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Doming the top - where’s the peak?
http://www.luthiersforum.com/forum/viewtopic.php?f=10102&t=9910		 Page 1 of 2
Author:  Mark Tripp [ Wed Dec 20, 2006 2:34 am ]
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For those that build with a domed top, where do you place the peak of the dome? At the intersection of the X-brace? At the bridge location? Elsewhere? Any theory behind the placement? Just something I've been wondering about for a few days.

-Mark
Author:  Dave White [ Wed Dec 20, 2006 2:49 am ]
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Mark,

I've had my wrist slapped on this subject before. If it is a spherical surface then technically there is no "peak" as wherever you are on the surface, every other point drops away from you - think of a ball.
Author:  Mark Tripp [ Wed Dec 20, 2006 2:50 am ]
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Hesh:

I do my my braces in a jig with a pattern routing bit. I've been placing the x-brace intersection at the apex of the dome radius (25') when routing. What if I decided to shift the apex on the X-braces even with the bridge location? Wouldn't this change the relative location of the apex of the top itself? Am I just looking at this completely catywompusly?

-Mark
Author:  JohnAbercrombie [ Wed Dec 20, 2006 2:59 am ]
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If the workboard is truly hemispherical, there really is no 'peak'...it's like asking where the 'peak' is on a basketball- so it wouldn't matter where you placed the soundboard.

I suspect with my home-made workboards it's safer to follow Hesh's advice and keep the soundboard centred. With the slight curve in most tops, it probably doesn't make a huge difference by the time it is all finished.

John
Author:  Michael Dale Payne [ Wed Dec 20, 2006 3:02 am ]
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The vertical quadrant of the dome. (you can call it the peak if you wish as far as I am concerned) is really up to you in theory, Where you place it will affect the bridge height to neck/neck block intersection relationship. I set mine on my SJ size 2" back from the centerline of the waist radius. This gives the proper incline for a 1/4 thick fretboard plane to be between 1/16" to 1/8" above a 3/8" thick bridge at the center of the saddle slot on a 25.4 scale. This position will change according to scale length and body shape. Locating it otherwise will not mean that it is located wrong. It just means that other considerations will need to be made to achieve the needed fretboard plane/ bridge relationship.MichaelP39071.4642013889
Author:  Mark Tripp [ Wed Dec 20, 2006 3:06 am ]
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Dave:

I think I remember a thread about this sometime ago. I'll try to explain my thinking, but might have to draw something here. If I have a radius dish I am building with - to glue my braces using a gobar deck, and I place the intersection of the X-brace dead center of the dish, assuming I also shaped the braces with their apex at the intesection, when I remove the bars and flip the top, the apex of the dome will be at the X-brace intersection. (Is this correct?)

Now, if I shift the braces when I shape them, creating the apex even with the bridge location, AND when I place the top in the dish to glue the braces, I shift it so the bridge location is now dead center, won't that cause the apex of the dome in the top to shift also? (Relative to the length of the guitar body.)

Certainly not trying to be confrontational, just trying to understand.

-MarkMark Tripp39071.4647106482
Author:  Mark Tripp [ Wed Dec 20, 2006 3:10 am ]
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Thanks, Michael, this is what I was wondering...

-Mark
Author:  letseatpaste [ Wed Dec 20, 2006 3:11 am ]
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There is a definitely a peak, I got into a tedious geometry argument over on mimf a couple years ago about it before I had put a top on a guitar. I'm pretty sure I was on the right side of the math/geometry, but practically speaking I think it doesn't really matter. The easiest thing, I think is just to make the tops of your tail and neck block at the same height, and then the apex lands at the center.
Author:  Mike Mahar [ Wed Dec 20, 2006 3:18 am ]
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You can move the effective "peak" of your dome by tapering the sides. In the first of two images that follow, the top is mounted to the side such as to make a 90 degree angle from a line from the head to the tail. This put the peak half way between the head and the tail. In the second image, the top is slightly tapered. This moves the peak further back towards the tail and also make for a larger neck set angle.

