Reply by RClark

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Posted on Questions about flattening workbench top/bottom

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126 posts in 3266 days

#1 posted 02-28-2021 07:13 PM

I recently dealt with this issue on a very large table slab (about 40 X 78 X 1 3/4).

To get a work surface that large, I used a 4X8 sheet of OSB. I edge jointed two dry 2X4 to get straight edge, and then attached them to the underside of the OSB sheet. I then laid this table assembly on top of saw horses. Since a table of this size will still flex, I used long straightedges to check for flatness, and then put shims between the work table and the saw horses as needed to flatten it all out.

We smoothed one side using a router sled suspended over the workpiece. Once that side was done, we flipped it over and spent a couple hours shimming all of the various components (workpiece, rails, router sled), but after we finally ran the router sled over the work piece, we still couldn’t get the top and bottom exactly parallel. Seeing that we were beginning to go into the “death spiral” in search of perfection, we decided to attack it slightly differently.

- On the bottom, we marked the areas of the bottom where the leg attachment points were going to be.

- We measured the thickness at those points. The thinnest attachment point was the new target thickness, BUT ONLY where the legs/leg assemblies would contact the table top.

- Using the router, we brought the other three contact points down to that thickness.

With such a large bench top in excess of three inches thick, what is going to make that 1/8th difference stand out? If you do make the whole bottom exactly parallel to the top, I’m thinking that any unevenness in the floor surface is still going to have to be dealt with with leg leveling devices of some sort.

For 1/8th inch over that size top, I’d be very tempted to do what I described above, or even work to shim the contact points of the leg assembly.

-- Ray

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