flattening workbench top

[Matt Day]

I've got a used maple workbench (3 1/2" thick) top that has a bit of twist and cup in it. The cup and twist of course are present on both the top and bottom. I'm going to use a router sled (as demonstrated by TWW) to flatten the top.

My question is, is it necessary to flatten the bottom at all or should I just leave it alone and shim it so it doesn't rock on the base? I think I'd rather do that than loose any more thickness.

[Mark Bolton]

If you leave the top thick and thin you will likely have recurring issues with twist and cup. It would be best to flatten the entire top in my opinion though you could of course be less fussy with the bottom face.

[Paul Murphy]

First check if in unlevel base has caused an unlevel top.

If your base is level one option is to rip the top along gluelines into 3 or 4 strips, face joint, plane, edge joint, and reglue. You'll loose less thickness this way, and only about 3/4" of width [you could even glue in an extra strip if desired].

[Matt Day]

Good suggestions guys. I did a little planing of the bottom where the base contacts the top, and a little skip planing (if that's what you call it) of the high spots in the other areas. The bottom is high in the middle, and the top is high on the ends.

I'll check that the base is level tomorrow morning. The previous owner had a twin screw vise on the end and installed the rear face with bed bolts, so cutting up the top would be a lot more difficult since I'd have to get the bed bolt holes to align again. I'll use that idea as my last resort though.

[Myk Rian]

so cutting up the top would be a lot more difficult since I'd have to get the bed bolt holes to align again.
Not to mention it might be full of nails/screws. So that could nix ripping it.

[Jim Matthews]

First check if in unlevel base has caused an unlevel top.

That's clever, right there.