John is working on completing the Trestle Table
I used my tax stimulus money to buy a flat screen TV. The problem is that I have to walk past the TV to get to the shop. So I was enjoying TV in all it flatness when I saw the end of the table sticking out of the shop. It seemed to beckon me as it lay there cupped and in need of work. But, I am a man, so I grabbed the remote and ignored it. Now I am back at it and dealing with the cup across the width of the table.
The trestle table top (say that three times fast!) is made from two book matched cherry boards. They finished out at 103 inches long and 13 inches wide. I added a middle board to the two book matched pieces to get a final width of 31 inches. This was going to be a kitchen table but with the length, it goes into the living room. I could have cut the top…only kidding, I would never cut boards like that. Would you?
The thickness of the top is just under ¾ inch so it flattens when forced without that nasty cracking noise that indicates you just learned another lesson. The cup, which you can see in the picture, is a ¼ inch. For the picture I clamped on side of the table flat so you can see the total cup that I needed to flatten.
Now the bread board ends create a cross grain situation and wood movement becomes an issue. I like wood movement! There I said it. I like feeling the non flush edges that arrive with the seasons. My friend has a table from the great Christian Becksvoort and he insisted on bread board ends. He can feel with his fingers that the top has moved. I like these oddities and have added divots and ridges under the arms of chairs for a person to finger when sitting. These subtle aspects humanize a piece.
First thing to do was to determine the size and layout of the tennons. Since we have a cross grain situation I am only going to glue the middle tennon,. I decided on three tennons because it gives me a middle. I think five tennons would work if the width would accommodate it.I also used a stub tennon on the entire width of the top. This stub is 3/8 inch in length. The thickness of all tennons is a ¼ inch. A rule of thumb in deciding the widths of the tennons is the have all of them equal ½ the total width of the table. For me, I made them 5 inches for a total tennon width of 15 inches. It is also important to have enough wood at the ends of the bread boards so I started the tennons ¾ inch in from the edge. I marked the mortis locations from the tennons.I increased the mortis for the end tennons by 1/8 inch on each side to allow for movement.
One thing about mortis and tennons that is finally sinking in is that they are related. What you do to one has an effect on the other. For example, my first idea was a tennon that looked like the tongue from the Rolling Stones symbol. From the tennons’ perspective I was right, but what about the mortis? Chris pointed out that the walls of the mortis are just as important as the tennon. If the walls are too thin, the tennon will crack them and that old sinking feeling arrives.At least this time I learned a lesson without cutting wood!
Regards John
John – There’s a way to flatten that top without the work required to plane it flat. 1/4" isn’t very much, but clamping it flat might well result in a crack down the middle of the table. Instead, use a sponge or a plant mister to wet the concave surface, and set this surface down on damp (not wet) towels on your bench. Then, use a hair dryer (or a heat lamp if you have one) to evenly heat the convex side, checking the top frequently for its progress towards flat. When it’s flat (should take 5 mintues at the outside – DON’T leave it unattended or it may cup the other way), either glue and pin the breadboard ends immediately, or put the top in panel clamps.
I typically use the panel clamp method. After about 24-48 hours in the clamps, the panel has re-equilibrated with the atmosphere and "set" into a flat position at that humidity, much like a steam-bent part will hold its shape after its dried around a form.
The top must be completed and installed at the same ambient humidity that you flattened it in, otherwise the top will continue to cup in one direction or the other.
I’ve had individuals suggest that planing the surface flat is a better method because it permanently flattens it, but this turns out not to be the case. The top will continue to cup (often in the other direction) with changes in humidity even after it’s been planed flat.
David,
Thanks for the post. I have been late getting back because I have been pondering your post. I think you are completely correct, I do wonder if the top would want to go back to it’s orginal position of a cup when it hits equalibrium again?
I tell you what I did. I modified your idea because the size of the top is too long to work on my bench. What I did was to clamp the top down to the trestle base and wet the cup side. My goal is to introduce moisture on the cup side making it expand while the convex side is drier. I am keeping it in the clamps and hope that I will have the results that you described.
If that doesn’t work I think I will do the same thing again but add using a hair drier from underneath. That would allow me to put damp towels on the top as you suggested.
Thanks for your post
John
John – One other comment. Bob Flexner presented a very interesting argument related to table top flatness a while back. The question was, why are antique table tops always "cupped" (i.e., their sides turned upward), no matter what the grain orientation (heartside down, heartside up). The argument is that after repeated wipings with water, the table top wants to "bow" with the sides turned down. Since the fixing of the top to the apron prevents this, the wood fibers on the top surface get compressed beyond their elastic limit, and when the wood drys back out, the wood fibers on the top surface shrink, and cup the sides.
It’s a convincing argument, and a good argument for putting a film finish on a table top.