Milling rough lumber flat and square

Every accurate joint starts with stock that is flat, straight, square and of consistent thickness. Lumber rarely arrives that way. Even boards surfaced at the yard are usually cupped, bowed or twisted a little by the time you get them home. Milling fixes that, and the order you do it in matters as much as the tools.

Before you mill

• Let it acclimate. Bring the lumber into the shop, sticker it (stack it with spacers so air reaches both faces) and give it time to reach the shop’s humidity. A week or two is common; longer for thick stock or lumber that has been stored outdoors.
• Pick parts from boards. Lay out the cut list on the boards in chalk, keeping show faces, grain direction and defects in mind. See board feet for how much extra to plan.
• Rough cut to size. Crosscut to length plus 1 to 2 inches, and rip to width plus about 1/2 inch. Shorter, narrower pieces lose less thickness when you flatten them, because a small bow over a short length is a small correction.

The sequence

1. Joint one face flat. On a jointer, place a cupped board with its concave face down so it rests on its edges and does not rock. For bow, likewise put the concave side down so the board rests on its ends. Take light passes until the whole face is flat. Without a jointer, use a planer sled, a router sled or a hand plane.
2. Joint one edge square to that face. Keep the flat face against the jointer fence. That edge is now straight and at 90 degrees to the reference face.
3. Plane the second face parallel. Run the board through a thickness planer with the jointed face down. Take material from both faces over several passes if you need to remove a lot, alternating sides, so the board stays balanced.
4. Rip to width. Rip on the table saw with the jointed edge against the fence, leaving about 1/32 inch.
5. Clean up the sawn edge. A light pass on the jointer, or a hand plane, removes saw marks and brings the part to final width.
6. Crosscut to length. Square one end, then cut to length against a stop so parts that must match do.

Mark the reference face and edge as you go, for example with a face mark and an edge mark in pencil. Later layout and joinery should always reference from those surfaces.

Mill in two stages

Removing wood releases stresses in the board, and it can move again after milling. For parts where it matters, such as table aprons, door frames and anything that must stay straight over a long length, mill in two stages:

1. Mill all parts to about 1/8 inch over their final thickness and width.
2. Let them rest, stickered, for a day or more.
3. Take the final passes to finished size.

Mill all the parts that must match in the same session, at the same settings.

Grain and tearout

Feed boards so the knives cut “downhill” with the grain, in the direction the fibres rise toward the surface. Tearout on the first pass usually means the board is going the wrong way. Where grain reverses, take lighter passes, feed slowly, or finish with a hand plane or scraper.

Snipe

Many planers cut slightly deeper at the start and end of a board, a defect called snipe. Leaving parts an inch or two long lets you cut it off. Supporting long boards level as they enter and leave the planer, and feeding boards end to end, reduces it.

Checking your work

• Flat: a straightedge laid across and along the face shows no light underneath.
• Twist: a pair of winding sticks, straight strips laid across each end, sight parallel.
• Square: a reliable square shows no gap between the jointed face and edge.
• Thickness: calipers read the same at several points along the board.

When the stock is thin

Boards that start close to finished thickness have little room to be flattened. If a 4/4 board is badly cupped, it may not yield a flat 3/4-inch part. Either use it for shorter pieces, where the cup is smaller, or plan a thinner part. Remember that the finished thickness in your design is what matters for joinery; see nominal and actual sizes.

In GrainDraft

A GrainDraft model uses finished dimensions, and the stock list reports each part’s finished length, width and thickness with its grain direction. Treat those as the targets for the last stage of milling, and add your own milling allowances when you rough cut.

Related: wood movement, box joints.