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Bevel edges on radius disc before adding top and back
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Author:  WudWerkr [ Wed Sep 14, 2022 3:34 pm ]
Post subject:  Bevel edges on radius disc before adding top and back
I see people with radius dishes that bevel sand the sides before gluing the top or back on. I have never done this and seem to be doing fine as far as glue joints holding.
To me it seems to be such a small amount does it really help that much ?

What is your take on that particular step in the process .

Yes why? idunno

No Why ? idunno

Thanks

Wud
Author:  meddlingfool [ Wed Sep 14, 2022 4:52 pm ]
Post subject:  Re: Bevel edges on radius disc before adding top and back
What do you mean by bevel sand?
Author:  John Arnold [ Wed Sep 14, 2022 5:06 pm ]
Post subject:  Re: Bevel edges on radius disc before adding top and back
Yes, because it increases the surface area of viable glue joint. Wood glue favors tight joints, on the order of 0.001".
If your back has anything more than the slightest radius, you need to angle the kerfing to match.
However, I do not do this with a radius dish. I have curved cauls (15 foot radius) that I use for the back braces. These were made from 2 X 4's, so they are 1 1/2" wide. These allow me to shape the kerfing, using the caul as a template. Most of that shaping is done with a block plane. Sandpaper wrapped around the caul finishes the job.
I do not favor radius dishes because my backs are made like older Martin guitars, which used a flatter radius lengthwise. That radius is closer to 25 feet, and it is automatically generated when the sides are profiled prior to bending them.

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Author:  Colleen_M [ Wed Sep 14, 2022 5:17 pm ]
Post subject:  Re: Bevel edges on radius disc before adding top and back
I’m awaiting delivery of a pair of radius dishes (15 foot back and 30 foot top radius), and I intend to do this. I’m sure you’re right about it being a minuscule amount, but I’m thinking it will help ensure a proper fit for the top and back, which of course have the same radius built into them by the radiused braces.
Author:  DennisK [ Wed Sep 14, 2022 8:27 pm ]
Post subject:  Re: Bevel edges on radius disc before adding top and back
You mean gluing a radiused plate to a flat rim versus radiused rim? You certainly can do either, especially if you use spool clamps or any other clamping method that squeezes out any gap there may be.

The rim plays a part in the overall plate shape, especially with ladder bracing which doesn't support any longitudinal curvature by itself. Spherical and cylindrical back bracing are no different until glued to the rim. But even with X bracing, a flat rim will change the curvature, mostly bending it flatter around the waist area.

Personally I arch my top braces individually and glue them on with cam clamps, and then glue the sides to the top with dentellones. That gives a mostly flat rim, but not as flat as if you glued linings with the sides in a mold and then sanded it on a flat sanding board (especially since I angle the tops of the dentellones a little for a better fit). I do it that way because I like to build new shapes all the time, and I don't have space to store a bunch of molds. And I figure you can build good sounding guitars with any combination of plate and rim radius, so I just focus on improving the sound within that style rather than worrying whether other rim shapes would be better.
Author:  Clay S. [ Thu Sep 15, 2022 8:03 am ]
Post subject:  Re: Bevel edges on radius disc before adding top and back
I radius sand the sides both before and after gluing the linings. My dish is motorized so it only takes a minute or so to do it. Radiusing before hand allows me to fit the linings better and radiusing after the linings are installed allows for a better fitting glue surface. Without radiusing there might be a small gap left on the inside edge of the linings. The tightest part of the joint will be cut away when the bindings are installed.
Does it make any difference - probably not. The top and back to sides joint has quite a bit of surface area for the stresses that are placed on the joint, so "less than perfect" is probably O.K.
Author:  TerrenceMitchell [ Thu Sep 22, 2022 1:20 pm ]
Post subject:  Re: Bevel edges on radius disc before adding top and back
To my mind, the joint between the kerfing and the top is the most critical, as the joint between the top and side will typically be removed for bindings. Unless you floss the whole rim somehow, I don't know how you'd get this joint satisfactory without just leveling it and forcing the top and back to join the sides at 90 degrees. But, as Dennis pointed out, using a flat kerfing surface, and clamping to it will change the shape of the places around the edges... probably introducing some unnecessary tension into the top.

Another advantage to the radius dishes is sanding the braces to final curve using them. Makes for better fitting (with less tension) braces. And, finally, gluing the braces in without a radius dish and a go deck would be tricky, unless you were strictly ladder-style, which could be done with backing cauls (much like Goodall does on their backs - See You tube Video).

Of course, if you are using a truly flat or around 50' radius for your tops, maybe the flat kerning would make absolutely no difference. I do 15' backs and 30'-40' tops, and the final bevel is noticeable to my eye on both. So, I like my dishes, and use them for multiple steps to insure accuracy.

Build on!
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