I try to make my guitars like the first image and let the dome determine the neck set angle. I flatten the upper bout so that the fretboard extension lays flat.

Author:  letseatpaste [ Wed Dec 20, 2006 3:18 am ]
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Doesn't matter where you sand or glue stuff in the dish... then it's all the same because it's the same radius everywhere in a sphere. The difference is in how you profile your sides that sets up the orientation of the top to the rest of the guitar.
Author:  Michael Dale Payne [ Wed Dec 20, 2006 3:19 am ]
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When you radius brace a top, you turn that top into a segment of a given sphere. You can then rotate that segment to any point around the center of that given sphere. Nothing in the geometry of the top changes. However if you offset the two spheres and then rotate the top around the center of the given sphere (The sanding bowl) you affect the rise/fall of a theoretical tangent between the now two separate spheres. Doing this is what I was talking of in my last posting.

Mike showed this well in his example sketchMichaelP39071.4735648148
Author:  Mark Tripp [ Wed Dec 20, 2006 3:33 am ]
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Thanks all! I get it now thanks to the pictures and great explanations.

Thanks again.

-MarkMark Tripp39071.483912037
Author:  Dave White [ Wed Dec 20, 2006 4:18 am ]
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Mark,

I hope I didn't come across as being confrontational - I didn't mean too. Your sanding dish is - I suspect - part of the surface of a sphere (usually done with a 20-28' radius for the top). By putting the X brace intersection at the centre of your dish you are giving most support to all of the braces keeping the surface spherical. If you moved to the edge of your dish with the X brace intersection, the surface is still a sphere but a lot of the bracing/top will not fall on your sanding dish and it is hard to glue it

If you just glue X braces to your top with the intersection in the centre of the dish and then when dry take it out. Hold the top so that the X brace intersection is at the highest point and put a straight edge on the top here. This will be a "peak as the top will drop away on all sides from here. Now rotate the top so that the X brace intersection moves down (say as if it were moving towards the bridge). Now put the straight edge on the new part of the top that is highest. Surprise surprise, this is now a "peak" and all points fall away from here with the X brace intersection below it.

I think when Hesh talked about the geometry "relative" to the plane of the neck fingerboard he put his finger on it (so to speak). Tops don't end up domed - they are not braced evenly everywhere and as Mike pointed out the way you shape the sides and fit them changes the tops surface. Plus some people deliberately try to keep the top surface flat above the soundhole. They end up as comples curved surfaces.

Hope this helps
Author:  Michael Dale Payne [ Wed Dec 20, 2006 4:36 am ]
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IMO when you glue up the bracing to the top. You create a segment, a portion of a given sphere. It knows no relationship to the taper of the rim at that point. Like Mike showed in his sketch when you taper and where you start the taper of the rim affects the cord of the sphere segment. By tapering the rim you define a rotation of the top around the axis of the theoretical sphere by rotating the cord line off the true horizontal or zero "X" plane

I may have miss read Marks question. The only time that you affect the where the peak of the dome will be is when you taper and dome sand the rim. That is when the angle of the sphere segments cord line is defined relitive to zero "X" plane.MichaelP39071.5302430556
Author:  Steve Saville [ Wed Dec 20, 2006 5:02 am ]
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I think the peak could be defined in relation to either the plane that the fingerboard is on or in relation to the sides. In those terms there is a peak.

My question is this -
Is it best practice to establish this peak directly under the bridge?
Author:  crazymanmichael [ Wed Dec 20, 2006 5:09 am ]
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i try to have the "peak", and the x-brace intersection about halfway between the front of the bridge and the sound hole; it helps reduce the bridge bulge and corollary sound hole dip. varies a bit between styles based on string tension.
Author:  Mark Tripp [ Wed Dec 20, 2006 6:02 am ]
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Dave:

Oh no, I didn't take your comments at all that way - was just trying to grok the geometry of it all, and didn't want my follow-up questions to be misinterpreted. Thanks for your explanation and everybody else too! Very helpful!

