CNC Machining Glossary

This glossary defines cnc machining terms that commonly appear in aerospace and defense RFQs, purchase orders, travelers, and inspection reports. It focuses on practical meaning—what the term changes in manufacturing risk, cost, schedule, and compliance—so engineers, procurement teams, and program managers can write clearer requirements and compare quotes on an apples-to-apples basis.

Where helpful, terms are framed in the context of modern hybrid workflows (e.g., additive manufacturing (AM) parts that are HIP’d and then finish-machined), and regulated quality expectations (e.g., AS9100 systems, NADCAP special processes, ITAR/DFARS controls, and complete certification packs).

Machine and operation terms

CNC machining: Material removal using computer numerical control to execute programmed toolpaths. In RFQs, specify what is being machined (e.g., “finish machine sealing faces to final size”), the material condition (e.g., annealed vs hardened), and any special constraints (thin walls, datum scheme, burr limits).

3-axis / 4-axis / 5-axis machining: Number of simultaneously controlled axes. 5-axis machining typically reduces setups and improves access to complex geometry, but may require tighter control of workholding, tool reach, and collision risk. If you need positional accuracy of features around a complex surface, stating “5-axis required” can prevent suppliers from quoting a risky multi-setup approach.

Mill (milling) vs lathe (turning): Milling uses rotating cutters; turning uses a rotating workpiece. Many RFQs include both via mill-turn or multi-task machines. A part that is “mostly turned with milled flats” often benefits from a mill-turn process plan to minimize concentricity stack-up.

Setup (op) / number of setups: One clamping/workholding event and the machining performed before removing the part. More setups can increase cost and increase accumulated error. When tolerances are tight across multiple sides, it’s worth discussing datum strategy and whether features must be cut in the same setup.

Workholding: Fixtures, vises, chucks, soft jaws, vacuum, or custom tooling that locates and clamps the part. For thin-walled aerospace parts and AM lattices, improper clamping can distort geometry; RFQs should clarify “no deformation allowed” requirements or allowable clamp features.

Datum / datum scheme: The reference surfaces/axes used to locate the part in manufacturing and inspection (defined by the drawing’s GD&T). A strong RFQ includes the latest drawing revision and, when needed, notes that inspection will be reported relative to the drawing datums.

Toolpath: The programmed motion of the cutter. Toolpath choice affects surface finish, burr formation, heat input, and dimensional stability—especially in titanium and nickel alloys. For difficult materials, specify whether tool marks are acceptable and whether any critical surfaces require a specific finish or lay.

Roughing vs finishing: Roughing removes most material; finishing brings surfaces to final size and finish. RFQs for precision parts should clearly identify which features are “as-machined” final and which will be left with stock for later operations (grind, hone, lap, or EDM).

Stock allowance / machining allowance: Extra material intentionally left for a later operation. This matters in hybrid workflows—e.g., PBF (DMLS/SLM) + HIP + finish machining. A common practice is to add machining stock to critical surfaces to clean up as-built texture, remove HIP canning witness, and ensure full dimensional control after densification.

EDM (Electrical Discharge Machining): Material removal via electrical discharges. Wire EDM cuts profiles; sinker EDM forms cavities with electrodes. EDM is useful for hard materials and sharp internal corners, but RFQs should address recast layer limits, required cleanup, and whether EDM surfaces are acceptable in fatigue-critical areas.

Drilling / reaming / boring: Hole-making operations with increasing control of size and finish. Reaming refines diameter; boring improves true position and roundness. If a hole is used for precision alignment, specify the fit class, surface finish, and whether it must be produced in one setup with related datums.

Tapping / thread milling: Internal thread creation. Thread milling can improve control in tough alloys and avoids tap breakage, but may cost more. RFQs should specify thread standard (UNC/UNF, MJ, metric), class, and whether gaging with go/no-go is required.

Deburr: Removal of sharp edges and burrs from machining. “Deburr all edges” is ambiguous; better language includes a numeric edge break (e.g., “break edges 0.005–0.015 in unless otherwise noted”) and any prohibited conditions (no rolled edges on sealing surfaces, no media entrapment in AM lattice regions).

Coolant / MQL / dry machining: Cutting fluid strategy. Coolant affects heat, tool life, and contamination risk. For oxygen-sensitive materials or parts with cleanliness requirements, specify whether water-soluble coolant is allowed and whether post-cleaning is required.

Post-processing (AM context): Secondary operations after AM build, such as support removal, stress relief, HIP, machining, surface finishing, and inspection. A clear RFQ states which party owns each step and what documentation must follow the part (heat treat/HIP charts, CoC, inspection data).

HIP (Hot Isostatic Pressing) / PM-HIP: High-pressure, high-temperature densification. For AM parts, HIP reduces internal porosity and can improve fatigue performance; for PM-HIP (powder metallurgy + HIP), it forms near-net shapes in a can. Important RFQ details include alloy spec, cycle requirements (if dictated), post-HIP heat treatment condition, and how HIP dimensional change will be managed (machining stock and post-HIP inspection gates).

