The WordsOn the Drawing.
Machining Glossary
63 machining terms defined in plain language — written by the shop that uses them, not copied from a textbook.
At a glance
- Terms /
- 63 defined
- Categories /
- 8
- Written by /
- The shop that uses them
- Updated /
- August 2026
Machining has its own vocabulary, and most of it is never written down anywhere a customer can find it. A “tenth” means 0.0001″, not a tenth of an inch. A tolerance in TIR is a total, not a plus/minus. These definitions exist so that the person specifying a part and the person cutting it are talking about the same thing.
Every term, A–Z
AnnealingBoringBroachingCaliperCase HardeningChamferChatterChuckClearance FitCMMColletConcentricityCounterboreCountersinkDatumDeburrDial IndicatorFacingFilletFirst Article InspectionFixtureFlatnessGD&THardnessHeat NumberHeat-Affected ZoneInterference FitJob ShopKeywayKnurlingLine BoringMachinabilityMandrelMicrometerMillingMTRNCRNormalizingNPTObsolete PartQuenchingReamingReverse EngineeringRunoutSplineStress ReliefSurface FinishTapTaperTemperingTenthThouThread PitchThreadingTIRToleranceTrammingTravelerTrue PositionTurningUndercutWeldmentWork Hardening
Tolerance & Measurement
How close is close enough, and how it is written down.
- Tolerancetolerancing · tol
- Tolerance is the amount a finished dimension is allowed to differ from the dimension on the drawing and still be accepted.
- Thouthousandth · thousandths
- A thou is one thousandth of an inch, written 0.001", and it is the everyday unit of precision on a machine shop floor.
- Tenthtenths · ten thousandth
- In machining, a tenth means one ten-thousandth of an inch — 0.0001" — not one tenth of an inch.
- GD&Tgeometric dimensioning and tolerancing · g d and t
- GD&T is a symbolic language on engineering drawings that controls the form, orientation and location of features rather than just their size.
- Datum
- A datum is the reference surface, axis or point on a part that all other measurements are taken from.
- True Positionposition tolerance
- True position defines a circular zone within which the centre of a hole or feature must fall, measured from the stated datums.
- Runoutrun out · TIR
- Runout is the total movement shown on an indicator when a part is rotated one full turn, measuring how far a surface wobbles relative to its axis.
- TIRtotal indicator reading · full indicator movement
- TIR stands for total indicator reading — the full span between the highest and lowest values an indicator shows across a measurement.
- Concentricity
- Concentricity controls how closely the centre axis of one feature matches the centre axis of another.
- Flatness
- Flatness controls how much a surface may deviate from a perfect plane, independent of any other feature on the part.
- Surface FinishRa · roughness
- Surface finish describes the fine texture left on a machined surface, most often stated as Ra in microinches or micrometres.
Machining Operations
What the machine is actually doing to the metal.
- Turning
- Turning is machining on a lathe, where the workpiece rotates and a stationary cutting tool removes material to produce round features.
- Milling
- Milling is machining with a rotating cutter against a workpiece that is held still, used to produce flat faces, slots, pockets and prismatic shapes.
- Boring
- Boring is enlarging an existing hole with a single-point tool to bring it to an exact size, position and straightness.
- Line Boring
- Line boring machines two or more bores in one pass along a single rotating bar so that they share one true axis.
- Facing
- Facing is cutting across the end of a part to produce a flat surface perpendicular to its axis.
- Reaming
- Reaming finishes a previously drilled hole to a precise diameter and a smooth surface using a multi-fluted tool that removes only a small amount of material.
- Threading
- Threading is cutting a helical groove on a part so it can screw into or accept another threaded component.
- Knurling
- Knurling presses a crosshatch or straight pattern into a cylindrical surface to create grip, usually with a hardened rolling wheel rather than by cutting.
- Broaching
- Broaching pushes or pulls a long, stepped cutting tool through or across a workpiece so that every tooth removes a little more material and the finished shape is produced in a single stroke.
- Deburrdeburring · burr
- Deburring removes the sharp raised edge of displaced metal left behind wherever a cutting tool exits the workpiece.
Part Features
The shapes on a drawing and what they are for.
- Chamfer
- A chamfer is an angled cut across an edge or corner, most commonly at 45 degrees, that replaces a sharp corner with a short flat.
- Filletradius · corner radius
- A fillet is a rounded internal corner where two surfaces meet, specified by its radius.
- Keyway
- A keyway is a rectangular slot cut into a shaft and its mating hub so that a key can sit in both and transmit torque between them.
- Spline
- A spline is a series of ridges machined along a shaft that mate with matching grooves in a hub to transmit torque across many teeth at once.
- Counterbore
- A counterbore is a flat-bottomed enlargement at the top of a hole that lets a bolt head or washer sit flush with or below the surface.
- Countersink
- A countersink is a conical enlargement at the top of a hole that lets a flat-head screw sit flush with the surface.
- Undercutrelief · thread relief
- An undercut is a narrow groove machined at the base of a shoulder or the end of a thread so the mating part or the cutting tool can run out cleanly.
- Taper
- A taper is a gradual, uniform change in diameter along the length of a part, specified either as an included angle or as a ratio of diameter change per unit length.
