Milling Rough Lumber: Flat, Square, and In Order
The fixed order for milling rough lumber: one face flat, one edge square, then thickness and width. Why a planer alone cannot flatten a board.
Rough lumber arrives cupped, bowed, twisted, or all three. Milling is the process of making it flat, straight and square, and it has a fixed order. Do it out of sequence and you get a board of consistent thickness that is still curved, which is the most common outcome for anyone working this out for themselves.
Why the Order Is Fixed
Every machine in the process references a surface. A jointer references the surface riding on its tables. A planer references the surface against its bed. A table saw references the edge against its fence.
If the reference surface is not flat or not straight, the machine faithfully reproduces the defect. The order exists to create one good reference and then build on it.
| Step | Machine | References | Produces |
|---|---|---|---|
| 1. Face | Jointer, or planer sled, or hand plane | The jointer tables | One flat face |
| 2. Edge | Jointer with fence, or table saw with a straight edge | The flat face | One edge square to that face |
| 3. Thickness | Thickness planer | The flat face against the bed | Second face parallel to the first |
| 4. Width | Table saw | The square edge against the fence | Second edge parallel to the first |
| 5. Length | Crosscut sled or miter saw | The square edge | Ends square, parts to final length |
Step One: One Flat Face
On a jointer, set a light cut and feed the board with the concave side down, so it rests on two points rather than rocking on a crown. Press down over the outfeed table once enough board has passed, not over the cutterhead, because pressure over the cutter pushes the board into a curve you are trying to remove.
Take repeated light passes until the face is continuous, with no rough patches left from the original surface. Those rough patches are low spots. The face is flat when they are gone, not before.
Check for twist with winding sticks: two parallel straight pieces set across the board at each end, sighted across their tops. If the far stick is not parallel to the near one, the board is twisted and needs more work at the diagonal high corners.
Without a jointer, a planer sled does the same job. Fix the board to a flat carrier board with hot glue or wedges, shimming under every point where it rocks, so the assembly is rigid. Run the sled through the planer and the cutter references the sled rather than the board.
A hand plane also works, and on a wide board it is often faster than building a sled. Traverse across the grain first to remove the bulk, then work diagonally, then along the grain for the finish. Winding sticks tell you when to stop.
Step Two: One Square Edge
With a flat face established, put that face against the jointer fence and joint an edge. Check the fence is truly square to the tables first, with a reliable square, because a fence a degree off produces a parallelogram in section that shows up in every joint later.
On a table saw, the same result comes from running the board against a straightedge or a jointing sled, since a rough edge against the rip fence just copies the rough edge.
The result is a board with a flat face and an edge square to it. That pair is the reference for everything that follows.
Step Three: Thickness
Put the flat face down on the planer bed and take material off the top. The rollers press the board against a flat bed, the cutter removes an even layer, and the top face comes out parallel to the bottom.
This is the step people try to do first, and it is why so many boards end up evenly thick and still bowed. The planer makes faces parallel. It does not make them flat.
Take material from both faces if the board allows it, flipping between passes once the second face is clean. Wood is not perfectly symmetrical in its stress, and removing everything from one side encourages movement.
Snipe, the slightly deeper cut at the start and end of a pass, is managed by supporting the board level on both sides of the machine and by running boards through nose to tail so the snipe lands on scrap.
Step Four: Width
Set the rip fence to the finished width, put the jointed edge against the fence, and rip the second edge. It comes out parallel to the first because the fence and the blade are parallel.
If the sawn edge will be glued, joint it afterwards or use a blade that leaves a surface clean enough to glue. A ripped edge with saw marks is not a glue surface.
Step Five: Length
Crosscut last. A part cut to final length before the board has finished moving may not be the right length when it is needed, and an end cut before the edges are straight will not be square to them.
Use a crosscut sled or a miter saw with a stop for repeatability, referencing the jointed edge against the fence.
Letting the Wood Move
Removing material releases stress that the board has held since it dried. The board responds by moving, sometimes within minutes, more often overnight.
Mill oversize first, in thickness, width and length. Sticker the boards with spacers between them so air reaches every face, leave them a day where they will live, then mill to final dimension. The second milling removes what moved.
This step is skipped more than any other and it is the reason a project that was square at glue-up is not square a week later. It costs a day and nothing else.
What People Get Wrong About This
The first mistake is running a bowed board through the planer and expecting it to come out flat. It cannot. The rollers flatten it, the cutter cuts it, and it springs back.
The second is jointing the face with the crown down. A crowned board rides on its centre and rocks, and every pass removes material from the ends while the middle stays. Concave side down gives two contact points and a stable ride.
A third is pressing down over the cutterhead on a jointer. It bends the board into the cut and the flat you produce is the shape of your own hand pressure.
A fourth is a jointer fence that is close to square rather than square. Everything downstream inherits the error, and it compounds in a glued panel.
And the last is milling everything to final dimension in one session because the shop was set up that day. The stock has not finished moving.
Reading the Board First
Sight along each board before any machine touches it. Note which way it cups, which way it bows, and whether it twists. That reading decides which face goes down, which end goes first, and sometimes whether to cut the board in half before milling, since two short boards have less bow to remove than one long one.
Cut oversize parts from the rough board around defects before milling. Milling a whole board flat and then cutting out a knot wastes the effort spent on the part you discarded.
Afterwards
Milled stock keeps moving if it is stored badly. Stack it flat and supported, or stand it on end, but never leave a board leaning against a wall on one corner for a week.
Once stock is flat and square, joinery becomes predictable. The fit of a mortise and tenon depends on reference faces being real, and wood movement explains what the board will keep doing after you have finished milling it.
Common questions
What is the correct order for milling lumber?
Flatten one face, square one edge to that face, thickness the board by planing the opposite face parallel to the flat one, then rip the second edge parallel to the first. The order is not a preference. Each step uses a reference created by the step before it.
Can a thickness planer flatten a bowed board?
No. The feed rollers press the board flat against the bed as it passes, the cutter removes material from the top, and the board springs back to its original curve on the way out. The result is a board of even thickness that is still bowed. A flat reference face has to be created first.
Why should you let lumber rest between milling passes?
Removing material releases internal stress, and the board moves in response. Milling to final dimension in one session often produces a board that is no longer straight the next morning. Taking stock close to size, resting it, then finishing the job lets that movement happen before the final cuts.
How do you flatten a board without a jointer?
Use a planer sled: fix the board to a flat carrier with shims under the high spots so it cannot rock, and run the sled through the planer. The cutter references the sled rather than the board and produces a flat face. A hand plane and winding sticks do the same job without machinery.
How much extra should you leave when milling?
Enough thickness and length to clean up after the board moves, and enough width to remove the first edge. Mill oversize, rest the stock, then take it to final dimension. Cut parts to final length last, because a board that moves in length moves least and a part cut early may not fit later.
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