Issue with preparing rim for gluing the guitar top

[Chuck Dvorak]

You know how they say, "Measure twice, cut once?" Well that's me, except more like, "Think a dozen times, cut once." My kit guitar project is making progress, albeit slowly. I guess I could begin every post in the same way. At least it is still progressing.

I am ready to profile the kerfed lining of the top rim to accept the guitar top. Following Bill Cory's "Building Martin-style Acoustic Guitar Kits" manual I glued the tail-block and kerfed lining 1/32" proud of the guitar rim, and I glued the neck block flush with the top of the rim. Now I feel this approach may have created a problem.

John Hall has a YouTube video, "Prepping the Rims to glue the back and top" where he first flattens the top with a flat sanding disc before creating a radius in the rim using a radiused disc. Should I be sanding the kerfed lining on the top down to be in the same plane as the neck block? I wonder why Bill Cory would suggest a different approach? And why wouldn't a builder just make sure the top of the rim is level, then glue the neck block, tail block, and all kerfed lining flush with the rim?


Thanks for any advice.


Chuck

[Ben-Had]

If you're gonna radius the top (25', 28' or whatever) and the kerfing isn't a little proud then when you sand the radius in you also sand the depth of the sides down and you want to keep that to a minimum.

[Kevin Sjostrand]

Hey Chuck,
What Bill Cory is telling you to do allows for a radius to be sanded onto the top and bottom of the neck and end blocks. If you glue them in flush and flat with the sides, there is no material to radius. What I do is pre radius the top and bottom of my blocks before I glue them in, with the outside edge just a shosh proud of the final height and this way there is very little material that has to come off as I radius the rims with the kerfing installed.
If you are flush with the rim you can easily glue on a piece of mahogany around 1/8" thick to the top and bottom of the block so you have material to sand a radius to. You need a radius, or slope at the neck block on top to compliment your neck angle.
Let us know how it goes!!

Kevin

[deadedith]

It's good to remember the difference between 'radius' and 'slope'.

Radius has to do with the kerfing.
Slope has to do with the rim.

It is not necessary to radius the kerfing on the top; the Martin factory does not do it. So if the kerfing is level with the top rim, you're fine as long as you don't mess with the 'radius'.

The 'slope' is the slight - 1 1/2* or so (the slope added to the neck angle must equal 90*) - drop off of the rim from the soundhole to the rim/headblock intersection. This can be done in a number of ways, with a flat disk or a flat sanding stick or even a router, and it is critical.

It's good that you are asking questions! And good luck to you.

[Chuck Dvorak]

So, I should have glued the neck block proud of the rim, just like the kerfed lining--is this correct?

Also, it is okay to shim up 1/32" piece on top of the neck block to meet the lining? Without getting the "whole picture" I wonder how this affects the fact that the block has already been mortised to accept the neck joint.

Thanks--

Chuck

[Ben-Had]

That's what I do for my tops. Sand the kerfing flat, sand in the slope and my top braces are radiused to 25' to get the complimentary angle.

[Ken Hundley]

I don't see an issue with adding a shim. There isn't a lot of stress on the guitar right here, so structurally, a shim shouldn't harm it. If you are careful, it shouldn't even effect it cosmetically if you are looking in the hole.

[Zen]

I don't see an issue with adding a shim. There isn't a lot of stress on the guitar right here, so structurally, a shim shouldn't harm it. If you are careful, it shouldn't even effect it cosmetically if you are looking in the hole.

I'm in the exact same place as Chuck--my top is meeting the blocks perfectly after sanding the kerfing with a sandstick. but when I turn the guitar over I notice that both blocks are a little below the sides/rim. And i have already sanded the kerfing on the back but the blocks are still about 1/16th lower--maybe slightly more. Do I shim both of them to meet the back ?
I posted this problem on the beginners thread as well so hope I'm not confusing anyone
Many Thanks again for all the advise guys

[tippie53]

the key here to set up the geometry in the sides so that the fretboard would remain flat and true. If you mismatch the geometry your neck angle will not be proper for a good action height and bridge and saddle height.
There are many ways to do this . You as a builder have to decide what process you will use . If the angle is too high you will have a fall off on the fretboard and too low your fretboard may rise. In my vids I stress that the angle is there for the target height of a 1/2 inch.

[Zen]

That's what I do for my tops. Sand the kerfing flat, sand in the slope and my top braces are radiused to 25' to get the complimentary angle.

When you say you "sand in the slope " thats where I get confused ?. I'm a little worried about the angle now especially having read tippie53 s post about the geometry and the fact that this step can really affect the action at a later stage. Do you have a method of sanding in the slope that you could tell me about ? What do you consider the slope should be and how would I check it ?
Thanks in advance for any advice
Zen