GD&T basics

GD&T (Geometric Dimensioning and Tolerancing): A standardized language (ASME Y14.5 is common in the U.S.) that controls geometry relative to datums. In RFQs, GD&T often drives inspection method, fixturing, and whether a shop can realistically meet requirements without excessive cost.

Profile of a surface: Controls a 3D surface within a tolerance zone relative to datums. Common on contoured aerospace parts and AM-to-CNC blended surfaces. If profile is tight on a freeform surface, confirm that the supplier can program and verify with CMM/scan data tied to the CAD model and datum structure.

True position: Controls location of holes/features relative to datums, usually for fastener patterns. The practical impact is setup strategy: the supplier may need to drill/ream in the same setup as datum features to avoid stack-up and maintain bolt-hole alignment.

Flatness / straightness: Form controls that do not require datums. Tight flatness on thin parts can be sensitive to stress relief, machining sequence, and clamping. If flatness is critical, consider specifying material condition and whether intermediate stress relief is allowed.

Perpendicularity / parallelism / angularity: Orientation controls relative to a datum. These are commonly driven by functional assembly needs. In RFQs, clarify which datum feature is primary for inspection and whether mating surfaces will be machined in the same operation.

Runout / total runout: Controls variation of a surface when rotated about a datum axis (typical for turned parts). Misunderstanding runout is a common quoting issue; it is not the same as “concentricity” in casual language. If you mean the rotating functional surface must stay within a certain wobble, runout is usually the correct callout.

MMC/LMC (Maximum/Least Material Condition) & bonus tolerance: Modifiers that adjust allowable positional tolerance based on feature size. This impacts inspection and can reduce cost when used correctly. If procurement sees large inspection time, ask whether MMC could be applied without affecting function.

Datum targets: Specific contact points/areas used to establish a datum on irregular surfaces (common in castings and AM parts). If your datum feature isn’t a clean plane or cylinder, datum targets can reduce ambiguity and improve repeatability across suppliers.

Finishing terms

Surface finish / roughness (Ra, Rz): Quantified texture requirement. RFQs should specify the parameter (Ra is most common) and units (µin or µm). Tight finishes often require fine toolpaths, grinding, honing, or lapping; they also change inspection requirements (profilometer traces) and can affect lead time.

As-machined: Surface state directly from machining operations. Clarify whether tool marks are acceptable and whether direction of lay matters for sealing or fatigue.

Bead blast / vapor hone: Abrasive finishing to reduce glare and remove minor burrs. It can change dimensions slightly and can embed media in porous surfaces—particularly risky for AM lattice or internal passages. If used on flow paths, specify cleaning requirements and whether blasting is prohibited on critical mating surfaces.

Anodize (Type II/Type III): Aluminum conversion layer for corrosion resistance (Type II) or wear resistance (Type III hardcoat). Anodize grows both inward and outward, affecting dimensions. RFQs should call out masking requirements for threads, bearing fits, and electrical bonding surfaces.

Passivation: Chemical treatment to improve corrosion resistance of stainless steels by removing free iron. Common specs include nitric or citric methods depending on alloy and customer requirements. Important in RFQs: material grade, any prohibited chemistries, and whether a test report is required.

Chemical conversion coating: Often used on aluminum to improve corrosion resistance and paint adhesion. Clarify if it is allowed on bonding surfaces or if specific conductivity requirements apply.

Plating (e.g., electroless nickel, cadmium alternatives): Adds thickness and changes fits. Defense/aerospace programs may restrict certain platings due to environmental and embrittlement concerns; the RFQ should specify the exact finish and any bake requirements for hydrogen embrittlement relief where applicable.

Paint / powder coat: Protective/identification coatings. For precision parts, specify masking and thickness limits to avoid interfering with assembly.

Shot peen: Surface treatment to induce compressive stress and improve fatigue life. It is typically a special process and may require NADCAP-accredited processing depending on program requirements. RFQs should include intensity, coverage, media, and any prohibited masking approaches.

Heat treat: Alters microstructure and mechanical properties. In regulated environments, RFQs should specify the governing spec and the required documentation (furnace charts, load thermocouple data, hardness results). Note that heat treat can move parts; plan for finish machining after heat treat when tight tolerances are required.

Inspection terms

FAI (First Article Inspection): A formal verification that the first produced part meets drawing requirements. In aerospace, AS9102-format FAI is common. RFQs should state whether FAI is required, whether partial FAI is acceptable for repeat builds, and what constitutes a “first” (new supplier, new process, new revision, or new machine/program).

CMM (Coordinate Measuring Machine): Precision measurement using a probing system to verify dimensions and GD&T. For complex profiles, CMM programs often require the CAD model and clear datum scheme. If profile tolerances are tight, ask the supplier how they will align the CMM coordinate system to the drawing datums.

CT scanning (Computed Tomography): Non-destructive evaluation (NDE) using X-ray CT to inspect internal features and porosity—especially relevant for AM parts, castings, or parts with internal channels. RFQs should define what is being evaluated (internal geometry vs defect size limits) and how results will be reported (images, 3D volume data, acceptance criteria).

