Major warpage

[myxpykalix]

I have a 2" thick slab that is 8 ft long X 24" wide and it has a major warp in it where it is bent up 1 1/2". Is there any way to flatten this and salvage it and end up with a board that is flat and about 1 1/2" thick.

[steve_g]

Jack…
Geometry says you’re not going to get a 1.5” thick, stress free board, from the parameters you’ve given… if it’s going to be used where you can hold it down, like a countertop, maybe.
This is how I level my slabs…


Attach two runners to the bottom side of the slab. Generally, I place these about 20% of the length in from the ends.
Put the assembly on the Bot, shim to get as stress neutral a position as possible and screw down. You’re nothing ahead if you force it flat, only to have it warp again when you release it. Using only two runners really simplifies finding the natural bent to the lumber.
Machine level
Turn the slab over, It should lay dead flat on the spoil board now. To machine this side, I use the “Bernie Tempest special edge clamp”… Cam clamps don’t work well on rounded or live edges! Both the BT clamp and the runners from step one assume screw holes are acceptable in the bottom and edges.
Surface the back side… if your total thickness after surfacing is critical, you may choose to not completely flatten this side. If I choose to leave the back side partially or completely “wild”, I’ll machine flat spots at leg locations if this is going to be something with legs, like a table or bench…



This method only works on wood that is quite thick and won’t vibrate when being milled, due to the unsupported areas of wood in the first step…
Good Luck!
SG

[gerryv]

If you want to minimize loss of thickness, some furniture makers have been known to counter-stress it with a jig and clamps mounted in a steam box. Typically this would mean pushing the moisture content up over 25% right through then progressively increasing the counter-stress. It could takes several weeks including the steaming and initial re-drying for such a heavy hardwood slab. If it is a natural warp it will want to come back but if it was induced, as in your case, it should work. I recall learning of this from a fellow who has a small, personal kiln that he could control the heat and humidity in who had success doing this but that was some years ago so the details are fuzzy.

Then, of course, there's kerfing but that's a mighty heavy slab!