CNC Machining Glossary

Definitions for common CNC machining, sheet metal fabrication, cutting tool, and metrology terms — from backlash to Z-axis.

Ae (radial depth of cut): The width of engagement between cutter and workpiece in the radial direction, measured perpendicular to the feed direction. Reducing ae below 50% of tool diameter triggers chip thinning — feed per tooth must be increased to maintain designed chip thickness. See chip thinning.
Ap (axial depth of cut): The depth of cut in the direction of the tool axis — how deep the flutes engage in Z. In milling, increasing ap while reducing ae (HEM strategy) maintains MRR while reducing radial forces.
ATC (Automatic Tool Changer): A mechanism that swaps tools in a CNC machine's spindle automatically under program control. CNC machining centers use carousel, umbrella, or arm-type ATCs. Capacity ranges from 10 to 300+ tools.
Air bending: A sheet metal bending method where the punch presses the material into a V-die without the material touching the bottom of the die. The bend angle is determined by punch penetration depth. Tonnage is lower than bottom bending; springback is higher but predictable. Most modern CNC press brakes use air bending. See tonnage calculation.

Backlash: The mechanical play or lost motion in a leadscrew or ballscrew system — the distance the screw must rotate before it begins moving the axis carriage when reversing direction. Ballscrews: <0.015 mm acceptable; >0.030 mm indicates wear. Backlash causes positioning errors, particularly on inside profiles and when reversing direction under load.
Ballscrew: A precision lead screw with recirculating ball bearings running between the screw and nut, replacing the sliding contact of an ACME screw. Ballscrews offer low friction, high efficiency (~90%), and positioning repeatability of 0.005–0.010 mm — standard on all production CNC machines.
Bar feeder: An automatic device that feeds bar stock through the spindle bore of a CNC lathe. Magazine-style bar feeders hold 10–20 full-length bars and advance each bar automatically when the previous one is consumed. See bar feeder vs bar puller.
Bend allowance (BA): The arc length of material consumed in the bend zone when sheet metal is formed. Used to calculate the correct flat blank length: Flat = Leg1 + Leg2 + BA. BA depends on material thickness, inside bend radius, bend angle, and K-factor. See bend allowance explained.
Bend deduction (BD): An alternative to bend allowance for flat pattern calculation. BD is the amount subtracted from the sum of the two outside-dimension legs: Flat = (Leg1 + Leg2) − BD. Tabulated in press brake manuals for common thickness/die combinations.
BUE (Built-Up Edge): A phenomenon where workpiece material welds to the cutting tool's rake face at low cutting speeds, forming a false cutting edge. BUE causes poor surface finish, unpredictable dimensions, and rapid tool wear. Most common when machining aluminium, stainless steel, and low-carbon steel at insufficient cutting speed. Prevented by increasing speed, using positive rake geometry, and applying cutting fluid.

Chatter: Self-excited vibration between tool and workpiece during machining. Chatter produces characteristic wave patterns on machined surfaces, loud noise, and accelerates tool wear. Causes include tool overhang, insufficient workholding rigidity, incorrect cutting speed, and worn spindle bearings. See how to reduce milling chatter.
Chip thinning: The phenomenon where the actual chip thickness (chip load) is less than the programmed feed per tooth when the radial depth of cut (ae) is less than half the cutter diameter. The RCTF (radial chip thinning factor) quantifies this: programmed fz must be increased by RCTF to maintain the designed chip thickness. See chip thinning explained.
CMM (Coordinate Measuring Machine): A machine that measures the physical geometry of a part by probing its surface at discrete points with a touch-trigger or scanning probe. Produces XYZ coordinates that can be compared to nominal CAD geometry. Used for first-article inspection, statistical process control, and reverse engineering.
Collet: A tapered split sleeve that grips a tool or workpiece shank when compressed inside a matching holder. Collets provide better concentricity and higher clamping force per unit area than drill chucks. ER collets (ER8 through ER50) are the most common standard in CNC milling.
Crowning: In press brake operation, the controlled upward deflection applied to the bottom beam (bed) to compensate for deflection under load. Without crowning, the centre of the bend would be shallower than the ends, producing a tapered angle. CNC press brakes use hydraulic or wedge crowning systems.

