CNC Operator’s Terms


1-2-3 Block A precision ground block that is 1x2x3 inches. Useful for helping set up the machine.
2D A prismatic part of constant thickness.
2-½ D A prismatic part with multiple thicknesses. All planes are parallel to each other.
3D A part that is not completely prismatic, including molds and organically shaped parts.
Burr A sharp edge or flashing.
Block A line of code in a CNC program.
CAD Computer Aided Design
CAM Computer Aided Manufacturing.
CDC Cutter Diameter Compensation.
CNC Computer Numerical Control.
Chip Load The amount of material removed with each pass of a tool cutting edge.
Datum Point on the part from which all coordinates are referenced.
Drill Chart A table that shows drill sizes, their decimal equivalent, and what size drill to use for a specified tap.
Feed The linear speed at which a tool advances through material.
First Article The first part machined with a new program and setup.
G-M Code The CNC machine tool language.
Gouge An error that causes an overcut on the part.
High Speed Machining (HSM) Toolpath strategies that minimize machining time and tool breaking. Includes several technologies such as Adaptive Roughing that maintain a constant amount of tool engagement.
Home Location of machine after it is turned on and moved to its start position.
ID Inside diameter or contour, like a pocket.
Insert Carbide composite cutting tool used extensively for turning operations.
Job A sequential list of one or more machining operations that share the same setup and fixture offset.
Machine Coordinate System Coordinates in reference to the machine Home position.
Mill A machine tool that removes material by spinning a tool and moving it in relation to the part.
OD Outside diameter or contour.
Overcut Describes a machined feature is too big because not enough material was removed during machining.
Post Processor The part of a CAD/CAM system that actually creates and formats the CNC program for a specific machine and control.
Rapid The fastest linear feed rate at which a machine tool can move.
Reference Point Some point on the part that can be found reliably by mechanical means.
Rigid Tapping The capability of a CNC machine to control a tap without the use of a special tapping attachment.
Scallop Ridges left on the part, usually from a ball or bull nose end mill on a 3D sculpted surface.
Setup A complete set of work holding to machine one side of a part.
Speed How fast a tool turns in RPM.
Spindle Part of CNC machine that grips and spins the cutting tool.
Stepdown Distance the tool moves down in Z between machining passes.
Stepover Distance the tool moves over in the XY plane between machining passes.
Stock Allowance Amount of material left after a machining operation or pass to be removed by a finish pass.
Spring Pass Two or more machining passes that follows the exact same path. Used to straighten walls and improve surface finish.
TLO Tool Length Offset. Used to tell CNC machine the length of a tool.
Tool Changer A device on a CNC machine that automatically takes tools from a carousel and places them in the machine spindle.
T-Slot A slot in the machine table into which a special T-nut can be inserted. Used to fasten clamps or parts to the machine table.
Turret Lathe assembly that attaches the lathe tool holder to the machine.
Undercut Describes a machined feature is too small because too much material was removed during machining.

CNC Operator’s Terms