Wrong radius dish for top-15ft What to do

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Wrong radius dish for top-15ft What to do

Postby Bob Howell » Tue May 07, 2019 11:27 am

Looks like I have made this mistake 1- or maybe two times. I see it can mess up the neck angle and mating with bridge and saddle. If I work this out what else should I look out for down the road?

Can only say distracted with life. I am going to label them .
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Re: Wrong radius dish for top-15ft What to do

Postby Barry Daniels » Tue May 07, 2019 12:21 pm

Just change the neck angle and make it work out. Shouldn't be a problem if you allow for it.
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Re: Wrong radius dish for top-15ft What to do

Postby Dave Meyrick » Tue May 07, 2019 3:10 pm

That does mean quite an arch on the top. As Barry says you are going to have to work out the geometry quite carefully.

A few things come to mind. In no particular order:

If you presize your sides based on a different radius they will likely be the wrong shape. Particularly if it is a cutaway.

As you will know it is normal to flatten out the arching in the upper bout, sometimes just in the middle where the fretboard lies. Normally by using a flat or near flat upper tranverse brace(s) and the head block. This will be a bit more extreme in your case.

Depending on the type of neck to body joint you are using there may be more work round there to get things right. Your neck is probably going to need to be set back a little compared to the bigger radius.

The trick is to keep measuring at each stage of the process and adjust joints appropriately. You can't just say oh that'll probably do. Don't ask how I know this!

I use a bolted straight mortice and tenon joint so can adjust the neck angle vertically and horizontally relatively easily. With my classicals I usually then have to taper the fingerboard extension to get it sitting flat on the upper bout.

I will admit I have found this one of the hardest areas to get right but with a decent instruction manual and patience I can usually get it about right.

Best of luck with it

Dave
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Re: Wrong radius dish for top-15ft What to do

Postby Bob Howell » Tue May 07, 2019 3:45 pm

It is a parlor based on Scott Antes plans .
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Re: Wrong radius dish for top-15ft What to do

Postby Bryan Bear » Tue May 07, 2019 4:34 pm

Draw it out from the side with all the parts and figure out where you need to be. I'm assuming that this parlor is a 12 fret, if it has a normalish scale length I would guess that you find that the plane of the top (under the fretboard extension) will need to extend out to about 2.5 mm (about 0.1") over the top at the bridge location. You can probably get away with putting a 2.5 mm (or whatever it works out to be) and sand the top with a straight edge that spans from the shim to the tail block (if the top is thick enough). If you haven't done the binding yet, it will be easy to keep an eye on how thin the topwood is getting.

I'm sure others will have different approaches to getting the geometry right.

I've never seen those plans but (I think I remember hearing that) they are drawn fairly heavy. A tighter top radius will also stiffen up the top some. You may find that you want to reach in and carve some of the braces a bit more.
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Re: Wrong radius dish for top-15ft What to do

Postby Bob Howell » Wed May 08, 2019 10:41 am

IMG_0334.JPG
This is the first parlor. I finally got the neck angle satisfactory this week. Initially the end of the fret board was pushed up at the roseate, causing buzzing from 8 -19 fret . Filed the bottom of the fb in that area, the top just under it, and blocked the fb up with veneer as it meets the body about 1/32".

I reduced the x brace to 1/4 x 1/2 at the x. and then quickly thinned to 1/4x 3/8 ". Other areas built lighter also. Bridge 5/16 high.

I realized what I was doing last week as I finished a second parlor sound box.

Notice the gap as the fb/neck meets the body.
Letting it settle in for a week and then I will lower the action a 1/32" or so.
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Re: Wrong radius dish for top-15ft What to do

Postby Bryan Bear » Wed May 08, 2019 1:06 pm

Reading this again, I think I may have overestimated the height of the sanding shim. I know the OP's problem is solved but just in case anyone is reading this in the future. . . I originally estimated a 2.5 mm shim at the bridge location with a sanding beam spanning that shim and the heel would get you close to the angel you need. Thinking about it more, it seems like it would be closer to 2 mm. But again, DRAW IT OUT; that is just an estimate based on a lot of assumptions.
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Re: Wrong radius dish for top-15ft What to do

Postby Bob Howell » Wed May 08, 2019 1:31 pm

I am only guessing on this guitar that I used 15' radius on top because of the problem I incurred. Last week I caught myself doing it on a current build. This is the 1st of 3 Parlors I am making.

On #3 I know I did it. From comments I understand it will not encounter any other oblivious problems.
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Re: Wrong radius dish for top-15ft What to do

Postby Chuck Morrison » Mon May 27, 2019 8:31 pm

I've been using a 15' radius on my tops and backs for several years on both classical and acoustic steel string guitars. The arch goes from block to block with no flattening of the upper bout. I pre-arch double tops (classicals usually) to that 15' radius, which avoids the stress inherent in bending a solid top over arched bracing. These tops resist the "S" shape that many flat tops develop. I know that some see this as a positive thing, but I don't.

For the neck, I start with 90 degrees for the fretboard to side(s) angle. That leaves a gap between the fretboard and top that needs to be filled with a wedge of the same material as the neck.
NBNOM2.jpeg
Neck Angle on 15' radius top

I use a bolt on mortise attachment and can adjust the angle fairly easily if it's needed or simply use a thicker or thinner wedge under the fretboard extension. I wait until I have it pretty close to where I want it before drilling the bolt hole(s) in the upper bout block.

I don't know that any of this is helpful, but It's how I deal with the neck angle issue on high arch tops.
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