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I'm a bit perplexed. Cumpiano's workboard is flat, yet the braces are arched. The workboard is flat and is used as a clamping surface. Doesn't this force the top and braces flat? Negating the point of arching the braces? Perhaps I missed something in the text but I don't believe he addresses this, or I'm missing something entirely.

My goal is to create a top with a 25' radius that's under tension.

Thanks,

-j

I clamp on a flat board with note card backing cauls.

Ken has some great info about how easy, and affordable it is to use this method.

http://www.kennethmichaelguitars.com/contourcaulkitinstrction.html

http://www.kennethmichaelguitars.com/topassembly.html

It has been a while since I have read that section of C&N because that is not how I do it but doesn't he use cork around the perimeter of his workboard?

You say that you want to a 25' radius under tension, I don't think that is his method. You have to be careful mixing methods without understanding all the hows and whys. IIRC his method also builds the neck angle into the box (again, I could be remembering that wrong since I don't do it that way). Make sure you have your box construction, bracing and neck angle methods all planned out, some may not easily be made to play nice with one another.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

He does use 1/8th cork on one side. Hardly seems like it's enough and it seems like it will flatten the brace. Maybe I just need to try it and see.

I think you're correct Brian in that I'm trying to mix a bunch of methods. I need to regroup and rethink some things. Thanks. Btw, I did switch to the metric system for guitar making. It truly makes things easier. I did make one error with a conversion that was monumentally dumb but recovered. There's now a handy dandy chart of common conversions on my wall.

-j

The workboard is flat, but there are shims around the perimeter to obtain the domed top.
If you want a 25' radius under tension, the technique would be to glue flat braces in a 25' radius dish.
Actually, Cumpiano's method gets the neck angle with the way he carves the heel. The shoulders are angled so that the mating surface is along the outside edge, and as the heel gets narrower the mating surface is not perpendicular to the neck.

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

If I recall correctly when I built using the method the flat braces were glued first using a flat work board as the backing, then all the arched braces are glued using a flexible backing board (1/8" thick hardwood slat) placed on the outside of the top or back. This allowed the clamping pressure to spread along the braces length, but since it was thin it would bend with the brace and leave the arch in place. It also served to protect the outside of the top or back from clamping marks. Once all the curved braces were glued in place you started using the workboats with the cork spacer.

Thanks Rodger, I knew there was something a bit different (from what I do) with his neck angle method but couldn't remember. Still, the message of caution for mixing methodologies applies.

Jimmyjames, you might be surprised at just how much dome 1/8" of deflection across, a say 15" lower bout, produces. That actually seems like it might be a tighter radius than needed. Go look at the section where he describes how to get the radius for the braces using a spline curve. I think he had something like an 1/8" deflection between 18" or something like that (but go look because I could easily be way off base with my memory). If that is true and considering that the cork would interface with the top inside the lower bout (making it less than 15") his method very well could be using producing a top with somewhat sprung in braces. But like I said, it has been a long while since I read it. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

kjaffrey, I just saw what you posted (after my reply) and that sounds vaguely familiar. I should just shut up about that section of the book since I clearly don't remember!

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Having never read Cumpiano's book I'm probably not in a position to comment, but from what I've gathered from questions asked on here, it seems like a very complicated method of building. If you just put 1/8th inch shims around the edge of the soundboard, the radius will be changing from tail to waist as the waist is closer to the centre of the arc than the tail. You'd then have to try and match that shape when planing the rims, which sounds pretty hard to me.
Why not just make a radius dish? That way everything fits together perfectly and you get a very accurate, predictable result. It only takes an hour or so and a couple of MDF boards. You don't even need to make a load of mess with a router - this method worked fine for me http://forum.ukuleleunderground.com/sho ... adius-Dish
Also go-bars are far cheaper than a load of clamps!

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"I am always doing that which I cannot do, in order that I may learn how to do it."
Pablo Picasso

https://www.facebook.com/FenskeGuitars

Hey Jimmy,
I've read that book at least ten times.
I am honing my woodworking skills before I try an acoustic build.
Sure would like to see some photos of your build!
Thanks,
Dan

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wah
Wah-wah-wah-wah
Wah

Hi Dan,

Well here's a few photos of the workboard in question. I'm not really sure how other people are doing this and well this is what I came up with. It rotates 360 degrees and tilts. My inspiration was a pattern makers vise. I don't know but it seems like being able to rotate the board in every direction might be of some use in the future, like when I do the bindings and such.

The workboard is cedar. I have a veritable motherlode of the stuff.

I'm in the process of making a mold that is the exact shape and slightly larger size of my guitar and has a fence around it so I can drop a top or bottom on it and plane it any direction without a clamp getting in the way. The fence will also make it easy to see high spots sighting across the board.

I want to carve an inverted top and bottom mold but don't yet have the visualization of exactly how to do it. I find the transition to upper bout flatness troublesome. Perhaps after this guitar I'll be able to do it.