Inspection & Quality
Proving the part is what the drawing asked for.
- First Article InspectionFAI · first article
- A first article inspection is a full dimensional check of the first part off a new setup, verified against every dimension on the drawing before the rest of the run is made.
- NCRnonconformance report · non-conformance
- An NCR is a nonconformance report — the written record raised when a part or process does not meet its specification.
- CMMcoordinate measuring machine
- A CMM is a coordinate measuring machine — an instrument that touches a part at many points with a probe and calculates dimensions and geometric relationships from their coordinates.
- Micrometermic · mike
- A micrometer is a precision measuring instrument that uses a calibrated screw to measure external, internal or depth dimensions, typically reading to 0.0001".
- Caliper
- A caliper is a sliding measuring instrument that reads external, internal and depth dimensions, usually reliable to about 0.001".
- Dial Indicator
- A dial indicator is a contact instrument that shows small movements of a plunger or lever on a dial, used to compare a surface against a reference rather than to measure absolute size.
- MTRmaterial test report · mill cert
- An MTR is a material test report — the mill's certificate stating the chemical composition and mechanical properties of a specific heat of material.
- Heat Number
- A heat number identifies the specific batch of molten metal a piece of material was poured from, linking the physical stock back to its material test report.
Materials & Heat Treatment
What the part is made of and what was done to it.
- Machinability
- Machinability is how easily a material can be cut — combining tool wear, achievable surface finish, chip behaviour and the cutting speed the material will tolerate.
- Work Hardeningstrain hardening
- Work hardening is the increase in a metal's hardness caused by deforming it, which in machining means the surface gets harder as it is cut.
- Annealing
- Annealing is heating metal and cooling it slowly to soften it, relieve internal stress and make it easier to machine.
- Normalizing
- Normalizing is heating steel above its transformation range and cooling it in still air to produce a uniform, refined grain structure.
- Quenching
- Quenching is rapidly cooling heated steel in water, oil or air to lock in a hard structure.
- Tempering
- Tempering is reheating quenched steel to a lower temperature to reduce brittleness and internal stress, trading a little hardness for toughness.
- Case Hardeningcarburizing · nitriding
- Case hardening produces a hard outer layer on a part while leaving the core relatively soft and tough.
- Stress Relief
- Stress relieving is a low-temperature heat treatment that reduces the internal stresses left in a part by welding, forming or heavy machining, without changing its hardness.
- HardnessRockwell · Brinell
- Hardness is a material's resistance to permanent indentation, measured on scales such as Rockwell C (HRC) for hardened steel and Brinell (HB) for softer or coarser material.
Workholding & Setup
How the part is held while it is cut.
- Chuckthree jaw · four jaw
- A chuck is the clamping device that grips a workpiece on a lathe, using jaws that move to centre and hold the part.
- Collet
- A collet is a sleeve that grips a workpiece or tool evenly around its full circumference as it is drawn into a taper.
- Mandrel
- A mandrel is a precisely sized shaft that a part is mounted on by its bore so that its outside features can be machined true to that bore.
- Fixturejig · tooling
- A fixture is a purpose-built device that holds a workpiece in a known, repeatable position for machining or inspection.
- Tramming
- Tramming is the process of aligning a milling machine head so its spindle is exactly perpendicular to the table.
- Chatter
- Chatter is self-sustaining vibration between the tool and the workpiece that leaves a rippled surface and shortens tool life.
Threads & Fits
How parts screw together and how tightly they sit.
- Thread PitchTPI · threads per inch
- Thread pitch describes how far apart the threads are — stated in inch practice as threads per inch (TPI) and in metric as the distance in millimetres between adjacent crests.
- Tap
- A tap is a cutting tool that forms an internal thread in a drilled hole.
- NPTnational pipe thread · pipe thread
- NPT is the American tapered pipe thread standard, where the thread itself tapers so that the joint seals as the threads are pulled tight together.
- Interference Fitpress fit · shrink fit
- An interference fit is one where the shaft is deliberately larger than the hole it goes into, so the two are held together by elastic pressure once assembled.
- Clearance Fitslip fit · running fit
- A clearance fit is one where the hole is larger than the shaft, leaving a deliberate gap so the parts can slide or rotate relative to each other.
Shop Floor
The language used between the office and the machines.
- Job Shop
- A job shop is a manufacturer that makes parts to each customer's specification in varying quantities, rather than producing its own catalogue of products at volume.
- Travelerrouter · job packet
- A traveler is the document that moves with a job through the shop, listing every operation in sequence and recording what was done, by whom and when.
- Reverse Engineering
- Reverse engineering in machining is producing a manufacturable drawing and a replacement part from an existing component when no original documentation is available.
- Weldment
- A weldment is an assembly built by welding several pieces of material together, often machined after welding to bring critical features to size.
- Heat-Affected ZoneHAZ
- The heat-affected zone is the band of parent material next to a weld whose properties have been changed by the heat of welding without being melted.
- Obsolete Partdiscontinued part · no longer available
- An obsolete part is a component the original manufacturer no longer supplies, whether because the product line ended, the manufacturer no longer exists, or the tooling was scrapped.
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