YA question about wood movement

[Phil Stone]

First of all, I apologize if this post is in the wrong forum -- it doesn't specifically have anything to do with hand tool technique -- but I just can't bring myself to post in the "other" forum, y'know, the one where people are unironically suggesting using a ROS to clean up finger joints. (Just kidding! -- no offense intended to ROS users!)

Edit: ugh, can't believe I accidentally put a smiley in the uneditable post title!

I am designing a simple corner TV table for a flat screen. Originally, I was going to use white oak plywood for the top and shelf, to match the QSWO frame. However, I found I'd have to buy a full sheet of the stuff at the local hardwood supplier ($$$), so I opted for a solid top. Found some nicely-figured ash, which I think will go well with the white oak. When I was thinking of using plywood, I designed the top and shelf to sit inside the frame, to hide the edges. Now that I've switched to a glued-up top/shelf, I think this is a recipe for disaster, no?:

tv_table.jpg

I think that wood movement will cause the top/shelf to blow the frame apart in this configuration. The top/shelf are essentially wedged by the angled design, right?

It's a shame, because I wanted to show the interesting geometry of the frame at the top, perhaps even highlighting it with a reveal and shaped leg tops. But I think now that the top and shelf will have to rest on top of the frame members in order to allow seasonal movement.

I'd be interested to hear any comments or ideas.

[Shawn Pixley]

I think you could have the top and shelf nest in a groove in the horizontal members similar to how a raised panel sits in a door frame. You can then leave enough room for seasonal movement.

[Andrae Covington]

I agree with Shawn, make it a floating panel.

Doing some rough calculations with my LV wood movement chart... making guesses here, like the top being about a foot wide, 4% annual humidity change, flatsawn ash... looks like it's only going to move about 1/8" - 3/16". If it's wider than that or your indoor humidity changes more then of course there would be more movement.

[Phil Stone]

I agree with Shawn, make it a floating panel.

Doing some rough calculations with my LV wood movement chart... making guesses here, like the top being about a foot wide, 4% annual humidity change, flatsawn ash... looks like it's only going to move about 1/8" - 3/16". If it's wider than that or your indoor humidity changes more then of course there would be more movement.

I thought about this, but I'm a little concerned about losing span strength in the panels (i.e, if they are thinner at the edges to fit into grooves on the horizontal members). The table is 16" wide (and just under 48" long), so good eyeball estimate, btw.

I suppose I could make full 3/4" grooves, but the panels would have to be at least an eighth of an inch or so below the frame so that the top of the frame members wouldn't be too weak. That's a bit more reveal than I'd like.

Definitely worth thinking about more though, and I appreciate the suggestions.

[Doug Trembath]

Phil, I understand your desire for the boxed in look, but one alternative is to execute the boxed in portion, use that as an apron, and place a shaped solid top above it. No movement worries, and a reasonable choice for a piece like this. I am fond of solid tops, myself, so this is what I would have originally gravitated toward, anyway. You must do what makes you happy, however. Just a suggestion from one of the guys...

Doug Trembath

[Phil Stone]

Doug, that's probably the way I'll go, most likely. The edge of the ash might actually make a nice contrast.

Thanks for the ecouragement.

[Karl Andersson]

You could run "hidden" stretchers/ bearers between the front and back "rectangle" legs (i.e., not the end legs) for the panel to lie on and still use the tongue and groove idea to conceal seasonal movement - that way, a 1/8 or 3/16 groove would be enough and you could place it anywhere, as its main function would be to hide the edge and also help keep the panel edges flat. The bearers would support the top, not the tongue/ edge - if it's 3/4 or 4/4 ash, it would be stiff enough to not flex under the load.

Of course, that would make the joinery at those 4 legs even more complicated...
Karl

[Pat Barry]

Since you have the same issue with both the top and the shelf the frame and panel method is a good solution, particularly since you want to show off the detail in the joinery / top of legs for example. In order to accomplish this you need to have the top and shelf set down a bit, maybe around 1/4" from the top of the rails. In the end this gives you the clean look you are after. I don't feel you need to have the hidden stretchers / supports Karl has mentioned because there really isn't much weight on top of this table with a flat screen LED TV.

Note - I really don't understand what Doug is talking about - it seems he too is saying go with frame and panel style, not exposing the edges of your ash top. He is suggesting an additive way to accomplish that, but it doesn't solve the lower shelf problem.

You may want to draw up the same type of design but implement a more traditional overhanging top (maybe about 2 inches overhang) to see what the finished look will be and compare to you original. You could have the same sort of implementation on the shelf with notches in the legs to allow for movement front to back which should be manageable.

[Doug Trembath]

Actually, Pat, he had my comments right. Your suggestion of the overhanging top is similar to mine. I don't like frame and panel tabletops due to their tendency to capture dust and grime. I have an inexpensive coffee and end table set I purchased before I got the new shop set up (and haven't yet replaced) which has exposed joints. Maybe I'm messy, but it is harder to keep clean than a solid top.

I prefer a solid top for aesthetic reasons, as well. I just think they look better. But, to be honest, I was thinking about the top only when I made my comment, so maybe I was off base there. Regarding the shelf, any movement you experience with a solid shelf will generally be taken up by the legs spreading that small amount. In practice, I haven't had any joinery come loose as a result of that minimal movement in any of the tables with shelves that I have built. I usually just mortise the inside corners of the legs for the shelves. The shelf method Phil has used in his design might blow out the rails he has incorporated. May need to rethink that approach if he goes with a solid top and shelf.

My Red Oak kitchen table top moves about 1/4 inch during the year here in Western PA, so a table like we're discussing shouldn't move more than half that, I would guess. I suppose we could research the amount of movement if we had dimensions and material species. I have seen reference tables for wood movement on the net. It might behoove Phil to check that out prior to making a decision.

Here's a link to a reference table from Woodworker's Source. There are a number of them available...

http://www.woodworkerssource.com/movement.php