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| Understanding Radius Dishes HELP http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=23019 |
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| Author: | YJ John [ Thu Jul 09, 2009 7:15 am ] |
| Post subject: | Understanding Radius Dishes HELP |
I tried the search engine. It led me down a dark cold alley...... I have a 15', 24' and a 28'. On my first top I used the 15' (dred) and found out I was wrong! What do you use for your top and back on a Parlor, OM, Dred. THEN why???? 2nd question. How do I figure out the raduis of an existing guitar? I am photographing and blueprinting to the best of my abilities a 90 year old Gibson L1. I sure would look smart walking in and measuring the radius. Please help my smarts 
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| Author: | Tom West [ Thu Jul 09, 2009 9:27 am ] |
| Post subject: | Re: Understanding Radius Dishes HELP |
Use the 15 ft for the back,and any of the other two for the top. I would use this for any of the guitars mentioned. The Gibson I'm not sure about,but Gibsons are noted for being built with a domed top.A guitar that old has an arch induced by string tension.Think you would be better served by looking for a plan of this guitarthan trying to copy the arch in the top.Good luck. Tom. |
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| Author: | Jim_H [ Thu Jul 09, 2009 9:50 am ] |
| Post subject: | Re: Understanding Radius Dishes HELP |
John, I use a 15' for the back and a 28' for the top. I chose those because that's what the people who taught me used. As far as measuring the radius of an existing guitar, especially an old one, I think you are going to have some issues. There is very likely some doming/bellying behind the bridge on the existing guitar, especially a 90yo one. You can use a straight edge across the top and measure the gaps on the sides. I don't have the forumula handy to convert those gaps to a radius, but I'm sure someone here can help. The problem is that this is only going to tell you what the existing radius is, with any doming/bellying included. I double you will be able to determine was radius was used (if any) when it was built 90 years ago. |
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| Author: | YJ John [ Thu Jul 09, 2009 12:11 pm ] |
| Post subject: | Re: Understanding Radius Dishes HELP |
Good point it moved. but I will try the straight edge just to look. I will photo the inside extensivly. If it shifted much I would think the bracing would pull. Tongue in cheek thought; If I use its current curve won't I get the vintage sound 
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| Author: | Todd Rose [ Thu Jul 09, 2009 1:38 pm ] |
| Post subject: | Re: Understanding Radius Dishes HELP |
Hey John, As others have pointed out, an old guitar won't have a simple dome shape no matter how it was built, due to the distortion caused by string tension over time. That said, a common method for measuring a radius is to use radius gauges - pieces of wood, metal, plastic, cardboard, or whatever cut to various radii. Kind of like a straight edge, only the edges are radiused rather than straight. You can buy them from Tracy (sponsor here: Luthier Suppliers) or make your own. One simple way to make one from a radius dish you already have is to take a piece of 3/4" plywood about 4" or 5" wide and 20"-24" long, lay it on edge across the dish, scribe a line down its length that follows the curve of the dish, cut that line on a band saw, take the convex half and sand it in the dish till it matches the radius exactly, stick adhesive sandpaper on the edge you just sanded, then sand the concave half on that. If you do this carefully, you should end up with your concave piece having the same radius as the dish you began with. |
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| Author: | Todd Rose [ Thu Jul 09, 2009 1:39 pm ] |
| Post subject: | Re: Understanding Radius Dishes HELP |
(double post error) |
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| Author: | Rick Davis [ Thu Jul 09, 2009 2:33 pm ] |
| Post subject: | Re: Understanding Radius Dishes HELP |
Simply lay a straightedge across the body near the bridge, where the distortion is less. Measure the drop off at each edge and the body width at that point. You can calculate the radius from those measurements, within the tolerance of your measuring devices. All the warnings about distortion over time apply! And, I wouldn't worry about what was done then. Use a similar radius that you're familiar with and it'll be fine. |
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