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| 13 fret & top radius http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=11753 |
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| Author: | crich [ Mon Apr 23, 2007 1:30 am ] |
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I finished a couple of 14 fret,20 inch long (25.4 scale) guitars with a 30' radius and was thinking of changing to a 25' radius. At that neck angle I wasn't getting a lot of saddle to play with. Now I'm finishing up a 13 fret,19 inch long with the same 30' radius and now I have plenty of saddle. So does anyone use different radius tops depending on body length and frets to the body? Or am I the last one to catch on?  It wouldn't be the first time. Clinton
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| Author: | crich [ Mon Apr 23, 2007 11:14 am ] |
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Bump |
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| Author: | old man [ Mon Apr 23, 2007 1:25 pm ] |
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I hope there's not a lot of difference as I'm making my first OM with the same scale as you, and 14 fret. I haven't done anything different than I did on my dreads. My bridge is just shy of 3/8" and I use a 28' top. It is perfect for a dread. Ron |
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| Author: | old man [ Mon Apr 23, 2007 1:28 pm ] |
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Almost the same, My OM will meet at the 14th fret also. Ron |
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| Author: | crich [ Mon Apr 23, 2007 1:43 pm ] |
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Is it a shorter body? Remember I went from a 13 to a 14 fret to body.  Clinton
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| Author: | old man [ Mon Apr 23, 2007 1:49 pm ] |
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The body is about 1/2" shorter. Ron |
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| Author: | TonyKarol [ Mon Apr 23, 2007 11:25 pm ] |
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I have been putting some thought into the geometry around this ... hereis hwat i have figured out: at 28 feet, the FB can lie flat and give approx 3/8 bridge with no falttening of the upper bout - M Payne had a side cut view CAD drawing that showed this pretty well, and it was for a 25.4 14 fretter. I have never done the math, or even drawn this out, but it seems to work out very well in practice for me. If you change the body size, say to a Martin 0, the length may change by 1/2 - 3/4 inch for the body - no biggy here. The change in radius arc height isnt much from 20 to 19.5 body length. The width decrease of the body should have no impact at all - the body simply narrows into the radius of the bowl used to shape the guitars rim. Changing to a 13 fret shouldnt cause any grief either, becaus emocing the bridge back 3/4 or so on the top should change the bridge height by 10-20 thou at most. Now the kicker - I have acouple students who somehow have guitars which using a 28 bowl, are not getting the proper height at the bridge - both are shooting high .. I cant explain (one is coming around today and we will revisit), but one way to remedy is the sand the radius into the FB on a taper, from the nut to the FB end, thus dropping the lie of the FB at the bridge. I have never really bought into the flattening of the FB at the neck block, as my guitars never seemed to need it, but will take a look today at another I have ready to bowl for the top - I want to see how much block is removed if I were to do this. I was also contemplating that on smaller body guitars, the top may need more curvature, say use 18 foot, to raise the top at the bridge area, thus needing a thinner bridge. Hmmm .. lots to consider ... |
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| Author: | crich [ Tue Apr 24, 2007 2:01 am ] |
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Yeah sanding the FB after I have it the way I wanted it didn't appeal to me either. Let us know what you find out. Clinton |
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| Author: | crazymanmichael [ Tue Apr 24, 2007 2:06 am ] |
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have you ensured that your necks are fitted at the same angle and that your bridges are the same thickness? ceterus paribus, on any given sized body, the greater the radius, the smaller the rise there will be such that the bridge will sit lower. but the difference will be quite small though, and i am too lazy to do the math. the extra saddle gained might be due to other factors than the different radius of the top. |
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| Author: | crich [ Tue Apr 24, 2007 11:17 pm ] |
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You know, I got to thinking about another factor, my sides. As many times as my sides go in & out of the form my sides just may not be perpendicular to...well, where the top would be if it wasn't domed? After looking at my sides they look as if are creating another 1 deg. in my neck angle. I think I'm going to go with a solid form on the next one. Clinton
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| Author: | tippie53 [ Tue Apr 24, 2007 11:53 pm ] |
