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Joining table top to base.

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Forum topic by Okie72 posted 09-24-2017 04:23 AM 266 views 0 times favorited 2 replies Add to Favorites Watch
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Okie72

1 post in 29 days


09-24-2017 04:23 AM

Topic tags/keywords: table

Hello everyone. Based on these pictures do I have to have an apron to get longevity of this table top? I want to avoid cracking, cupping, etc. i plan to use clips. My thought was to use my biscuit joiner to make slots and use the mounting clips. My fear is that the yop will warp without an apron. I thought snout putting a piece of wood and screwing elongated holes with a screw and a washer . Thought? Any advice is appreciared. The top is 5-8 % mc soft maple.


2 replies so far

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stellakelly

5 posts in 36 days


#1 posted 09-24-2017 08:46 AM

Where are the images?

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JBrow

1242 posts in 704 days


#2 posted 09-24-2017 07:35 PM

Okie72,

Since the photo or sketch did not tag along with the text, I will only offer general comments. When I have designed tables, an apron is included when an apron compliments the overall design.

I doubt the top will twist, cup or crack because there is no apron. Cupping and/or twisting can occur when the rate at which moisture enters or leaves one face is greater than the rate at which moisture enters or leaves the opposite face. It can also be caused when expansion and contraction across the width of the top is restrained because the top is rigidly fastened to the base.

If the top is built flat and machined (planed and sanded) on both faces, ends, and edges equally; the same finishing schedule used on both faces, the ends, and edges; and air is allowed to freely move around the entire top, the rate at which moisture enters or leaves the top is balanced and the rate of exchange reduced (due to the applied finish). The top should remain flat throughout its life.

Cracking can be caused when moisture leaves a top that is rigidly fastened to the base. As the top tries to contract across its width while held firmly in place to the base, the top builds up internal stresses until those stresses are relieved with a crack. On the flip side, if moisture enters a top that is rigidly attached to the base, the top could cup across its width as the top expands.

If the top is attached to its base, with or without aprons, so that the top can expand and contract across its width, cracking and cupping due to internal stresses is unlikely to occur.

Ideally, the completed table will be placed where exposure to direct sunlight is limited and away from air flowing out of HVAC vents. These precautions help maintain balance in the rate at which moisture enters or leaves the top on both faces.

As far as fastening the top to allow for expansion and contraction of the top, I have used several different methods, one of which is similar to method you described. The first method is to run stopped dados about ¼” wide X 3/8” deep in a ¾” thick apron held down from the upper edge of the apron by about ¼” to 3/8”. Z clips (either shop made or purchased) are attached to the top with the tongue inserted into the dado, but not bottomed out in the dado. In the summer the clip is mounted a little deeper in the dado and in the winter, not so deep. However, the problem is ensuring there is enough room for the top to expand without the clip bottoming out in the dado while at the same time making sure the clip is engaged in the dado enough so that when the top contracts, the clips remain engaged in the dado. For me this is mostly a guess. The wider the top the more of a challenge positioning the clips just right becomes. This problem may explain why figure 8 fasteners, mounted a little differently, seem popular.

My preferred method is to install cross members in the apron running perpendicular to the direction of the grain in the top. The rabbet in the ends of the cross members are glued into a rabbet milled into the top inside of the apron. The cross members set about 1/8” to ¼” below the top edge of the apron. Slots are milled into the cross members. The top is then attached with screws through the slots in the cross members into the top.

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