Guitar top and back radius position

[Martin]

Hi,

I've been searching everywhere online to find an answer... and have not found it yet.
It may be so obvious no one has bothered to state it.
I have finished joining the back of my first build. I have made a 15 ft radius dish for shaping the back.
Simple question is... where do i place the centre of the radius when it comes to bracing.
Should I line up the deepest part of the dish with the bridge?, over the sound-hole?, dead centre of the guitar body length? at the widest part of the lower bout?
Picture is of the back, in purple heart (the only wood I have available at the moment) cut 5mm oversize all round.
Currently at 5mm thick but will be sanding thinner tomorrow.
Thanks for your input.
Martin.

[kiwigeo]

Hi Martin,

The dish is the same radius regardless of where the top sits....just position it so the entire back/top is within the edges of the dish and you should be fine.

[Martin]

I'm more thinking about where the radius is when fitted to the sides.
See picture.
If the deepest part of the radius is toward the neck end, the lower bout tapers in.
If the deepest part of the radius is toward the lower bout then the neck end tapers in.

[kiwigeo]

I don't have any strict rule on this one....generally on my instruments the widest point of the side is slightly aft of the bridge area.

[Allen]

Your choice.

For my ukes the tall block is 10mm deeper than the neck block as that suits the way I like them to look. As far as I'm aware, all the Martin guitar plans I've seen do domething similar.

[WJ Guitars]

I'm more thinking about where the radius is when fitted to the sides.

I not sure whether this information is useful regarding the guitar sides and top / back radius shaping of acoustic guitars.

In the stages before the sides are bent and fitted into the guitar mold re: sanding the sides using a radius disk with 80 grit sandpaper. I trace the acoustic guitar side shapes from a template I made from 4mm mdf board (Note: For cutaway a different template shape is required). I cut them slightly wider on the band saw along the guitar back side edge. The accurate shape of my templates has been set out from side contour calculations. The required data is as follows:

Box length
Chosen Back Radius
Tail end height
Neck end Height

Instructions and formulas for these calculations are found on this website:

http://www.doolinguitars.com/articles/sidecalc/

I have provided a pdf that is an example of how I make these calculations using 'Excel'.

Guitar Sides Cal.pdf

(920.1 KiB) Downloaded 596 times

Once the sides are cut out I then bend the sides and with the more accurate cut out shape I then find it is easier to sand the correct shape in the guitar mold that will fit to the final design of the domed required for the braced top radius and the braced back radius of a guitar ready for the next stage to glue and close the box. There other methods that can be use to achieve the same result. However, I now have my templates and I find it easy during the guitar building process to get the right size and shape when building guitars.

Do what best works for you.

Wayne
https://wjguitars.wixsite.com/mysite-1

[blackalex1952]

Check out the way the old school Spanish construction methods work and are executed. My findings are to place the back in such a way that someone holding the instrument with a strap doesn't choke the lower bout with their body.
There are high end US builders, eg Linda Mansell who has made instruments for Pat Matheny who do interesting things with great results, Google her build pictures. Get the Gore/Gilet books and understand that the top (soundboard) in a "non" live back does most of the work in the lower frequency response, and that a "non live" back still takes part a little in the recipe. Some high end builders also do very heavy thick laminated non live backs. Not a good idea to build so that the player can choke the back, either way most people hang a guitar around their neck with a strap...avoid the chance of that kind of player choking the lower bout back with their gut and that the guitar sits or is held in such a way that the lower bout is choked. Having said all that, I like guitars which have the deepest part of the radius under the bridge when not building experimentally, for most folks. One much important point is to use laminated liners between the soundboard and the sides. Cheers! Ross

[WJ Guitars]

I agree that laminated side linings definitely for the top is recommended and also for the back side linings is good (kerfing lining for the back is still ok). The closed box is structural stronger and helps reflecting sound back on the top as explained in the Gilet / Trevor Gore books.

If you check out Robby O'Brian's video you tube listing you will find a a tutorial on shaping the radius curve for bracing using a radius dish that has 80 grit sandpaper glued onto the dish.

John Hall, 'Blue Creek Guitars', has a you tube tutorial video showing how he shapes the sides in a guitar mould using a radius dish.

Wayne
https://wjguitars.wixsite.com/mysite-1

[kiwigeo]

Sanding the radius into the sides by "driving the porcelain bus" is an absolute pain in the arse job. There was one builder in the US who had rigged up his sanding dish on a potters wheel. He got the wheel turning and then just held the sides down on the dish and hey presto. It's something I plan to work on once my workshop is back in action.

[johnparchem]

If the side template\profile is correct and the are sides cut very close to the profile and the waist is bent in the correct place, one can have the sides very close to the correct size and radius with minimal sanding by driving the bus.

[WJ Guitars]

Sanding the radius into the sides by "driving the porcelain bus" is an absolute pain in the arse job. There was one builder in the US who had rigged up his sanding dish on a potters wheel. He got the wheel turning and then just held the sides down on the dish and hey presto. It's something I plan to work on once my workshop is back in action.

Martin your reset up of your workshop is going to spin well!

Wayne
https://wjguitars.wixsite.com/mysite-1

[Dave Higham]

If you're working from a plan, or even if you've drawn your own, you will have a body depth at the neck block and at the tail block; (usually) deeper at the tail block. These two dimensions and the back radius you choose determine where the deepest part of the body is. You don't need to worry about it.

[ArmstrongLutherie]

If you're working from a plan, or even if you've drawn your own, you will have a body depth at the neck block and at the tail block; (usually) deeper at the tail block. These two dimensions and the back radius you choose determine where the deepest part of the body is. You don't need to worry about it.

Agree 100%

[simso]

There was one builder in the US who had rigged up his sanding dish on a potters wheel. He got the wheel turning and then just held the sides down on the dish and hey presto.

That is how I do mine

Steve