Making a Radius Dish

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.

Route away.

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.


About Alex

Love making sawdust and turning wood into objects of practical beauty.
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8 Responses to Making a Radius Dish

  1. Pingback: Radius methods for a beginner -

  2. Mike Vineyard says:

    Instead of cutting with the bandsaw save time. Easy way, you can bend an one inch mdf strip and Sheetrock screw it down in the curved shape. Like between 32″ bend up .5 to form the bend of a 25′. screw this down solid and make a rectangle box and spin the disc while routing. Hope I helped I spent too long trying to get a decent arc. Wished I’d known this earlier. Mike Vineyard

  3. Phil says:


    Just one not of caution – this method does not produce a perfect circular radius profile, rather it will be a hyperbola, so the curve will be straighter near the outside than in the middle – it’s a product of the way a solid beam reacts to a bending force – it tends to bend most near the point of contact and stays straighter as you get further away. At these shallow depths of curve, it’s unlikely to be very critical, but it could make a difference when using the dish to profile bracing strips. As you push them back and forth, you imagine they are following a perfect circular surface, but the edges would cut less than the middle, being straighter.

    Despite what I’ve said, I think it’s the way I would make a radius dish but I’d run a random orbital sander around a bit halfway to the edge just to add that little bit of extra relief. I haven’t calculated it, but my guess is the difference is perhaps one or two thicknesses of paper for a dish intended to be a 20′ radius.


  4. Brend says:

    Creating a jig for cutting a Radius dish is an interesting problem.
    Firstly, bending up a 32″ spline by 0.5″ for a 25′ radius is not quite correct; this dimension (known as the sagitta of the arc) is actually a little less than 0.5″, 10.845mm in fact (it’s easier to measure in mm at this small dimension). I know I’m being picky here and in practice we would round to 11mm to measure this off a tape.
    Moreover, Phil is correct in what he says. A more accurate option would be to bend the spline across 3 points instead of 1. Say the mid point and two other points each 8″ in from each end of the arc. The height at these two other points is actually 8.135mm.
    How do I know this?
    A very interesting site,, shows the formula for calculating these dimensions and also has a handy calculator for those who do not wish to do the math.
    It strikes me that bending the spline over 3 points on the arc would possibly be more accurately achieved than trying to cut a 32″ long arc on the band saw unless one has an extension table and very steady hands.

  5. Alex Alex says:

    Thank you Brendan and Phil for your input and informative post. I will most certainly take this into consideration for my next luthier project.
    Brendan also thank you for leaving your blog URL. I paid a visit to your site. I was very impressed with your collection of planes. While I consider myself a user more than a collector I do have a several hand planes and am always on the lookout to add to my collection.

  6. Brend says:

    Thanks Alex,
    I became interested in hand planes about 15 years ago after purchasing a few wooden antique planes while on holidays. I began reading up on the history of the plane and became intrigued. Then I began making them. I also started collecting and now have a fairly extensive collection. Of all woodworking tools, I think the plane, in its various forms and guises, epitomizes the craft more than any other hand tool. I like the unplugged lifestyle and try to use hand tools as much as is practical, so many of my planes are often used. They go back into the glass display cases after use; I never leave one on the bench overnight. Oh! to be loved like my hand planes are….Ha!Ha!


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