Back Strip

[jeffhigh]

Up till now I have always done the back reinforcement strip in cross grain material. but I am considering doing my next ones in long grain.
I don't do a centre inlay so that area is not weak and it seems to me that the cross grain strip is working against the long grain back when it comes to humidity changes.
Seems to me that whilst the crossgrain strip is stronger adross the joint, a long grain strip would be quite adequate.
Anyone tried this?

[Kim]

Have not tried it Jeff but thought the point of a cross grain strip was to support the glue joint during movements with humidity by the fact that it does 'not' expand in the same direction as the rest of the back wood.

Cheers

Kim

[jeffhigh]

What I am concerned about is that particularly with a thin responsive back, increased humidity acting on the crossgrain strip will tend to bow the centreseam inwards between the braces. at the same time as the panel as a whole is tending to expand outwards

[kiwigeo]

Seems to me that whilst the crossgrain strip is stronger adross the joint, a long grain strip would be quite adequate.
Anyone tried this?

IMHO I agree with this.

[Kim]

What I am concerned about is that particularly with a thin responsive back, increased humidity acting on the crossgrain strip will tend to bow the centreseam inwards between the braces. at the same time as the panel as a whole is tending to expand outwards

I do see your logic Jeff but I don't know that a 2mm thick 'softwood' crossgrain reinforcement strip that is 19 to 25mm wide expanding over its length with humidity could have enough influence on a braced hardwood back to be that much of a concern unless the plates were to be 'paper' thin. Mind you I can't see any reason not to try other than to say you will need to choose your centre strip material carefully, preferably a strip from the back plates themselves so you can be confident that everything will move with humidity at the same time and by the same degree.

Cheers

Kim

[jeffhigh]

What I have tended to see on a few instruments, is that when finish sanding the back, the centre seam area seems to be a bit low in between braces and takes more sanding to the surrounding areas to get a good surface for finishing. I am attributing this to the crossgrain strip.
I have a crossgrain longgrain strip set up in my garage as a pointer to indicate RH and it moves significantly.

[Kim]

What I have tended to see on a few instruments, is that when finish sanding the back, the centre seam area seems to be a bit low in between braces and takes more sanding to the surrounding areas to get a good surface for finishing. I am attributing this to the crossgrain strip.

I'm wondering if that could all be attributed to the back strip Jeff. Don't forget when it comes to volume of wood the braces themselves would have more substance than the back strip alone...softwood have larger cells and expands more, but they also compress more too so mass is a consideration.

Regardless I see the use of shellac to seal the inside of the guitar as a good move on the back and sides. It will not technically 'seal' the wood but it will offer a buffer against the swings so that movement is restricted to a lesser degree between the extremes...looks good and makes the inside easy to clean out with compressed air also.

Cheers

Kim

[charangohabsburg]

Hi Jeff,

Why not make a laminated centre seam strip? Start with gluing on a cross grain strip as usual, plane it down to "thinner as usual", and then glue a thin long grain veneer strip on top of it.

I have done something similar (although with braces) on the upper bout part of my charango after I had a problem with cupping. After gluing a 0.5 mm cross grain strip on top of the braces that piece never again moved (it was several weeks with great humidity changes before I finally glued it to the body rim). I plan to do that on all of my future instruments (with the transverse braces, "thick diagonal" braces and centre seam, but not with the fan braces of classicals).

[Allen]

Switched to long grain back strips a few years ago Jeff. I noticed the same thing you are talking about on uke backs which must be very thin and responsive. It made a huge difference to the issues you describe. I usually use the same wood that the back is made from so that it has a more uniform look to the back.

The back strip can also be thinner than spruce. On ukes I plane them down to about 1.8mm or a bit less.

[kiwinoz62]

Hi Jeff,
I am presently making 2 dreadnaught size cutaways & have done exactly what you've asked with the back strip.
One build is IRW b/s, sitka spruce top, other build Qld maple b/s (thx xray, beautiful wood) engleman spruce top.
All dimensions are thicknessed to 2.5 mm.
All bracing including the Back Strip is spruce & I am building to Scott Antes dreadnaught plan.
Both builds have been boxed up for the past week awaiting binding, so far I have'nt seen any cupping or irregularities with the backs.

With the active humidity in Sydney, both builds appear to be stable, but then, time will tell. . .

[Dominic]

I'm wondering if that could all be attributed to the back strip Jeff. Don't forget when it comes to volume of wood the braces themselves would have more substance than the back strip alone...softwood have larger cells and expands more, but they also compress more too so mass is a consideration.

Kim

But then not all softwoods are the same, WRC changes much less from humidity changes than spruce. And some hardwoods are much more prone to moving than other hardwoods and some softwoods so its hard to get a rule that fits all circumstances.

What do you mean by the last bit, 'they also compress more so mass is a consideration'. What are you getting at?

Cheers
Dom

[kiwigeo]

Is conventional back bracing part of the problem?

I've gone to using cross bracing on my steel string and classical backs. Had a classical back go awry on me a few years back and ended up pulling off the back and rebracing. Haven't had any issues with cross braced backed instruments.

[charangohabsburg]

Is conventional back bracing part of the problem?

I think that usually it is not a problem.

[kiwigeo]

Is conventional back bracing part of the problem?

I think that usually it is not a problem.

Youre trying to keep the back in a dome with a radius that's tighter than the top. Bearing this in mind and without getting into the detailed engineering surely a cross brace makes more sense than conventional bracing?

[ozwood]

Hi Gents,

Just playing devils advocate for a Minute , but could the problem also be because many people sand their braces in a radius dish ,which would possibly unduce some cupping in between the braces also, I have heard it suggested that it's best to shape braces with a router and a Jig , and to clamp them on to the back in a Jig that is radiused only across the grain (side to side) and left dead flat along the grain ( front to back) I witnessed this approach when I visited Gerards workshop , and the explanation was because they wanted to avoid exactly what you are describing.

Cheers,

[Kim]

What do you mean by the last bit, 'they also compress more so mass is a consideration'. What are you getting at?

Cheers
Dom

A brace has more mass than a section of backstrip and therefore, as it expands across its width, it does so with much greater force than a section of backstrip. Not only does it expand with greater force, it does so 'across the full width with the guitar',unlike the backstrip which is restricted to only 10mm either side of the centre seam.

What I am getting at is that perhaps it is this series of isolated areas of greater force which is having the 'most' influence upon the longitudinal movement of the back plate. If one thought this was a plausible thing, or even considered it a significant factor in that movement, then they would need to consider using narrower taller back braces to reduce the effect yet still maintain structural integrity.

Cheers

Kim

[kiwigeo]

Here's how most people seem to approach framing on a dome....multiple cross braces??

[jeffhigh]

Going with a Trevor Gore style back bracing on the current Pair
ie one brace across the centre of the lower bout with diagonal braces stiffening the surrounding areas

Plus transverse braces across upper bout and waist

[Kim]

Hi Gents,

Just playing devils advocate for a Minute , but could the problem also be because many people sand their braces in a radius dish ,which would possibly unduce some cupping in between the braces also, I have heard it suggested that it's best to shape braces with a router and a Jig , and to clamp them on to the back in a Jig that is radiused only across the grain (side to side) and left dead flat along the grain ( front to back) I witnessed this approach when I visited Gerards workshop , and the explanation was because they wanted to avoid exactly what you are describing.

Cheers,

There is a lot of good sense in that Paul, enough to warrant taking the time to make a back brace gluing base with a radius in a single direction across its width as suggested.

Thanks Mr Ozwood

Cheers

Kim