CNC Workholding & Inspection Glossary

Definitions of the terms that come up when specifying workholding, tooling and inspection fixtures. Each entry has its own anchor, so you can link straight to a single definition.

This glossary is kept in English only. Terms such as datum, locator, pull stud and GR&R circulate in English in engineering practice, and translating them introduces more confusion than it removes.

Zero-Point & Workholding

Terms used when a shop moves from dial-in setups to a shared reference interface.

Automatic Pallet ChangerAPC: A machine feature that swaps a loaded pallet into the work zone and the finished one out, usually in well under a minute. It only pays off if the fixture on the pallet is already correctly positioned.
Base Plate: The plate bolted to the machine table that carries the zero-point receivers. It converts the machine table into a grid of known positions and is usually indicated in once, then left alone.
Clamping Force: The force holding the workpiece against cutting loads. Too little allows movement; too much distorts thin-walled parts, which then spring back out of tolerance after release.
Dedicated Fixture: A fixture designed and built for one part number. It is usually the fastest to load and the most rigid, and the hardest to justify at low volume. Read more →
Direct Mount: Fitting receivers straight into the machine table rather than into a separate base plate. It saves stack height but ties the interface to that one machine.
External Setup (Off-Line Setup): Preparation done while the machine is still running the previous job — loading the next fixture, presetting tools, checking the part. Converting internal setup into external setup is the main mechanism behind SMED.
Grid Pitch: The centre distance between receivers in a base plate, commonly a round metric figure. Choosing one pitch across the shop is what makes fixtures interchangeable between machines. Read more →
Hydraulic Clamping: Clamping actuated by pressurised fluid, used where higher force is needed in a small envelope. It requires a power unit and lines, and the circuit has to be considered in the cell layout.
Locator: Any element that constrains a part or fixture in a defined direction. In zero-point systems a locator usually fixes position in X and Y; in checking fixtures it establishes a datum contact.
Modular Fixturing: Building fixtures from a kit of standard elements — plates, risers, clamps, locators — instead of machining a dedicated fixture for every part. It trades some rigidity and cycle time for far shorter lead time.
Pallet: A carrier that holds the workpiece or fixture and is exchanged as a unit. Pallets let the operator load the next job while the machine is still cutting the current one.
Pneumatic Unlocking: A design in which spring force holds the clamp closed and shop air is used only to release it. If air is lost the clamp stays closed, which is why this arrangement is common on machining centres.
Pull Stud: The male locating and clamping element fitted to the underside of a pallet or fixture. It is drawn into the receiver, providing both the position and the clamping force.
Pull-In Force: The axial force with which a receiver draws the pull stud onto the seating face. It determines how firmly the fixture is held against cutting loads and how well the seating faces stay in contact.
Receiver (Clamping Module): The female half of a zero-point interface, mounted in the base plate or tombstone. It pulls in and holds the pull stud, and provides the seating face the fixture rests against.
Repeatability: How closely a fixture returns to the same position after being removed and re-clamped. It is a property of the whole assembled system — interface, seating faces, cleanliness and clamping force — not of a single component in isolation. Read more →
Seat Check (Air Seating Sensor): An air-pressure test that confirms the fixture is fully down on its seating face before the cycle starts. A chip under the seat lifts the fixture by a few microns, which the sensor detects as a pressure drop.
Setup Time: The time a machine is stopped between the last good part of one job and the first good part of the next. Zero-point systems attack it by moving preparation off the machine rather than by making operators work faster. Read more →
SMED (Single-Minute Exchange of Die)SMED: A changeover-reduction method that separates work which must happen with the machine stopped from work that can happen while it runs, then converts as much as possible into the second category.
Soft Jaws: Vise jaws machined in place to match the workpiece profile. Cutting them on the machine that will hold the part is what makes them locate accurately.
Stack Height: The vertical space consumed between the machine table and the bottom of the workpiece by the base plate, receivers and fixture. It comes directly out of the machine's Z travel.
Tombstone: A rigid column mounted on a horizontal machining centre pallet, presenting two, three or four working faces. Fitting each face with zero-point receivers multiplies the number of parts loaded per pallet change.
Workholding: Everything between the machine table and the workpiece: vises, chucks, fixtures, clamps and the interface that positions them. It sets the practical accuracy limit of any process.
Zero-Point Positioning System: A quick-change workholding interface in which pallets, fixtures and tombstones all locate to the same reference on the machine table. Because the reference is shared, a fixture set up off-line drops onto the machine already in position, removing the dial-in step. Read more →

Tooling & Angle Heads

Spindle interfaces, driven tooling and the vocabulary of reaching awkward features.

