TECHNICAL REFERENCE • CNC MANUFACTURING
From drawing language to manufacturing decisions
An engineering glossary for buyers, designers and quality teams sourcing CNC components and workshop equipment. Explore 42 terms covering machining, tolerancing, metrology, tooling and technical procurement.
Start with the distinction: precision turned parts are manufactured components. CNC equipment supports the workshop. Each entry below explains the term and what to clarify when specifying or sourcing it.
A–Z term index — 42 entries
Machining processes
CNC — Computer Numerical Control
Programmed control of machine axes and functions. CNC describes how a machine is controlled; turning, milling and drilling describe how material is removed. A CNC designation alone specifies neither achievable tolerance nor an inspection method.
Specification & sourcing note: For a quotation, define the finished component through its released geometry, material, tolerances and acceptance requirements.
CNC components vs CNC equipment
CNC components are manufactured parts such as shafts, housings, brackets and manifolds. On this website, CNC Equipment means workshop infrastructure for storing and transporting tools, including CNC tool storage cabinets and CNC tool transport trolleys.
Specification & sourcing note: A precision turned part is a CNC component when produced by CNC turning. A tool cabinet supports the manufacturing process but is not the machined component being purchased.
Precision turned parts
Parts produced predominantly by controlled turning, typically with cylindrical, conical, threaded or faced features. The workpiece normally rotates while the cutting tool feeds. Driven tools can add milled flats and cross-holes. “Precision” has no universal numerical tolerance attached to it.
Specification & sourcing note: Specify the critical diameters, datum relationships, surface texture and inspection requirements; do not substitute the word precision for measurable acceptance limits.
CNC milling
Material removal using a rotating cutter moving relative to the workpiece. Tool access, cutter diameter, reach and workholding constrain machinable geometry; cutter engagement changes through pockets and corners.
Specification & sourcing note: Review internal corner radii and deep features before release. A CAD model with sharp internal corners does not by itself define a feasible milling route.
3+2 machining
Indexed machining uses two rotary axes to orient the part or tool, followed by cutting with the rotary orientation held fixed. It provides access to multiple faces without requiring all five axes to interpolate during the cut.
Specification & sourcing note: Ask whether indexed access meets the geometry and setup needs before specifying simultaneous machining.
Simultaneous 5-axis machining
Coordinated linear and rotary motion can change tool orientation while cutting. It supports complex access and surfaces, but the machine kinematics, tool assembly, workholding and validated NC program remain decisive.
Specification & sourcing note: A five-axis machine is not a guarantee of a particular part tolerance. Confirm the actual manufacturing and inspection strategy.
Boring
Enlarging or finishing an existing hole with a boring tool. Rough boring prioritises stock removal; fine boring controls the final size and surface. Tool overhang, bar stiffness and cutting forces become especially important in deep bores.
Specification & sourcing note: Define bore diameter, depth, form, datum relationship and surface texture separately. A small adjustment increment on a boring head is not a guaranteed finished-part accuracy.
Chatter and tool overhang
Machining vibration can degrade surface texture, dimensional results and tool life. Long overhang increases sensitivity to tool-system flexibility; damped tools, suitable geometry and stable clamping can help.
Specification & sourcing note: Review the complete spindle–holder–tool–workpiece system. Reducing feed alone is not a universal cure for vibration.
Programming & cutting data
CAD, CAM and NC code
CAD defines the design geometry. CAM plans manufacturing operations and toolpaths. Post processing converts those operations into NC instructions for the intended machine and control. These are separate stages with different validation needs.
Specification & sourcing note: Supply the released model and drawing together, or an explicitly agreed model-based definition. A visually correct toolpath does not establish that the final NC program is correct.
Post processor
Software that translates CAM output into machine-specific instructions. Axis configuration, controller functions, tool changes and coordinate handling must match the target machine.
Specification & sourcing note: Record the machine and post version used for program release. A generic post name or successful code export does not constitute machine prove-out.
Cutting speed v_c and spindle speed n
Cutting speed is the relative surface speed at the cutting edge; spindle speed is revolutions per minute. For diameter D in mm: v_c = πDn/1000, with v_c in m/min. At constant rpm, surface speed changes with cutting diameter.
Specification & sourcing note: Illustrative calculation: D = 50 mm and n = 1000 rpm give v_c ≈ 157 m/min. This is a unit example, not a recommended cutting condition.
