Dealing with a twisted slab

I just acquired a large (9' x 14" wide x 2.25" thick) white oak slab from a local sawmill destined for my inlaw's mantle. I hadn't noticed before I dragged it home that it's got a pretty severe twist (about 2" at the ends). It's mostly in the last foot or so of the board I can probably shorten it up to close to final length and get some out this way. I can also loose about a .25" on each side to untwist a bit, but I'm still looking at a pretty big effort to true it and keep 8'.

I've been looking at how to take it out without loosing too much thickness. The best advice seems to be "buy another slab" but I'm not sure that's viable in this case.

The best solution seems to be:
1) rip on bandsaw, (which is gonna be a challenge with a piece this big)
2) then true up the pieces,
3) then glue back together
4) then retrue.

Does that work for twist?
Most of the (rip and rejoin) technique seems to be focused on cup, not twist. The "kerf the back and press out" method probably wont work since it's a thick thick piece.

I should also mention that my main jointer is 2.5" wide as it's a 607. I do have a power planer.

I would cut close to final length, removing the part with the most twist, and trim to width, then see what you have to deal with.

Then you have to decide how much thickness you really need. I would be inclined to keep the slab intact, unless there was a very important reason for it to be absolutely as thick as possible.

But you can certainly save some thickness by ripping in two- the twist should be about half as much on each half after ripping, unless there is some weird tension in the board that is released after ripping. That is a possibility especially when the original slab has that much twist.

Is there any way you can return it, and select a more appropriate slab?
You know what needs to happen,and even then it's a crap shoot, but it doesn't seem as if you have the machine(s) to even your odds of success.
Big slabs of wood are definitely special, and nice, but they can present some special challenges. I always try to get them as close to the finished dimension as possible. When they decide to move, nothing within reason is going to stop them.

Unless Grampa's clock ,or some other heirloom , requires that width; I would just make it narrower. 8 inch mantel is pretty big for traditional designs. I remember telling a client it was too big. Changed her mind after actually seeing it installed. I've always tried to make them no more than 7, and that is too much for real formal stuff.

<quote>8 inch mantel is pretty big for traditional designs.</quote>

Alas, the supports stick out from the wall 10" as they used to stick out 9" and then had a inch thick stone veneer applied over the ends (and sides).

I'll try to get photos today. I will probably end up buying another slab as I like the oak and can probably use it for something else.

Make sure you know the codes about fire safety and distance of wood from the opening. A stone slab will not twist not will a cast concrete one.
Bil lD

Twisted wood has internal stress that will always try to relieve itself by twisting more. I would be afraid that after you get the board straight and installed the heat from the fire will cause more twisting. In my opinion, it is better to eliminate a problem rather than fix it, get another board.

Generally when a plank is twisted only at the ends it is because there are inconsistent stresses at the ends of the plank, as compared with the remainder of the plank.

Typically logs are bucked to length in order to cut out defects such as limbs. If the buck is made too close to the limb location, there will be stresses at the end of the log due to the different cellular orientation between the main trunk cells and the limb cells. These stresses are then transmitted to the lumber as it dries.

It is very difficult to remove this type of stress from within a board.

Here is a quartersawn sycamore board that had a limb location near the end of the plank. You can see where it both twists as well as bends up.

Attachment 370391

Do you know anyone with a bandsaw sawmill? It is a trivial thing to put a small slab like that back on a mill and flatten it. The sawmill you got it from might be willing to do it.

HOWEVER, unless it is kiln dried or has already been air drying for 3-4 years it is almost guaranteed to twist some more. If still wet inside and the twist was from shrinkage while drying, it will likely continue to twist as it continues to dry. (thick wood can be damp inside for years - if fresh from the sawmill it is probably dripping wet inside) If the existing twist was from internal stresses it may not twist much more if they are significantly relieved.

I studied twist once and found out twist from drying is a significant problem if the slab is cut at an angle to the axis of the pith. If aligned perfectly parallel, a board that is plain-sawn will still cup instead of twist (absent internal stress due to reaction wood, etc.) since transverse shrinkage is about twice that of radial shrinkage. Cutting a slab to minimize warp requires a log with straight grain and special care (and will likelly cost more).

