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I have been going thru the Cumpiano book in order to build my first guitar. I am using this method as it is the only resource available to me other than the internet and also because I am a hand tool based shop.

My question is about gluing the radiused back plate on the box. Im not quite understanding how the sides and kerfing is shaped properly in order to create a proper glue joint between the two. His description shows him planing down the sides starting just behind the waist down to the headblock height creating a ramp. Then he uses his large sanding board to round out the facets from front to back creating a curved back. What I don't understand is how the radius from side to side is created so the kerfing along the sides receives the same contour as the back plate in order to get a good glue joint there. Obviously a radius dish creates this perfect fit but I am not prepared to purchase the dishes at this point in my attempt at building a guitar. Not sure if this is a one time thing or not.

So what am I missing here on this step? Even gluing down the top plate seems as though it is being forced flat on the kerfing due to the cork lined work board. Just seems like I'm missing something.

I noticed this, too, and do not like it. I think for the top, because the arch is larger, IE; flatter, and because some of the braces are radiused and some are not, this is fine. In fact, though I could not name them, I know from past threads I've read that there are well known builders here who glue a radiused top to flat linings. As for the merits/disadvantages of this method, I'll let those more experienced than I fight it out. I know what I'm doing re: the top, and I'm sticking to it.
For the back I plan to make a radiused sanding stick similar to the one used in the Stew Mac dread kit assembly instructions to radius my back linings, as I think the radius for the back is much tighter and I'm not comfortable with flat linings there.
http://www.stewmac.com/freeinfo/i-5295/i-5295.pdf
Edited to add: The top is indeed forced flat to the linings because it is forced against the workboard and shim. The back, however, is attached via the roping method, and I just don't see a lot of concentrated downward force at the top edges of the back plate forcing it to conform to the flat linings. So I think for this reason also, the back linings need some type of radius.(Also edited for spelling)

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Nope, you're not missing anything. The plates and sides are flexible enough to be forced together even if they don't match shape precisely.

I profile my back rim a bit differently, so I don't need the large sanding board. Start by planing it flat, and about 1/8" higher than the headblock/tailblock. Lay it on the bench and shine a light inside to see where the high spots are as you approach this. Then mark a pencil line all the way around, 1/8" down from the edge... this is the line of where the rim would be if it was flat and level with the blocks. The goal is to have it dome up a bit from there as you go inward toward the center. So then plane it down to the blocks, and smoothly blend up to the waist, which remains 1/16"~1/8" above the line, depending on how heavily domed your back is (the book does a heavy dome, so 1/8"). Glue linings a little proud, plane them down, but angled just a touch to support the dome, and you're done. Sometimes I give it a final pass with a small sanding block to smooth out any ridges from the finger plane jittering as it runs over the kerfs in the linings.

You gotta force that sucker flat around the edges! Or you can plane it freehand to get it close to the correct angle. Both choices suck if you ask me.

My suggestions are as follows

A. When making your template for the back brace arches go ahead and create a "negative" just as wide as you wish maybe an inch or two. Then coat it with sandpaper. You now have a radiused sanding block without the trouble of making a whole dish. It's not going to make the kerfing as perfect as a dish but it'll get closer than by hand. And will look much better than forcing it flat.

B. temporarily attach sandpaper on the inside edges of the already braced back an use it as a radius dish. This could be a major pain in the butt. But if done with patience would make a very nice fit.

I know just what you mean. It bothered me also.

Now I follow the Cumpiano procedure to get the sides ready then I finalize the rims with a radiused piece of wood and a sanding block. You can see where you need to round the rims slightly to match the radius. I use 16' on the back and 28' on the top. I leave the sound board flat from the end of the fingerboard to the heel. Sand paper could be put on the bottom of the radius board and used like a shaped sanding block.

I do it like this

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Joe Beaver
Maker of Sawdust

Which I actually have read a number of times and makes pretty good sense to me. Again I'm still concerned about the flat edge elsewhere, especially after creating the rabbet for the purfling/binding. If I don't get a great glue joint on the flat areas it seems as though those parts would just pop loose after/during the creation of the rabbet.

I may end up creating some sort of dish to radius the flat spots as a 1/4" rise on each side of the back seems like a hefty radius not to do anything with on the sides of the rim.

I do like this idea. Curious why not just apply sandpaper to the radiused piece of wood? Is there any reason why you couldn't do this?

Anyone have any idea what the actual radius is on Cumpiano's guitar? And what is a good way of converting a known radius (say 16') to an arc on a 2' long board? It would be a bit hard to find a 16' compass and area to do this in.

This thread has that info, and a link tioa calculator for the radius you'll need.
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=38784

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

16' rope taped to the floor with chalk on the end.

You laugh, but I actually contemplated a variation on this theme.
Fortunately it wasn't necessary.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

I actually drew my 15' curve based on that

I profile my #1 based on the Cumpiano method.
The back is taper down after the waist.
The back fits the rim lining ok, no worries about that.
The only drawback is there is a slight dip at the waist.
Usually when using a radius dish the waist should be slightly higher than the rest of the rims.

Currently I don't use a radius dish but a sanding stick instead.
The back braces are sanding on the rims before gluing to the back.

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Sen CL Goh
http://senguitar.blogspot.com

I asked Cumpiano bout that when I was at his tutorial.

