Official Luthiers Forum!

I’ve been following Brune’s very detailed 1937 Hauser plans fairly closely for body and bracing dimensions. Unless I’m missing something there is no specific guidance for the amount of arching for the back. Anyone know what it is?

As with doming of the soundboard, there seems to be no consensus amongst authors on the amount of back arch.

Courtnall suggests arching the back braces at:
3mm – top bar
4mm – center bar
3mm – lower bar
The different arcs create a lengthwise arch as well as an arch across the back. The 4mm arch of the center bar equates to a radius of roughly 8.4 ft.

Cumpiano’s book recommends arch offsets that also vary to create a lengthwise arch as well as across the back. The no.3 brace (there are 4 total) provides the most arch having a roughly 10.8 ft radius.

Bogdanovich uses a 15 ft radius on 3 of 3 bars. I guess this doesn’t produce a lengthwise arch (?) Bogdanovich also shellacs the center patch and braces for the back. Is the shellac important?

Can anyone familiar with Romanillos tell me the back arch he generally builds with and how he shapes the bars? I understand that the rounded edges (as opposed to the glued edges) are contoured along their length with the middle being somewhat shorter. Can someone elaborate on this?

Thanks for any insight on the subject.

Joe

David LaPlante could probably tell you the Romanillos number. I build Romanillos style guitars, and I radius the back at 25', and it seems to work fine, but I do not know if it's authentic at all.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

I use 25' for all 3 of mine.
I would think that a different arch in braces so close together would cause stress in the wood.
my heel to butt arch is the same 25'
just my 2 cents

_________________
Mike Collins

"The different arcs create a lengthwise arch as well as an arch across the back. The 4mm arch of the center bar equates to a radius of roughly 8.4 ft.

Cumpiano’s book recommends arch offsets that also vary to create a lengthwise arch as well as across the back. The no.3 brace (there are 4 total) provides the most arch having a roughly 10.8 ft radius.

Bogdanovich uses a 15 ft radius on 3 of 3 bars. I guess this doesn’t produce a lengthwise arch (?) Bogdanovich also shellacs the center patch and braces for the back. Is the shellac important?


Joe[/quote]"

Joe, it is important to understand that the legthwise arc is NOT created by the varying arch heights on the braces, It is created by the profiling of the sides that the back attaches to.
If this is done by sanding on the radius dish that is used for profiling the braces, then you will have a 2 way arch ie a dome.

If you just sand a contant slope to the sides, you will tend towards a straight longitudinal profile to the portion of the back between the first and last braces.

Thanks everyone for the replies. It’s taking me a while to visualize the concept here. I am looking at the method of bracing the back separately, outside of the instrument (as opposed to fitting the braces to the ribs first.)

It seems that varying the amount of arch in the braces would produce both a lengthwise and crosswise curve to the braced back prior to it being assembled to the body. I can see where profiling the sides will also want to impart a lengthwise arch. For a good fit with minimal internal stress shouldn’t the two be in harmony? Or am I over thinking this?

Joe