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PostPosted: Thu Jan 10, 2008 11:50 am 
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Cocobolo
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Hi All,

I'd like to tap into this vast pool of knowledge again if I could.

I made this sanding jig to sand the upper bout to the correct angle so I can have a 1/2" saddle height from the top plate. 3/8" bridge and 1/8" saddle above the bridge.



The guitar rests on the center piece of cork at the bridge location. The 2 outer pieces of cork keep the box from tipping from side to side.

Here it is in "use".



On my sanding board I have 80 grit on one end and 220 on the other. The sanding board is the same thickness as the board with the 3 pieces of cork.


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PostPosted: Thu Jan 10, 2008 11:54 am 
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Cocobolo
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Oops, hit the post button instead of preview.



I'm trying to get about .050" gap at the bridge location when I hold this straightedge on the fingerboard gluing surface.

right now it's about .080"


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PostPosted: Thu Jan 10, 2008 12:03 pm 
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Cocobolo
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Trying to get this all in before my dial up connection craps out.

So, basically the thickness of the center piece of cork determines the gap I get with the straightedge (less the thickness of the sandpaper).

Does any of this make sense?

Any comments about my setup would be greatly appreciated.

As always, thanks so much.



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PostPosted: Thu Jan 10, 2008 12:15 pm 
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Cocobolo
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Oh yeah,

25' top radius.
1/4" fingerboard thickness (at center).
.043" fret height.

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PostPosted: Thu Jan 10, 2008 12:52 pm 
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Koa
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Kirt,
It seems like that will work just fine. I accomplish the same thing but via a
slightly different route. I place a shim of determined thickness at the bridge
location. I then take a sanding board that "rests" atop the shim and then
proceed to true up the upper bout ramp.

I find truing the upper bout to be absolutely essential in getting a perfectly
straight fretboard.

All the best,
Simon


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PostPosted: Thu Jan 10, 2008 1:00 pm 
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Koa
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Kirt,
I forgot to mention that the setup I use might be a bit easier to use since
there is no guesswork involved. However, I have never tried it the way you
are and it might work great for you. If you find that it takes you more than 2
mintues to get the upper bout ramped correctly then you might want to try
it my way and see if that works any better for you.

Guitar building is all about finding what works best for YOU. Let us know
how it turns out.

--
Simon


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PostPosted: Thu Jan 10, 2008 4:27 pm 
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Cocobolo
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I'm not getting this, are you sanding a 'notch' into the top of the guitar to fit the fretboard??


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PostPosted: Thu Jan 10, 2008 4:50 pm 
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Brazilian Rosewood
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Another option is sanding the angle into the rimset before attaching the top.  Then the sanding technique you show can be used for fine tuning and not remove as much wood from the top.  Here's a jig that works for me for doing that.
http://kennedyguitars.com/Kennedy%20Guitars/Upper%20Bout.htm l

Terry


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PostPosted: Thu Jan 10, 2008 5:02 pm 
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Here's the link fix


http://kennedyguitars.com/Kennedy%20Guitars/Upper%20Bout.htm l


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PostPosted: Fri Jan 11, 2008 12:17 am 
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Cocobolo
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Thanks Simon,

I thought of doing it that way, but my sanding board was hitting on the bottom part of the sound hole (bridge side). I didn't see any need to sand all of that area. Maybe a 25' radius is just a tad too sharp.

What thickness of shim do you use at the bridge location?

Anyway, I raised everything up so that area would clear. That way I can concentrate on the area above the soundhole. I got this idea from Jim Olsons website.

Here's his:



He moves the guitar on his but I'm moving the sandpaper on mine.

There shouldn't be any guesswork involved once I settle on the correct gap at the bridge. It should all depend on the thickness of that center piece of cork.

Does .050" seem like enough???

I've heard that when the fingerboard is on with the frets in, a straightedge on top of the frets should just kiss the top of the bridge. Taking my previous numbers into consideration I calculate .082" gap.

.375   bridge height
-.043   fret height
-.250   fretboard thickness
______
=.082

If .082" is a better number then I'm pretty much there without any sanding, and that would be great.

Terry, I like that idea to get the rimset trued up before hand. Thanks for the link. I radius the ends of my transverse brace but leave it flat in the center.

If I can get this jig set right it should really help me zero in on that angle with repeatable results.

So what do you-all use to get this right?

Thanks again.



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PostPosted: Fri Jan 11, 2008 12:27 am 
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Cocobolo
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Blackheart,

No, not sanding a notch just tweaking the angle on the upper bout so the fingerboard makes a smooth transition at the neck/body joint. Just need a flat wide enough at the correct angle to glue the fingerboard to the top.

Cheers

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PostPosted: Fri Jan 11, 2008 2:31 am 
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Brazilian Rosewood
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This is what I use to sand after the top is on.  There are many variations on this theme.
Terry




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PostPosted: Fri Jan 11, 2008 2:49 am 
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Kirt,
What you are doing will work - it is just a matter of how well it will work
for you. I can't imagine Olsen doing anything that wasn't well thought-
out and highly efficient...

You also have to take into account the bridge area "rising" up a small
amount under string tension. It isn't substantial for my guitars but I do
take it into account. I like to have a good 1/32" air space above the
bridge going from the top of the frets.

With a 25' radius you may or may not need to sand that area. I use a 28'
radius and my upper bout needs just a little touching up. But with a 25' I
would most definitely be sanding off some of the rosette first.

Once again, results are all that matter so go with what works best for you
- but if you find you are sanding a lot of material from the upper bout
region, then something is not set up correctly.

All the best,
Simon


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PostPosted: Fri Jan 11, 2008 3:01 am 
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Old Growth Brazilian Rosewood
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Yep lots of variations on this theme is right-on.

