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More on WRC for steel
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=20396		 Page 1 of 1
Author:  Alan Carruth [ Thu Jan 08, 2009 4:02 pm ]
Post subject:  More on WRC for steel
I've tried to post this on the original thread five times, and gotten dumped every time. Let's see if this works.

I've used a lot of cedar over the years, and except for the dents, I love it.

The only tips I'd add refer to the bridge. Cedar has lower splitting resistance and peel strength than spruce. If the wood starts to let go along the back edge of the brdige it will just peel off, even if the glue joint is perfect. I've seen a couple of bridges (not mine, thank goodness) that have peeled off with a complete layer of top wood on the underside; the glue line didn't fail, the top did.

One of them was a production instrument, and there was no top wood torn out beyond the bridge outline; it was an absolutely clean and sharp break. When they scribed around the bridge to remove the finish from the top the worker cut into the wood rather deeply, and the glue didn't get into the cut enough to stick it back together. My partner and I suspected that whoever it was had just put a new blade in his/her X-Acto knife, and wasn't used to how sharp it was. The remedy here is to always use a new or sharp blade, and get used to that aspect of it so that you only cut into the finish.

The other major thing you can do is to make the bridge wider than usual, so that the length of contact along the direction of the string pull is greater. When you load a glue line in shear the stress is not even all through the joint. The stress level through the middle of the joint is uniform, established by the nature of the glue and the wood, and how well the joint was made, but the stress rises rapidly at the edges. It's the maximum value at the back edge of the bridge that sets the load limit for the joint. Making the bridge wider means that more of the total stress is taken up in the low-level part in the center of the joint, and this reduces the maximum stress at the edges.

Martin went to the belly bridge when they switched to higher tension steel strings. The actual footprint of a Martin bridge is smaller than a typical classical bridge, but they stay stuck down better because the area is distributed in such a way as to reduce the maximum stress along the back edge.
Author:  Dave Ellingsworth [ Thu Jan 08, 2009 4:10 pm ]
Post subject:  Re: More on WRC for steel
Thanks Alan,
That's an excellent explanation of counteracting the forces affecting the bridge.
I have some WRC that I want to use in a future build and this info will come in handy, no doubt!
dave
Author:  walnut47 [ Thu Jan 08, 2009 4:28 pm ]
Post subject:  Re: More on WRC for steel
Those of you who have been building with cedar tops, what are your thoughts regarding fingerboard and bridge woods?
Thanks,
Walter
Author:  MaxBishop [ Thu Jan 08, 2009 5:34 pm ]
Post subject:  Re: More on WRC for steel
Alan,

A question: are there similar problems with WRC on a classical/flamenco?

Thanks,
Max
Author:  Alan Carruth [ Fri Jan 09, 2009 3:23 pm ]
Post subject:  Re: More on WRC for steel
Max:
Wood and glue don't know the difference. I use a slightly wider bridge on my cedar topped classicals too.
Author:  Mike Collins [ Sat Jan 10, 2009 4:56 pm ]
Post subject:  Re: More on WRC for steel
Just one warning;
Cedar does not hold a glue joint as well as spruce.
As Al has stated .
Meaning braces separate easier from it and this can be a problem.
I have a late 80's Taylor in the shop with loose braces;it's cedar & rosewood.
The ends of the finger braces(next to the X) all let go .

This is not the first time I've seen loose braces on cedar tops-I've
fixed hundreds over the years!
This does not mean it's a bad tonewood-you just need to tuck the brace ends well or put a dab of epoxy on those ends..
Mike
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