Wood Apron on an MDF Table

Furnituremakers discuss moisture-related movement issues for an MDF table-top with a Maple apron. March 28, 2010

Question
I'm building a round table out of an MDF veneered panel. The panel will be edge banded with 5/4 x 6" maple. The diameter is 5'. I want to attach the apron to the solid edge and attach cross members to strong back the panel, using screws. Is wood movement an issue here? I could put all of the frame on the MDF, but this table will be heavy. I could see the edge band breaking off the MDF if anyone had to move or lift the table.

Forum Responses
(Furniture Making Forum)
From contributor J:
Any wood movement issues you have will be unrelated to the way the top is connected to the frame. MDF does move a little with humidity changes, as will the solid edging, so the joints between pieces of edging might not remain absolutely tight. I would not rely on glue alone here - some biscuits between edge segments, and between the edge and MDF center, would be appropriate. If you do a good job of that, I doubt you have much to worry about in terms of it being broken when (not if) it is moved.


From the original questioner:
This is what I thought too. But are you suggesting I attach the apron to the substrate, and not the solid edge. Can I strong back across the substrate and onto the solid edge? I prefer the legs to be closer to the perimeter. I plan on using splines all the way around the substrate, and at each joint of the edging.

From Gene Wengert, forum technical advisor:
MDF does not move much with changes in MC while the maple will move considerably more. The real problem here is that when the maple dries (such as in the wintertime with low interior RH), it wants to decrease in diameter. However, the MDF prevents that, so stress and cracking are likely. Conversely, in the summertime with higher RH’s and swelling, the diameter will increase and leave gaps.

I am not sure why biscuits will help as they are used for positioning a joint and do not add to the strength. Even so, the stress developed will exceed the strength of any joint. To use cross pieces, as you suggest, the screws must be in slotted holes so that they can move; do not drive the heads into the wood which would prevent movement. Bottom line: wood movement is an issue and expect some opening of the joints due to the design and not to the construction.



From contributor J:
Gene, I'm puzzled by a couple parts of your response on this one.
1. Given that, as you just posted in another thread, "...lumber does not shrink or swell along its length; only width and thickness," how is the solid maple edge around the perimeter going to decrease in diameter when the RH drops, or increase in diameter when the RH rises? I can't think of any way that the diameter of a ring made up of sold wood segments can change without the length of each segment changing.

2. While I understand that biscuits don't add strength to edge-grain joints when gluing up a slab, I'm pretty sure they can add quite a lot of strength to end-grain - to - end-grain joints such as those between segments of the ring around the table, as they act much like loose tenons.


From Gene Wengert, forum technical advisor:
I am delighted that you are thinking about the answers.

Here is my response:

1. I am assuming that the ring of hardwood is 6 inches wide. So, although the length of a series of segmented pieces will not change, the wood will shrink or swell in width...about 1% for a 4% MC change or about 0.06" or 1/16". This would be on both edges.

2. Good point indeed. I was thinking only of side to side joints when I wrote the response - biscuits joining the rim to the core and not the individual pieces in the rim. An additional thought is that a good moisture vapr resistant finish should be used and applied to all exposed wood evenly and at the same time to help avoid moisture differences that can cause warp.


From contributor J:
Thanks for the follow-up, Gene. I would think that, as the segments tended to shrink in width during dry seasons, each would be prevented by its neighbors from moving inward. A chord line connecting the outer corners of any given segment would tend to stay in the same relative position; as the segment shrunk, material inside that line would move outwards towards the line, while material outside the line would move inward towards the line. In other words, the ID of the ring of segments would actually increase in the winter time, not in the summertime as you wrote, which is part of what got my attention.

Hypothetically, if there were 10 segments cut from 6" wide maple, then each would occupy 36 degrees of the circle, with about an inch and a half outside the chord line and four and a half inches inside the chord line. Seasonal shrinkage across four and a half inches of maple might be about 1/32," which is the sort of gap I'd expect to eventually develop if the table were built in a humid season. I suggested biscuits between segments and MDF so that a mechanical interlocking joint would remain even of the segments loosened from the MDF. However, since this particular table is apparently being built in the wintertime, it seems conceivable to me that such gaps between MDF and segments may never develop.


From the original questioner:
Thanks for your thoughts. Here is a picture of the top in question. I am going to rout this into a circle. Maybe I will use a smaller radius to decrease the size of the edge band.


Click here for higher quality, full size image

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