He said that in his experience the wood is flexible enough so that it doesn't matter and in his career he hasn't had a back seperate from the ribs due to a perceived radius mismatch, so in his view it's a non-issue, after 300 plus guitars completed.

Indeed in my builds when looking in the soundhole at the back joint the linings are firmly attached to the back with no gaps.

I don't rope my backs, I use cam clamps. I don't like the roping method. The stupid rubber ropes snap a lot and it feels clumsy to me.

I'm pretty good with the clamps and I think they do as good of a job mating the surfaces as roping.

You could probably do the same thing by making clamping cauls and use fewer clamps, but I think not having the cauls forces the back onto the sides better.

I'm not convinced it's that big a deal but I can see why people would think it could be.

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Old growth, shmold growth!

Also as far as the different radii are concerned, Cumpiano's braces are curved. but the back isn't actually "domed" per se the way the tops are, and it doesn't seem that there would be a constant radius anyway, as the curvature at the heel and headblock wouldn't necessarily match the curvature along the sides.

But again, the wood is likely flexible enough for it to be a non issue anyway. With the backs being arched the way the are you have to force the wood into place and glue it down, forcing it to conform with whatever you do.

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Old growth, shmold growth!

The actual radius of your build is not needed. Just make an extra back and top brace using the same method. Then use those braces as templates to make reverse radius form. I do believe you could put sandpaper on the radius form and use it to set the rim radius. Should work fine.

_________________
Joe Beaver
Maker of Sawdust

Whisperer
Wish I had a quarter of those clamps. Way cool

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Joe Beaver
Maker of Sawdust

what bothers me and i hope someone can put me straight (sorry for hijacking the thread but it is related) is; if you taper the back toward the headblock BEFORE installing kerfed lining, when you come to install them, do you have to break the linings at the point where the taper begins so that they follow the line of the rim?

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We are all in the gutter but some of us are looking at guitars.

Im not so sure that the broken kerfed lining is a big deal, especially considering some people glue individual blocks in place. I could be wrong though.

Ok, so I was thinking about the process a little more. So because you have to bend the back a bit to conform to the lengthwise arch created in the sides, this in turn should flatten the back out slighty shouldn't it? That could be the reason for the heavy initial curve of the back braces.

thanks for reply, Todd. So, do you glue the kerfed lining on before tapering the back? And so the kerfed linings become reduced in size as they reach the head block.

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We are all in the gutter but some of us are looking at guitars.

I could be wrong but I don't think that is what he was saying and if I remember the Cumpiano/Natelson method correctly the sides are bent at full width across the lengthe of the board so you really cannot glue the linings in first. I use a similiar method and get everything close then glue the linings in and finish it off. That bend at the waist you are talking about is no big deal with normal kerfing it just goes with the flow. I have no experience with reversed linings though.

Yeah, on my build I cut the kerfed linings at the waist and glued in the next strip at the new angle.
Wasn't sure if that's the best method but how else to get a straight strip around a curved rim? Your method of making the kerfed linings wide is good also, Todd. Might try next time.
Anthony, sorry for ignoring your original post and butting in with my own problems.

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We are all in the gutter but some of us are looking at guitars.

AnthonyE, thank you for the thread! There were a lot of questions I had that I got answered here. I appreciate all those who responded with their experience and knowledge.
I was talking with someone else on another forum and mentioned how amazed I am at the willingness of most builders to share and to help. So glad to be a part of this community, and hope I have plenty of opportunity to pay it forward in time.
The only negative is that now I have clamp envy, lol.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Cumpiano does things the way he does for two reasons:

1) It's the method he was taught and, where he thinks it was deficient he's made some changes.

2) He enjoys the process.

For instance, he was taught to rope the back to clamp it.

I told him I had a crap load of clamps and he said that would be fine if I could clamp them fast enough, so that's what I do, and I actually like manipulating clamps and finding their optimum positions. (I'm quite good at clamping broken headstocks so that all they need are a light finish touch up)

He said that there were probably better ways of attaching the back, but that he actually likes roping the back down, it's fun for him.

Same reason for profiling the back the way he does, although currently he omits the extra profiling of the tailblock as being unnecessary.

So sometimes the reasons for doing something a certain way are that

1)It's good enough (meaning there are no structural or aesthetic problems)

2)It's fun.

So it may not be necessary to bevel the edges to match the radius of the back, but it indeed may be more fun, and possibly more satisfying.

Me, I like the fact that I get good results without feeling like I MUST match the edges. (However, I must poit out that the edges DO match under clamping pressure.)

Tooling and skills have nothing to do with it.

If you think about it, it takes less skill to spin a radius dish around on the back of the instrument and call it done than it does to meticulously sand around the edges, blending various planes, angles, and domes by hand to produce a good and aesthetically pleasing result (IE glued tightly around the circumference with no gaps, and no unsightly dimples!).

I'm not trying to rankle anyone's feathers here, I'm not offended in any way, just my point of view.

I'm sure Cumpiano possesses the correct skills necessary to radius a back using dishes if he really wanted to.

So it's not fair to say that a deficient skill factor would be a reason for NOT using radius dishes.

However, they certainly look faster and more efficient than what Cumpiano is doing, which would be a very good reason to switch.

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Old growth, shmold growth!

I like to choose methods that are easier and more accurate. Different strokes, eh?

For different folks!

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Old growth, shmold growth!