Official Luthiers Forum!

Hello All,

I am a high school shop teacher out here in Montana and very amateur guitar player.  I have a couple of students who want to build acoustic guitars.  I have the Kinkead and Cumpiano text as well as a lot of internet research.  For our first guitars I am going to stick with the guitar that Kinkead builds.  We will be using a go-bar and a workboard (radius dish) to glue braces (25’ and 15’).  We have not built either at this point.  I am running into two problems. First off, what radiuses are Kinkeads soundboard and back?  I could go off of his radius dimensions in the book and router out the workboard, however this would make for a ton of chiseling and scraping, as I would only be able to make the router passes at his given points.  I would prefer to build the workboards with a 25’ radius and 15’ radius.  If I do this will it work ok for Kinkeads plan.  Which leads me to the second question?  Where would the center point be on the guitar soundboard and can I still use his radius dimensions for the braces.  I think if someone could tell me the radiuses for Kinkeads guitar and where the center point would be on the soundboard I could get started.  Its all new territory for me so hopefully its clear as to the problem I am running into.  Thanks people.

 

-Jay

 

Hi Jay, don't have a Kinkead book. But welcome, anyway.
Sounds like a neat project for your students. Bruce

_________________
http://www.dickeyguitars.com

Jay, you can use the 25 and 15 radiuses without any problem, but the brace radius needs to match it. If you make radiused dishes or the glue up of the bracing to the top/back, just put self stick sandpaper in the dish and sand the proper matching radius onto the braces. I'm building the Kinkead OM right now, too. I used 28' and 15' because that is what my dishes are.

Ron

_________________
OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

[QUOTE=jsampson] ...and where the center point would be on the soundboard -Jay[/QUOTE]

Hi Jay,

Your workboard (if it is constructed as a typical "radius dish") will be a section of a sphere. So, wherever your student places the top within the workboard, it will have the same radius. I'm not sure if that answered your question or not.

Disclaimer: I am exactly one guitar beyond your students, but I'll offer some other tidbits, knowing if they are wrong, some real luthier here will correct me.

A different way of thinking about the center of the top:
Think of a "flattop" guitar as having a flat top: the top of the neck block and the top of the tail block being in the same plane. If you now create a three point arc, using the edge of the guitar body at the neck and the tail as the beginning and ending points, then the highest point of the dome (arc) would be halfway between those endpoints. Most modern steelstring guitars do not have a perfectly uniform dome shape to the top, due to the attachment of the fingerboard extension.

Because it would be so difficult to undercut the fingerboard extension with a concave arc to match the convex arc of the top (no one does that), the top is usually flattened out a bit under the fingerboard. So, the top starts out as a section of a sphere when most of the curved top braces (such as the X-braces) are added, and then a stout brace (upper, transverse brace) with a straight edge is glued on the top beneath the fingerboard extension, pulling that area flatter.

Hope this helps,

Dennis

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Hi, Jay-
Welcome to the OLF!
I'm a retired high school (science) teacher, and one year in the 80s I was persuaded to teach a guitar-building course as part of the shop program at my school. I'd only built 5 or 6 guitars at the time, but it all turned out OK (amazingly enough).
At the time, the 'best' instructional book available was the David Russell Young book "The Steel String Guitar-Construction and Repair". It might be worthwhile for you to pick up a copy (used on amazon or abebooks, or borrow from library). His method of neck construction (no truss rod) and attachment (epoxy butt joint) are not much used these days, but otherwise it's a good reference.
The reason I mention this book is that it details building a guitar with arched back and soundboard as well, though using clamps rather than a dish and go-bars for brace gluing.. As Dennis has pointed out, it's important to make the transverse brace under the fingerboard flat, or nearly so. Young explains this quite clearly, and obviously you want your top out of the radius dish when you do this brace.

BTW, the Kinkead book is excellent in my opinion- though I've only taken bits of his method for my builds.

Cheers
John

Hi, Jay, and welcome!

A few months ago our own Bob Connor from down under generously put together a whole passel of tutorials here. You might want to bookmark it.

You might have already found builds on the web, but look at Proulx Guitars, wellsguitars, luthierforum (without the s). Many of the builders here have build photos on their web sites. I have a bunch of photos documenting the construction of my total lifetime guitar output of 2.5 guitars. Short on text but long on visuals, might be helpful.

