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Loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface. Frequently occurs in steel exposed to air at high temperatures, resulting in loss of hardness and strength at the surface.

degrees of freedom

Number of axes along which a robot, and thus the object it is holding, can be manipulated. Most robots are capable of maneuvering along the three basic Cartesian axes (X, Y, Z). More sophisticated models may move in six or more axes. See axis.


Removal of ions from a water-based solution. See semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.

depth of cut

Distance between the bottom of the cut and the uncut surface of the workpiece, measured in a direction at right angles to the machined surface of the workpiece.

depth-to-diameter ratio

Ratio of the depth of a hole compared to the diameter of the tool used to make the hole.


Cubic crystalline form of carbon produced under extreme pressures at elevated temperatures. The hardest natural substance, it has approximately five times the indentation hardness of carbide. Its extreme hardness, though, makes it susceptible to fracturing. See superabrasive tools.

diamond bandsawing

Machine operation in which a band with diamond points is used to machine carbides, ceramics and other extremely hard materials.

die casting

Casting process wherein molten metal is forced under high pressure into the cavity of a metal mold.


1. Spreading of a constituent in a gas, liquid or solid, tending to make the composition of all parts uniform. 2. Spontaneous movement of atoms or molecules to new sites within a material.

diffusion coating

Any process whereby a basis metal or alloy is either coated with another metal or alloy and heated to a sufficient temperature in a suitable environment, or exposed to a gaseous or liquid medium containing another metal or alloy, thus causing diffusion of the coating or of the other metal or alloy into the basis metal or alloy with resultant changes in the composition and properties of its surface.

direct numerical control (DNC)

Method of transferring CNC code from the CAD/CAM system to the machine tool.

disc grinding

Operation in which the workpiece is placed against the side of a wheel rather than the wheel’s periphery. See grinding.

disc-cutting attachment

Adjustable device for a contour bandsaw that positions stock to allow the sawing of arcs and circular shapes on a contour bandsaw.


Form of relief given to the face of an endmill to prevent undesirable contact with the work. Similar to clearance.

dividing head

Attaches to a milling-machine table and precisely indexes the workpiece. Allows equally spaced cuts to be made when machining gear and sprocket teeth, spline keys, serrations, etc.

dovetail cutter

Cutter for milling dovetail slots. See milling cutter.


Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.

drill jig

Accessory that holds a workpiece securely while guiding a drill or other tool into the workpiece. Ensures accurate, repeatable location.

drill-grinding gage

Used to check a drill’s entry angle into a workpiece. Also used to check accuracy when grinding drills.


Operation in which a rotating tool is used to create a round hole in a workpiece. Drilling is normally the first step in machining operations such as boring, reaming, tapping, counterboring, countersinking and spotfacing.

drilling machine (drill press)

Machine designed to rotate end-cutting tools. Can also be used for reaming, tapping, countersinking, counterboring, spotfacing and boring.

drilling tool (drill or drill bit)

End-cutting tool for drilling. Tool has shank, body and angled face with cutting edges that drill the hole. Drills range in size from “microdrills” a few thousandths of an inch in diameter up to spade drills, which may cut holes several inches in diameter. Drills may have tapered shanks with a driving tang and fit directly into a spindle or adapter, or they may have straight shanks and be chuck-mounted. The rake angle varies with the material drilled. Styles include twist drills, straight-flute drills, half-round and flat drills, oil-hole drills, indexable drills and specials.

drive plate

Attaches to a lathe spindle and has a slot or slots that engage a driving dog to turn the work. Usually used in conjunction with centers. See centers; driving dog.

driving dog

Device having a ring or clamp on one end that slips over the workpiece to be turned; a screw secures the workpiece in place. The dog’s opposite end (tail) fits into a drive plate attached to the machine spindle. See centers; drive plate.

ductile cast irons

Ferrous alloys in which graphite is present as tiny balls or spherulites. The spheroidal graphite structure is produced by adding one or more elements to the molten metal, among which magnesium and cerium are commercially important. Approximate composition of ductile cast irons is: 3.0 to 4.0 percent carbon, 0.1 to 1.0 percent manganese, 1.8 to 2.8 percent silicon, 0.1 percent (maximum) phosphorus and 0.03 percent (maximum) sulfur. Typical ductile cast iron grades are D-4018, D-4512, D-5506 and D-7003 by definition of the Society of Automotive Engineers; 60-40-18, 65-45-12, 80-55-06, 100-70-03 and 120-90-02 by definition of the American Society for Testing and Materials. Also known as nodular cast irons.

ductile fracture

Fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy.


Ability of a material to be bent, formed or stretched without rupturing. Measured by elongation or reduction of area in a tensile test or by other means.

dynamic stiffness

Measure of a machining system’s ability to dampen vibration from a forced input. If the dynamic stiffness of a system is not sufficient to dampen vibration, chatter occurs. See static stiffness; stiffness.


When drilling, a device for measuring the generated torque and axial force (thrust). When milling, a device for measuring the generated torque and feed force. When turning, a device for measuring the tangential, feed and radial forces.