I’m making a round table out of a beautiful walnut crotch slab. Table will be 48″ diameter. Thickness is 1-7/8″. I have the slab and it is in the 10 percent moisture or less range. I checked flatness – there’s about a 3/16 crown on the under side when I put a 48″ level on it. I ordered some C-channel and plan to install that, hoping to pull it flat. I don’t have access to a wide planer, sander or CNC (like Blacktail Studio does on youtube)
So, how hard should I work on the flattening aspect? Would really appreciate any comments, suggestions, especially things to do/not do on this and general construction issues.
Replies
Ignoring the underside... how flat is the top surface? If it is out by the same 3/16 you can flatten it with a router sled setup and leave the underside as is... depending on the table's base design. At almost 2" thick pulling it flat using screws seems unlikely.
Yup. Ignore the bottom. Unless you have a hydraul8c press at a shipyard, you won't pull a slab that size flat.
I wouldn't ignore the bottom, unless the structure of the base can ignore the crown. A router sled would be a good way to flatten out both sides. If you plan to keep the slab flat, you need to have the base structure go across the width of the crotch, which likely is the way it is crowned. A scrub plane and a straight edge would also work, and might be enjoyable if you like using hand tools. I agree with others that you won't have much luck pulling that flat, and even if you do, it is much more likely to crack or pull loose from the base over time; not what you want with a fine table.
Another option I just thought of is to make a pair of straight dados in the bottom to fit the base structure into. They would not need to be at all deep at the ends, and would be deeper at the crown. This would allow you to attach straight pieces to the underside. You would still need to flatten the top.
Just a head's up: I always smooth and sand the underside of tables that don't have skirts for at least 4" from the edge of the table top, as people who approach a gorgeous table often unconsciously touch the underside instead of the top that intimidates them.
I hope the table is lovely!
As jharveyb mentions, don't rule out handplanes if you enjoy using them as much as I do. You'd save on gym membership.
The underside has to be flat. If it's not it'll pull the understructure into twist.
Get the part that will register to the aprons/understructure flat before you plane the other side to finished thickness. If the part exposed to the world is a little bit wavy, most people will never notice. If you pull the understructure into twist and have a leg 1/2" off the ground, well, you have a major problem on your hands. And God forbid the thing have drawers, runners, and kickers. You'll never get anything to work properly.
Anybody telling you otherwise doesn't have a clue about making furniture.
That's really not true. At all.
People made furniture for hundreds of years with the bottoms of tabletops hewn with an axe. They bore no resemblance to "flat" the way most current woodworkers think of it. Legs were leveled in the last step of making the piece, which is still best practice, whether it's a chair or a table.
If the bottom is really wonky, it needs a little straightening. But flat enough to put a straightedge across? Nope. Nope.
The OPs underside has a 3/16 crown over a 4 foot length. You won't find a piece in a museum with a bottom that flat.
Floor boards in old houses, Willamsburg vintage, were only flattened on the bottom side and parallel to the top sides at the floor joist locations. The rest was left wild. The bottom of the table could be treated the same way, only the area in contact with the support structure flattened and coplanar with the top. A lot less work, especially with hand tools.
I agree with everyone else that attempting to pull it flat won't work. I know this from destroying a beautiful table top, attempting to do the same thing, about 40 years ago. I life in a major city so I would pay my local cabinet shop to flatten it for $50 in less than an hour. If you love using a plane, great but it's a ton of work. Do both sides - you'll like it better. If you can get/build a router sled that's your best option outside of a major cabinet shop.
I think the bottom does need to be flat if you're planning to use steel.
C channels are going to bend, don't bother with it. If you want to put some steel that will really work, use T steel - the stuff they use under bowling alley lanes. It can be imbedded in the wood if desired.
My suggestion is get it reasonably flat - don't shoot for perfection right now. Router sled is the best method. No way I would touch crotch wood with a hand plane!!
Then make some VERY stout cauls, 2x4 hardwood, maybe even 2x4 steel channel, and clamp the top up to hold it - VERY IMPORTANT!
You're just about there moisture wise. If this is out in a shop with uncontrolled humidity, I would either move into your house, or into an enclosed room with a dehumidifier for a few months.
Then check for flat and to a final skim coat with router sled, get some finish on both sides, get the understructure mounted ASAP, and cross your fingers. Crotch is prone to cracking and splitting. :-)
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