Tear-out

Also called: tearout, tearing out, grain tear, breakout, spelching

Tear-out is what happens when a cutting edge, whether it belongs to a plane, a saw, a router or a drill, fails to sever the wood fibres cleanly and instead rips a ragged patch of them away, leaving a torn hollow where a smooth surface should be. It is not one fault with one cause: wood tears out for different reasons in different situations, and the fix depends on which situation is in play.

Tear-out ahead of a plane

Wood does not cut the way metal does. Hoadley (1980) explains that wood tissue does not shear cleanly at a blade's edge but yields and tears ahead of it, both above and below the actual plane of the cut, which is why a shaving can lift a torn scrap of surface with it instead of parting company cleanly. This happens most readily when the tool works against the grain, or when the grain is interlocked or keeps changing direction through the board, so that no single feed direction is truly with the grain for the whole surface. Several features of a plane exist to fight this tendency. The cap iron sitting just above a plane iron is there to break the shaving as it lifts, taking the strength out of it before it can drag a splinter from the surface ahead of the cut. A tightly set mouth, the gap in the sole just in front of the edge, works the same way by holding the wood down right up to the point of cut, so the fibres have less room to lift before they are severed; Ellis (1908) credits a very fine mouth with preventing tear-out on this basis. Setting the iron to take a very thin shaving, so that it slices rather than gouges, has the same effect. Writers do not agree on what to do with the cutting angle itself. Joyce (1987) notes that on moulding planes, which cannot carry a cap iron because the bevel of the iron faces the wrong way for one, the angle is raised instead, to around fifty five degrees, to compensate for having no cap iron to break the shaving. Porter (2004), writing about difficult, interlocked grain such as is often found in quartersawn stock, recommends the opposite: a reduced cutting angle, said to lower the risk of tear-out and of the surface picking up rather than cutting cleanly.

Tear-out where a cut breaks through

A different problem shows up wherever a tool cuts across the grain and then exits the far side of the wood, whether that is a drill coming through the back of a board, a router running off the end of an edge, or a saw finishing a crosscut. There is no fibre beyond the exit point to hold the surface together, so the last of the cut tends to blow out into a rough chip instead of parting cleanly; Hylton (2005) notes that this can happen at the end of a cut on a router table almost regardless of which way the work is fed past the cutter. The usual answers all work by supplying that missing support. Clamping a scrap piece of wood behind or beyond the workpiece gives the fibres something to tear into instead of open air, a method Noll (2002) describes for backing up work on a radial arm saw. Scoring a line with a marking gauge along the cut before sawing or chiselling gives the fibres a clean place to break at, rather than letting the tear run wherever it likes; Duginske (1999) makes the same point about a gauge line marking the cut depth. Where a cut must cross the grain and a torn edge cannot be tolerated, one option, described by Rogowski (2002), is simply to cut oversize and trim the torn edge away afterwards. Noll (2002) also points out that a joint such as a box joint, whose notches run parallel to the grain rather than across it, only tears at the top of each notch rather than at both sides, because most of the cut is not fighting the grain at all.

Tear-out as a joint failure

Tear-out is not always caused by a cutting tool. Tredgold (1871) describes a mechanical version of the same fault: when a peg is driven through a bored mortise and tenon joint to draw the shoulder up tight, a technique called draw-boring, pulling the peg too far risks "tearing out the bit of the tenon beyond the pin". Here the timber fails under the strain of assembly rather than under a blade, but the result looks much the same, a torn-away piece of end grain, so the same word is used for it.

Reducing the risk

Besides matching feed direction to the grain and keeping edges sharp, grain that will not plane cleanly in any feed direction can often be handled with a scraper instead of a plane. Bird (2004) reports that a scraping plane can true difficult joints and smooth strongly figured panels without tearing them, in situations where a bench plane alone would tear the surface. Where a lower cutting angle is not enough on its own, backing boards, gauge lines, and cutting oversize then trimming remain the standard ways to keep a torn edge from ever reaching the finished surface.