your radius? If you flex the sides at all I would think that you would lose your radius, or at least make it wrong. I am always in my molds, and I draw a line to keep it in the same place in my mold, while I sand to ensure a good fit. If I can do it out of the mold that would be easier. (This is my least favorite step in all of guitardom.)

If you are using a relatively flat radius (60 ft.) for the top as some do, having a flat rim won't make too much difference. The flatter "doming" of the top can give a bit more bass response to the instrument as a dreadnought usually has. The sanding stick Freeman has shown should get you close enough for the back. As Alan mentioned, in a quarter inch you don't get that much rise from one side of the lining to the other.