Adjustable Angle Head: An angle head whose output axis can be set to a range of angles. It covers compound features that a fixed head cannot reach, at some cost in stiffness and setup time.
Angle Head: A driven attachment that redirects spindle rotation, most commonly through 90 degrees, so a 3-axis machine can cut side faces, cross holes and deep pockets without re-clamping the part. Read more →
Automatic Tool ChangerATC: The magazine and arm that exchange tools without operator intervention. Oversized attachments such as angle heads may need adjacent pockets left empty.
BT Taper: A 7:24 steep-taper tool interface widely used on Asian-built machining centres, in sizes such as BT30, BT40 and BT50. The number refers to the taper size, not the tool length.
CAT Taper: The North American 7:24 steep-taper standard, dimensionally close to BT but with a different retention knob. Knobs are not interchangeable between the two.
Driven (Live) Tooling: Powered tool stations on a lathe turret that let milling, drilling and tapping happen without moving the part to a second machine.
ER Collet: A widely used clamping collet series, designated ER11 through ER50. Each collet covers a small diameter range, so a set is needed to span common cutter shanks.
Fixed Angle Head: An angle head with a permanently set output angle, usually 90 degrees. Fewer joints generally means more rigidity than an adjustable head of similar size.
Gauge Length: The distance from the spindle gauge line to the tool tip, used for length offsets. Angle heads change the effective gauge geometry, so their offsets are set differently from straight holders.
HSKHSK: A hollow short taper that clamps on both the taper and the spindle face. The dual contact improves rigidity and repeatability at high spindle speeds compared with a taper-only interface.
Reach: How far a tool or attachment can extend into a feature before the body interferes. On angle heads, reach and rigidity pull in opposite directions.
Retention Knob: The stud screwed into the back of a steep-taper holder that the drawbar grips. Its geometry is specific to the machine standard and must match the spindle.
Runout: The amount by which a rotating tool deviates from its axis of rotation, measured as total indicator reading. It shortens tool life and shows up directly as hole size and surface finish error.
Shank: The plain portion of a tool or holder that is gripped. Shank type and size determine which collet or holder is required.
Spindle Taper: The internal taper that receives the toolholder and defines what tooling a machine can accept. It is the first thing to confirm before quoting any attachment.
Stop Block (Anti-Rotation Arm): The feature that reacts the torque of an angle head against the spindle housing or a fixed post. Without it the head body would simply rotate with the spindle.
Through-Spindle CoolantTSC: Coolant delivered through the spindle and tool to the cutting edge. Whether an attachment can pass it through is a real selection criterion for deep-hole work.
Torque Rating: The maximum torque an attachment is designed to transmit continuously. Exceeding it shows up as accelerated gear and bearing wear rather than immediate failure.

Inspection, GD&T & Metrology

How a part is located, measured and judged against the drawing.

