How can I be sure my workpiece feet are level, when my workshop floor isn't?

Question

I'm building a coffee table with four feet and have realized that my workshop floor is not level at all. It is not sloped, but rather has an uneven surface. While it doesn't make a difference for general use, I have found a variation in the surface of a few mm, which make it difficult to judge if all legs are precisely the same length. My table may be sure in my garage, but wobble in my living room.

Since I'm not in any position to re-do my workshop floor, nor move my woodworking to my living room, is there an easy way to ensure my table legs are the same without having a level floor to work on?

Another side effect of this is that also a lot of the other workshop fittings are also not level (workbenches, etc) which make for interesting woodworking!

Answer 1

I've never tried this but it should work. It won't gurantee all legs are the same length but it should verify all the ends are in a plane and your table won't wobble on a flat floor. It does rely on your table being rigid. It also assumes your legs are pretty close in length to begin with.

Place your table upside down on your workbench. Maybe shim any corners not in contact with the bench (unimportant if your table is really rigid). Fasten a piece of non-stretchy string between diagonal corners such that it makes contact with the bottom of each leg and is taught as possible without pulling the legs together. Call these legs 1 and 3. Now fasten another string to one of the other legs, say 2, and pull it taught to leg 4. If, as you bring the second string down just to the point of contact with leg 4, the string just touches the first string, your leg ends are all in a plane.

If, once the string makes contact with leg 4, the second string is above the first string (not making contact), you need to trim leg 2 or leg 4. Measure and choose the longer leg if there is one.

The third possibility is that the second string makes contact with the first string before making contact with leg 4. In this case, start over but with the first string between legs 2 and 4.

This relies on the fact that two intersecting straight lines define a plane. One caution is that the two strings should have similar tension.

Edit: My answer was directed at trimming legs of an already assembled table but it could be equally applied to legs clamped onto the rails previous to fastening.

Edit 2: If you need do this frequently, you could substitute two straight edges for the string. One of the straight edges, say 1 x 4 pine, would be cut in two in the center. Spread the pieces apart and splice a short piece along the bottom such that it maintains the straight edge.

Edit 3: As @aaron points out, the string method is made more precise by shimming the string on legs 2 and 4 by a string thickness and gauging levelness when the strings barely touch.

Answer 2

My table may be sure in my garage, but wobble in my living room.

That's one 'secret' to this kind of thing, after levelling the top of the table in the living room (or wherever it's going) mark the legs for length there. Then take it back into the workshop to cut them to length.

You can scribe the legs for length using a huge variety of tools, including a bunch of fancy, overly complicated (read: expensive) gadgets sold to fleece gullible woodworkers into throwing money at a simple problem that never had an expensive solution before. Or you can use a basic child's compass or dividers.

For scribing table legs specifically probably your simplest option is a pencil laid on a thin block of wood that will put the tip even with the bottom of the shortest leg. If your table's legs are already very close to even you may even be able to get away with laying the pencil directly on the floor and using half its own thickness as the scribing amount.

Or, if you have a table saw you may prefer this method posted by Popular Woodworking back in 2012, Level Legs the Easy Way by Steve Shanesy

Another side effect of this is that also a lot of the other workshop fittings are also not level (workbenches, etc) which make for interesting woodworking!

This is a side issue that you should address in another Question if necessary but in short, a non-level floor should not at all lead to non-level working surfaces — pack out or shim legs, or add adjustable feet, so that everything IS level.