"Flat tops" vs arched bracing Lead

215 Pages·2009·7.82 MB·English

by  Mike Miller

Author Comment
Steve Frost "Flat tops" vs arched bracing Lead [-]
Tags : None
I've been under the impression that Gibson always used a radius on their top braces, while Martin originally
used straight braces to produce a true "Flat-top", until at some point they switched over to the current 28'
r adius top. I've posed the question before on other forums, and never got a definitive answer. What's the real
story on this?
(01/23/07 3:08
PM)
Re: "Flat tops" vs arched bracing #1 [-]
I believe Gibson started radiusing their top braces around 1935. I also believe that Martin has always used
Arnoldgtr
radiused top bracing, but is is difficult to determine exactly how much. Over the years, the combination of
shrinkage and string tension tends to flatten the tops in the soundhole-bridge area.
Also, Martin has always clamped the top braces flat, relying on the resilience of the spruce to spring back into a
curve once the gluing is done.
One other change that Martin has done in recent years has to do with the way the sides are profiled where the
b ack joins. Today, you see a lot more longitudinal curvature of the back, resulting in deeper sides in the waist
area. This is a 'modern' guitar building technique, where the profile is generated with a large radiused sanding
(01/23/07 5:55
dish.
PM)
Although a back with a constant spherical radius is easy to generate, it is not traditional.
John
Steve Frost Re: "Flat tops" vs arched bracing #2 [-]
Thanks, John- as I said, it was an impression, and I have no idea where I might have gotten it. I built my first
guitars using Irving Sloan's and David Russell Youngs books. Sloan used a truly flat top, and Young advocated
the domed top. I know I'm not telling you anything new, but mention it just for general illumination. Sloan
seemed to be traditional, versus some of Young's more "radical" ideas. So, perhaps I assumed that the dome

was the modern method. Thanks for the information! Does anyone have a figure for the Martin arch radius?
(01/23/07 8:54
Just curious.
PM)
BHguitars Re: "Flat tops" vs arched bracing #3 [-]
Quote:

I believe Gibson started radiusing their top braces around 1935

(01/24/07 2:56
Correct. While those pre '35 had a less arch the typical Gibson arch started with the beginning of the
AM)
codeletters meaning in 1935. But the pre' 35 Gibsons already had an arch although this did not include the
Author Comment
important place around the sound hole and above the bridge.
Quote:

I also believe that Martin has always used radiused top bracing, but is is difficult to determine exactly how
much

I double this basically also but in my opinion Martin gained top arch more in using humidity and moderate heat.
While humidity always was (and still is) a big matter especially while bracing a guitar at Martin the tops were
stored dry and put next to the heating stove before braced. The top grain shrinks before the braces are glued
on first. Now when normal humidity returns the grain swells back somewhat and results in a slight top arch.
Additional this treatment prevents from future dryness cracks. This technique is old and widely used at
manufacturers (even at cabinet making these days) working with hide glue. The Spanish guitar makers at
example hang up their tops before they are braced on a line right under the ceiling near the heating stove
where the dehumidifying is working the best.
I have noticed that Martin tops have a different top arch depending on the season they were build. While
winter guitars have a better arch summer guitars mostly are flat and show more often a subsidence between
bridge and sound hole. This is also an indication for the "humidity" arch technique rather than the radiused
brace technique at old Martins. I'm sure that Gibson heated or dried down their tops as well additional.
Quote:

Today, you see a lot more longitudinal curvature of the back, resulting in deeper sides in the waist area. This is a
'modern' guitar building technique, where the profile is generated with a large radiused sanding dish.

This technique is a result of mass production. While old Martins mostly had a nice uniform back arch with
raised waist Gibson didn't pay attention to the waist area of the back. The result is a less longitudinal arch in
general but not uniform as consequence. At the waist there is much more arch crosswise while the lower bout
shows very little arch only.
But surprisingly the back arch at nicely arched '30s Martin backs is crosswise nearly the same than lenghtwise .

Willi
Steve Frost Re: "Flat tops" vs arched bracing #4 [-]
Were these Martins built with their kerfed linings beveled slightly to facilitate joining the arched braces to a
level rim? Trying to get an image of this. Is that what's going on with the Huss & Dalton TDR series?

