I have to build a solid oak dining table 48" by 10' for a client. I got some 6/4 slabs; the finished thickness will be 5/4 or so. What size should I rip the boards for a stable top? I'm thinking 5". The top will be floating on the apron. I do not use oak very often.
(Furniture Making Forum)
From contributor P:
Why rip them down? I would just max out the width of each board to get your 48. It will expand and contract seasonally no matter what. Just make sure there is a lot of room for it to move, and make sure you seal the bottom equal to the top.
There seems to be two schools of thought regarding grain orientation in panel layup. One says alternate the growth rings, the other says keep them the same. In the first case, while alternating growth rings provides for a flatter overall top, it will be wavy as each board cups. The second case can result in the entire panel bowing, but if your design has robust bracing (like a wide apron or trestle style table), that can minimize the overall bow (almost eliminate it in some cases), and then you don't have the wavy surface to deal with. I've made them both ways, and don't really have a preference except for final appearance. When you lay out your boards, in either a same or alternate ring plan, look at the overall panel. Depending on the actual grain pattern of the individual boards, it may look much better one way than the other. If the best look is with a same ring orientation, then you need to critically examine your design to determine if the cross members are strong enough to restrict the bowing to a manageable level. The amount of eventual bow (or cup for individual sticks) is dependent on MC as Gene was saying. Also a function of humidity variations in your locale.
Having said all that, I would probably approach it as follows:
- rough mill all lumber to 1/16" over thickness.
- lay out an alternating growth ring pattern using an odd number of boards (I suggest an odd number so that you can have the outside two boards oriented the same way - and I prefer to orient them so that any cup would be upwards. My theory is that 'tis better to cup up and not have pencils rolling off the table, or any spills running more quickly over the edge).
- board width at this time would be in the 5-6" range, and ultimately determined by how many are necessary to get to your final width. Be sure to take into account your machine capabilities - in your case it would be nice to stay under 24" so you can lay up two 24" panels, run them through your shiny new 24" widebelt, then join the two halves. With an odd-number layup as I just described, that might not work if you need 48" final width. But you could fake that by ripping the center board and using each half in its own layup. If you have an 8" jointer, you could maybe push that to 7" wide boards.
- after a suitable acclimatization period (several days?), run the last 1/16" off on your widebelt to re-flatten.
- final glue-up of the two halves, then a little hand work, or off to the neighbor's 52" widebelt for a final pass (obviously, if you go that way, don't take all 1/16" off before the final join).
And lastly, let's not ignore the customer here. If they understand wood and its true beauty, they may understand the fact that wood moves, and be quite happy to have a product that behaves that way. However, if all they want is pretty and flat, and can't be talked into veneered MDF, then you may have to be more conservative and go with narrower boards - like 3-4". Let's hope for the former.
For what it's worth, I made a set of four tables for a lady that loves wood - mahogany. All wood was cut from a single stick 22" wide, 4" thick and 9' long. The tops are 20"x20" (two tables) and 20"x22" (two tables) by 1-1/4" thick. And to top it off - no aprons! Almost two years and no unacceptable movement yet... and they look great with the tops made from a single stick. But the point is that she was willing to risk the top warping to have the benefit of a single slab. Not a lot of tables can boast that, at 20".
I'm not sure where it says that the narrower the strips of wood are, the less chance of misbehaving. Apparently Nakashima and Maloof both missed woodshop the day the instructor taught that lesson.
My only suggestion would be to have it screwed down to the base every foot or so (across the grain) with a screw that will give for expansion/contraction. Going across the grain… I'd use four of them - 1 at each apron, and 2 others 1/3 way across from each apron. Just drill a hole a little wider than the screw to let it move, then snug the screw into the top.
Further, my initial posting said that if there is no (or very small) moisture change, the entire question is not too important, as there will not be any significant change. With no great change, the width is not important. This is how some people are able to successfully (most of the time) use wide strips.
When using oak and a brace or skirt that has the grain at 90 degrees to the face, you will get quite a bit of movement difference, *if the moisture changes*. The idea of making the hole slightly oversize is not going to be adequate. For example, a 48" wide table made of flatsawn red oak will change width by 1% with just the normal summer to winter moisture changes. This means that the width will change nearly 1/2" overall, so you need at least 1/4" extra room in the holes on the edges. Further, the screws cannot be so tight that they cannot move or slide.
As a professional trouble shooter for over 40 years in the wood industry, I can tell you many stories about tables made with wide pieces and braces or skirts that did not have adequate room for movement. Anyone who ignores the movement of wood, which will occur from season to season, is asking for trouble.
In addition, anyone who does not build with the correct MC lumber is also asking for big trouble. Check the MC yourself. Do not rely on the supplier to give you the correct MC.
Regarding alternating the grain (rings up or down on adjacent pieces), as you get lumber that is sawn closer to the pith of the log, you will get pieces that are more prone to cupping with MC changes. Therefore, this technique can indeed help in some cases (when lower grade lumber is used, for example, or when the rings have a lot of curvature when viewed from the end grain) keep the table flatter overall. When the pieces are alternated, the "cathedrals" of flatsawn oak lumber will also alternate direction. To avoid this contrast, people will also alternate the pieces end for end and then the cathedrals will all align, if this is the effect desired.
I am not sure that a top with 3" maximum width versus a top with 5" will actually appear that much different. But if you do go to the more expensive furniture stores including PA House and those making antique recreations, you will see that even they use narrow strips. It is not just the cheap furniture.
To avoid the narrow planking affect from using narrow pieces, oftentimes the table top has a veneer laminate. With veneer, there are many beautiful grain patterns possible, including book matching. Veneering to achieve a fancy look with expensive furniture has been done for centuries.