Official Luthiers Forum!

Another radius dish question for you all.

My understanding is that the top of the rims is generally designed to be flat along the edge (ignoring the upper bout for the moment which generally gets adjusted to the neck angle-discussed in another topic. Also, I'm talking about along the length not the radius that gets sanded into the kerfing). The bottom of the rims (for the back) is flat (or close to it) from the tailblock to some point along the side where it starts to taper down until it reaches the neck block.

I've read in several places that some people sand the sides in the radius dish before putting the kerfing on and then sand again after the kerfing is on to get the proper radius on the kerfing which I understand.

Here is my question: When you sand the top and back in the dish doesn't this change the profile of the top and back of the rims along it's length to the profile of the dish itself? Instead of getting the profile discussed above you get a bowl shape. I guess if you wanted to keep the taper as discussed above on the back you would need to sand the upper and lower bouts separately (by tilting it in the dish)?

Sam buddy you may be over thinking this.

The rim, both the top surface and the back surface, are sanded in the dish so as to meet the radius of the back and top of the guitar that was presumably built in one of the two respective dishes that you choose to use for your back and top.

The rim, prior to installing the kerfed linings, is also sanded in the dish so that when you install the kerfed linings you can do so with the linings just proud ( 1/32") of the edge of the rim. Then the rim, with linings installed, is again sanded in the respective dishes. If you install the linings prior to profiling (sanding) the rim in the dish you will have more work to do because you will be sanding through a lot more of the kerfed linings slowing down the process.

When you are finished with the dish sanding the guitar sides and the kerfed linings will have a smooth, radiused mating surface for the domed top and back. You are gluing the top and back to the sides as well as the kerfed linings.

Now I think that you are asking about flattening or taking the radius out of part of the rim? Again I only do this for the upper bout of the top and the back is left profiled/radiused by the dish sanding. Also, not everyone takes the radius out of the upper bout of the top either. It's just what I do.

You could simply radius the rim, both sides, in the dishes and attach the top and back. This is when some folks will sand the area under the fret board extension flat to mate nicely with the fret board. There are many ways to accomplish the same things here. What ever method that you use may employ another technique down the road to get the end results that you want.

Does this help?

Sam,

This may or may not be right, and I am sure that peeps with some radius dish sanding experience will be along momentarily, but I am under the impression that most people will tilt the radius dish to touch the head and tailblock and then sand, which will keep the taper intact. Because you are sanding in a portion of a sphere and the side is not bent in a circle, but undulates across this sphere's plane you would have a slight lazy "S" shape to the side if you were to straighten it out.

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Brad Tucker

I use 18 and 28 bowls .. to set the taper, simply pick the tail block and neck block height you want, set to the finished dimensions of the guitar, and they can be square (ie non angled top/bottom faces). Install. then sand away until the radius has been put onto the sides and blocks. You will see that with the top and back put on, you are pretty much back at the height you wanted to be for the finished guitar again.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

This is kind of what I was asking about. I'm not questioning the process at all, just trying to understand it.

When the sides are sanded in the dish it seems the profile will change. Whatever profile you have seems like it will change into a curved shape. I have attached a picture of what I am talking about. It is a crude drawing but I think it gets my point across. The first drawing is before you sand and the second is after you sand.




(I'm having trouble posting the picture so we will see what happens.)[img][/img]

yep ..thats kinda what you will get Sam ... and all will be fine.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Excellent. Thanks. I was just trying to visualize it before I go through the process so that I know what I will get in the end.