3-2-1 Location: The classic scheme for fully constraining a rigid part: three points define the primary plane, two the secondary direction, one the tertiary. It removes all six degrees of freedom without over-constraining.
3D Scanning: Capturing a dense point cloud of a surface and comparing it with the CAD model. It covers whole surfaces where a fixture checks specific features, and the two are often used together.
Attribute Gauge: A gauge that returns a pass/fail judgement rather than a value. Fast on the line, but it cannot show a trend before parts go out of tolerance.
Body-in-WhiteBIW: The welded vehicle body structure before paint, trim and powertrain. Its checking fixtures are large, and datum strategy dominates their design.
Calibration: Comparing a gauge against a known reference and recording the deviation. Checking fixtures are normally calibrated on a schedule and after any repair or crash.
Checking FixtureCF: A dedicated fixture that holds a part in its drawing-defined datum position so that dimensions can be checked quickly and consistently. It answers pass or fail faster than a CMM, without producing a full coordinate report. Read more →
Coordinate Measuring MachineCMM: A machine that measures point coordinates on a part and evaluates them against the model. It is the reference method for accuracy and traceability, but slower and less suited to line-side checking than a fixture.
Datum: A theoretically exact plane, axis or point from which other dimensions are measured. Physical datum features on the part contact the fixture, which is why fixture and drawing must agree on which surfaces those are.
Datum Reference FrameDRF: The set of three mutually perpendicular planes established by the primary, secondary and tertiary datums. Every located dimension is measured from this frame.
Degrees of FreedomDOF: The six ways a free body can move — three translations and three rotations. A locating scheme is complete when all six are constrained exactly once.
Flatness: A form tolerance requiring a surface to lie between two parallel planes a stated distance apart. It refers to no datum, because form is independent of location.
Functional Gauge: A hard gauge built to the maximum material condition of a feature. If the part accepts the gauge it will assemble, which makes it a fast go/no-go check rather than a measurement. Read more →
Gauge R&RGR&R: A study that separates measurement variation into repeatability (same operator, repeated) and reproducibility (between operators). It tells you how much of the observed spread comes from the gauge rather than the process.
Geometric Dimensioning & TolerancingGD&T: A symbolic language for stating how much a feature may deviate in form, orientation, location and profile, and relative to which datums. It replaces ambiguous plus/minus notes on features whose function depends on relationships rather than size alone. Read more →
Master (Setting) Part: A reference part of known geometry used to verify that a fixture or gauge still reads correctly. It is how drift is detected between formal calibrations.
Maximum Material ConditionMMC: The condition in which a feature contains the most material — largest shaft, smallest hole. Tolerances stated at MMC may gain bonus tolerance as the feature departs from it.
Measurement System AnalysisMSA: The broader assessment of a measurement system's bias, linearity, stability and variation. GR&R is one component of it.
Net Pad: A small machined contact pad that touches the part at a defined datum point. Using discrete pads rather than a full surface keeps contact predictable on parts that are not perfectly flat.
Over-Constraint: Locating a part in the same direction more than once. The extra contact forces the part to conform to the fixture, so the fixture measures its own error instead of the part's.
Production Part Approval ProcessPPAP: The automotive submission package that demonstrates a supplier can make the part to drawing, repeatably. Fixture and gauge documentation is normally part of the submission.
Profile Tolerance: A tolerance zone that follows the true profile of a surface, controlling form, orientation and location at once. Widely used on sheet-metal and moulded automotive parts.
Statistical Process ControlSPC: Charting measured values over time to distinguish normal variation from a real shift. It depends on a measurement system whose own variation is small relative to the tolerance.
Toggle Clamp: An over-centre clamp that locks with light hand effort and releases quickly. Common on checking fixtures because it applies repeatable force without a torque wrench.
Traceability: An unbroken chain of calibrations linking a measurement to a national or international standard. Without it a number is a reading, not a measurement.
True Position: A location tolerance defining a cylindrical zone around the theoretically exact position of a feature. It is stated relative to datums and usually expressed as a diameter.
Variable Gauge: A gauge that returns an actual measured value. Slower than an attribute check, but the numbers support SPC and early drift detection.

Automation & Pallet Handling

What workholding has to provide before a cell can run unattended.

Chip Management: Keeping chips off seating faces and locators. It is the most common practical cause of lost repeatability in an otherwise sound zero-point installation.
Clamp State Confirmation: A signal proving the clamp actually reached the clamped or unclamped position. In an unattended cell this is a safety interlock, not a convenience.
Collision Avoidance: Verifying, usually in simulation, that tool, holder, fixture and robot paths never intersect. Angle heads and tall fixtures make this check non-optional.
Cycle Time: The time to complete one part, from the start of a cycle to the start of the next. In an automated cell it includes load, unload and any waiting, not just cutting.
Gripper (End Effector): The tooling at the robot wrist that holds the part or pallet. Its envelope constrains fixture design, because it has to reach in and out without collision.
Lights-Out Machining: Running production without an operator present, typically overnight. It requires the cell to detect its own faults, because nobody is there to notice them. Read more →
Overall Equipment EffectivenessOEE: The product of availability, performance and quality for a machine. Setup reduction improves the availability term, which is often the largest single loss.
Pallet Identification: Tagging each pallet, usually by RFID or barcode, so the control knows which program to run. Without it a pallet pool has to be loaded in a fixed sequence.
Pallet Pool: A store of loaded pallets that a machine draws from automatically. The pool sets how long the machine can run unattended before it needs a person.
Part Presence Detection: A sensor confirming a workpiece is actually in the fixture before the cycle starts. It prevents machining air and crashing into an empty fixture.
Recovery Logic: What the cell does when something goes wrong at 3am — stop safely, skip the pallet, or alarm and wait. Deciding this before installation is what separates a cell that survives a night from one that does not.
Robot-Tended Cell: A machine loaded and unloaded by a robot rather than an operator. The workholding has to present the part in a position the gripper can reach and confirm it is seated. Read more →
Takt Time: Available production time divided by customer demand — the rate at which parts must be completed to keep up. Cycle time has to be at or below it.

