problem resawing

[Barry Lyndon]

I just tried resawing for the first time and had a pretty big issue. I started out with a nice straight (no cupping, no bowing) 5/4 Walnut board about 7 inches wide and 6ft long. I resawed it in half so I'd have two boards about 1/2 inch thick. After resawing both boards have a huge bow in them. If I stand them back up together (with the resawed faces together) the are together at the ends but in the middle there is like a 1/2" to 3/4" gap. In fact, the force of them pulling apart was so string that when I got to the end of the board while resawing it just snapped apart while I was sawing.

What is the issue here? Is there something I'm doing wrong? Am I buying the wrong lumber? Did I just get a bad board?

I bought it at a good lumber supply, and used a 14" Delux Rikon with a 3/4" timberwolf resaw blade if any of that metters.

[Mike OMelia]

Does the wood have a lot of figure? Resawing can cause heavily grained and figured wood to change shape. Just put it under something heavy at a proper humidity setting for a while. Maybe even apply a heating blanket. I cut boards to 0.12" for guitars. It can be scary. Borson Resaw actually glues the board to a stable "sacrificial" plank so that released stress will not affect the remaining cuts.

Mike

[Myk Rian]

For a 14" saw, I recommend a 1/2" 3tpi blade.
I've had the best luck with Timberwolf blades, although there will be others with their own favorites.
Use the flutter method to set the tension.

Wood moves when you cut it and release the tension in it. You never know what it will do until you make the cut.
Did you buy it at a Borg, a lumber yard, or from a local sawyer?

[Mark Singer]

Its the internal stress in the wood.... Look at the edge when you select wood to resaw... symmetry around the centerline is a good choice

[John TenEyck]

The bowing could just be due to the moisture content being different in the middle vs. the outside. Just wait a day or two and see if they straighten out. Leave them leaning against a wall or on edge, not flat on one face. If the moisture content was different inside vs. out, they will straighten out. I've seen it happen in as little as over night. If they stay bent then moisture was not the problem; it more likely was internal stress.

[Barry Lyndon]

Mike, it doesn't seem to have any more figure than any other walnut but apparently there was something going on in there.

Myk, would a 1/2" vs. a 3/4" blade have made any difference in this situation? A couple of others have suggested a 1/2" as well but I already had the 3/4" on hand so I used that. I'm assuming this has no affect on the issue I had. Maybe I'm wrong though?

Mark, thanks that's a great suggestion! I'll be sure to look at the grain on the edge when I'm looking at a piece to resaw.

[Dave MacArthur]

Purely internal stress, nothing to do with re-saw per-se. This is discussed in just about every thread in "stock preparation" (you might use the search with that phrase for some good ideas). A lot of folks buy boards that are thicker than needed, or mill their own, and prepare their stock from same. The process is usually to:
1. cut the boards to rough length first, reducing all the bow/curve a long board might have that will cause machining/safety issues as well as wood loss when trying to straighten. Length meaning distance along the grain.
2. rip boards to rough width next, width being distance across the grain, usually on a flat-cut board. This width can change with moisture, so it's best to oversize. Boards can also curve after cut, so that you have to joint the edges (one concave and the opposite edge convex) straight, thus losing width.
3. re-saw boards to rough thickness. Thickness is usually talking about "quarter-sawn" or radial grain direction, that is from the center of the tree towards the bark; but this is most common only because most wood is "flat sawn" or "plain sawn", making it's thickness radial. However, in a quarter-sawn board the grain directions of width and thickness are reversed.
4. Let the wood sit for a few days to move
5. Mill square by: joint one face, joint one edge, plane opposite face, rip opposite edge. Still slightly oversize maybe.




I've been meaning to write this anyways so this may be a useful time. Here you go, wrote this just for you ;)
Wood Movement and internal forces (http://www.sawmillcreek.org/showthread.php?162619-Wood-Movement-and-internal-forces)

[Harvey Pascoe]

A lot of good advice here, but you did not mention whether the winding of the board was straight. That could have done it. On the other hand, six feet is a long length to resaw and expect perfect results. If you're final length will be less than six feet, I'd suggest you that you size the board closer to the final length and width you will be using before resawing.

If both boards are of equal thickness at both ends and the middle, you did nothing wrong, its the board.

[Jim Finn]

as Dave said....having the wood sit in your workshop a few days before cutting worked for me when I had this problem.