Feed per revolution and feed per tooth
Turning feed f_n is commonly expressed in mm/rev. Milling feed f_z is expressed in mm/tooth. Milling table feed follows v_f = f_z × z_c × n, using the effective tooth count for the operation.
Specification & sourcing note: Example: 0.05 mm/tooth × 4 effective teeth × 6000 rpm = 1200 mm/min. Confusing mm/min with mm/rev can change the intended cut dramatically.
Depth of cut a_p and radial engagement a_e
In milling, a_p describes axial cutting depth and a_e radial engagement. These affect chip thickness, tool loading and heat; a programmed feed per tooth is not always the maximum chip thickness.
Specification & sourcing note: Compare cutting data only with the same engagement and cutter geometry. Full-slot cutting and low-engagement side milling impose different conditions.
Tolerances & drawing interpretation
ISO GPS and ASME GD&T
Two specification frameworks used to communicate dimensional and geometrical requirements. ISO 1101 addresses geometrical tolerancing within ISO GPS; ASME Y14.5 defines the ASME dimensioning and tolerancing system. Similar symbols do not make all default rules interchangeable.
Specification & sourcing note: State the governing system and edition. Resolve conflicting title-block, model and drawing requirements before manufacturing. References here explain concepts; the controlled drawing and invoked standards govern acceptance.
Datum, datum feature and datum system
A datum is an ideal reference established from specified real features. A datum feature is the physical feature used to establish that reference. A datum system combines references in a defined order to constrain the relevant degrees of freedom.
Specification & sourcing note: Distinguish the design datum system from a convenient machining zero. Changing the order of datum references can change how a part is aligned and evaluated.
Size tolerance and fit
Size limits constrain the permitted size of a feature. A fit describes the relationship between mating internal and external sizes: clearance, transition or interference. ISO 286 provides a code system; a designation such as H7 requires a nominal size to resolve numerical limits.
Specification & sourcing note: Specify both mating features and functional requirements. Size limits alone do not fully control orientation, location, texture or assembly behaviour.
Flatness
A surface-form requirement evaluated between two parallel limiting planes. Surface flatness does not use a datum and does not establish parallelism to another surface.
Specification & sourcing note: Define the controlled surface and whether the part is evaluated free or restrained where relevant. A thin part may measure differently when clamped.
Roundness vs cylindricity
Roundness evaluates circular cross-sections. Cylindricity controls the cylindrical surface as a whole between coaxial limiting cylinders. Both are form controls; neither alone locates the feature relative to another axis.
Specification & sourcing note: Choose the control from the functional requirement. A satisfactory diameter reading at a few points does not prove satisfactory roundness or cylindricity.
Position tolerance
Controls the permitted location of a feature relative to its theoretically defined location and any specified datum references. The zone shape, feature definition and modifiers determine how the requirement is evaluated.
Specification & sourcing note: Do not replace a diametrical position requirement with independent ±X and ±Y limits without engineering approval; their permitted regions differ.
Circular runout vs total runout
Circular runout evaluates variation during rotation at individual circular elements relative to a datum axis. Total runout applies across the controlled surface during rotation and traversal.
Specification & sourcing note: Runout can combine the influence of form and datum-axis relationships. It is not simply another word for roundness or a universal substitute for an axis-location requirement.
General tolerances and legacy drawings
General specifications cover applicable features without individual requirements. ISO 2768-2:1989 is withdrawn and ISO identifies ISO 22081:2021 as its successor; the latter provides rules for general geometrical and size specifications.
Specification & sourcing note: Do not silently replace a legacy drawing reference or assume its tolerance classes transfer unchanged. Review the intended requirement and issue a controlled drawing revision when a change is agreed.
Surface texture
Ra and Rz surface roughness
Ra represents the arithmetic mean magnitude of profile height departures; Rz characterises peak-to-valley profile height using the prescribed evaluation convention. The same Ra can occur on surfaces with very different peak and valley structures.
Specification & sourcing note: There is no universal Ra-to-Rz conversion factor. State the parameter, limit, applicable standard and measurement settings rather than requesting only a “smooth finish”.
Surface texture and ISO 21920
Profile texture evaluation depends on the defined filtering and evaluation procedure. ISO 21920-2:2021 specifies profile terms and parameters and succeeds legacy documents including ISO 4287. Profile and areal measurements are different approaches.