General Machining

Background terms that keep coming up in workholding and fixture discussions.

3+2 Machining: Positioning the two rotary axes, locking them, then cutting with the three linear axes. Simpler to program than continuous 5-axis and rigid enough for most tilted-face work.
5-Axis Machining: Machining with two rotary axes in addition to the three linear axes, letting the tool approach the part from almost any direction. It removes setups but raises machine, programming and fixture cost. Read more →
Anodising: An electrochemical surface treatment for aluminium that adds a hard, corrosion-resistant oxide layer. Common on fixture plates that are handled frequently.
Burr: Raised material left at an edge after cutting. On a locating surface a burr becomes a positioning error, which is why deburring is part of fixture maintenance.
Chatter: Self-excited vibration between tool and workpiece, leaving a characteristic surface pattern. It often traces back to fixture or attachment stiffness rather than the cutter.
Deflection: Elastic bending of tool, part or fixture under load. It shows up as a dimensional error that disappears when the part is released, which makes it easy to misdiagnose.
Electrical Discharge MachiningEDM: Removing material by controlled sparks between electrode and workpiece. Zero-point interfaces are common in EDM because parts move repeatedly between machining, EDM and inspection.
First Article InspectionFAI: A full dimensional check of the first part from a new setup or process, verifying every drawing requirement before production continues.
Hardened Steel: Steel heat-treated to increase hardness and wear resistance. Locating elements and seating faces are usually hardened so that repeated clamping does not wear the reference away.
Horizontal Machining CentreHMC: A milling machine with a horizontal spindle, usually with pallets and tombstones. Chips fall away from the cut, which suits long unattended runs.
Indicating (Dialling In): Using a dial or test indicator to bring a part or fixture into position on the machine. It is the step zero-point systems are designed to eliminate from the changeover.
Lead Time: The elapsed time from order to delivery. For custom fixtures it is driven by design approval and material availability rather than machining hours.
Minimum Order QuantityMOQ: The smallest quantity a supplier will accept for an order. For catalogue workholding components it is often a single piece; custom fixtures are quoted per project.
Request for QuotationRFQ: A buyer's request for price and terms against a defined scope. For fixtures the quality of the RFQ inputs — model, GD&T drawing, machine, volume — determines how firm the quote can be. Read more →
Rigidity: Resistance to deflection under cutting load, across the whole chain from spindle to fixture to part. The least rigid element sets the achievable depth of cut and surface finish.
Surface Finish: The texture of a machined surface, usually stated as Ra. On seating faces it affects how consistently a fixture repeats.
Thermal Growth: Dimensional change caused by temperature, in the machine, fixture or part. It is a common explanation for measurements that drift through a shift with no other cause.
Thin-Wall Machining: Cutting features whose walls are thin enough to move under clamping or cutting force. Clamping strategy usually matters more than cutting parameters.
Tolerance Stack-Up: The accumulated effect of individual tolerances along a chain of features or components. Every additional interface between machine table and part adds to it.
Toolpath: The programmed route the tool follows through the material. Fixture geometry constrains it, which is why fixture design belongs in the process planning conversation.
Tramming: Aligning the spindle square to the table. It is a machine-level check that must be right before any fixture accuracy discussion is meaningful.
Vertical Machining CentreVMC: A milling machine with a vertical spindle. Chip evacuation is worse than on a horizontal machine, which matters for zero-point seating faces.
Work Offset: The stored coordinate shift between machine home and part zero, such as G54. With a zero-point interface the offset for a given fixture position becomes a fixed, reusable number.

104 terms · Shanghai LinkMaster International Trading Co., Ltd.