Datum: A theoretically exact point, axis, line, or plane used as a reference for dimensional measurement and GD&T tolerances. In machining, datums are established by contacting the part against precision surfaces (datum simulators). Datum selection determines which features are measured relative to which reference — errors in datum setup propagate through all measurements.
Depth of cut: See Ae (radial) and Ap (axial).
Dwell (G04): A G-code command that pauses spindle movement for a specified time or number of spindle revolutions. Used to ensure a complete final revolution at the bottom of a boring cycle, improving hole finish and roundness.

EDM (Electrical Discharge Machining): A non-contact machining process where material is removed by a series of electrical sparks between the electrode and workpiece through a dielectric fluid. Wire EDM cuts 2D profiles through conductive material. Sinker EDM creates 3D cavities using a shaped electrode. EDM is unaffected by material hardness — commonly used for hardened tool steel and complex mould cavities.
Endmill: A rotary cutting tool with cutting edges on both the tip and the periphery, used in milling machines. Unlike drills, endmills can cut laterally as well as axially. Common configurations: 2-flute (for aluminium and plastics), 4-flute (general steel), variable-helix (chatter reduction).

Feed per tooth (fz): The thickness of chip each cutting flute removes per revolution, measured in mm/tooth (or inches/tooth). The fundamental chip load parameter in milling: too low causes rubbing and work hardening; too high causes chip packing, edge failure, or breakage. Correct fz depends on cutter diameter, material, and operation type.
Feed rate (Vf): The linear velocity of the cutter or workpiece in mm/min (or in/min): Vf = fz × z × n, where z is the number of flutes and n is spindle RPM. Feed rate is the value entered into the CNC program (F-word). Confusingly, "feed" is used for both feed per tooth and feed rate — context determines which is meant.
Fixturing: The hardware and methods used to locate and clamp a workpiece in a defined position on the machine table. Proper fixturing prevents movement under cutting forces, establishes repeatable part location, and ensures datums are correctly established.
Flatness (GD&T): A GD&T form tolerance specifying that all points on a surface must lie between two parallel planes separated by the tolerance value. Unlike parallelism, flatness has no datum reference — it controls only the form of the surface, not its orientation in space.

G-code: The standardised programming language for CNC machine tools (ISO 6983). G-codes are preparatory function codes (G00 = rapid, G01 = linear feed, G02/G03 = arc, G41/G42 = cutter compensation). M-codes (miscellaneous functions) control machine auxiliaries: M03 = spindle on CW, M08 = coolant on, M30 = program end.
GD&T (Geometric Dimensioning and Tolerancing): A symbolic language (ASME Y14.5 / ISO 1101) for defining the allowable variation in form, orientation, location, and runout of part features. GD&T replaces ambiguous ± coordinate tolerances with mathematically precise tolerance zones. Key symbols: ⊙ (position), ⌀ (cylindricity), ⌢ (circularity), // (parallelism), ⊥ (perpendicularity), ∠ (angularity). See true position and GD&T.
Guide bushing: A precision bushing mounted close to the cutting zone in a Swiss-type lathe, providing radial support to the bar stock. Eliminates bar deflection under cutting forces, enabling very precise turning of long slender parts from bar stock. Requires bar stock ground to close tolerance (h6 or better). See Swiss lathe vs CNC lathe.

HAZ (Heat Affected Zone): The region of material adjacent to a weld, laser cut, or plasma cut that has been altered in microstructure and properties by the heat of the process without melting. In laser cutting, HAZ depth is typically 0.05–0.3 mm; in plasma cutting, 0.5–2 mm. HAZ can cause hardness changes, distortion, and reduced corrosion resistance in stainless steel.
HEM (High Efficiency Milling): A milling strategy using small radial depth of cut (5–25% of tool diameter), large axial depth of cut (up to 2–4× tool diameter), and increased feed per tooth (corrected for chip thinning). Maintains consistent tool engagement angle and chip load, dramatically extending tool life and enabling higher MRR. Also called trochoidal milling or dynamic milling. See chip thinning and RCTF.
Honing: An abrasive finishing process using bonded abrasive stones to improve the geometry and surface finish of a bore. Produces a characteristic crosshatch pattern that retains lubricating oil in cylinder bores and hydraulic components. Typical Ra: 0.05–0.4 µm.

