Chisel
Also called: firmer chisel, bevel edge chisel, mortise chisel, mortice chisel, paring chisel, sash mortise chisel
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A chisel is a hand tool made from a flat steel blade with a sharpened edge at one end and a handle at the other. A joiner cuts and shapes wood with it by pushing the blade into the timber with hand pressure, or by driving it in with blows from a mallet.
The main kinds
The firmer chisel is the general purpose tool of the set: Audels (1923) described it as suited to either paring, driven by hand pressure, or light mortising, driven by mallet blows. The paring chisel is kept for finer work; Nicholson (1832) noted that only the shoulder or hand is used in forcing it into the wood, never a mallet, and Hasluck (1897) likewise described it as guided entirely by the hand. A mortise chisel clears the waste out of a mortise, the rectangular slot cut to take a tenon. Alexander (1978) described it as narrow and deep bladed, with a handle built to be pounded, used to chop the rectangular mortises for the back posts of a chair to take their slats. Its blade is made thick through the body to survive repeated mallet blows, and Nicholson (1832) recorded the resulting bevel, the sloped face ground behind the cutting edge, which he called the basil, at about twenty five degrees; a paring chisel's edge is ground shallower, and Bird (2004) put that at around twenty five degrees too, sometimes as little as twenty. Heavier again is the framing chisel, which Audels (1923) noted could double as a deck chisel aboard ship. Some chisels are built for one job only: Ellis (1908) named the swan's neck, or mortise lock chisel, evidently made for cutting the mortise that takes a lock. A gouge, whose blade is curved rather than flat, and a side chisel are sometimes used not to cut but to scrape, held close to flat against the wood and drawn across it; Jones (1920) advised beginners to do more of this scraping and less outright cutting with the two, since they can be used safely with comparatively little skill.
Cutting a mortise by hand
To keep the sides of a mortise even in width from top to bottom, the chisel has to be held square: Hasluck (1897) insisted its face stand at right angles to the work throughout the cut. Audels (1923) warned against cutting parallel with the grain until the waste in the centre of the mortise has already been cleared, because cutting along the grain too soon can split the wood at the sides. Wearing (2010) set out a practical routine: before starting, put a depth mark on the blade using a strip of masking tape so you can see when to stop; clamp the timber firmly to the bench; then chop in near one end of the mortise with the bevel facing the middle of the slot. Pull the chisel back out, spin it around, and chop in again from the other direction, this time with the bevel facing the hole, so the waste comes away from both ends toward the middle. Frid (1979) was firm that a chisel should always be struck with a mallet, never anything else. The same tool does finer work on a joint like a dovetail: Frid (1979) recommended running the chisel along a marking gauge line to deepen it before chopping out the waste, so the grain does not tear away behind the scored line, and cutting a small V shaped nick behind the dovetail's shoulder for the chisel to register against as it pares home. Bird (2004) cautioned against paring straight down onto a scored baseline from the very first cut, since this can push the chisel under the line and ruin the joint; better to saw a shade above the line and pare down to it afterwards.
The chisel in a machine
A hollow chisel mortiser mechanises the job. Its chisel is square in section with an auger bit, a screw tipped boring tool, turning at speed inside it: the bit bores the round hole and clears the chips, while the square corners of the chisel around it square off the sides of the mortise as the assembly is pushed down (Audels 1923; Rogowski 2002). Rogowski (2002) noted that a machine of this kind is built for one job only, mortising, and that the mortise is first marked out on the timber with a gauge set to the chisel's width, so the fence can then be set to place the chisel correctly over the line. Porter (1999) described setting bit and chisel up together: the auger bit is tightened in the machine's chuck, the clamp that grips the bit, a couple of millimetres short of the chisel's shoulder, the chisel is then lifted to close that gap, and only then is it tightened in turn, so the two work in step. In an older design, Audels (1923) explained, a plunger carrying the chuck and chisel runs in guides on a pivoted casting, and the depth of cut is set by adjusting the table the timber rests on: raising the table makes the chisel bite deeper.