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| Flattening the top under the fb extension http://www-.luthiersforum.com/forum/viewtopic.php?f=10102&t=3420 |
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| Author: | Pwoolson [ Tue Oct 04, 2005 12:55 am ] |
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Hi all, I know we've had this conversation somewhere before but I can't remember where. So let's revisit. I'm doing a lot of rethinking and redesigning of my guitars. 90% due to necessity. Most of the redesign is in the neck/neckblock area to guarantee perfect projection every time. My current method of flattening the top under the fb extension is that of Charles Fox and Lance. I use the paddle and sand it flat. But I'm not convinced that that is the most accurate/predictible way. I would like to sand the linings flat and then mount the top flat in that area. But I'm having troubles figuring a way to do this. any thoughts? Thanks in advance. Paul |
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| Author: | Brock Poling [ Tue Oct 04, 2005 1:02 am ] |
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I do it the FF way. after I profile the entire top in a 28" dish to get the linings flush, I switch over to a flat "dish", and chalk the neck block and linings down to the widest part of the upper bout. I put a .150" stick under the tail block and sand the upper portion by swinging/sanding the rim from side to side until the chalk is removed. My upper transverse brace as well as the patch are also flat (no radius). If everything works the way it should (which it does most of the time... well... this part anyway) you should get about 3/32 clearance at the bridge location when you put a straigtedge on the centerline. |
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| Author: | John How [ Tue Oct 04, 2005 1:35 am ] |
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My method is this. After I shape the sides with linings in my dish and while the sides are still in the form fresh out of the dish, I use a straight edge to bridge across the ribs just behind the neck block to check the height of the block. It is usually about a 32nd to a 16th taller than the ribs. I use a small hobby plane to taper the block so that the straight edge will slide up unto the block while still resting oon the ribs. Then I know I'm close. Then I will use a larger flat board with a sheet of sandpaper to finish sanding the upper bout area above the sound hole flat. Just make sure you stay above and do not sand completely to the waist. Mr Hoffman's idea looks quite interesting though and I might look into that a little more. |
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| Author: | Rod True [ Tue Oct 04, 2005 5:22 am ] |
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I use the Hoffan meathod. Two tapered beams located on eather side of the ribs on the side of the mold. Run the router on a sled to create a flat taper from the point at the top of the sound hole to the front of the body. Once the top is on and bindings installed, I elevate the tail of the guitar and sand the tappered, flat portion of the body on a flat surface covered with 120 grit. |
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| Author: | Pwoolson [ Tue Oct 04, 2005 6:20 am ] |
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Rod, can you explain this a little more. I'm curious why you should need to sand it differently if you have straight braces going onto straight linings. |
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| Author: | John How [ Tue Oct 04, 2005 7:42 am ] |
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My guess is he is just trueing up the flat surface prior to finishing so thaat he will be assured of a flat surface to attach the FBE. just a guess. |
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| Author: | Rod True [ Tue Oct 04, 2005 8:49 am ] |
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John, your correct. Also I cut the tapper before I installed the linings, and now that I think about it, this is such a small tapper that I don't need to cut the tapper till after the linings are installed and I think I will do that next time. Just translate what I do to the back on to the top. So once the top is glued on, I do the sanding to true up the flat portion under the FBE as John suggested. |
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| Author: | Rod True [ Tue Oct 04, 2005 9:00 am ] |
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I'll post a mock up pic if that would help Paul. I just have a body with the top being glued on as we... uh... er speak er type. I want to make a small wheel that clips onto the form to create the slight tapper to do the final truing of the FB area. |
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| Author: | BruceH [ Tue Oct 04, 2005 10:53 am ] |
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Hey Rod, How much taper from top of sound hole to the front of the body are you talking about? 1/16" ? 1/32" ? Thanks. |
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| Author: | Sylvan [ Tue Oct 04, 2005 11:05 am ] |
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Here is how I do it. I divide my top into two sections: soundhole towards the neck and the soundhole to the rear of the guitar. Initially I sand the back portion of the top (soundhole to tailblock) with a 25' radius dish. I then flip the dish over and use the flat side to sand the front of the top (soundhole to neckblock). Before I sand the flat portion, I place a 1/2" stick which is wider than the guitar under the side of the dish closest to the tail block so that the flat sanding dish is now canted toward the neck block. Chalk up the neck block and the linings to the soundhole with colored chalk and start sanding. When the chalk is gone you will have a flat top that slants back at the slight angle I use for my neck and fretboard. What you will find is that the soundboard drops down about 1/32-1/16" at the neckblock. You may have to experiment slightly with the thickness of the stick for your neck angle but doing it in this manner insures that the angle of your fretboard and neck will match the angle of the top exactly. Once it is finished your top will sit in two planes - flat to the soundhole and radiused from the soundhole to the tailblock. |
