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HI -

I know I have seen this question here - and I searched - but I do not have an answer still....

I am building using the Cumpiano book - I am curious if I use his method to create a radius in my brace then glue the sound board to it - what radius will my sound board be ? He puts two pins at a distance then pulls the center in a bit with another pin to get a radius on a piece of rod that is stretched between the pins. So does any one know what this radius equates to in real numbers ?

I want to make a radius template using some online calculators - should I use 15' for top and back or 25' ? or what ? I don;t want to mess up the radius so that all the other dimension form the Cumpiano book are "off" - does this make sense ?

Thanks in advance !
John

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It's this new idea from recent decades that everyone gets a participation award. - MUX

I've seen this done a few times when doing research into radii myself.

If anything like what I have seen, it is probably done completely by eye. A jig I have seen has a line on the workbench so he can pin the brace in the same place every time. He has another line for back bracing. There are a few people who will get a block plane and again just plane a radii to eye, must be years of practice to know what you are doing I guess, as my attempts end up in a doorstop .

I am using Cumpiano too (still not done with my first) and when I put the curve in the braces it was mostly done by eye. I used Stephen Boone's little mini plane and shaved off the ends until I got it to about where Cumpiano says it should be. I would put the brace curved side down on the table and look at how much space I had on each end between the table and the brace, and when I got to where it was supposed to be I sanded it a bit to smooth out the curve and that was it.

It seems to have ended up ok that way.

I don;t thik I will have a problem making the brace the CUmpiano way or any other way -

My question really is - what is the radius of the Cumpiano brace for classical, I doubt he even knows or cares to, but some math wiz should be able to figure out a number to put on it? It would be useful to know so you can make templates and dishes and still stick to the plans in the book with little variance.

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It's this new idea from recent decades that everyone gets a participation award. - MUX

Have you seen how Mario does his? I think Alan Carruth has a simular way too.

John-
The 'depth' number is called the Sagitta of the arc..
R.M. Mottola has provided the info you need and a calculator to do the math at:
http://liutaiomottola.com/formulae/sag.htm

Mottola's website has lots of good resources- worth browsing.

Cheers
John

Thanks everyone - I have no problem make the arcs - yet (that's next week's question)
I was wondering what John had answered for me - thanks ! the Sagitta

John A

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It's this new idea from recent decades that everyone gets a participation award. - MUX

The top and back aren't any shape remotely close to a spherical dome... If you look at his instructions, some braces are left flat, and others are curved... He also springs braces into the plates -- which leaves you with a surface that doesn't match the curve you started with anyway....

Why bother worrying about what exact Radius would reasonably correspond to his different fair curves? Make yourself a fair curve template according to his specific instructions and go with it.. It takes about 3 minutes to make up one of those templates with a metal ruler, some tacks, and a pencil.

Here's what you aren't thinking of....

There are a whole bunch of different measurements that all have to be about right for the string height and the action to be in the right range.... Top dome, neck set angle, fretboard angle/shape, bridge size and position, etc..... and there are a million little things that effects this... Say you use a 25' radius instead of his offset -- you put more radius into it than he built, so the bridge sits too high..... Now, you gotta change the way you profile your fingerboard or your bridge height.. but how much? Say you radius one of the transversal bars instead of leaving it flat... Same thing -- you change those relationships...

When you go changing stuff -- you mess up those spatial relationships... and you end up with a bunch of extra work and tail chasing at the end of the build when you are trying to get everything to come together properly...

and when you change the bridge height -- you are going to change the bridge weight and stiffness... which makes a pretty large change to the sound balance of the instrument...

Don't over think it too much -- just try following his instructions this 1st time... If you don't like it, you can always change it on the 2nd....

Thanks

John

Truckjohn (John Cox) makes a good point about following a plan/method/book.
Though Cumpiano&Natelson wouldn't be my choice for a classical building book, you should stick to that method if you have started with it.
Most classicals don't use a 'fully domed' soundboard- the doming is usually just in the lower bout area. So instead of a radius dish, a 'solera' with a hollowed area is used. Building on a radius dish will probably cause action/geometry problems unless you really plan ahead.

Stephen Boone has some good pics of making a solera in the 'Flamenco video' thread- he uses his 25' radius template for part of the shaping. Good read.
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=27124

Cheers
John

Thanks guys - good advice by all.

I like the Cumpiano method - straight forward and simple -

Thanks !