Indexable insert: A disposable cutting tip clamped mechanically to a tool body (holder). When one cutting edge wears, the insert is rotated to expose a fresh edge (indexed), then discarded when all edges are worn. ISO 1832 standardises insert shape, clearance, tolerance, chipbreaker, and geometry coding. See carbide insert grade selection.
Interpolation: The continuous, coordinated movement of two or more CNC machine axes to produce a desired path. Linear interpolation (G01) produces straight lines; circular interpolation (G02/G03) produces arcs. 5-axis simultaneous interpolation moves all five axes together to maintain tool normal to a complex surface.

K-factor: The ratio of the neutral axis position to the material thickness (t_neutral / t) during sheet metal bending. K-factor determines bend allowance for flat pattern calculation. Typical values: 0.30–0.50; mild steel air bend = 0.44. See bend allowance and K-factor.
Kerf: The width of material removed by a cutting process — the gap left in the workpiece. In laser cutting, kerf is typically 0.1–0.3 mm depending on material thickness and power. In plasma cutting, 1–3 mm. Kerf must be accounted for in CAM nesting to maintain correct part dimensions.

Lapping: A precision abrasive machining process using loose abrasive compound between a lap and the workpiece surface. Produces very flat, low-Ra surfaces (Ra 0.006–0.1 µm). Used for gauge blocks, precision flat surfaces, and lapped valve seats. Material removal is extremely slow.
Linear encoder: A position-feedback device attached directly to the machine axis (as opposed to a rotary encoder on the motor or leadscrew). Linear encoders measure actual carriage position directly, eliminating ballscrew backlash and thermal growth from the feedback loop. Typical resolution: 0.0001 mm (0.1 µm).

MMC (Maximum Material Condition): A GD&T modifier (symbol ⓜ) indicating that a tolerance applies when the feature contains the maximum amount of material — smallest hole, largest shaft. MMC allows bonus tolerance equal to the departure from MMC, enabling more parts to pass inspection while maintaining functional assembly requirements. See true position and GD&T.
MRR (Material Removal Rate): The volume of material removed per unit time, measured in cm³/min or in³/min. MRR = (ap × ae × Vf) / 1000. Maximising MRR while maintaining tool life and surface finish is the primary objective of feeds and speeds optimisation.

Neutral axis: The theoretical plane within a bent sheet metal part where material is neither stretched (tension) nor compressed — its length remains constant through the bend. In sheet metal, the neutral axis is not at the geometric centre of the thickness; its actual position is described by the K-factor.
Nesting: The process of arranging multiple flat-cut parts on a sheet to minimise material waste. CNC nesting software optimises part placement considering kerf width, material grain direction, and lead-in/lead-out positions. Utilisation above 85% is considered good; above 90% is excellent.

Overhang ratio: The ratio of tool (or boring bar) stick-out to tool diameter. Stiffness decreases as the cube of overhang length — doubling overhang reduces stiffness 8×. Rule of thumb: keep overhang below 3× diameter for solid carbide endmills, 4× for HSS, 5–6× for dampened or heavy-metal boring bars.

Parting off: A turning operation that cuts a finished workpiece free from the bar stock by feeding a narrow parting blade radially into the rotating workpiece until the part separates. The most vibration-prone operation in turning — minimum overhang and slow feed are critical.
Press brake: A machine tool that forms sheet metal by bending it between a punch (upper tool) and die (lower V-die). CNC press brakes use servo-controlled back gauges and ram position feedback for repeatable bend angles. Types: hydraulic, servo-electric, mechanical. See hydraulic vs electric press brake.
Profile tolerance (GD&T): A GD&T tolerance specifying that all points on a surface or line must lie within a zone offset equally inside and outside the true profile. Used for complex curves, airfoil surfaces, and any surface that cannot be adequately controlled by size tolerances alone.

