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| Author: | Andy Zimmerman [ Thu Jun 16, 2005 12:18 am ] |
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Newbie again This is the 1st time I have used radiused dish molds for sanding the sides and bracing the soundboard and back. When I glued all of the back braces on via a go bar deck, the back did not maintain the shape of the 15' dish. The braces were radiused to match the dish. It seems that once removed from the dish mold the back springs back to a straighter shape. Is this normal? Do you not worry about it? Willl I be fine when I glue the back to the radius sanded sides?? Thanks again Andy |
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| Author: | Michael Dale Payne [ Thu Jun 16, 2005 12:50 am ] |
| Post subject: | |
If the bottom of the braces were shaped to a 15'radius and glued up in a 15'radius bowl there should be no spring back to speak of. Now the back wants to pull the braces back some. But unless you made your braces too thin or too short they should have the strength to resist this. Now I have seen someone hand sand brace stock on a dish pushing firmly in the middle of the brace stock and sanding. This will causes the ends of the brace stock to flex up as pressure is exerted in the middle and leads to a flatter sanded surface than intended. I built a fixture that hold and supports my brace stock evenly across the entire length and cuts the shape by means of a guide template on my router table. You can see one example of these fixtures in the library of plans. It is the X-Brace Profile Fixture. |
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| Author: | Josh H [ Thu Jun 16, 2005 1:10 am ] |
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Andy I know what you are talking about. If you are using a common ladder bracing pattern on the back (which is what I also use) it will seem to flatten a little once you unclamp it from the go deck. But you should find that when you glue the back on to the rim it will have the proper radius. My experience is that once the back is on the rim it is fine.Providing your braces have been cut properly that is. I use a router setup to cut braces like Michael suggests. They are easy to make and save a ton of time. Josh |
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| Author: | Mattia Valente [ Thu Jun 16, 2005 1:15 am ] |
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Short version: Yes, that's normal. 1) make sure the brace bottoms, when they sit in the dish, conform to the shape of the dish without any pressure. I plane the bottoms, sand to final shape with the dish, and then scrap the gluing surface smooth. Pretty quick, and yet another excuse to break out the plane. 2) Assuming you've ladder braced (standard Martin-type back bracing) the back, the back should conform to the radius dish tightly when you've got the go-bars in place. 3) As soon as you take the back, braced and all, out of the dish, if you've ladder braced it, it WILL 'straighten out'. You should maintain the side to side 15' radius arch (ie in the direction the braces run) but you won't maintain the lengthewise one. Why? Simple. The braces are only imparting a side-wards arch. The sanded profile on the back of the rims will define the 'lenthways' arch. Now, if you X-brace back, the portion with the X-brace in it will keep the 15' radius arch (on my last one, I did an X in the lower bout, so that stayed arched, no dish needed, and the upper bout was ladder braced, so that didn't, not quite). You should be able to use fairly gentle finger pressure to push the back into the dish and have good contact all around, though. |
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