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It's this new idea from recent decades that everyone gets a participation award. - MUX

another side point about following books.

i, like a dummy, was building from cumpiano's book and i had an LMI dread plan too. I got into some wierdness dimensionally. Lesson learned: if you're going to follow a book, follow the WHOLE book, and nothing but the book. It'll seem difficult at times.

One drawback of cumpiano's book is that, on a very rare occasion, the pictures don't quite show enough to really get an idea of what the text is saying. Maybe if i was smarter....i dunno. Like where the scarf joint is cut and how, and then how the headstock is reglued on. I got pretty confused there. His website clarifies alot of those little quirks. especially the heelblock dimensions. make sure you read that online.

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brendan 3.0 new and improved, with better interpersonal subroutines

Alan's technique is very different, although it could be used with a shooting board to keep the arch perpendicular to the side of the brace.

Alan puts a small shim on the back of his smoothing plane. The shim has the right height and it is placed far enough back from the blade to cut the arch he wants. A bit of super glue holds the shim ot the plane. He has a couple of marks scribed on the bottom of the plane that mark where to put the shim for top or back braces.

Thanks John, I knew it was something to do shims and a plane.

I have a really easy method that makes 4 perfectly radiused braces in 5 minutes (or so).

First, take a quartersawn board the length of your longest brace and the height of your highest brace X 4 times the thickness of the 4 braces plus 3 saw kerfs and a little more.

Then bandsaw and sand (I use a 6 X 89 sander with a perperndicular table) the radius on the blank.Make sure to keep the edge at 90 degrees.

Then simply cut the braces to the desired width on the bandsaw and clean up the bandsaw marks with a drum sander or by hand .(I send mine through my wide belt sander).

This has been the quickest,most accurate method, I have found.

If I can remember, I will photograph the process next time I do it.

Brad

The plane looks something like this with the shim on it:



It works and it's cool. I can't say that I truly understand exactly how it works- if someone wants to show the math I'd love to see it.

It sure seems that someone should sell an attachment like this for a plane to turn it into a convex surface planer.

I don't actually use this method any more, I find a shooting board- the method shown above, works really well.

John

Lately, I don't see much notice of Jon Natelson (co-author with Cumpiano) ; I think he had a lot to do with the classical guitar sections of the book (the dust jacket describes him as a classical guitar builder). So perhaps it should be 'the Natelson method' or perhaps you are just referring to the brace shaping info ?

In any case, it would be a good idea to print out the corrections listed at
http://www.cumpiano.com/Home/Book/textbook.html
and keep them with your book, if you have an earlier edition.


Cheers
John

I did not find the book a good "method" for building a guitar. Too many short cuts, and too much back and forth between SS and Classical, for easy understanding. It's probably true of most books, including Courtnal's book, but I liked it better. I think John must be right about Natelson, too. Frankly, I've never heard anyone rave about the great sound of a Cumpiano classical guitar. Maybe I just don't travel in the right circles, but word does get around when guitars are exceptional.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Yup- my opinion also. But, it gets recommended all the time....it's a mystery to me. (is it a 'Made in USA' thing???)

For classicals a good choice IMO is Rik Middleton's 'Guitar Makers Workshop' but it doesn't get mentioned often.
http://www.amazon.com/Guitar-Makers-Workshop-Rik-Middleton/dp/186126707X/

Courtnall is OK, but expensive, and I find the omissions from the 'plans' in the book irritating.

Cheers
John

Probably because there wasn't any other books out on the subject then. It does have a lot of good info in it for the beginner and shows lot's of info. There didn't seem to be too much out there 10 or so years ago.
A dish also puts the radius on the perpendicular as well where as a plane doesn't. Although it really doesn't matter in the real world.

I like how simple Mario's method is. But I wonder how many times he has taken off part of a finger tip with the fingers holding that thin brace as he zips his plane back and forth? It does not look safe enough for me (my hold down finger(s) may slip - ooch!). Maybe clamping the brace down to do the planing would be my ticket.

Ed

....

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It's this new idea from recent decades that everyone gets a participation award. - MUX

waddy, i agree. they should have either issued it in two volumes: steel string vol 1 and classical vol 2, or split the book into two distinct sections in one volume, each section having a materials list etc.

but who am i to say? I was at his shop last summer having a guitar repaired by Harry Becker, who basically is the main reason i started building. The shop is super cool. Old school. Cumpiano was out of town but it was a great experience. I was thinking of asking if i could sit quietly in the corner while they work one day....

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brendan 3.0 new and improved, with better interpersonal subroutines