Contouring sides to match top

[Kim Strode]

I am currently making an acoustic guitar and decided to build a 25' radius top which I've completed. I am now trying to contour the sides of the body to match the top. The consensus seems to suggest shaping the sides of the body with a 25' radius dish using sandpaper, but to flatten the sides and neck joint above the waist. When I try to achieve this the contoured dish also contours the area at the neck. How can I achieve a radius at one end and flat at the other?

[Mark McLean]

Do it in the radius dish to get the slight contour established all round. Then take a length of flat/straight timber - I use a bit of 4x2, but anything like a floorboard or a fence paling would do. Glue some sandpaper onto it. Use this to span across the rim at the upper bout and the neck block and give that area a flat profile. In fact, you can use this straight beam to do a 'radius' also, and I find this easier than trying to sand in the radius dish. If you glue a wedge with about a 2 degree angle onto one end of your sanding beam, when you lay it across the rim it will be angled slightly towards the outside. You then get a sanded edge with a slight (actually imperceptible) angulation. Although it is flat, not radiused, it will match well with the shape of your curved plates at the point where they meet the rim. My sanding beam has one side which sands a perpendicular edge, and the other side sands an angled face. It has 80 grit glued on to it and you can do the whole rim of the guitar in a couple of minutes.

In truth, I wonder if creation of that curved profile on the sides makes much of a difference. If your side is only 2mm thick, and the kerfing or lining maybe 5mm, that is not much width on which to impart a 25' radius. You won't be able to see the difference between the part that is radiused versus the bit that is flat. And does it make any difference when you come to glue the back and top on? Probably minimal.

[kiwigeo]

I do it pretty much how Mark does it. I have a sanding bar that has a slight radius in it and a flat central area that corresponds with the area covered by the upper fretboard. Trevor's books have good coverage on this.

[Steve.Toscano]

In truth, I wonder if creation of that curved profile on the sides makes much of a difference. .

If clamping the edges flat on a fully radiused top i suspect you might leave yourself open to potential cracking due to the stresses.
I do as mark mentioned. I have a 'sanding stick' that is radiused to my back* radius with some 120grit glued on the curved surface.
The same could be done with a block plane or flat sanding block and a keen eye.

*Back because I only dish the middle part of the lower bout of my tops, therefore where it meets the sides is flat.

[routout]

Hey that's a good explanation how many times do I read how too's and get confused (it's just me rite)thanks for that

[Kim Strode]

Thanks for that simple explanation Mark. I made a new sanding stick today, as suggested, and it worked a treat.