Cupped slab for Nakashima style bench

[Robert Johnson]

I'm a fairly new handtool woodworker and I'm building a Nakashima style bench as my first project. (I'm basically copying the George Nakashima's R bench.) I have a large 8/4 slab that my wife and I recovered from her grandparents' Pennsylvania farm about 20 years ago. I think it's oak, but I'm not sure. It's about 8 feet long and tapers from 14 to 16 inches in width. I plan to crosscut the wide end to make a leg for one end of the bench.

The 2-inch thick slab has a 3/8-inch cup and I'd like to flatten the top and leave the bottom the way it is. Both sides are currently quite rough. I can't decide whether to plane the concave side or the convex side for the top. I'm leaning towards flattening the convex side and making that the top. Are there any advantages or drawbacks for choosing this approach? I'd like to avoid causing more cupping or perhaps encourage the non-planed side to be more flat.

— Robert in Albuquerque

[David Weaver]

I'd knock the high points off of the edges on the concave side first. Figure that's the side pulling together, and you want to relieve it.

[Chris Vandiver]

I'd first look at the slab and determine which side is from the outside of the tree. This side is typically the show face and it is also the side that will cup more readily than the inside face. Nakashima was greatly influenced by Japanese woodworking and for the most part, a Japanese craftsman would never use the inside face as the show face(unless it were facing in), if you get my drift. Another thing to keep in mind is that if you plane one side down you need to do something to the oppisite side to act as a counter balance(or the slab may continue to cup). Carvers use this technique often. The cupping you're experiencing may be due to the slab sitting cupped side up for a period of time, so that it is drier than the side facing down(sunlight, etc.). As David Weaver says, the concave side is the easiest to flatten.

[Jeff Heath]

I agree with all that is already mentioned ^^^^^, and would add that a moisture meter will really help you tell what's going on within the wood. I work with slabs alot, and I can tell you that cupping is usually the result of uneven drying (unless, of course, you've got a piece of timber loaded with reaction wood). You do need to work both sides, though, if you want it to stay reasonably flat.

[Sean Hughto]

Flatten both sides and choose which is the "top" based upon which looks more appealing to your eye (grain, color, knots, live edge, etc.).

[Brian Holcombe]

I think Nakashima mostly uses the outside face as the show side.

[Prashun Patel]

It is the leg side that requires more critical flattening.

[Chris Vandiver]

It must be remembered that Nakashima had access to the highest quality slabs found anywhere(he built mainly for Rockefeller) and his lumber inventory was(is)legendary. I'm sure his meticulous drying and storage methods made cupping less of a problem, for the most part. His designs don't lend themselves to the use of unstable lumber. In fact, they rely heavily on the stability of the slab. Any cupping on the top will move the underside as well, bringing the legs with it.
What goes around comes around.

http://www.nakashimawoodworker.com/furniture/9/43

[Brian Holcombe]

His tables often utilize battens or joinery that serves as a batten.

[Jim Matthews]

A bench need not be flat, to provide comfortable seating.

If this is your first effort, solid joinery for the base
and stability are paramount. If the curve is subtle,
why not keep it?

Adrian McCurdy bench.jpg

http://www.adrianmccurdy.co.uk/stools-and-benches.html

[Chris Fournier]

It must be remembered that Nakashima had access to the highest quality slabs found anywhere(he built mainly for Rockefeller) and his lumber inventory was(is)legendary. I'm sure his meticulous drying and storage methods made cupping less of a problem, for the most part. His designs don't lend themselves to the use of unstable lumber. In fact, they rely heavily on the stability of the slab. Any cupping on the top will move the underside as well, bringing the legs with it.
What goes around comes around.

http://www.nakashimawoodworker.com/furniture/9/43

A slab, especially of the scale that George Nakashima used is inherently unstable. Nakashima went to lengths to stabilize his projects without them tearing themselves apart.

I helped sell a quantity to livesawn lumber to his daughter Mira and there was nothing exceptional about the dimensional stability of the lot. It was large and nicely figured, simple as that.

Personally I'd flatten both faces and choose the show face as Sean has suggested. If this is not your cup of tea then I would machine as you see fit and watch for movement - taking several weeks to do so. Joinery is always easier when using flat, square surfaces.

[Chris Vandiver]

A slab, especially of the scale that George Nakashima used is inherently unstable. Nakashima went to lengths to stabilize his projects without them tearing themselves apart.

I helped sell a quantity to livesawn lumber to his daughter Mira and there was nothing exceptional about the dimensional stability of the lot. It was large and nicely figured, simple as that.

Personally I'd flatten both faces and choose the show face as Sean has suggested. If this is not your cup of tea then I would machine as you see fit and watch for movement - taking several weeks to do so. Joinery is always easier when using flat, square surfaces.

Nakashima has been dead for 23 years, so I seriously doubt you sold him wood while he was alive and certainly not during his heyday(1950's-1960's) when most of his work was done for the Rockefellers. The wood his daughter buys might have a different criteria and of course. really premium, old growth slabs are very hard(and expensive) to come by.

[Sean Hughto]

I'm interested to know what premium old growth slabs are like. Do they not cup and breath like other slabs?

[Chris Fournier]

Nakashima has been dead for 23 years, so I seriously doubt you sold him wood while he was alive and certainly not during his heyday(1950's-1960's) when most of his work was done for the Rockefellers. The wood his daughter buys might have a different criteria and of course. really premium, old growth slabs are very hard(and expensive) to come by.

I was very clear in my post about whom I helped sell wood to. I very seriously doubt that you read my post.
Wood moves in predictable ways old growth or less so. Large scale pieces will move considerably and at different rates from edge to edge due to grain orientation.

The more things change the more they stay the same.

[Kees Heiden]

I have made a slab table last year. I used two 40 cm wide ones and a narrow one in the middle, so the slabs weren't super wide. 4cm thick, 2.10 meters long, and no glue joint in the middle, they are about 1 cm apart. Sturdy undercarriage with thick stretchers sideways and two lengthwise.

I flattened both sides. Which was a huge amount of work with handplanes. One of the slabs decided to twist within a week after I flattened it. The other only bowed a little , which was no problem with the sturdy under carriage. But the twisted one was a problem, because it wanted to lift a leg. In the end I deepened the offending place with a gouge, so the underside is not so nice anymore. The table has been through a wet summer and is now in the middle of the dry inhouse winter climate and it behaves perfectly well.

We discussed this a bit back when I was working on the table and several people reported middle age tables made with very thick and very wide boards. These were only flattened on the top, the underside was cut out where the tables were attached to the stretchers. So, these didn't selfdestruct either.

Conclusion? Hmmm, no idea really.