Let us know how it goes!

_________________
now known around here as Pat Foster
_________________
http://www.patfosterguitars.com

Call me this weekend. We have been working with high schools that put together building programs. I am sure we can help you out with information and help getting started.
It is a great opportunity to apply the math and physics to something they can actually touch and feel
john hall
blues creek guitars

Welcome to the OLF !
Another good site is Charlie Hoffmans.

Hoffman

_________________
Anderson Guitars
Clearwater,Fl. 33755

Hey Dennis and everyone else who replied,

Thanks for the help and the great websites to check out. So, it sounds like the highest point (center) of the arch will be at the center of the guitar body.  Correct??? For some reason in all of my research, I thought the arch centered over the sound hole.  It seems that I have read so many different articles, books, and websites on the entire process of building the guitar, that I get lost.  I believe I will slow down and take it one step at a time.  Thanks again people.

-Jay

 

Hi Jay,

Your workboard (if it is constructed as a typical "radius dish") will be a section of a sphere. So, wherever your student places the top within the workboard, it will have the same radius. I'm not sure if that answered your question or not.

Disclaimer: I am exactly one guitar beyond your students, but I'll offer some other tidbits, knowing if they are wrong, some real luthier here will correct me.

A different way of thinking about the center of the top:
Think of a "flattop" guitar as having a flat top: the top of the neck block and the top of the tail block being in the same plane. If you now create a three point arc, using the edge of the guitar body at the neck and the tail as the beginning and ending points, then the highest point of the dome (arc) would be halfway between those endpoints. Most modern steelstring guitars do not have a perfectly uniform dome shape to the top, due to the attachment of the fingerboard extension.

Because it would be so difficult to undercut the fingerboard extension with a concave arc to match the convex arc of the top (no one does that), the top is usually flattened out a bit under the fingerboard. So, the top starts out as a section of a sphere when most of the curved top braces (such as the X-braces) are added, and then a stout brace (upper, transverse brace) with a straight edge is glued on the top beneath the fingerboard extension, pulling that area flatter.

Hope this helps,

Dennis[/QUOTE]

The trick to making radius dishes is to mount the stock on a lazy susan, and run the router on a sled in a track of the right radius over it. Once you see it, it's so obvious that you'll kick yourself if you made one the hard way. I did.

There has been a lot of discussion on line about where the high point of the dome is, or should be. Suffice to say it's not written in stone. You can get the neck, fingerboard, and bridge geometry to work out right with or without a fully domed soundboard, but sometimes you end up having to shim or trim the fingerboard extension.

I've had several high school kids as students over the years. A couple of them have been duds, but the good ones have been a real pleasure to work with. One of them has since turned pro, gets higher prices than I do, and has sold instruments to some major names. Maybe I did something right.   

Hesh, my friend, you said

You can make yourself a template for these two respective arcs simply by fixing a string either 15' or 25' respectively at one end to a fixed point and then attach the other end to a pencil and draw the arc.

Conceptually, you are perfectly correct. But, have you ever really tried doing this? String is pretty stretchy stuff. At least the string that I tried was. And at 25', it also tends to drag on the ground. Holding the pencil perpendicular with the line taut was more than I could manage. The result; no two lines were the same. My field trial was a bust. 

I ended up using a little math, plotting a few points, and using a flexible plastic ruler to connect the dots and profile a 25' radius - and then bought the form from Roy Noble.

Life is short.

Larry

 

 

I doesn't matter if your 15' radius dish is not a true 15', it could be 15.5' or 14.5 feet, it matters not. As long as it is the reference point for all "15'" radius parts of the guitar. Unless made on a CNC machine I doubt that many of the dishes we make ourselves are exactly what they pretend to be.

I'm sure my 15' and 25' dishes are not exact, but they work as long as the radius is consistent in each dish.

