Step Five: Plane the Second Face Parallel to the First
Mark the finished thickness of the piece around all the edges of the board using the flattened face as the reference, then lay the flat face down on the bench and finish with a scrub plane and a smoother. You could, if you prefer, use a thickness planer however. With the flattened face side down on the table, adjust the thickness planer to take off just a little bit, then run the piece through. Continue this until you have reached the finished thickness that you want, which I prefer to measure with a set of calipers. Now you have a piece that has two perfectly parallel, flat, straight faces that are exactly the thickness you want, separated on one side by an edge that is perfectly flat and square to both faces.
In practice I take a few precautions first to eliminate problems while planing. I use my calipers to find the thickest part of any of the pieces I am running through the planer at this time. Any project is made up of smaller assemblies - a door, a drawer, a face frame, or a table top for example. Unless you run a production shop where you are building the 4,685th copy of a piece of furniture, it is inadvisable to cut out all of the parts of your project at once. Recreational woodworkers usually work to a plan, one that is being tested for the first time. For this reason it is prudent to cut out all of the parts for each subassembly rather than the whole project. This is important because when you are building something, like the frame in the example here, all four parts must be the same thickness. Hence, you do not run each board through the planer to finished thickness separately. It is best to set the planer to the greatest thickness you have, pass everything through, then readjust the planer and repeat. Then, when you are done, you have every board passed through at exactly the same setting, thus they are all exactly the same thickness. You will never, no matter how careful you are, get the planer to exactly the same thickness by readjusting the cutterhead.
I have been asked what I do if I have three of four boards all nicely cleaned up on the back and one left that has some more to come off. Shouldn't I just run the last board through once more and leave the others? No. Flush, tight fitting joints require uniform thickness of all the boards, so I run them all through till they are all done.
Back to our example, you will see that after planing, my final thickness is about a sixteenth shy of 3/4 of an inch. This is the chance you take with backer board and, as it happens, is not a big problem considering the project I am using the lumber for.
A final word about snipe. This is the extent of the snipe I get. Proper adjustment of your tool should allow you to reduce the snipe to this, or less, without having to build sleds or extensions for your thickness planer. This amount of snipe is easily removable with one pass of a No. 5 jack plane and is perfectly acceptable.
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