Mortising
Also called: chopping a mortise, cutting mortises
One page. Other spellings and regional names stay on this URL.
Mortising is the work of cutting a mortise, a rectangular socket cut into a piece of wood to receive a matching tongue, called a tenon, on another piece. Since most timber framing is held together this way, in a mortise-and-tenon joint, mortising is one of the most common operations in the trade.
Cutting a mortise by hand
Timber to be mortised by hand is marked out and scribed, has fine lines scored into it with a marking gauge, along both edges, the same as it would be for any hand joint. The chisel used to remove the waste is a mortising chisel: thicker and more robust than an ordinary paring chisel, and the only type of chisel meant to be struck with a mallet, its handle carries a heavy iron ferrule, a metal band, to protect the butt end from the mallet's blows. A firmer chisel, a general-purpose tool, can also be used for light mortising as well as for paring, pushed by hand for the one and struck with a mallet for the other, but certain lighter chisels are the wrong tool for mortising altogether. Japanese mortising practice uses a finer chisel again for fine work, with a blade about a quarter of an inch thick and an edge ground to around 20 degrees, thinner than an ordinary timber mortising chisel. Good technique leans heavily on shoulder pressure, forcing the chisel down mainly with the weight of the body rather than by striking it, keeping the actual chopping to a minimum. This matters even more on green, unseasoned wood: mortising it by shoulder pressure gives more accurate results than mortising it by chopping. Cutting a post's mortise starts with chopping cuts made between the marked layout lines, stopping short of chopping right to the mortise's very end. One craftsman was seen chopping a mortise by alternately tapping a wedge tighter and chiselling deeper to clear the slot, and a whole mortise cut by hand only comes out accurate once the technique has been practised. Japanese joinery uses a further alternative, cutting stop cuts to control and limit how much wood is removed within the marked-out layout. On the order of operations, Hasluck (1907) recorded two different practices. Many joiners doing an entire mortise by hand prefer to trim the splayed shoulders, the angled cut where a rail meets a stile, close to the line first, considering it a saving in labour. Hasluck himself thought the stronger method was probably the reverse: cutting all of a stile's mortises before its splay shoulders are cut at all.
Protecting the wood
In framed-up work, furniture or joinery built from separate rails and stiles jointed together, it generally pays to leave the stiles over-length at both ends, forming a horn: a short stub left standing proud beyond the joint. A stile's horn helps stop softwood splitting during deep mortising, and shields the corners from incidental damage.
Machine methods
A boring-and-mortising machine bores or routs out the holes that form a mortise, working on a table fitted to hold the material. Because most framing relies on mortise-and-tenon joints, both mortising machines, which cut the rectangular holes, and tenoning machines, which cut the matching tenons, see heavy use. Several distinct mechanisms do the cutting. A hollow chisel mortiser drives a freehand revolving auger bit inside a square hollow chisel, the chisel squaring off the round hole the auger cuts. A chain mortiser instead uses an endless cutting chain, each link carrying a cutter on its outer face; a guard sits over the mortise on top of the timber while the chain itself stays free to travel up and down, and that chain must stay fully guarded throughout the operation. A lathe can also be fitted with a mortising attachment that works horizontally, with the wood fed up to the chisel rather than the chisel moving; alternatively, a revolving mortising bit is plunged straight into the wood, and then either the bit or the workpiece is moved sideways to complete the mortise's length. A rotary miller bit of this kind leaves the mortise's ends rounded, so for cabinet work those ends need squaring off afterward by hand. Spiral-fluted bits give better chip removal for mortising than plain ones, and purpose-made mortising bits are another option; drilling a line of overlapping holes is a further way to form a mortise. A strip of tape stuck onto the chisel before mortising marks the target depth. A mortising jig can also be built for machine work: its fence needs three doubled fence blocks glued together, and moving the jig's spacer to the outside shifts the cutting bit further away from the block.
Holding the work
Work-holders for mortising come in horizontal and vertical versions to grip the workpiece. Larger work is fixed on a mortising stool or horse, a low, sturdy stand built for the purpose. One craftsman held work for boring or mortising by wedging it between three rectangular pegs fitted into holes bored in the bench surface; on old workbenches, these dogholes, the sockets that take bench dogs or holding pegs, were likely bored first and only squared off afterward, whether by hand tools or by machine. On a hand mortising machine, the workpiece itself is set up on a strip of scrap deal, softwood board, so that the chisel is kept off the machine's iron bed.