Ra (Arithmetic mean roughness): The arithmetic average of the absolute surface profile deviations from the mean line, measured in µm or µin. The most universally specified surface finish parameter. See surface finish Ra chart.
RCTF (Radial Chip Thinning Factor): A correction factor applied to feed per tooth when ae < D/2. RCTF = √(D / (2 × ae)). The programmed fz must equal the desired chip thickness × RCTF to produce the correct chip load at the reduced engagement angle. See chip thinning explained.
Runout: The total deviation of a rotating surface from its true axis of rotation, measured radially (radial runout) or axially (axial/face runout). Causes: bent tool, out-of-spec holder, dirty taper, worn bearings. High runout causes one flute to cut deeper than others, reducing effective tool life. Maximum acceptable spindle runout on a production machining center: 0.003–0.005 mm TIR.
Rz: Mean roughness depth — the average of five peak-to-valley heights across five sampling lengths. More sensitive to isolated deep scratches than Ra. Rz ≈ 4–7× Ra for most machined surfaces. See surface finish Ra chart.

Soft jaws: Chuck jaws made from a machinable material (typically aluminium or mild steel) that are bored to match a specific part diameter while mounted in the chuck. Provide a full 120° contact arc per jaw, improving concentricity and preventing part deformation. Essential for second-operation gripping and thin-wall parts. See how to machine soft jaws.
Spindle speed (n): Rotational speed of the machine spindle in RPM. Calculated from cutting speed: n = (Vc × 1000) / (π × D), where Vc is in m/min and D is tool diameter in mm. Modern CNC machining centers: 8,000–50,000 RPM depending on machine type and spindle design.
Springback: The elastic recovery of sheet metal after bending — the slight unwinding of the bend angle that occurs when the punch is retracted. Springback increases with material yield strength and decreases with tighter inside radius. CNC press brakes compensate by over-bending slightly. Springback for mild steel air bend: 1–3°; high-strength steel: up to 8–12°.

TIR (Total Indicator Reading): The total range of indicator deflection during one full rotation — the sum of the positive and negative deviations from nominal. Used to quantify runout, concentricity, and flatness in shop practice. Equivalent to full indicator movement (FIM) in some standards.
Tolerance: The total permissible variation in a dimension or geometric characteristic. Tolerance = upper limit − lower limit. Tighter tolerances require more capable processes, longer setup times, and higher cost. Standard machining tolerances without special effort: ±0.05–0.10 mm; precision turning/grinding: ±0.005–0.010 mm.
Tonnage (press brake): The force applied by a press brake punch to bend sheet metal, measured in tons (US or metric). Required tonnage formula: T (t/m) = (t² × Rm) / (8 × V), where t = thickness, Rm = tensile strength, V = V-die width. See press brake tonnage calculation and tonnage calculator.
True position: A GD&T location tolerance specifying that the centre of a feature (hole, pin, slot) must fall within a cylindrical (or circular) tolerance zone centred on the true position defined by dimensions from datums. Calculated as: TP = 2 × √(ΔX² + ΔY²). See true position and GD&T.

V-die: The lower tool in a press brake setup — a block with a V-shaped opening into which the sheet metal is pushed by the punch to form the bend. V-die width (V) is typically 6–10× material thickness for air bending. Narrower V-dies require less tonnage and allow tighter inside radii; wider V-dies produce larger inside radii and require more tonnage.
Vc (cutting speed): The peripheral speed of a cutting tool or workpiece in m/min (or SFM in imperial). The fundamental parameter from which spindle RPM is derived. Cutting speed recommendations are material-specific: aluminium 300–1,000 m/min; mild steel 150–300 m/min; titanium 25–80 m/min; hardened steel 80–200 m/min (CBN).

Work holding: The collective term for all devices, methods, and fixtures used to locate and secure a workpiece on a machine tool. Good workholding provides: correct datum establishment, rigidity against cutting forces, repeatability (part-to-part location consistency), and fast setup. See workholding hub.

Zero-point clamping: A modular workholding system where a standardised pull stud built into the workpiece pallet engages a spring-loaded receiver on the machine table. Provides <5 µm location repeatability and sub-minute pallet changes. Enables the machine to run lights-out across multiple setups without a measuring routine between each part. See zero-point clamping systems.