(01/24/07 10:49
PM)
Author Comment
Re: "Flat tops" vs arched bracing #5 [-]
Quote:

Were these Martins built with their kerfed linings beveled slightly to facilitate joining the arched braces to a
BHguitars
level rim?

Steve,
I'm not sure if I understand correct. Do you mean if the kerfing matches the arch of the braces and the arch of

the back? Yes. When the kerfing has been glued to the sides and are levelled for the back they will follow the
(01/25/07 2:33 back arch. Same is with the top side kerfing and with the top bracing. Also the main braces will be tucked or
AM) mortised into the kerfing with an exact cutout for the braces. The back of the kerfing could be left with its glue
surface to the sides. This way the brace ends are locked in and won't get loose. The brace ends shouldn't hit
against the sides directly since when the arch will move down a little because caused by low humidity you
wouldn't want to find the braces to be pressed against the sides.

Willi
Re: "Flat tops" vs arched bracing #6 [-]
Quote:

But surprisingly the back arch at nicely arched '30s Martin backs is crosswise nearly the same than lengthwise .

Arnoldgtr

Willi, that has not been my experience. I use side profiles from prewar Martins, and they don't have enough
'rise' in the waist area to produce a spherical arch. I have checked the back arches on many prewar Martins,
and I have yet to find one that has a true spherical arch. They generally have the most curvature across the
waist area, less curvature across the lower bout, and even less curvature longitudinally.
If you look at modern Martins, you will see what I am talking about. Virtually all modern Martins have a much

more pronounced longitudinal curve in the back when viewed from the side. This is because the sides are
(01/25/07 3:22 deeper in the waist area, necessitiated by the increased curvature produced by the concave sanding disc.
AM)
If you don't have a new Martin in hand, you can look at the side views of new Martins on the Martin website.
Martin used to bevel the kerfing to match the back curvature by using a block plane. An arching template is laid

across the kerfing to 'eyeball' the arching angle. I still use this method, because I don't believe in the spherical
arch, for several reasons.
Here are the radii that I use:
Top = 25 feet
Back, longitudinal = 20 feet
Back, lower bout (wide braces) = 18 feet
Back, waist brace = 15 feet
Back, upper brace = 25 feet
Author Comment
This produces a 'fair curve' when coupled with older Martin side profiles. The back is fairly flat in the upper
bout, which results in more stability for the neck block.
John
Re: "Flat tops" vs arched bracing #7 [-]
Quote:

Arnoldgtr
Is that what's going on with the Huss & Dalton TDR series?

I believe that Huss & Dalton (like today's Martin) uses the concave sanding disc to arch the linings and profile
the sides. This is what I mean by a spherical arch, where the radius is constant over the entire surface.

The problems with a spherical arch in the back are threefold:
(01/25/07 3:39
1) It doesn't take into account the anisotropic nature of wood. Wood shrinks and swells across the grain, but
AM)
very little along the grain. This means that the back arch needs to be more across the grain, so that the back can
rise and fall when the moisture changes. A flat back will crack or sink concave if it shrinks.
2) A back with too much longitudinal arch will cause a shift in the neck block angle when the moisture changes.
This will change the neck angle, requiring frequent action adjustments.
3) A spherical surface is the most resistant to vibration.
John
Re: "Flat tops" vs arched bracing #8 [-]
Quote:

If you look at modern Martins, you will see what I am talking about
BHguitars

John,
Sure, I know. I have seen such machines but also I guess the contemporary use of tall lower bout back braces
benefit a more pronounced arch especially at the lower bout. We also use various arching templates (I guess

every small maker does?) for various arches and cross and lenghtwise arches.
(01/25/07 4:13 We recently checked over the side template for our 12 fret 0 size and so I re-checked some Martin back arches
AM) of various 0, 00 and 000 size Martins I had at hand. All differed less or more but I also found some really fine
uniform arches length and crosswise with no bump . I'll see if I can get some pics later to demonstrate. I believe
the less of back arch at the lower bout depends on the use of the flat back braces that keep the arch solid but
slightly less curved than tall braces.

