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Flatsawn Bridge?

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Flatsawn Bridge?

Postby Michael Baresi » Tue Jul 19, 2016 2:38 pm

What would be the likely negative effects of using a flatsawn rosewood bridge.
Which direction might it warp and how would it interact with the top?
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Re: Flatsawn Bridge?

Postby Alan Carruth » Tue Jul 19, 2016 4:36 pm

Generally, as wood goes from green to dry, it warps in a way that tends to straighten out the annual ring lines as seen on the end. This is due to the fact that, for most woods, tangential shrinkage is greater than radial shrinkage. A flat face on the 'bark' side will tend to go concave, and on the 'heart' side it will go convex. The less ring line curvature there is, and the smaller the difference in shrinkage rates, the less trouble you'll run into. Quartered wood pretty well avoids curvature in the grain lines for relatively thin sections, and that's one reason we use it.

Flat cut wood also has somewhat more of a tendency to split in my experience. Partly this is due to the greater shrinkage, but also partly to the fact that the medullary rays somewhat reduce the strength of the bond between cells. Anybody who has split a lot of fire wood knows that it's usually easiest to split along the radius, perpendicular to the flat cut face.

I like to use skew cut wood for bridges, with the annual ring lines as close as possible to forty five degrees on the end. This has the highest shear modulus, and the highest splitting resistance. It does tend to have a low cross grain Young's modulus as compared with quartered wood, but the difference is less in hardwoods than it is in softwoods, and, at any rate, is not really an issue in a guitar bridge IMO.
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Re: Flatsawn Bridge?

Postby Michael Baresi » Tue Jul 19, 2016 4:56 pm

Alan thanks for the knowledgeable reply.
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Re: Flatsawn Bridge?

Postby Eric Knapp » Tue Jul 19, 2016 4:57 pm

Alan Carruth wrote:I like to use skew cut wood for bridges, with the annual ring lines as close as possible to forty five degrees on the end. This has the highest shear modulus, and the highest splitting resistance. It does tend to have a low cross grain Young's modulus as compared with quartered wood, but the difference is less in hardwoods than it is in softwoods, and, at any rate, is not really an issue in a guitar bridge IMO.

Does it matter which direction the 45 degrees is pointing? This is a good tip.

Thanks,

-Eric
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Re: Flatsawn Bridge?

Postby Ryan Mazzocco » Tue Jul 19, 2016 6:55 pm

Yep. Skew cut all the way. Think about it. If your bridge is quartersawn all of your grain lines are running perpendicular to the surface. And so is your saddle slot and bridge pins all in a row. this weakens your bridge in all the wrong places. While quartersawn seems to be the preferred cut for most anything on a guitar the bridge is the exception. IMO, flat sawn is better for a bridge but also has the problems Alan mentioned above. Skew cut is a nice balance.
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Re: Flatsawn Bridge?

Postby Eric Knapp » Tue Jul 19, 2016 7:22 pm

Ryan Mazzocco wrote:Yep. Skew cut all the way. Think about it. If your bridge is quartersawn all of your grain lines are running perpendicular to the surface. And so is your saddle slot and bridge pins all in a row. this weakens your bridge in all the wrong places. While quartersawn seems to be the preferred cut for most anything on a guitar the bridge is the exception. IMO, flat sawn is better for a bridge but also has the problems Alan mentioned above. Skew cut is a nice balance.

Do you align the rings like this?

Code: Select all
Neck ---> \\\\\\
          Bridge


Or this?

Code: Select all
Neck ---> //////
          Bridge


Thanks,

-Eric
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Re: Flatsawn Bridge?

Postby Ryan Mazzocco » Tue Jul 19, 2016 7:43 pm

Eric, Good question. I've honestly never noticed or cared. But there may be a right answer and if so I'm looking forward to hearing it as well.
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Re: Flatsawn Bridge?

Postby Alan Carruth » Thu Jul 21, 2016 1:28 pm

I'm not sure it matters which way the ring lines run in a skew cut bridge. I've never had any trouble with them, so I can't say which is more likely to split out.

Aside from the bridge, I also like to use skew cut wood for the bridge plate, liners, and the tail block, for the same reasons.
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Re: Flatsawn Bridge?

Postby Eric Knapp » Thu Jul 21, 2016 1:44 pm

Alan Carruth wrote:I'm not sure it matters which way the ring lines run in a skew cut bridge. I've never had any trouble with them, so I can't say which is more likely to split out.

Aside from the bridge, I also like to use skew cut wood for the bridge plate, liners, and the tail block, for the same reasons.

Thanks, Alan. I'm filing this tidbit away for when I make my first non-archtop acoustic. I'm hoping that will be sooner than forever from now.

-Eric
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