Colin

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

[QUOTE=LPMc]Conceptually, you are perfectly correct. But, have you ever really tried doing this? String is pretty stretchy stuff. At least the string that I tried was. And at 25', it also tends to drag on the ground. Holding the pencil perpendicular with the line taut was more than I could manage. The result; no two lines were the same. My field trial was a bust. [/QUOTE]

I drilled a couple of holes in a tape measure.  put a nail through one in the grass at the edge of the driveway, pencil through the other and using a piece of plywood on the driveway, drew an arc on poster board, and on a piece of MDF.  Seemed to work pretty well, after getting things lined up.  You have to align the edges first, or have a piece way longer than you need and just cut to length when done.  This eliminates the stretch thingy, and since the tape measure is curved a little, makes it glide over the surface pretty well.  I don't think if you're off by 1/8 or so that you have created a problem. Bending a rod with a known deflection does not create a perfect arc either.  If you don't have a driveway, use the street, preferably not the interstate.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Ok People,

I'm confused again in regards to routing out the workboard (raduis dish) which will be used in the go-bar press.  My original confusion was where the highest point of the sphere would be on the guitar soundboard.  Dennis you say it would be centered between the neck block and tail block.  I'm good with that. If I look at the dish in Kinkead's book on page 56, it shows a great picture. But wouldn't the dish shown have to have had two different center points when it was routered out.  One near the bottom and one near the sound hole.  I was under the impression that I could just router out one sphere and then center the soundboard at the highest point of the sphere when we clamped it in the go bar.  If did router one big sphere, the dished portion would not reach all the way to the neck.  I hope thats clear. Thanks.

Your workboard (if it is constructed as a typical "radius dish") will be a section of a sphere. So, wherever your student places the top within the workboard, it will have the same radius. I'm not sure if that answered your question or not.

Disclaimer: I am exactly one guitar beyond your students, but I'll offer some other tidbits, knowing if they are wrong, some real luthier here will correct me.

A different way of thinking about the center of the top:
Think of a "flattop" guitar as having a flat top: the top of the neck block and the top of the tail block being in the same plane. If you now create a three point arc, using the edge of the guitar body at the neck and the tail as the beginning and ending points, then the highest point of the dome (arc) would be halfway between those endpoints. Most modern steelstring guitars do not have a perfectly uniform dome shape to the top, due to the attachment of the fingerboard extension.

Because it would be so difficult to undercut the fingerboard extension with a concave arc to match the convex arc of the top (no one does that), the top is usually flattened out a bit under the fingerboard. So, the top starts out as a section of a sphere when most of the curved top braces (such as the X-braces) are added, and then a stout brace (upper, transverse brace) with a straight edge is glued on the top beneath the fingerboard extension, pulling that area flatter.

Hope this helps,

Dennis[/QUOTE]

jsampson wrote:
"Because it would be so difficult to undercut the fingerboard extension with a concave arc to match the convex arc of the top (no one does that), the top is usually flattened out a bit under the fingerboard."

My students and I use a fully domed top, and fit the fingerboard extension to it. It takes some time, but it can be done, and it's no different from the usual fitting step on classical guitars with up-set necks.

Start out with the sides bent such that the upper edge, which was staight before you bent them, will now sit flat on the bench top. In this case, all of the curves are cylindrical sections, and the sides are perpendicular to the bench top everywhere when they are sitting top down.

Dress off the upper edge to fit the dome in such a way that you take the same amount of material off both ends (neck block and tail block) of the top edge. The easy way to do this is to put the rim set in the outside form on the dish, with both ends touching, and scribe around it with a pencil at the height of the biggest gap between sides and the dish surface, then trim to the line. In this case the high point of the dome will be halfway along the length of the top.

Now take a similar set of sides and put them on the dish, top down, but this time shim the neck block end up off the dish by 5mm or so. Scribe around in the same manner, and trim to the line. Now you've taken 5mm more wood off the height of the side at the tail end than at the neck end, so that, overall, the dome is sloped toward the tail a bit. This moves the high point up toward the neck block, since raising the neck end is the same as dropping the tail.

This is the way I do it. Romanillos points out that this essentially puts the bridge further 'downhill' from the high point of the dome, and this seems to add some resistance to dishing the top.

If you think about it, it's just what you do on the back when you taper it, only the other way. With the neck block 1-1.5cm lower than the tailblock the whole back slopes toward the neck when you set the guitar on it's top on the bench. If the sides are perpendicular to the bench top, the high point of the back will be 'way down in the lower bout someplace, and quite near the tailblock. If it's easy to see this, why is it so hard to visualize the high point of the top dome as being somewhere other than the center point of the length of the top?       