If you can get a board that is perfectly quartersawn it is unlikely to twist, especially if stickered until dry. But even if quartersawn I would want it completely dry first. If thick wood is not completely dry before putting up as a mantel it will likely warp from the heat and accelerated drying on one side from the fireplace. BTW, I have seen several big mantels in houses, 5-6"x8-10" that were quite twisted. Maybe the homeowners didn't notice... On the other hand, I have a 6x8 piece of mahogany that was a mantel in a friends family for over 50 years and it's straight as an arrow. Oak, unfortunately, is known to be especially unstable.

I just thought of one thing that may work: Saw thinner boards from all surfaces of the slab and fasten them to a stable core of MDF or something, perhaps leaving the back unsheathed for mounting. Careful glue lines might be invisible. A friend of mine did this for a 10x10 support post in his timber frame house. I resawed a rough 2x12 douglas fir and he used the 1/2" boards to cover a dry pine post to match the other posts in the house.

If you lived closer I could probably cut a quarter-sawn chunk of white oak that size. I have plenty of logs. I'm going to cut slabs for outdoor steps and benches - I didn't think of mantels.

JKJ

Not knowing how long, or how well the slab was dried at the sawmill when you picked it up, chances are that it may still be working and will continue to do so, especially if there is still significant moisture in it. moreso from the heat of the fireplace. It's possible that a sawmill will accept returns so approach them about exchanging this slab for one that is more thoroughly dried and perhaps they could rough cut the slab for you. Who knows, they might even have a stash set aside for their own use & willing to give up a piece.
Generally speaking a "laminated" beam with alternating wood grain would be more stable in the long run. Could you build up a laminated beam & then glue a facia board to the "exposed" side to show off the natural grain?

The most efficient way to handle this is using a router with a flat bottomed planer bit.

Set up a couple rails and a sled. Woodwhisperer has a good video about this on his workbench build.

Here was my set up to flatten a workbench top.

Attachment 370397

I've found with twisted boards you never get the thickness you think you will.
Once you get one side flat its matter of planing it down and you'll see what thickness you'll get.

Keep in mind if twists are caused by stress and can reoccur even after the board is flattened.

Worst case scenario you can have it resawn into planks.


If they will exchange the slab, I recommend this, and they are really nice guys they will let you.

Without seeing the slab, it's hard to recommend to return it and buy another.

Slabs can be notoriously unstable because they are often wide flatsawn swaths, or two nicely rift or quartersawn halves separated by a region close to the pith that may be prone to cracking.

If your goal is a relatively narrow mantle, then I would explore ripping out the center section of the slab and joining the two outer sections, assuming the grain match and ray fleck is acceptable to you. You'll still be able to retain a live edge if that's important to you.

Narrower pieces will be much simpler to flatten - even if you have to joint them again for re-gluing.

I deal with this like John suggested. However, I have a sawmill so I have the capability to put the kiln dried slab back on the sawmill and make a skim cut so that one face is flat. That becomes the reference face on the planer, and I can get the slab perfectly flat and it stays that way because I do it only on slabs that are fully kiln dried to less than 10%. You put the slab on the mill and shim it so that it does not rock or move, then take off the least amount of wood that will flatten the top face. Then plane the other face.

Maybe you can locate a bandsawmill like a Woodmizer and ask the Sawyer if they will flatten one face for you. Craigslist is one good place to look. If you were close to me, I would do it for you.

Quote:

---

Originally Posted by Danny Hamsley

You put the slab on the mill and shim it so that it does not rock or move, then take off the least amount of wood that will flatten the top face. Then plane the other face.

---

My Woodmizer will saw down to 1" thickness so it might be possible to skim both sides while at it. (I have to make a skimming cut very slowly or the blade can flex and rise - sharp or new blade is better.) But as mentioned, I'd really want it dry first or it might just warp again!

JKJ

Would this be possible? - could the OP build a plywood chamber around the slab and steam it for two and a half hours? He could have a 2x4 frame in place under the steaming slab that is secured to prevent it twisting. After steaming he places another 2x4 frame over the slab and he clamps the frames together, flattening much of the twist, and keeps it all clamped up for a couple of weeks.

Having steam bent chair parts I know this would take a huge amount of clamping, with several friends to help get those clamps on, and I won't be surprised to hear it's just not possible. Which would be a pity because I have a eucalyptus slab that could do with a spa treatment...