Materials · Wood behaviourintermediate~8 min read

Grain direction and wood movement

Short answer

Wood is a bundle of straws. Cutting along them shears cleanly, cutting across them can tear, and cutting into their ends is hardest of all. Wood also moves across the grain with humidity, which is why a board you flattened in summer can cup by winter.

The straws

Wood is made of long cells running the length of the tree. Think of a tight bundle of drinking straws. Almost everything about how wood machines follows from that one picture.

Surface	What the cutter meets	Machining behaviour
Face grain	The sides of the straws	Cuts cleanly. What most flat work uses.
Edge grain	The sides of the straws, from the edge	Similar to face grain, slightly harder
End grain	The open ends of the straws	Hardest to cut cleanly. Fuzzes and chips. Absorbs finish heavily.

One board, three completely different surfaces. Face and edge grain present the sides of the fibres to the cutter, which shears them cleanly. End grain presents their cut ends, and that is the one that fuzzes, chips and drinks finish.

An end grain cutting board is not a harder piece of maple, it is maple presented to the tool in its most difficult orientation. Expect fuzz, plan a light finishing pass, and use a very sharp bit.

Grain direction within a cut

When the cutter travels along the grain, it can either lift fibres away from the surface or press them into it, depending on which way the fibres run relative to the cut. Lifting them is what causes tearout: a fibre gets picked up and splits ahead of the cutting edge rather than being severed at it.

This is why the same toolpath can cut beautifully on one side of a shape and tear on the other. The tool direction relative to the grain reversed, even though the settings did not change.

The practical answer

You cannot orient a closed shape so every side cuts with the grain. What you can do is leave a small allowance and take a light finishing pass, because a light cut has far less force available to lift a fibre. That single habit fixes most tearout on most jobs.

Figured grain

Curly maple, birdseye, quilted figure and crotch wood all have grain that changes direction constantly within a small area. They are beautiful and they are genuinely difficult:

No cut direction is right everywhere, because the grain direction changes within the cut.
Tearout is likely no matter what you do.
Very sharp tooling, light finishing passes and higher RPM all help.
Accept that some sanding will be part of the process.

How the board came off the log

Every board is a slice through a set of concentric rings, and the angle those rings make with the wide face is what decides how the board behaves. That angle is set at the mill, long before you see the wood, and reading it off an end grain is one of the most useful things you can learn to do at a lumber rack.

The four ways a log gets broken down. The board under each log shows what the rings look like on its end, which is the part that matters: the closer those rings are to standing on end, the less that board will move across its width.

Cut	Rings to the face	Also called	Movement	Look and cost
Plain sawn	0 to 45 degrees	Flat sawn	Stable in thickness, moves most across the width. Cups readily.	Cathedral figure. Best yield, cheapest, most common.
Quarter sawn	45 to 90 degrees	Vertical grain	Stable across the width, moves more in thickness. Barely cups.	Straight grain, ray fleck in oak. More waste, more expensive.
Rift sawn	30 to 60 degrees	Bastard sawn	Very stable, sitting between the other two	Very straight and uniform, no ray fleck. Most waste of all.
Live sawn	Every angle, within one board	Through and through	Mixed. The middle of a wide board behaves differently from its edges.	Full log width and all the character. Highest yield, least waste.

The bands overlap, and that is not a mistake

Rift sits across the boundary between plain sawn and quarter sawn, so a board with rings at 40 degrees can honestly be called two different things depending on who is selling it. The angles above follow the National Wood Flooring Association definitions. Other sources split them into three tidy non-overlapping bands instead. When it matters, look at the end of the board rather than at the label.

