Parabolic Back Bracing

How to do parabolic shaping on the back braces of a new acoustic guitar.

Category:

Education

Tags:

• luthier
• lutherie
• guitar making
• acoustic guitar building
• braces
• back braces
• parabolic braces
• parabolic bracing

[dandeliontea7]

why are you calling them parabolic braces? they arent parabolic at all...

Still, thanks for the video :)

[CambrianGuitars]

@dandeliontea7 - Agreed. I tried to draw real parabolae using Corel Draw, but the curves are unrealistic. I draw then carve them by hand in a kind of fair curve arc across the length of the braces. Across the width, they are like Catherdral window arcs. Luthiers have been calling this kind of brace shaping "Parabolic Braces" for quite some time now, so I just went along with the flow..... :)

[JoylessBrotato]

I don't know if someone has already asked, but why did you do the rim in that saw pattern?

[CambrianGuitars]

@JoylessBrotato - This back was a failed experiment. I tried making a back and sides with finger joints between them. I used my laser to cut the castellated edges. However, even though I'd allowed a little slop for where the sides are bent, the two pieces would not come together to glue up. The little fingers broke off too easily. I decided to re-use the wood for a smaller guitar for another experiment (a heel-less neck idea, which was also a failure!) Cheers, Dave F.

[CambrianGuitars]

@donepearce - My gobar rods are fiberglass with silicone rubber tips on them, so it would be difficult to shape them appropriately. It's a good idea though, and I will certainly give it a try to see if I can shape them and make them work. I'll post back later when I've tried it. Cheers, Dave F.

[donepearce]

@CambrianGuitars Have you thought about carving vee grooves into the ends of the gobars so they fit over the braces?

[CambrianGuitars]

@donepearce - Thanks for the translation! The reason is that it's almost impossible to get the gobar rods to stay in place on the top of a curved (in two directions) brace. I plan to move to a vacuum press system at some point in the near future - this will allow me to pre-carve the braces more accurately before gluing them in place. Until then, I'm stuck with gluing on full height/width braces and carving them post glue-up. Cheers, Dave F.

[donepearce]

@CambrianGuitars He asked why you don't plane the braces before you put them in - it would be easier to get them the right shape.

[CambrianGuitars]

@donepearce - The absolute strongest shape would, of course, be an I-beam, but this is not practical (difficult to glue up etc.) and adds a lot of weight - one day, I'm going to try it using carbon fiber! Parabolic shaped beams have the highest stiffness to weight ratio of all. Currently, I'm experimenting with a 3-piece all-spruce top that has no bracing (beams) at all - I'm laser cutting a honeycomb pattern in the center piece to remove about 50% of the overall weight.....

[donepearce]

@CambrianGuitars Ok, another question. Why a parabola? Why not a catenary, an ogee, the list is endless.