For me it starts with how I construct the guitar.  The upper transverse brace is not radiused at all which flattens my upper bouts.

In addition, prior to putting the top on, I sand the rim on a flat board, face down, with a 1/8" shim under the tail block.  I make pencil marks on the "kerfed linings" from the center of the waist all the way through and including the neck block.

With the rim face down I move it back and forth on the flat sanding board a couple dozen times until the marks are gone.  This effectively takes the radius out of the upper bout and flattens it.  Then the top, with the flat upper transverse brace goes on.

IMHO the advantage here of building the correct angle into the box is that you don't have to sand flat the top in this area and if it needs any attention at all it is very minor.

In the event it needs attention for the fret board to sit nice and flat I use the fret board as the sanding caul as below.

























Notice that this is very fast and easy - just the way I like it.....

Often, because I build the upper bouts flat I don't have to do this at all.

And then of course when the fret board is leveled on the neck/guitar I can also sand in a slight drop away from the 14th to the 21st frets.



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PostPosted: Fri Jan 11, 2008 3:33 am 
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Cocobolo
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A little sketch in Autocad shows that the difference between a 25' and 28' radius is about .018" in the 20" span from headblock to tailblock. So that is probably why I'm hitting the rosette area.

I've seen this top move that much or more during some of our humidity swings we've had lately. Need to make sure my humidity is right when I do this.

So, NOW I'm thinking that maybe I need about .110" gap at the bridge location, when checking with the straightedge. That's .080" from my figures above plus .030" belly rise under tension. Sounds like alot.

My first guitar ended up with a thin bridge (about 5/16") and a saddle a little under 1/8". It turned out alright, but I'd like to avoid this in the future. It doesn't give much to work with years down the road when the action starts to rise. And I think you get more tone with a taller saddle.

Someday I'd like to go with an adjustable neck and floating fingerboard extension as some do here. Seems that would take care of everything.

So Terry, what's the difference in thickness of the 2 pads on the rig in your last post? What do you shoot for as a saddle height?

Sorry if I'm being a PIA but I want to get this right.



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PostPosted: Fri Jan 11, 2008 3:40 am 
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Oh, thanks Hesh. I didn't see your post.

That does look easy. I like that too.

So when you take your fingerboard back off, what gap do you get at the bridge position when you lay a straightedge on the surface you just sanded?


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PostPosted: Fri Jan 11, 2008 4:18 am 
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Old Growth Brazilian
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One thing to keep in mind when calculating this based off using data from specific dome radii.

Depending how and if you sand the dome radius into your rim and linings the incline formed by the dome can be more than you expect.

For instance say you sand the dome fully into your linings, neck block and all. Now if you have a positive means of insuring the central axis of the dish is always parallel to the sides and in the center of the guitar you can reasonably predict the incline formed by the dome along the centerline of the guitar.

On the other hand, if the dish is tilted a little towards the neck the the incline will be more than you had calculated, and you FB plane will be high. If the dish tilts toward the tail block a bit the then too low. Don't as how I know this . (one revers wedge is one too many)

That said if the plane of the dome is maintained plumb. Then a 28' radii requires very little sanding tweaking to fit a 1.5 deg neck angle. Mostly just a matter of taking the rise formed by the dome out so that there is a flattish plane from FB extension edge to the apposing FB extension edge. Of course scale length, and body length effects this some because of the bridge location in relation to starting point of the dome at the neck and the central axis of the dome, but is true for a 25.4 or 25.5 scale with the central axis of the dome centered in the center of the body along the centerline of the guitar.

After having dealt with the tilted dome issue once or twice I may have convinced my self that not sanding the dome into the tops linings is not such a bad idea after all. Despite the added stress on the lining/top joint. For me that is still up in the air.


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PostPosted: Fri Jan 11, 2008 4:57 am 
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Cocobolo
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Thanks Michael,

I radius everything on the rim, linings and blocks to 25'.

After I get this flat correctly sanded for the fingerboard extension I plan on measuring the angle between that surface and the neck mounting surface, and then cut my neck at that angle, whatever it is.

Seems like that should take care of that situation if I read you correctly.


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PostPosted: Fri Jan 11, 2008 5:04 am 
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Old Growth Brazilian
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yep that will take care of it. I cut the basic set angle when I cut the tenon then fine adjust by flossing.


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PostPosted: Fri Jan 11, 2008 5:55 am 
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Cocobolo
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Thanks all, I love this place.

I wish I had you guys around back in 78' when I first got interested in building guitars. It took another 15 years before I finally started one, then another 10 to finish it.

Course I got married and had to get a real job in the meantime.

I admire and envy those that had the drive and means to start back in those days.

Cheers

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PostPosted: Fri Jan 11, 2008 8:38 am 
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Brazilian Rosewood
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Kirt,
The pad under the pivot post on the second jig is a little over 3/8" which is just a little over my bridges which are around 11/32".  The sanding pad is 1/4" plexiglass to match the fretboard.  My guitars seem to move between 1/32 and 1/16" strung up.  I like to have a straightedge on the frets just clear the top of the bridge under string tension.  This seems to do it for me.  If I get the angle on the rimset right I don't have to do much with the second jig.
Terry


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PostPosted: Fri Jan 11, 2008 9:13 am 
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Hesh, you win the award for simplicity. I dig it.

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"The Dude abides. I don't know about you but I take comfort in that. It's good knowin' he's out there. The Dude. Takin' 'er easy for all us sinners. Shoosh." The Stranger


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PostPosted: Sat Jan 12, 2008 4:36 am 
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Thanks alot for all the help, everybody. I think I have it figured out.

"The Dude abides".    I love that movie.

Cheers

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