blowout
The damage done to the exit side of a drilled hole or sawn edge when no sacrificial backerboard is used during the drilling or sawing: the drill bit's or sawblade's exit on the far side causes chips of wood to be broken from the edge.
blowout: splintered exit damage on the back of drilled wood
A blowout occurs when a drill bit or saw blade breaks through the far side of a workpiece and tears away a ragged, splintered crater of wood fibers. The tool's sharp edges have nowhere to go but outward when they exit, and without support underneath, the wood grain simply fractures and lifts away instead of shearing cleanly. The damage is worst on the exit side because the tool is moving fastest there and the wood has no backing to hold it together.
Drill bits cause blowouts when the lip edges cut through and emerge into open air. Twist bits, spade bits, and Forstner bits all do this; the smaller the bit diameter, the easier it is for the bit to grab and tear. Saws create similar damage at the underside of a cut, whether using table saws, circular saws, or band saws. The exit surface splinters because the tool's teeth or flutes have broken the wood fibers free but nothing is there to support them as they leave the material.
The standard fix is to clamp a scrap board directly underneath or behind the workpiece. The bit or blade then exits into this sacrificial backing board instead of into air, and the backing absorbs the tear-out damage. Many shops use melamine or MDF for this because they are cheap and produce less splinter than solid wood. Alternatively, drilling or cutting from both sides and meeting in the middle, or using a sharp tool at a slow feed rate, can reduce but not eliminate blowouts on critical work.
For drilled holes, Forstner bits are less prone to blowout than twist bits on plywood because they have a flat bottom that shears rather than grabs. Reverse-rotating the bit as it exits can help. For saws, a fine-toothed blade, proper tension, and climbing-cut techniques reduce damage on some applications, but backing remains the most reliable defense.
Blowout severity depends on wood species, grain direction, and moisture content. Brittle hardwoods and plywood blowout worse than soft, stable woods. The term itself comes from the visual result: a violent, explosive-looking rupture at the exit point, as if the material blew apart from the inside.