Hello everyone,

I'm posting this second question again because I think I messed up when I posted it.

I'm confused again in regards to routing out the workboard (raduis dish) which will be used in the go-bar press.  My original confusion was where the highest point of the sphere would be on the guitar soundboard.  Dennis you say it would be centered between the neck block and tail block.  I'm good with that. If I look at the dish in Kinkead's book on page 56, it shows a great picture. But wouldn't the dish shown have to have had two different center points when it was routered out.  One near the bottom and one near the sound hole.  I was under the impression that I could just router out one sphere and then center the soundboard at the highest point of the sphere when we clamped it in the go bar.  If did router one big sphere, the dished portion would not reach all the way to the neck.  I hope thats clear. Thanks.

I don't have Kinkead's book, but if this helps then great. My radius dish is just that. A dish of a 25' radius for the top, and another dish of 15' for the back. It's 24" across made up of 2 layers of 3/4" MDF and has 80 grit sandpaper used for large floor sanders glued to it.

The body of the guitar easily fits inside this size of dish. I radius the top edge of the rims first. I rub a piece of chalk of the edge of the rim and then rotate the rim (still in its mold) back and forth in the dish. Checking regularly to see where there is chalk left. Sometime a little bit of a block plane here and there and some more chalk helps speed things along. Since the  dish is just part of a sphere the top of the guitar is going to end up being just part of a sphere as well.

I think what may be confusing you (if I understand you correctly) is that the bridge may visually end up being at the "highest point of the sphere", but in reality every point would be the highest point. It just is a matter of perspective. I just lay my top, or back for that matter in the middle of the dish and glue my braces.

For what it's worth, I don't use the radius dish to glue up the braces in front of  the sound hole. I make them flat and just use a flat piece of MDF as a backer while glueing on those braces. I read somewhere that it made fitting the fret board easier than dealing with the radius there. It worked and I haven't tried any other way as of yet. There are lots of ways to do this, and I'm sure you will have read all kinds of variations of gluing up the sound board.

_________________
Allen R. McFarlen
Barron River Guitars & Ukuleles
Facebook
Cairns, Australia

Jay,

OK. You've got me, a guy who has completed one guitar, and Alan Carruth, a true legend in lutherie and especially left-brained lutherie, giving you advice. Which of us will you pick?

Pick me! Pick me! {hehehehhe}

John from Blues Creek Guitars is also willing to help you out with developing proven methods that work for high school students.

So, you'd be nuts to listen to me.

I don't have Kinkead's book, but I know that a very few luthiers use a cylinder rather than a radius for either the top or top and back. Perhaps that's what you're seeing in Kinkead's book? I think most luthiers nowadays use either a pair of radiused dishes for the top and back, or use a flat workboard ("solara") and place shims (or even a guitar-shaped shim) beneath the rims/sides of the box when building, to lift the sides so only the tip of the dome touches the workboard.

As for what Alan and Allen are saying about the position of the bridge relative to the "high point" of a sphere, remember that the bridge will probably not be in the middle of the guitar (half way between the neck block and tail block); its position will depend on the scale length chosen and which fret you say will align with the neck/body joint (12th or 14th, or rarely 13th fret.) So the bridge will more than likely not be at the pinnacle of the dome, but behind the pinnacle.

You and you students will probably pick a guitar to "copy", for example a Martin dreadnaught with a 25.4" scale length, joining the neck/body at the 14th fret. It just happens that the 14th fret at that scale length is about 14" from the nut, so in that case the saddle will be about 11.5" away from the 14th fret body/neck joint.

(This is a bit simplified, but...) after the body/box is built, you and your students can determine the neck set angle that will be required with each box, to get the string action reasonable, using a typical thickness of bridge and height of saddle. In other words, if the top's dome is a little higher or little lower than the plan, you can compensate with the angle you cut in the neck where it attaches to the body.

You may want to pick up yet another book on constructing guitars, "Guitarmaking: Tradition and Technology" by Natelson, which I see for $17 used at Amazon. It may help you and your students visualize the process.

And with that, I think I should drop back and let one of the experienced luthiers guide you from here.

Best of luck in the class!

Dennis

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Oops, I now see in your first post you already have that other guitarmaking book.

Dennis

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia