How to Plane the Edge of a Board Straight

I’ve been writing a series of posts about milling lumber by hand. Today, in continuation of that series, I want to look at the process of planing the edge of a board dead straight from end to end.

As discussed in the last segment of this series, the edge of a board can be in various states relative to the board’s face. The edge can be square to the face but not straight from end to end. Conversely, it can be straight from end to end but not square to the face. It can be both square to the face and straight from end to end. Or it can be neither square to the face nor straight from end to end.

Sometimes an edge needs to be perfectly square to the face and sometimes it doesn’t. I discussed some scenarios when this might be the case last time, so if you haven’t read that post yet, go back and read that one first, because I’m going to make several references to it today.

If it was necessary to plane the edge square to the face, then the process of straightening that edge has likely already been started. So after planing the edge square, check its length with a straight edge before doing any additional planing to see if additional work is even required. Just like with squaring the edge, it is critical to understand the starting point – if the edge isn’t already straight, is that edge convex or concave along its length? The answer to this question will dictate the process.

Just about every board is going to have some degree of crook along its length. It’s easier to straighten an edge when it starts out hollow or concave along its length rather than convex. So, I typically check both edges using a long straight edge to identify the concave edge before starting to plane. Once the concave edge is identified, planing can begin.

The process of straightening long board edges is made much easier by using a plane that has a long, flat sole. The longer the sole, the longer the registration surface, and therefore, the flatter the resulting edge will be. A shorter soled plane can absolutely be used to straighten the edge, but it will require that closer attention is paid to removing any hollow and not creating a crown.

I also prefer to straighten edges using a jointer plane with a straight rather than a cambered iron. Recall that the cambered iron was used to take a shaving that tapers in thickness to help plane the edge square to the face. The main reason for using a straight iron, rather than a cambered iron, for straightening the edge is that by using a plane with a straight iron, I don’t have to worry too much about inadvertently changing the angle of the edge relative to the face.

If the edge has already been planed square, great care will be needed when using a cambered iron to ensure that the center of the blade is directly over the center of the board’s edge, in order to avoid undoing the work of squaring the edge. However, the straight iron of the jointer plane will remove a shaving of consistent thickness regardless of where on the blade the shaving is taken. Therefore, even if the plane is not held perfectly centered on the edge of the board, the edge will remain square during the straightening process.

When the board’s edge is hollow along its length, I start at one end and plane the full length of the board. At first, the plane will only cut at the two ends of the board. That’s because its long sole keeps the blade riding above the low areas. As the edge gets closer to flat, the areas at the ends that the plane is cutting will get longer and longer.

Eventually, the two areas being planed will meet at about the center of the board and the plane will make one long continuous cut from one end of the board to the other. This is the last shaving I take. At this point, the edge of the board should be about as straight as the plane is capable of making it with that particular setting.

The finer the cut is, the flatter the edge will be. I try to make the last couple of passes with a very finely set plane in order to get the edge as close to perfectly flat as I can. Once the jointer plane has taken its final long, continuous shaving, I’ll confirm the condition of the edge of the board with a straight edge, and, if required, a try square.

If the edge is crowned along its length to start out, I approach it a bit differently. Put quite simply, if the edge starts out crowned along its length, I make it hollow before I make it straight. It should be fairly obvious from my description above how the hollow aids the process of straightening the edge by using the two separate shavings as a depth stop. When the edge is crowned, however, the plane will simply follow the contour of the board’s edge. So planing from end to end will only serve to create a narrower board that still has a crown along its length. Therefore, I remove the crown first, and turn it into a hollow that I can then use as a depth stop as described above.

To remove the crown, I start by planing only the center of the board’s edge. I start with only the middle 6″ or so and make passes with the plane until it stops cutting. Then I’ll lengthen the stroke of the plane so I’m planing the center 12″ or so, again, until the plane stops cutting. If you need a visual cue to judge your progress, making marks across the board’s edge with a pencil or marker will show you where you have planed and where you have not.

This process is continued, gradually lengthening the center area being planed until the entire edge, with the exception of perhaps the last 1-2″ at each end, has been planed and the edge is hollow along its length. Then it’s just a matter of planing from end to end and using the two shavings as a depth stop as described for the hollow edge above.

When the process is completed, I make one final check with the straight edge just to confirm that I have accomplished my task. As described above, I will also check the edge for square at this point, if it needs to be square. If the process was followed and a straight iron used in the plane, the edge should still be square.

With the first edge straight, the second edge can now be addressed, if required. If the second edge needs to be parallel to the first edge, a marking gauge is used to scribe the width by running the fence of the gauge along the planed edge. A gauge that uses a knife or pin is preferred to one that uses a pencil as the scribe line will be an aid in the process of making the second edge parallel.

Once the width is marked with the gauge, the second edge can be planed by watching the gauge line. As the edge nears completion, the wood adjacent to the gauge line will get thinner and thinner. Eventually, the thin whisker of wood will be planed off, and this signals that the second edge is parallel to the first edge and that the board is planed to the desired width.

Once the edges are addressed, the board can be planed to final thickness, if required. We’re almost there!