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The primary braces and secondary braces are well explained in Gore's books and these make sense to me. Please help me understand the need for the tertiary braces.

I don't see anywhere in the Gore books where an explanitation is given for the need for the 3 tertiary braces. 2 of them are in line with the front of bridge, so I don't see how they would help prevent bridge rotation and associated "roller coaster" of the top. So how do these two function?

The 3rd one crosses the centerline of the top at the big bout butt end with the trouble of adding carbon fiber above and below it. I don't see how this does much except strengthen the top's center seam glue joint, which a simple smaller & lighter cleat could do. What is the purpose of this brace in Gore's falcate scheme.

There’s one real good guy to ask…

I think that the main purpose of the tertiary braces is to impede the cross-dipole mode of vibration. This mode has roughly half of the lower bout vibrating in anti-phase to the other half, so it is self-cancelling and a waste of energy.

Happy to be corrected, but that's my current understanding. I'm in the process of modelling a falcate-braced classical body in CAD, so thinking a lot about how the design works.

Sorry to hijack this thread, but which CAD software are you using?

I really enjoyed Onshape, but I want to generate G-code for my CNC, and they wanted $2500/year for CAM. I suppose I could have just exported to STL or STEP and used a third-party CAM tool, like Carbide Create, Easel, or MeshCAM. Those also cost money, but a lot less. I guess BlenderCAM is free. I was using kiri:moto but it stopped producing usable G-code for me.

However, I also wanted to code some add-ons to do some calculations, and Onshape's programming language is missing some features I needed.

So I'm converting to FreeCAD, which is free, includes CAM, and programs in Python, which can do the calculations I need.

FreeCAD's UI breaks my expectations sometimes, so I've had troubles learning it. There are some good videos on Youtube though. However, it is still possible to do things that break the model and can't easily be undone or repaired. So I'm still struggling with it.

Perhaps we should take this conversation to the CNC and Lutherie forum.

Greg

SolidWorks has a makers package for $50/yr. Includes CAM.

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Jim Watts
http://jameswattsguitars.com

I’m using Rhino, and specifically the Grasshopper plugin which allows me to generate all of the geometry using nodal visual coding.

Amongst the available nodes is a Python node which I used to create my own CAM functions and generate Gcode.

I’m very happy and proficient in the Rhino and Grasshopper environment, but I don’t have much other experience to compare it with.

But yeah, CAD aside, it would be wonderful if Trevor Gore has time to comment on the design function of the tertiary falcate braces!

Section 7.3.2 in Build gives the guidelines for designing falcate bracing layouts. Point e) says that the "arms" of the braces should divide the lower bout into approximately equal angular segments. This is so that there are no large unsupported areas of top panel that can "flap like a flag" when the top vibrates and promotes a unified monopole mode. The inner ends of the laterals (and the fish-tailed CF) are positioned over what would otherwise be a stress concentration at the forward corners of the bridge. How you shape the laterals controls how active the cross tripole is (an efficient sound radiator second only to the monopole) and its resonant frequency. They also protect against splits along the grain at the edges of the lower bout.

The "3rd one" has similar "anti-flag" duties, helps promote the monopole and reinforces the centre seam in the lower bout.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au