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I use a 28 on the all 14 fretters 12 fretters. I also can tell you that the neck angle is key to this and the top set up for the neck angle. This all has to work together. I just don't place the radius willie nillie. In my early building years I found that often the small things early in the build can cause big things at the end. You need to be very repeatable in your processes. THe neck block has to be secure till the top and back are on. The math of the top has to match the neck angle for the neck you are using. Not all necks are the same. Martin uses 1 1/2 degree ( this equates to a 28 foot radius ) Some manufacturers use 1 degree and this hits to 30 foot. So you need to take this in consideration. I also do a pre fit of the neck when the body is glued up. If I have to tweak the top so I have a bridge and saddle height of what I want I can do it at this point in the build. In a perfect world I like .500 at the bridge area with the total of the saddle when set up. To attain this I will dry fit so the line of the neck to the top will be 1/16 3/16 off the top at the saddle location without the freboard on. This allows me to set things up with a .350 to .400 bridge and still have .100 of saddle exposed. With that line established I can tweak the fretboard area of the top so things are in a straight line and I can eliminate the 14th fret hump. This is one of the things that newbies often miss in thier early days. IF the neck block is not in a position that is square to the top you will be fighting this all the time. john hall |
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| Author: | TonyKarol [ Wed Apr 25, 2007 12:54 am ] |
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more thinking .... The FB and how it lies in relation to the top, and the gap it creates where the bridge sits is the only thing thats important. The angle created by the top and neck block is completely irrelevant .. except that you must cut the necks heel angle to match whatever it is. I checked five of the bodies in the shop yesterday - only measuring the top and the lie of where the FB ext will sit (not bothering with the heel angle, as mentioned above) - all of them gave approx 1/16 gap where the bridge will go, and they where all different scales and neck joints - some 13, some 14, and 24.9, 25.4 and a 28.5 baritone, all 28 ft rad tops. The curve of the top changes so little in moving the bridge back or forth 3/4 of an inch, that the bridge height can easily be compensated for, with a change in height of a few thou. I say this to my students all the time when cutting the heel angles - you dont say, well I want 1.5 degrees, and cut it for that. You measure the body directly, and transfer that angle to the table saw (or however you cut the heel in your shop) and go from there. Whether its 1.5 or 1.4 or 1.6 doesnt matter - its all in the lie of the top radius .... John is right on when he says keep the blocks steady as you bowl the top and back radii (ie -use spreaders), and when gluing on the top and back - but I would add, dont just cut the neck angle willy nilly - measure it - just in case something moved in the process. |
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| Author: | crich [ Wed Apr 25, 2007 2:39 am ] |
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I never thought of that before, the angle of the sides only matters in cutting the heel angles on your neck. The radius of the domed top is everything. That being the case the radius one chooses has to change the height of the dome in the bridge area. So then does placement in the dish. If I centered the bridge area in the deepest part of the dish, I raised the top in the bridge area. I try to be consistant in placing my plates in the dish and sanding the rims in the same area of the dish. You are right, If I move the bridge back on the top 3/4" for 13 fretter, I guess that would make little difference. Man, this means I have to make a new dish..  I'm just not getting the results from a 30'. Clinton
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| Author: | crazymanmichael [ Wed Apr 25, 2007 2:58 am ] |
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whilst it is correct to say that the radius of the top will change the height of the dome the change is really very small. and where you do your gluing or sanding in the dish is irrelevant. it is a segment of a sphere. every part is equidistant from the center of the sphere. putting the bridge area in one part or another will not affect bridge height. the radius of the will and the radius is the same everywhere on the dish. it is a sphere, a three dimensional circle. every point on its surface is equidistant from the center. the cutting of the neck heel cheeks at an angle to suit whatever height the bridge is at will determine the closeness of fit and the saddle height etc. |
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| Author: | Barry Daniels [ Wed Apr 25, 2007 3:02 am ] |
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So I take it you guys make the top completely domed instead of using a flat upper transverse brace. |
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| Author: | crazymanmichael [ Wed Apr 25, 2007 3:21 am ] |
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i dome mine, but sand in a flat spot so that the fb extension will sit with out gapsat the sides. i suppose it would be more technically correct to sand a dome into the underside of the fb extension to match the dome of the top but that is too much trouble for what to me is no gain. just as i don't like the idea of gluing a radiused top onto flat rims, i don't like the idea of the stress involved in the transition from domed braces to flat braces. i know many do it with good results, but it just doesn't feel right to me. |
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| Author: | TonyKarol [ Wed Apr 25, 2007 4:51 am ] |
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Top is completely domed on mine .. you need to flatten under the fretboard only -- but its minescule, a few thou and even then, its only in the very centre of the FB ext area, as it too, is its own little dome. Check it with a straight edge and see ... |
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