Guitar top and back radius position

Talk about musical instrument construction, setup and repair.

Moderators: kiwigeo, Jeremy D

Post Reply
Martin
Gidgee
Posts: 9
Joined: Tue Mar 27, 2018 9:06 am

Guitar top and back radius position

Post by Martin » Thu Jun 20, 2019 11:41 am

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.
Attachments
Back.jpg

User avatar
kiwigeo
Admin
Posts: 9746
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Guitar top and back radius position

Post by kiwigeo » Thu Jun 20, 2019 12:09 pm

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

Martin
Gidgee
Posts: 9
Joined: Tue Mar 27, 2018 9:06 am

Re: Guitar top and back radius position

Post by Martin » Thu Jun 20, 2019 2:19 pm

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.
Attachments
view.jpg

User avatar
kiwigeo
Admin
Posts: 9746
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Guitar top and back radius position

Post by kiwigeo » Thu Jun 20, 2019 2:48 pm

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.
Martin

User avatar
Allen
Blackwood
Posts: 5140
Joined: Thu Oct 11, 2007 5:39 pm
Location: Cairns, Australia
Contact:

Re: Guitar top and back radius position

Post by Allen » Thu Jun 20, 2019 3:25 pm

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.
Allen R. McFarlen
https://www.brguitars.com
Facebook
Cairns, Australia

User avatar
WJ Guitars
Blackwood
Posts: 143
Joined: Thu Jan 26, 2017 4:28 pm
Location: Sutherland NSW

Re: Guitar top and back radius position

Post by WJ Guitars » Thu Jun 20, 2019 6:20 pm

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 52 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
Blackwood
Posts: 646
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Guitar top and back radius position

Post by blackalex1952 » Fri Jun 21, 2019 11:30 am

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
"Everything I say on the topic is based solely upon inexperience and assumption!"

User avatar
WJ Guitars
Blackwood
Posts: 143
Joined: Thu Jan 26, 2017 4:28 pm
Location: Sutherland NSW

Re: Guitar top and back radius position

Post by WJ Guitars » Fri Jun 21, 2019 6:45 pm

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
P3090129.JPG
P3210162.JPG
P3200160.JPG
P3030089.JPG

User avatar
kiwigeo
Admin
Posts: 9746
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Guitar top and back radius position

Post by kiwigeo » Fri Jun 21, 2019 8:48 pm

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

johnparchem
Blackwood
Posts: 424
Joined: Mon Jan 21, 2013 2:59 am
Location: Seattle
Contact:

Re: Guitar top and back radius position

Post by johnparchem » Sat Jun 22, 2019 12:32 am

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.

User avatar
WJ Guitars
Blackwood
Posts: 143
Joined: Thu Jan 26, 2017 4:28 pm
Location: Sutherland NSW

Re: Guitar top and back radius position

Post by WJ Guitars » Sat Jun 22, 2019 1:16 pm

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

User avatar
Dave Higham
Beefwood
Posts: 24
Joined: Sun Apr 20, 2008 4:21 pm
Location: A Pom in S.W.France

Re: Guitar top and back radius position

Post by Dave Higham » Sun Jul 14, 2019 2:26 am

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
Beefwood
Posts: 25
Joined: Mon Jun 16, 2014 5:48 pm
Location: Brisbane, Australia
Contact:

Re: Guitar top and back radius position

Post by ArmstrongLutherie » Sun Sep 01, 2019 5:14 pm

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
Blackwood
Posts: 1676
Joined: Mon Apr 25, 2011 10:36 pm
Location: Perth WA

Re: Guitar top and back radius position

Post by simso » Sun Sep 01, 2019 5:31 pm

kiwigeo wrote:
Fri Jun 21, 2019 8:48 pm
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
Steve
Master of nothing,

Do your own repairs - http://www.mirwa.com.au/How_to_Series.html

Post Reply

Who is online

Users browsing this forum: No registered users and 27 guests