Reinforcing Gap in Back to Rim Joint

[PSmill]

Hi, I glued on a back and despite a lot dry fittings which all looked good, there is a slight gap at the waist on both sides. Can't explain that one. Anyways, looking for the best method to fill the gap in a way that ensures structural strength. I am thinking, after cutting the binding channel, but slide some thin shim stock or sliver in there and glue it in with titebond. An outside job as it's out of reach inside. Does anyone have a better method? Thanks a lot. P

[tippie53]

dry clamp and see if that closes it. if it does glue clamp your done.

[PSmill]

Thanks. The back plate must have shifted a bit so the ends of the braces, where passing through the rim, were a bit too thick, preventing the back from seating completed. I used a thin knife to remove a bit of material from the brace where it meets the side, and that allowed the gap to close up, don't use those spool clamps too often, but worked well for this. Thanks.

[Njohneer]

My problem is with the top but may be similar. I found while sanding the top with 28’ radius disc, there is a span of about 2 ½ inches that dips at the waist on each side by 0.080-0.100 inches below the plane of the rest of the rim/kerfing. The back sanding went fine. I’m a first time builder - Martin Jumbo kit that was in a box for about 30 years – so it’s all new to me. Trimming or sanding 0.100 inch off all the rest will be difficult and don’t know what this would jeopardize. I think I can glue a thin piece on top there and sand down to match. If I were to just let it go, I think purfling and binding(0.063”x0.205”) will cover but may cause acoustic issues? I had planned to use gobar for gluing the top as well as the back but this issue may mean I need to use clamps instead to try to draw the top down.

I would appreciate any advice from experienced builders. Thanks

[jread]

Basically that part of the waist is where the chalk never comes off during dish sanding? Yup. Me too. As John said, I just smooth it out with a small sanding block then clamp shut and there is no gap or dip. Good to go

[MaineGeezer]

When it happened on one of mine, I did as you suggested: I glued on thin shims and sanded them down with a radius dish so the ends of the shims tapered down to nothing. Seemed to work. Or as previously suggested, see if you can pull it closed with a clamp and, if so, just glue it,

[Kevin Sjostrand]

This is interesting. I sand mine in the radius dish and I just keep sanding until all the chalk is gone. I do pre shape this area before I even bend the sides.
Then the back and top fit nicely.

[tippie53]

the top should not be entirely radiused, I only radius the area to create the neck angle for the fretboard extension. If it is working for you , that is fine.
When I am done I want about 1/2 inch of string height off the top. IF you over radius you over stress the top. What radius are you using

so by doing it this way yes I know the waist may gap and here is where the go deck is go good. I can check the tightness of the joint and make adjustments. A flash light in the sound hole shows you a lot.

the other point is that , even is you glue up the top and back in the dish , they don't stay that radius. they do moce because of humidity so keep all this in mind as you close the box

[phavriluk]

I think the problem makes sense: The sides are profiled as if the back was flat, but it's domed. The farther the edge of the sides recede from the edge of the dome, the greater the gap between the domed back and the side, most likely at the narrowest part of the waist. I use a sanding bar that pivots at the center of the dome's diameter so that the sides end up being higher closer to the waist, following the dome.