-Mark
Author:  Todd Rose [ Wed Dec 20, 2006 6:07 am ]
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If you look at Mike's two drawings and imagine the neck attached to each of those two bodies, you'll see that you'd have to tip the neck down on the bottom drawing in order to have the fretboard tongue lay flat on the top and have your neck angle right. Now, imagine that neck attached and then lift the end of the neck back up to where it's parallel with the one imagined on the upper drawing. What's different between the two? Nothing about the "peak" of the top or the relationship between the fretboard and the top. What's changed is the geometry of the sides in relation to the rest of the guitar.

The only way to really give yourself substantial leeway for variation in the relationship between the fretboard and the top, or where the top's "peak" is in that relationship, is to use an "elevated fretboard" design.Todd Rose39071.9258680556
Author:  Shane Neifer [ Wed Dec 20, 2006 6:12 am ]
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I have to agree with Steve. He is thinking the same as I. The top developed from a dish is a section of a sphere. There is no 'apex' of a brace by shifting the centre, you will just end up with one end thinner than the other as the curve of the brace remains a "fair" curve and consistent along its entire length. The "apex" on a guitar built with a radiused dish is really determined by neck angle, the relationship of the neck and strings to the bridge. Changing scale length will change the neck angle if you want the same the string/bridge heigth. As does going from 12 to 14 fret etc. Non of these things work in isolation, they all have a bearing on each other and all require consideration when developing your designs. I am speaking from my design sample size of ONE!

Shane
Author:  Wade Sylvester [ Wed Dec 20, 2006 6:26 am ]
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Correct me if I’m wrong but shouldn’t we be thinking of the dome apex placement in relation to bridge and fret board angle?
Or, Todd, is this what you mean?

After playing with Mike’s Dome drawing, it appears the bridge and fret board (as a unit) also wraps or folds along the sphere of the dome. I guess if you have done it right, you would always have a slight apex in front of the bridge relative to the bridge and fret board angle in both 1 and 2. Does this seem right?
Maybe I’m missing something?

Shane, I did'nt read your post. I guess we are all thinking along the same lines here.
Thanks guys for a great discussion!

Wade
Wade S.39071.6210532407
Author:  Michael Dale Payne [ Wed Dec 20, 2006 7:02 am ]
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[QUOTE=Wade S.] Correct me if I’m wrong but shouldn’t we be thinking of the dome apex placement in relation to bridge and fret board angle?
Or, Todd, is this what you mean?

After playing with Mike’s Dome drawing, it appears the bridge and fret board (as a unit) also wraps or folds along the sphere of the dome. I guess if you have done it right, you would always have a slight apex in front of the bridge relative to the bridge and fret board angle in both 1 and 2. Does this seem right?
Maybe I’m missing something?

Wade
[/QUOTE]

Yep you are seeing right but keep in mind that in your example 1, the body is is the same depth at both ends. This puts the cord of the segment at zero in the "X" plane and equal to the length of the body. Example 2 the cord is rotated.MichaelP39071.6270023148
Author:  Michael Dale Payne [ Wed Dec 20, 2006 8:10 am ]
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As you can see with the arc's cord held at ZERO degrees in the "X" axis, the peak or "Y" axis quadrant of the dome is well forward of the bridge on my SJ.

This is for a 25.4 scale 19-15/16" long body with a 25' radius top. and achieves a 1.5 degree neck angle with little sanding.. It does require some sanding but not much.

Remember the side taper is on the back not the top.
Author:  Wade Sylvester [ Wed Dec 20, 2006 8:35 am ]
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I see!
Thanks Michael!
Yes and the sides are tapered for the back not the top. That would have made it tough to get a propper neck set.
You start to see how everything is interrelated.
I guess if it were easy, everyone would be Luthiers.
Thanks,
Wade
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