Specification & sourcing note: Keep the specified standard edition and evaluation conditions with the result. Do not assume a legacy roughness callout and a newer evaluation setup are automatically equivalent.
Lay and measurement direction
Lay is the predominant direction of surface marks. Measurement direction relative to that lay can materially affect the recorded profile. Surface appearance alone does not define sealing, friction or fatigue behaviour.
Specification & sourcing note: Identify functional surfaces and required measurement direction where relevant, especially for ground or turned sealing interfaces.
Measurement & metrology
Measurement accuracy
The closeness of a measured value to the true value of the measurand. In the VIM, accuracy is a qualitative concept, not a numerical quantity. Numerical specifications should identify the actual error limit, uncertainty or other performance metric.
Specification & sourcing note: A display resolving 0.001 mm does not establish 0.001 mm measurement accuracy. Ask what the quoted instrument specification actually represents.
Precision and repeatability
Precision describes agreement among repeated measurements under stated conditions. Repeatability concerns a restricted set of substantially unchanged measurement conditions. A tightly grouped set of readings can still be biased.
Specification & sourcing note: Specify the repeatability conditions and distinguish measurement spread from closeness to the correct value. Repeating the same measurement is not an independent proof of accuracy.
Measurement uncertainty
A quantitative description of the dispersion of values reasonably attributed to the measured quantity, based on available information. Contributions can include calibration, temperature, setup, probing and sampling.
Specification & sourcing note: Report the measured value with the relevant uncertainty and coverage information. Uncertainty is not the tolerance and is not the same thing as a known measurement error.
Calibration vs adjustment
Calibration establishes the relationship between reference values and instrument indications, including associated uncertainties, and supports deriving measurement results. Adjustment changes the instrument response. They are different operations.
Specification & sourcing note: A calibration certificate does not by itself prove a specific part measurement is fit for purpose. Review range, conditions, uncertainty and the measurement method used.
Metrological traceability
The relationship of a measurement result to a reference through a documented, unbroken chain of calibrations, with uncertainty contributions at each stage. It is a property of the result.
Specification & sourcing note: Distinguish measurement traceability from material heat-number or production-lot traceability. These records answer different questions and may all be needed.
CMM — Coordinate Measuring Machine
A system for acquiring coordinates and evaluating geometric features. CMM calibration and verification establish aspects of system performance; the part-specific result also depends on probing, alignment, point distribution, fitting and environment.
Specification & sourcing note: Agree the datum alignment and evaluation method for critical features. A CMM report is not automatically a complete inspection of every drawing requirement.
Reference temperature and thermal expansion
Dimensional metrology uses a standard reference temperature of 20 °C. Measurements away from the reference may need thermal correction and an uncertainty allowance. A first-order model is ΔL = αLΔT.
Specification & sourcing note: Illustrative assumption: a 100 mm part with α = 12 µm/(m·K), 5 K above reference, expands about 6 µm. Use the actual material coefficient and measured part temperature for real work.
Quality & process capability
Process stability
A stable process has a statistical behaviour that remains consistent over time. Control charts and time-ordered observations help distinguish routine variation from special causes. Specification limits and statistical control limits serve different purposes.
Specification & sourcing note: Establish stability before treating a capability number as predictive. A batch within specification can still come from an unstable process.
Cp and Cpk — Process capability
For the conventional normal-distribution model, Cp = (USL − LSL)/(6σ). Cpk = min[(USL − μ)/(3σ), (μ − LSL)/(3σ)]. Cp compares spread with tolerance width; Cpk also reflects centring.
Specification & sourcing note: State the sampling plan, stability evidence and method used to estimate σ. Required thresholds are customer-specific; a single measured part cannot establish process capability.
Capability example: why centring matters
Illustrative stable normal process: limits 9.90–10.10 mm, mean 10.04 mm, σ = 0.02 mm. Cp = 1.67, while Cpk = 1.00 because the nearer limit is only three standard deviations from the mean.
Specification & sourcing note: The example is calculated from the stated assumptions. It describes neither CNCsupplier performance nor a guaranteed rejection rate for real production.
FAI — First Article Inspection
A structured, documented inspection of an initial production article against applicable requirements. SAE AS9102C establishes aerospace FAI requirements when invoked; it complements customer and other applicable requirements.
Specification & sourcing note: Agree the required forms, characteristic coverage and supporting evidence before quotation. FAI does not replace routine production controls or establish supplier certification.
