Official Luthiers Forum!

+1 on tite bond. I only uses ca flood for shell which obviously won't swell

My only comment is that .110-.115 seems really thick to me. I do .095 on dreads for rosewood, and .090 for anything smaller. Always reserving the right to change my mind on any given piece...

I do back strips both ways- in a routed channel or between the two halves of the back. When using thinner strips inlaying in a channel is a little easier, although if needed to go between the back pieces wood can be glued to the back of the strip to bring it up to full thickness.
I sometimes glue the strip between the plates and use wider bindings and purflings to "grow" the back plates and make a slightly larger instrument from wood that would otherwise be too narrow.

If we use an interior support strip (BJB), why don't we use one for the top (TJB) ??

My first guitar with a Sitka top got a little hump at the joint, so I put in a couple of cleats.

Ed

Classical guitars often have a fan brace glued over the center seam. A couple of cleats might not be a bad idea.

Martin has done it that way forever. I have done it both ways, and gluing it between the halves is easier. It also gives you a bit more width in the back, if you need it.

I suppose the best answer is that spruce glues better than rosewood. The other answer is that a mosaic backstrip has little strength. Even when inlaid into the back, it weakens it. IMHO, a mahogany back with a plain wood backstrip or no backstrip at all does not need the reinforcement. Before 1930, Martin has used a diamond cleat on the most critical area of the top seam, which is just below the bridge.

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John

I glue the decorative strip between the halves, and I believe it actually becomes stronger than inlaying it in a routed rabbet. I plane the edges of the back halves like the soundboard, and glue them up with the center strip and clamps for a very tight fit. If you inlay it in a rabbet, the fit will probably not be as tight, and what's left of the back's long grain wood under the strip isn't very strong, so all in all a pretty flimsy joint. Not that any of this really matters, as the cross grain reinforcement on the inside will make the joint very solid again. The weakest area actually becomes the wood right next to the reinforcement ('stress riser?'), but if the wood is normally stable and the instrument is not abused, it usually isn't a problem.

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Rian Gitar og Mandolin

I do it the same as Arnt, leaving the plates a little thick for glue up and then sanding the whole back to final thickness.

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Good judgement comes from experience. Experience comes from poor judgement.

John said

"Before 1930, Martin has used a diamond cleat on the most critical area of the top seam, which is just below the bridge."

That is where mine developed a hump. Can't hurt to put on in just in case.

Thanks

Ed

I liked Arnt's answer and I was going to clamp the center strip between the two plates today on a classical I am building, but the strip I have is thinner than my back target thickness. So looks like I will inlay it.

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http://www.Harvestmoonguitars.com

I've used both methods. I tend not to use back strips anymore because I like the look of a simple joint but when I did do it I had my own personal ideas about it. If it was a mosaic then it would be inlaid into a routed out channel but if it was a solid strip then it would be joined together. My reasoning is that a mosaic is a bunch of pixels of wood glued together and doesn't have enough strength given certain climate changes. In hindsight it seems to me that routing would always be a stronger joint as you are creating a sort of lap joint. But I've never tested it nor have I ever read any tests on it.

I've never had a back joint with a strip of any method fail but I have had back joints with no strip fail so there is either something going on there or it's bad data for bad intuition.

Martins used to have thinner backstrips, too. They would glue a strip of Spanish cedar on the back of it to make it thicker, then glue it between the halves.

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John