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| Author: | Tim McKnight [ Tue Oct 04, 2005 11:43 am ] |
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I do mine the same way John How does except my sanding stick only has a piece of sandpaper in the center of it to sand the neck block extension level with the linings first. Then the the entire upper bout area is dressed flat with a tool similar to the level w/ sandpaper. |
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| Author: | Rod True [ Tue Oct 04, 2005 12:02 pm ] |
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Bruce, I cut 1/16" taper from front of sound hole to front of the body. Works out to be 0.88+/- degrees neck angle. |
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| Author: | L. Presnall [ Tue Oct 04, 2005 12:23 pm ] |
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Hey guys, just curious...why do you elevate the tail of the guitar before sanding the upper bout area? I've always just used the radius dishes on the guitar as it sat in the mold on the bench...(of course, I have always had to play with the neck angle on my guitars, and I'd prefer more predictability and repeatability)...what am I missing here? |
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| Author: | BruceH [ Tue Oct 04, 2005 1:30 pm ] |
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Rod and Sylvan - Thanks for the details. Now I've got a handle on what I need to do on my next build. |
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| Author: | Rod True [ Tue Oct 04, 2005 4:47 pm ] |
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Larry right? When I sand the upper bout area, I do it upside down. So picture the body all glued up, flipped so the top is lying on a flat board with sandpaper attached. Now raise the tail of the guitar and sand the upper bout area flat. There has been great debate over which way to ensure the FBE is nice and tight to the top of the guitar. I have chosen to flaten out the upper portion of the top to do this. Some just use the 25-28' radius for this. I have layed all of this out on cad some time ago and all of the angles end up about 0.75 to 2.2 degs. You be the judge as to which angle you want. There seems to be quite the difference here. |
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| Author: | Pwoolson [ Tue Oct 04, 2005 10:43 pm ] |
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Guys, thanks for the insight. If I may be so bold to ad a few comments: I think those of you that flip the dish and sand that are flat are in pretty much the same boat as I am right now. Sure, you can get it flat, but how are you assuring the angle? I know there was talk of a wedge under the tail block and then sand and if that works for you, great. The reason, however, that I'm rethinking this is to have perfect results EVERY time. I'm sick of doing neck resets on brand new guitars. So, I think Charlie has my vote on best design. The two rails can be cut to the angle you want the projection to be and just route away. I see no room for error (unless the router slips of course). Thanks again. Paul |
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| Author: | Josh H [ Tue Oct 04, 2005 10:51 pm ] |
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This has been a good discussion. I am in the same boat as you Paul and am rethinking this aspect of my guitars. I do the sanding method with the stick thing as well, but I want more consistent results. Josh |
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| Author: | Brock Poling [ Wed Oct 05, 2005 12:42 am ] |
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I don't see why my way isn't precise. The height of the stick ensures the correct angle in the body. And coupled with your tenon jig the neck angle ends up perfect every time. After combining the two systems this is a very easy way to go to get great results. |
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| Author: | Pwoolson [ Wed Oct 05, 2005 12:51 am ] |
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Brock, you have a good point. I was thinking of it in "reverse engineering" terms. I'm doing everything by the numbers and I was thinking that I NEEDED a 2.2? set on the top. When in reality, if I do the top first, as I would normally do, I can just match the neck to the top angle. |
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| Author: | Josh H [ Wed Oct 05, 2005 1:01 am ] |
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Hmmm....maybe my problem is with the height of my stick I am using under the tail block. I will have to go and check that. Josh |
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| Author: | John How [ Wed Oct 05, 2005 1:08 am ] |
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Brock and Paul, using your neck set jigs (and they are nice jigs), you should have no problem setting your necks to the correct angle but you still want to make sure that the top is cooperating by building in the the correct angle(of the top at the neck block) and there is a corret angle for every design. This angle will be the one that allows you to get the proper string height over the sound board for the string torque. It may vary on different designs so that is why it is important to set the angle of the top in the upper bout as compared to the radius of the rest of the top. |
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| Author: | Andy Zimmerman [ Wed Oct 05, 2005 2:13 am ] |
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This may sound like a stupid question....If you know you want a 2.2 degree angle of the top for a particluar design, why not just sand it in with your dishes. Mine has a hole in it and it spins around a center pole. I can lean on either the tail block or neck block side as I sand and can change the relative angle. I guess you are trying to just engineer a predictable way to do it. Andy |
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