Tool interfaces & workshop equipment
HSK toolholder interface
A hollow-taper interface with taper and flange-face contact. HSK forms and sizes define different interface configurations; the complete designation matters.
Specification & sourcing note: Specify the form and size, such as HSK-A63, plus tool mass and envelope when ordering CNC tool holder storage inserts. Matching the nominal number alone is insufficient.
SK, BT and CAT steep-taper interfaces
Families of steep-taper machine-tool interfaces. Flange geometry, retention arrangements and applicable interface specifications differ. Face-contact variants add further compatibility considerations.
Specification & sourcing note: Do not assume SK40, BT40 and CAT40 are interchangeable because they share a nominal size. Confirm the complete spindle and holder requirements, including the pull stud where used. For workshop organisation, review storage for SK tool holders and mixed tooling.
VDI toolholder interface
A cylindrical-shank toolholder system used on lathe turrets, referenced in SAUTER documentation to DIN ISO 10889, formerly DIN 69880. It is distinct from a milling-spindle taper interface.
Specification & sourcing note: Specify the shank size, holder orientation, turret compatibility and any driven-tool requirements. For VDI tool holder storage, check the complete holder envelope and mass as well as the shank.
Rated load and load distribution
The allowable load for a specified equipment configuration and operating condition. Drawer, pull-out, shelf, holder and complete-trolley ratings describe different limits.
Specification & sourcing note: For an equipment quotation, provide the heaviest tool, total stored mass, distribution and transport conditions. Adding individual shelf ratings does not establish the permitted total trolley load.
Engineering & procurement
DFM — Design for Manufacturing
A structured review of geometry, material, tolerances and process access to identify manufacturing constraints and cost drivers before release. DFM suggestions remain proposals until the design authority accepts them.
Specification & sourcing note: Review deep pockets, thin walls, internal radii, tool access, datum strategy and unnecessary tight tolerances. Record approved changes in the controlled technical definition.
RFQ — Request for Quotation
The technical and commercial package used to request an offer. For CNC parts, include drawing/model revision, material condition, quantities, tolerances, surface requirements, inspection scope and delivery destination.
Specification & sourcing note: Separate prototype quantity from annual demand. List special cleaning, packaging, certificates and first-article requirements explicitly so quotations cover the same scope.
Revision control
Identification and controlled release of the product definition and subsequent changes. Models, drawings, purchase orders, programs and inspection records need a clear relationship to the accepted revision.
Specification & sourcing note: If the model and drawing disagree, resolve the conflict before manufacture. Do not infer that the file with the newest timestamp is the approved master.
Put the terminology to work
Send the released drawing, model and requirements for a component review, or describe your tool inventory and workshop layout for an equipment inquiry.
Technical sources & further reading
References include standards publishers, the international metrology vocabulary, NIST and original technical documentation from machine, tooling and measurement specialists. Manufacturer references support the explanations and do not imply a product endorsement.
- ASME Y14.5-2018 (R2024) — Dimensioning and Tolerancing
- ISO 1101:2017 — Geometrical tolerancing
- ISO 5459:2024 — Datums and datum systems
- ISO 286-1:2010 — Tolerances, deviations and fits
- ISO 22081:2021 — General geometrical and size specifications
- ISO 2768-2:1989 — Withdrawn; successor ISO 22081:2021
- KEYENCE — Form tolerance
- KEYENCE — Geometrical tolerancing symbols
- ISO 21920-2:2021 — Profile surface texture parameters
- Mitutoyo — Surface roughness measurement guide (legacy ISO framework)
- JCGM VIM 2.13 — Measurement accuracy
- JCGM VIM 2.15 — Measurement precision
- JCGM VIM 2.26 — Measurement uncertainty
- JCGM VIM 2.39 — Calibration
- JCGM VIM 2.41 — Metrological traceability
- NIST — Process capability
- NIST — Assessing process stability
- NIST — Uncertainties in measurements at nonstandard temperatures
- Renishaw — CMM calibration
- SAE AS9102C — First Article Inspection Requirements
- Sandvik Coromant — Milling formulas and definitions
- Sandvik Coromant — Turning handbook
- Sandvik Coromant — Boring application guide
- Haas — 3+2 and simultaneous 5-axis machining
- Autodesk — Post processing manufacturing operations
- HAIMER — Overview of toolholder technology
- SAUTER — Toolholder operating instructions