To avoid transferring lots of lines and facilitate the accuracy of the sound hole I just made multiple copies of my plan and cut and pasted. The paper didn't like the glue much and in parts it's a tad warbly but where it counts is fine.

I also have it setup to change from a guitar body holder to a neck holder that will allow me to carve the neck and adjust it to any angle. I just made my headblock extra tall to accomodate a screw set. When the neck is finished I can cut that portion off.

I have no idea if my construction ideas will work out. I'm more nervous making jigs and fixtures than making actual items. This is all uncharted territory for me.

-j

I did make (not sure what to call them) 15' and 25' curves in wood. I just got some static cord, tied it to a stable object, measured out the distance, wrapped the line around a pencil and scribed and arc. So I've got a fair idea, well no not really. I'm just really unsure about what I'm doing. I just can't visualize it all yet.

-j

As kjaffrey says, the braces are glued using cam clamps and a flexible backing slat. No workboard involved. The workboard is only to hold the braced soundboard while you glue the blocks and sides to it, taper the rim, and glue on the back. The cork liner allows the domed soundboard to be supported all around the edges while doing that.

I don't like workboards and moulds, so I just assemble the whole thing freestanding on my flat bench. Using dentellones instead of kerfed linings, I only need one point supported at any given time. And I glue the back with spool clamps, so the soundboard doesn't need any dished surface to sit against.

For brace arching, I just mark a straight line along the brace 1/16"-1/8" from the edge, and plane away until it looks like a reasonably smooth curve, using the distance to the straight line as a visual reference for how high the arch is. No templates needed, and different braces can be arched different amounts, or even different curvature along the length of the brace if you want. For example, I make the upper transverse brace flat under the fingerboard, and then curve down 1/32" or a bit more to the rim.

Also, with this style of building, the soundboard rim is flat. This does have tonal implications compared to a radiused rim, plus it shouldn't be used with extreme soundboard curvature (like 12' cylindrical radius that some builders use) since it is sort of forcing two different shapes together.

And lastly, be careful when using an outside mould with the style of headblock described in the book, because he uses a 90 degree angle between the soundboard and sides (and between the neck surface and heel cheeks), but cranks the whole thing back by 1.5 degrees or so to get the neck angle right. So the sides are actually a bit twisted at the upper bout, unlike the modern outside mould/radius dish building style. You an cut the headblock with a 1.5 degree angle, and heel cheeks with matching 1.5 degree angle, to eliminate the need for the twist. Just be aware of what's going on.

I'm with Peter. Radius dish and go bars just make life easier, and get the right result.
C & N wrote a great book which got a lot of us started in guitar making - but it is pretty old now and some of those methods have really been superceded. Mr Cumpiano himself does a lot of things differently these days.

I am an Industrial Engineer by trade -- so the idea that a new process is being formulated prior to actually experiencing, seeing, the expected goal of a tool or procedure seems way odd. There are so many that have "been there done it" --- pretty difficult to "do it right right from the start", when it not even clearly understood what "right is" I may have it wrong but this is what glean from the OP. $.02

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Ken Cierp

http://www.kennethmichaelguitars.com/

It is a bit weird. He just glues on the top to the flat linings. I always had a problem with that but it does work. I like to slant the linings to the radius though.

When I first bought that C&N book, everyone did it that way. Only a couple freaks were experimenting with radius dishes. Most people don't seem to realize that this radius building thing is a relatively new innovation.

Anyhow, I still prefer the flat board + shim method. A dugout workboard does effectively same thing .

Yeah first came the solera. I guess that's where the idea of radius dishes came from. The last several guitars I've built have been dead flat, I just let the strings pull the arch into shape

I did always wonder if that's the way W&N actually build guitars or if they have well made fitted and arched work boards and just came up with the cork shim method for hobbyists. I've built many using that method and they all came out just fine but I've always felt that the cork shim thing is not quite the perfect long term solution. But it certainly works.

I think I remember one poster here who may have taken a class from Cumpiano maybe he'll chime in.

This is exactly correct. Before I do something I get quiet and still in order to visualize what I'm going to do, thinking through each step in great detail. Zen approach if you will. I just can't "see" it all yet and I don't like that. The back seems simple enough since the braces are not angled the same way the top is.

I've also wondered if the early Martin's were made flat and the combination of string tension and humidity slowly arched the top over time. I've heard stories, perhaps apocryphal, that Martin used a heater to bend the tops into an arch and the varying humidity is why there's so much variance in top deflection. Who knows?

Martin has "always" made flat top guitars and yes it is the string tension that creates the top dome. The main braces X and tone bars are contoured to 52' --- this is not a secret, just call the factory. And, it was not until the dawn of the cutaway models (1980ish) that the process included the current sloping procedure. In fact the current 28' rim contouring used by some, is a means to replicate the Martin factory 1.5 degree fingerboard extension slope. The key in all this, is indeed the slope which creates the straight line fingerboard plane and prevents the hump or ski jump problem at the neck/body joint.

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Ken Cierp

http://www.kennethmichaelguitars.com/