Before I can continue with the bracing on the top plate I need to make a jig so I can make another jig so I can glue up the bracing with a radius on the top plate.
I am not sure when this came into style but many acoustic flat-tops now have a radius on the top plate as well. Some say they sound better others say they are structurally stronger. The sound argument is likely one of preference. However the structural argument is likely to be factually correct. Case in point: I have an old flat-top that I always kept in tune. And quite recently while going over it I noticed the mid-section around the sound hole was dipping, while the lower bout below the bridge was arching. Closer examination showed the bridge to be at an exaggerated angle to the plane of the top. Obviously what was happening was the strings tension had pulled the bridge forward down, deforming the top. Over several years of string tension the X-bracing weakened. Probably the best fix is to remove the top and replace the X-bracing.
You can see the unnatural angle of the bridge even with the stings loose. It gets worse when it is tuned up.
At any rate with this flat-top guitar I have chosen to build it with an radius top.
Typically the most common radius is 25′ for the top and 15′ for the back. In order to put that radius in there we need to first calculate the arc of the bend we need. For that we solve for X using X=L²/2(R). Or you can use my nifty radius calculator I did in Excel today. Just choose the L and R value from the drop down list and it calculates X. All my values are in dreaded millimeters but I did a conversion table there for quick reference.
To make the radius jig I first glued up two 18mm mdf for extra thickness and added stiffness.
I then drill a 10mm hole in center and plug a dowel in it attached to my trammel.
The round, flat dish.
This is where that sound hole cutout comes in handy.
Support on the edges
Making my rails for the router carriage that will ride over top the round dish. My rails here are 790mm long. So I measure 790 and put a nail at each end point, mark center and using a piece of maple create an arc that is 17mm deep. pencil it in and then take it over to the band saw. Final sanding on the disc sander.
You can see my carriage sitting on the round board now. It already has 25′ radius rails on it and beside it are the newly made 15′ radius rails.
With my router in position I hold it down and rotate the round plate. Clamping the router is recommended. As the router slowly advances down the rails while I rotate, the radius arc is routed out.
Finally I scrape the dish smoothing out all the rough spots.
This dish is now ready to be used for putting a radius arc on the X-braces and gluing up the braces on the the top plate.