Willi
Author Comment
Re: "Flat tops" vs arched bracing #9 [-]
According to their website, Huss & Dalton uses 2 styles of guitar construction, one that dishes the top and back
rims to a radius, and their traditional series: "Our Traditional series features the same 25' radius prepared into
Steve Frost
the braces, but the sides are left flat for a more traditional build style. This build style tends to have a more
traditional tonal character, emphasizing a bit more bass."
BTW, I have no connection to H & D, never seen one, though they look very nice.
This is interesting, because in the builders forums that I frequent, nobody's really talking about this. The
prevailing attitude seems to be that if you're not sanding your rims on a radius dish, you're missing the bus.
Maybe not!

John and Willi- thanks for your perspective. Most of us don't have direct access to these vintage instruments,
(01/25/07 9:28
especially to the insides, so this is a priceless opportunity. Thanks!
AM)
Hey- Where's Henk? I figured he'd chime in by now.

Re: "Flat tops" vs arched bracing #10 [-]
John Platko
Great thread!

Does anyone know where and when the concave sanding dish started to be used? When did Martin start using
it?

Side profiles would be great to see Willi. I'd be especially interested in a OO-14 fret side profile.
(01/25/07 11:50
AM)

Re: "Flat tops" vs arched bracing #11 [-]
Quote:

FoolForWood
Hey- Where's Henk? I figured he'd chime in by now.

Sorry Steve, it's a great thread but I just overlooked it.

This topic is so big, I don't know where to start.
(01/25/07 2:54
PM) Mmm, let's do Huss and Dalton first.

In the GAL magazine some years ago, I read something that puzzled me. There was an interview with the guys
and the top arching was discussed. Here's my problem.
Author Comment
Normally, all other things being equal, the stronger the arch (no extra mass but a stiffer top), the less bass. So if
you want more low oomp, you'd increase the radius (=less arch). H&D use a 25 ft radius for their top bracing
(rounder than a Martin), and the tops are glued to the sides after these have been sanded in a 25 ft concave
dish. So, the fit should be perfect.
But they also have a traditional line of guitars. Here you would expect them to use flatter braces and a less
concave dish, or a more traditional method altogether. However, what they do is the following. They use the
exact same tops (with the 25 ft radius bracing) and glue them to a flat plane rim. You now have to force the top
to adapt to the different shape. This will cause extra stress (I think they even call it "pre-stressing"). I would
expect that this extra stress will cost bass response. However, they claim that these guitars sound MORE bassy,
and closer to the original Martin sound. I'm not saying this can't be true, but if it is true, I would like to know the
physics behind it. (Does the middle part of the top get a more efficient spring function, like the diving board
that needs a firm base? Just a wild guess.)

Henk
Re: "Flat tops" vs arched bracing #12 [-]
Quote:

they don't have enough 'rise' in the waist area to produce a spherical arch.
BHguitars
John,
I'd like to pick this up for some further discussions ... later. I don't find enough time now. Top and back arch,
arch profile and side shape are big matters at construction in general. Everybody has his own understanding
and interpretation what works best. Even my partner Rudie and I have somewhat different opinions in this
matter and discuss this often. But to demonstrate some details here are some pics. To sort arches one could list

as follows:
(01/25/07 4:01
1. The arch itself
PM)
2. Different arches at different places
3. Ratio between cross and lenghtwise arch
4. Side profile at the waist

The pics show a typical but stronger Gibson back arch of a '39 Recording King Ray Whitley. I used a simple tool
that is easily adjusted for different arches. This is only to check out and to compare arches and their ratio at
different places. Here I adjusted the tool similar to what the back arch lenghwise could be:
Author Comment

Clicke here for large image

Large image

Same tool adjustment at the lower bout:
Author Comment

Large image

... at the waist:

Large image

... upper bout:
Author Comment

Large image

Now the pics show following:
1. Gibson usually does not have a strong back arch. This one is more pronounced than usual. Especially
longitudinal it is stronger than most Jumbo bodies I have seen.
2. Adjusted the tool according the longitudinal arch the lower bout crosswise is slightly more arched. Not
shown is the arch near the 4th back brace were it is less than crosswise. At this place the longitudinal arch
shows a lowering. The waist shows an incredible strong arch. The upper bout is less arched and lousy looking
like the back of a camel but is also sunken in. Compare to the longitudinal arch. This is also the reason because
of slab cut wood.
3. The ratio lengthwise to crosswise is a stronger allover as it is typical for Gibson back arches.
4. It is visible nicely what John and Henk described above concerning the waist. The waist height stands in
relation to the back arch crosswise at the waist but also lenghtwise. Gibson does not care much about this so
the backs often have a "strange" arch. The waists are not raised, the short waist back brace shows a bigger arch
but often the arch lenghtwise at the waist is sunken in meaning either the arch at the waist is too small or the
waist not high enough.

There are other concepts of back arches and side profiles like vintage Martin or Larson. Larson is very
interesting since they seem to be the inventor of the steel string guitar top (and back) arch. Meaning Larsons
have a pronounced arch of both, top and back with raised waists at the sides for top and back. This construction
is considered (by some people) to be the earliest and best arch construction. But as I said: to be discussed ...

More later,

Willi
Author Comment
Re: "Flat tops" vs arched bracing #13 [-]
"Fascinating", Mister Spock would say. What I see in the pics is much like what beginning luthiers tend to
overlook, especially if they don't use the right books or teachers. I'm not a luthier so maybe I should hush, but
I've seen quite a few guitars made by beginners with the same mistakes. What happens is this. They brace the
back, all four braces with more or less the same arch. Transverse braces only, so the result is a cylindrical arch.
(Even if it would approach a spherical arch, there's still the same possible problem, although a little less
pronounced). Then they taper the sides, let's say from four inches at the heel to almost five at the tail. But no
curve! Just like e.g. luthier Donald Brosnac "teaches" in his handbook on steel string guitar construction. Of
course there's a bad misfit now. The tighter the waist, the stronger the misfit, as the sides should be
considerably taller in the waist area. They'll have to apply some serious force to glue the pieces together, and if
they succeed the back will have a considerably stronger arch in the waist area. If they also forget to bevel the
linings and the bottom of the blocks (heel especially), you'll get some odd shapes.

It's almost unbelievable that the Gibson seems to show some of the beginners mistakes. Pictures can be
deceiving, and in wide angle and/or close up mode straight lines outside the centre usually show some convex
distortion. But on my monitor, the second pic shows an almost dead straight back taper (side view of the
binding where it borders the side wood). It seems no wonder the crosswise arch is strongest at the waist.
FoolForWood
On the other hand the lengthwise arch looks quite fair, apart from the heel area (which can be flattened by
almost seventy years of string pull). So perhaps they did compensate somewhat by giving the middle braces a
stronger arch.

The nice thing about spherical arches and sanding dishes for the sides (and braces) is that the fit is always
perfect. But, as John pointed out, there may be other things to consider. A more traditional way is shown in

Cumpiano and Natelson's book "Guitarmaking". Here they start out with a preliminary arch that is nicely
(01/25/07 5:27 smoothed out.
PM)
Another neat way is how a friend of mine does it. A little difficult to explain, but I'll try. First, he braces the
(slightly oversized) back with arched braces. It is now curved crosswise, but not much lengthwise. He then
places a stiff stick or small beam over the centre backstripe and ties it to the back with rubber bands. Next he
puts wedges (small blocks wiil do, too) at the ends of the back, between the stick and the back. This forces the
back into a semi-parabolic shape. By choosing different thickness blocks, or simpler, by just sliding them a little,
he adjusts the curve to his liking. The arch in both directions is now set. He then places the back with stick etc.
on the guitar's (untapered) sides. The soundboard is faced down, of course. He lifts the back at the tail's side to
create the amount of taper he wants and puts a spreader block between back and tailblock. Then with a
spacer/spreader (?) and white pencil he traces the exact arch plus taper in one go onto the sides. After sawing,
the only thing that is not perfect yet is that the linings are still flat. But with a little experience this shouldn't be
a problem to fix.

Quote:

I use side profiles from prewar Martins, and they don't have enough 'rise' in the waist area to produce a
spherical arch.