=Cylinder-head=, _or_ =cylinder-cover.= A piece that encloses or seals the end of a cylinder.

D

=Dead centre.= The stationary centre of a lathe.

=Dead-smooth file=, _or_ =superfine file.= The finest or smoothest cut of file.

=Delivery-rolls.= Rolls that remove the work from a machine or from its cutters or knives.

=Describing-circle.= The circle or cylinder containing the pencil for rolling a curve.

=Diametral pitch.= A system of designating the pitches of gear wheels.

=Diamond-point.= A machinist's chisel, square in cross-section, having a diamond-shaped face at the end, and two cutting edges, one at a right angle to the other.

=Die.= 1. A tool for cutting threads upon external surfaces, such as bolts. 2. A chumpy sliding piece.

=Differential screw.= A screw having a coa.r.s.e and a fine thread, the difference between the two pitches enabling a more powerful strain to be endured by the thread.

=Dimension planer.= A wood-planing machine, for planing accurately to size.

=Disk= _or_ =disc.= A cylinder whose length is very short in proportion to its diameter.

=Dividers.= A tool having two legs with sharp points at their ends for measuring distances or drawing circles.

=Dog.= A work holding device.

=Dog-chuck.= A chuck containing independent dogs or jaws.

=Dog-head.= A hammer used in plate or saw straightening.

=Double-eye= _or_ =knuckle-joint.= A joint in which one piece is forked at its end, to receive the other, and a pin pa.s.ses through both.

=Double-thread.= A screw thread, having two spirals in the same bolt or body.

=Dovetail.= A groove that is wider at the bottom than at the top, or a projection thicker at the top than at the bottom.

=Draw-filing.= That cla.s.s of filing in which the line of file motion is in the direction of the width of the file.

=Drawn-down.= Decreased in diameter, width or thickness, by forging or swaging.

=Drawn-out.= Increased in length, by forging or swaging.

=Drift.= A tool that cuts the walls of a hole while it is driven through by hammer blows.

=Drift-pin.= A taper pin that is used by boiler makers to drive through holes that do not come fair, or match properly.

=Drill.= A tool to pierce holes.

=Drill-chuck.= A chuck used to hold drills.

=Drilling-machine.= A machine for drilling holes in metal.

=Driver.= 1. A device for driving work in the lathe and sometimes called a dog or carrier. 2. A gear wheel which drives another.

=Drop-hammer.= A forging or stamping hammer which is lifted by power and let fall of its own weight.

=Drunken thread.= A screw thread that is not a true spiral, but is waved in its course.

=Duplex slide-rest.= A feed motion in which there are two slide-rests in one slide-way.

=Dutchman.= A piece let in to restore a worn part, or to hide a defect.

E

=Edge-moulding machine.= A machine for dressing the edges of wood-work to shape, and usually for forming a moulding thereon.

=Emery grinder=, _or_ =emery-grinding machine.= A machine in which emery wheels are used to grind the work.

=Emery wheel.= A wheel composed of emery cemented together under pressure.

=Endless screw.= A short length of screw formed to drive the teeth of a worm wheel.

=End-mill.= A milling-machine cutter, having teeth on its end face.

=Engine-lathe.= A lathe having a feed motor for the cutting tool.

=Epicycloid= ([)e]p-i-s[=i]'-kloid). A cycloidal curve in which the rolling circle is rolled outside the fixed or base circle.

=Equalizing-file.= A file that is slightly thicker in the middle than it is at either end.

=Expanding-chuck.= A chuck that is capable of expanding to accommodate a slight variation of work-diameter and usually holding the work from its bore.

=Expanding-mandrel.= A mandrel whose diameter may be varied, usually by means of moving jaws or pieces.

=Expansion-joint.= A joint capable of permitting the parts it connects to expand and contract under variations of temperature.

=Extension-bit.= A bit in which a cutter can be set so as to bore different sizes of holes.

=Extension-lathe.= A lathe whose bed is in two longitudinal divisions so that the upper one may be moved endways and thus form a gap to let chucked work of large diameter pa.s.s.

F

=Face.= 1. The broadest surface of a piece, or that having the largest area. 2. The circ.u.mferential surface of a wheel or pulley. 3. A surface on a gear-wheel tooth.