=Involute.= A curve generated by the path of a given point in a straight line, as the line is rolled upon a circle. (Vol. I. p. 31.)

J

=Jack-plane.= A plane employed to rough out the work.

=Jig.= A device for holding work and guiding the operating tool.

=Jointing-machine.= A machine for truing the surfaces of wood-work that are to form a joint.

=Journal.= That part of a shaft that runs in a bearing which guides or limits the motion of the shaft.

=Jump= _or_ =upset.= To enlarge at the end by a forging process.

K

=Kerf.= The pa.s.sageway or slot cut by a saw.

=Key.= A rectangular wedge for locking two pieces together.

=Knife.= The cutting tool used on a wood-planing or wood-splitting machine.

=Knurling-tool= _or_ =milling-tool.= A tool used to press indentations into the edges or upon the surface of metal, in order to increase the hand grip of it.

L

=Land.= That part of a tap or a reamer that lies between its plates and carries the cutting edges or teeth.

=Lantern.= A primitive form of gear in which rungs are used instead of teeth.

=Lap.= A grinding device consisting of a lead or other soft metal surface, on which emery and oil is used.

=Lap-joint.= A joint in which one piece overlaps the other.

=Lap-weld.= A weld in which both pieces are beveled at the ends and one end overlaps the other where the two are put together to weld.

=Lathe.= A machine that revolves work to be operated by cutting tools.

=Lathe-bed.= The frame that carries the head and tail stock of a lathe, and that rests upon a solid foundation.

=Lathe-carriage.= The sliding piece that carries the tool rest of a lathe.

=Lathe-centre.= The piece or part of a lathe that enters the coned recess of lathe work that is held between centres.

=Lathe-saddle.= The sliding piece that carries the tool rest of a lathe.

=Lathe-shears.= The frame of a lathe that carries the head and tail stocks, and that rests on legs.

=Lead-screw.= A screw for a lathe that is used for screw cutting only.

=Left-hand thread.= A screw thread in which the nut must be revolved in a direction opposite to that in which the hands of a watch move, in order that the nut may screw upon the bolt.

=Leg-vise.= A machinist's or blacksmith's vise having legs.

=Line of centres.= A line, real or imaginary, pa.s.sing from one centre to another.

=Line out.= To mark on work lines denoting the depth of surface that is to be cut away.

=Liner.= A piece of iron put behind or upon a piece to take up its wear.

Line-shaft. A shaft employed to transmit motion from an engine or motor to distant points.

=Link.= A piece having holes or pins at its end to connect two other pieces together.

=Live centre.= The centre of the live spindle of a lathe.

=Live spindle.= The revolving spindle of a lathe.

=Loose.= A term used to denote a part of a plate or saw that is not under tension.

=Lost motion.= Motion that is not transmitted on account of the looseness of the parts.

=Lug.= A small projection.

M

=Machine-bolt.= A bolt and nut of the sizes kept in stock by machinery dealers, the bolt usually being black or unturned.

=Machine-screw.= A small screw made to the Birmingham wire gauge.

=Machine-tap.= A long taper tap used in threading nuts in a machine.

=Machine-tool.= A machine that performs work by means of cutting tools.

=Mandrel.= 1. A cylindrical piece which is driven into hollow work and holds it while it is turned in the lathe. 2. A piece or bar on which work is driven or forced. 3. A shaft running in bearings.

=Mangle-wheel.= A gear wheel whose teeth are so arranged that the wheel is reciprocated back and forth on its centre, and does not make a full revolution.

=Marking-gauge.= A tool used by wood-workers to draw a line upon work.

=Master-tap.= A standard tap used for producing thread-cutting tools, or kept as a standard of size.

=Matched.= A board that has a tongue on one edge and a groove on the other so that the edges of the boards will match or be fitted together.

=Matching-machine= _or_ =matcher.= A wood-working machine which cuts a groove on one edge and a tongue on the other edge of a board or piece of work.