What each one is actually for

Plain sawn. The default, and fine for most work as long as you know it will move. Avoid it for wide panels that have to stay flat.
Quarter sawn. Worth the money whenever flatness matters: wide tabletops, door panels, long straight parts, machine fixtures and jigs. The ray fleck in oak is either the reason you bought it or the reason you did not.
Rift sawn. Often specified in oak precisely to avoid that fleck, and the usual choice for table and chair legs, because all four faces then show the same straight grain.
Live sawn. Where wide character boards and live edge slabs come from. A single board can hold quarter sawn grain in the middle and plain sawn grain at the edges, which means it does not move uniformly across its width. Boards that include the pith will check and often split.

Cupping, and how to keep it out of your work

Rule of thumb Wood shrinks along its growth rings roughly twice as much as it shrinks toward the pith. That one ratio is the entire cause of cupping.

In a plain sawn board the rings run as shallow arches across the width. As the wood dries, each arch shortens along its length more than it shrinks in thickness, so the arches flatten out. The face nearer the bark ends up hollow and the face nearer the heart ends up crowned.

The same drying, two different outcomes. The plain sawn board on the left has curved rings that flatten as they shrink, so it cups. The quarter sawn board on the right has rings standing on end, so nearly all of the movement goes into its thickness, where you will never notice it.

The mnemonic that works at the lumber rack

Look at the end of the board. It will cup in the opposite direction to the curve of the rings, so rings arching upward means the top face goes hollow. Said the other way round: a board cups away from the heart. You can predict which way a board will move before you buy it.

Nine ways to keep a piece flat

Buy the right cut for the job. Quarter sawn or rift for anything that has to stay flat. This one decision beats every technique below it.
Avoid boards with the pith in them. The rings curve hardest there, it will check, and often it splits outright.
Use narrower boards. Cup grows with width. Four narrow boards glued into a panel cup far less in total than one wide board covering the same span.
Think about ring direction in a glue-up. Alternating the rings board to board gives you several small ripples instead of one large cup. Keeping them consistent gives one broader cup that is easier to pull flat with a fastening. Both are defensible. Doing it without thinking is not.
Orient the crown to work with the load. On a shelf, or anything that carries weight, put the crowned face up so the load pushes the board flat instead of deepening the cup.
Acclimate the stock in your own shop for a week or more before machining anything that matters.
Take equal material from both faces. This is the CNC specific one. Removing it all from one side unbalances the board and it will move. See slab flattening.
Finish every face and every edge, including the underside. A panel that can exchange moisture through one face only is a panel that will cup.
Let it move, and store it flat. Attach tabletops with slotted holes, buttons or figure-8 fasteners across the grain, and sticker boards flat rather than leaning them against a wall.

Flattening a cupped board does not cure it

It removes the evidence. The board cupped because of how it was sawn and how it dried, and machining it flat takes the most material off exactly where it was crowned. If the moisture content has not settled, or if you took it all off one face, it will move again. Dry it first, machine it second, and take from both sides.

Wood movement

Wood takes on and gives up moisture with the humidity around it, and it changes size when it does. The critical facts:

It moves across the grain, not along it. A board gets wider and narrower, not longer and shorter, in any meaningful amount.
It moves more tangentially than radially, which is why plain sawn boards cup: one face is effectively longer than the other.
Machining releases internal stress. A board that was flat can bow the moment you remove material from one face, because the tension holding it flat is now unbalanced.

The slab flattening trap

Remove all the material from one face of a slab and it will often move. Take roughly equal amounts from both faces, flip it, and give it a day between operations to settle. See slab flattening.

Practical rules

Acclimate your stock. Bring wood into the shop and leave it for a week or more before machining anything that matters.
Machine both faces when you are removing significant material, rather than taking it all from one side.
Design for movement in anything that joins wood to wood across the grain. A panel glued rigidly into a frame across the grain will crack.
Finish all faces, including the back and the edges. It slows moisture exchange and reduces cupping.
Expect end grain to behave differently in every way: cutting, sanding, glue-up and finishing.

Sawing methods and growth ring angle definitions · National Wood Flooring Association technical publications
Wood Handbook: Wood as an Engineering Material · USDA Forest Products Laboratory the primary source for shrinkage behaviour