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I joined the rosewood back plates together and applied my reinforcing strip which moved slightly to one side at the upper bout when clamped, but doesn't look to be a problem to me at least. But the real issue came whenever I was routing a channel for my decorative center strip. I had a makeshift fence set up for my router and it moved ever so slightly because I failed to secure it good enough. So instead of a straight channel I have a slight "S" pattern going on. It is hardly noticeable until you hold the back at eye level and parallel to the ground. I went ahead and inserted the decorative strip and filled in the gaps with epoxy and sawdust and it looks fine. I'm just worried about the integrity of the joint.

Why rout for the strip when it can be safely applied between the plates ? I don't see a reason to add another process to building a guitar. You will get a cleaner joint and not as much work. I use the same method CF Martin used for over a century . The cross grained spruce will support that joint very well.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I don't either, unless the grain lines in the wood don't go together well, then I think a thin strip of whatever wood is used for the binding breaks up the look. I have a cocobolo set that I will probably put a .080 strip of ebony between the plates to make it look better. I agree with John Hall, glue the strip between the plates.

Fred

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Fred Tellier
http://www.fetellierguitars.com
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The are used as decoration , that is all.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I assume you mean the back graft (reinforcement)? The only reason they are needed is because the decorative center stripe makes a weak spot, particularly if it's something like herringbone and runs all the way through. That and "Tradition"

edit: and backs are traditionally thinner so that may justify it as well.

I still put them in on backs without a center stripe though. It takes only a few minutes and is easier than answering the questions I'll get if I leave it out....

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I too put in a crossgrain reinforcement strip even if I don't use a decorative purfling / marquetry thing on the back. I can tell you from experience that they are a LOT more difficult to put in after the box is closed and you need to put one in to reinforce a repaired joint...

Whenever I want a decrative strip on the back, I glue it between the halves, like John Hall describes. One reason to use one can be to make the back just that much wider.

What I don't get is why you routed for the decorative strip after you glued the reinforcement strip, or did I misunderstand that part...? That sure seems backwards.

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Rian Gitar og Mandolin

Don't worry. As long as the joint is well made (tight) it will be just fine. It's just an aesthetic issue.


Some guitar builders claim that an inlaid center strip will result in a stronger joint than having the strip just between.
But I'm not sure if this is true.


As some told before: It looks nice, doesn't it? Guitar tops have a rosette instead

@ Todd: nice setup pictures.

For those of you that glue the decorative strip between the halves (as opposed to inlaying), how do you prepare the joint? When preparing the joint of two plates you 'close the book' as it were, put the stacked plates on the shooting board and keep shaving fine shavings until the joint passes the candling test. Now, with the decorative strip to be glued between the two plates, how do you handle the shooting board thing? Do you have the decorative strip stacked on the first plate, joint it so it passes the candling test, then do the same thing for the second joint on for the other side of the decorative strip? For me this would be impossible because when I'm trying to get that joint in the back (or top) plate perfect I end up shaving off quite a bit of wood (say 1/8" from both plates). This is not a big deal when dealing with the top or back plates but taking that amount of wood from a decorative strip would reduce it to nothing. I've been curious about this for awhile, any insight much appreciated.

Cheers,

Stefan

You could re-route the channel but go a tad wider than you planned, to straighten out the line. A good solid jig (like what Todd showed) is a great way to do it, especially if you plan to do more in the future. Trust me...having the right tools the first time will make a huge difference (I am a past pro at being in a hurry, half-assing something...then just having to re-do it the right way) .

As to the use of a 'center strip'...I would agree that it is an aesthetic choice...minimal to no structural need there. I like them, unless you do have a dynamite book matched set of course.

-Matthew

I did a few tests on back strips for strength years ago. Both samples used the same 28 martin style strip. One I inletted into the back plate the other glued in the middle and both received the cross grained spruce. I did a control without a perfling strip and cross grain and one with nothing. All 3 joints did about the same with cross grain support. The routed inletted did show a flaw as the joint may have had an integrity issue where the perfling strip may not had seated well in the bottom .
The plain joint cracked along the glue line and across it but the majority of the glue line held. In all the supported joints , the cracks didn't enter past the cross grain . The perfling ( 28 style checker board ) did show some stress related cracking .
The process I used was plates were 6 inch wide supported on the ends within about 1 inch and I kept adding weights till failure. The cross grained joints were within a few pounds of each other , I don't remember the exact weight but it was around 10 to 15lbs when they failed and the plain one went with the first weight I put on it.
I don't really think the perfling is more than decoration , but the cross grained spruce does make a big difference. The fact that the perfling strips showed stress , was something I couldn;t tell if it was from the weight or the fall shock. The back plates did bend more than I though before failure.
The fact that the joint strength was much stronger with the support sold me on using it . No bracing was used in conjunction with this test. I also use tite bond for the glue.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Older Martins used a cleat. There is more support on the top with the braces and plate then there is in cross section of the back. Also spruce tends to glue better than most of the hardwoods. Look at the back and you have spans of about 4 inches between braces , on the top the spans are much less. The top from the neck to the hole is glued by the fretboard popsicle and truss rod brace , then the bridge and plate area really supports this area along with doing the energy transfer. On most top cracks you see the top open behind the bridge , and that was the area you would see a diamond shaped cleat , between the tail block and lower tone bar. The rosette supports the joint as do the sound hole supports so you can see there is a system in place.
Also , and this is more of my opinion ,is that the added mass may not be a good thing on the top.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Also glue up the stripe when joining the back. If using a zipper or similar multi piece stripe after thicknessing the back I flood both sides of the stripe with thin ca glue. Then scrape clean. Not sure it makes it much stronger but it lets me sleep at night,
Tom.

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