NDE (Non-Destructive Evaluation): Methods such as dye penetrant (PT), magnetic particle (MT), ultrasonic (UT), radiography (RT), and eddy current. Many are special processes and may need NADCAP depending on contract flow-downs. RFQs should specify method, acceptance class/standard, and whether the supplier must provide certified reports.

In-process inspection: Checks performed during machining (probe, micrometer, bore gage) to prevent scrap. Tight-tolerance parts benefit when RFQs allow realistic in-process checkpoints (e.g., “critical bores to be verified after rough and before finish”) rather than only end-of-line inspection.

Gaging (Go/No-Go): Attribute inspection for threads, pins, and some features. It is fast and robust for production, but it doesn’t substitute for positional verification when GD&T requires it. RFQs should clarify which characteristics require variable data vs gage-only acceptance.

Inspection report / ballooned drawing: A “ballooned” drawing maps numbered characteristics to recorded results. Procurement-ready RFQs often require a ballooned drawing and a dimensional report for all characteristics or for a defined subset (critical-to-function).

Calibration: Verification that measurement equipment is accurate and traceable to standards. For defense/aerospace suppliers, calibration control is an AS9100 expectation. If measurement is critical, RFQs can require that calibration certificates be available upon request.

Material traceability: The ability to trace material back to a heat/lot and mill certification. For AM, traceability also includes powder lot, reuse strategy, build ID, and post-processing batch IDs (HIP/heat treat). RFQs should state traceability expectations, especially when DFARS specialty metals restrictions apply.

CoC (Certificate of Conformance): A supplier statement that parts meet requirements (drawing/spec/PO). In regulated workflows, the CoC is part of the certification pack and should reference revision levels, material certs, special process certs, and inspection status.

Quoting terms

RFQ (Request for Quotation): The package sent to suppliers to obtain pricing and lead time. A strong RFQ includes: drawing + revision, 3D model (if applicable), material spec, quantity and delivery schedule, finishing requirements, inspection requirements (FAI, CMM report, NDE), packaging/marking, and required certifications (AS9100, ITAR, DFARS flow-downs).

PO (Purchase Order) / flow-downs: Contractual requirements passed from the customer to the supplier. In aerospace/defense, flow-downs can include quality clauses, record retention, counterfeit parts prevention, and special process approvals. Quote validity often depends on confirming these up front.

Lead time: Total time from order release to ship date. For machined parts, lead time is influenced by material availability, programming, setups, special processes (heat treat, anodize, shot peen), and inspection/FAI. For hybrid AM + HIP + machining, add time for build scheduling, HIP batch windows, and post-HIP dimensional verification before finish machining.

MOQ / lot size: Minimum order quantity or minimum batch. Even for prototypes, some finishing and inspection steps have fixed setup costs; RFQs should clarify whether pricing should be for prototype vs production rates.

NRE (Non-Recurring Engineering): One-time costs such as programming, fixturing, tooling design, CMM programming, and first-article documentation. RFQs should ask suppliers to separate NRE from per-piece pricing so program teams can evaluate production scalability.

Alternates / “or equal”: Proposed substitutions (material, process, finish). In regulated programs, alternates often require engineering approval. Suppliers will quote more confidently if RFQs clearly state whether alternates are permitted and how deviations must be requested (deviation/waiver process).

Critical-to-quality (CTQ): Features with high functional or safety impact. Identifying CTQs in the RFQ (or via a characteristic accountability list) helps suppliers plan in-process inspection, choose stable operations, and prioritize risk reduction.

ITAR-controlled: Indicates export-controlled technical data and parts. Practical RFQ implications: controlled access to drawings/models, U.S.-person handling requirements, and limits on offshore processing. If ITAR applies, specify it clearly and confirm whether outside processing (plating, NDE) must also be ITAR-compliant.

DFARS compliance: Defense Federal Acquisition Regulation Supplement requirements that may include specialty metals restrictions and country-of-origin controls. RFQs should include DFARS clauses and require material certs that demonstrate compliance.

AS9100 / NADCAP: Quality management system and special process accreditations. AS9100 is often a baseline for aerospace machining suppliers; NADCAP applies to specific special processes (heat treat, NDT, chemical processing) depending on customer requirements. RFQs should state required certifications and whether current certificates must be provided in the cert pack.

Certification pack: The document bundle shipped with parts, often including CoC, material certs, special process certs, inspection reports, FAI, and traceability records. Clarify at RFQ time what must be included and whether electronic copies are required.

Common misunderstandings

“Tight tolerance everywhere” vs function-based tolerancing: Overly tight tolerances across non-functional features drive cost and scrap. A best practice is to apply tight GD&T only where it matters (fits, sealing, alignment) and keep the rest reasonable. If you need a supplier to quote quickly, highlight CTQs to prevent overestimating risk.

Surface finish is not the same as appearance: A visually uniform matte surface can still have unacceptable Ra, and a shiny surface can still violate lay or waviness needs. If a surface is functional (seal, bearing, fatigue), specify the measurable finish requirement and any lay direction constraints.

Deburr language is often incomplete: “Deburr” alone can lead to sharp edges left in place, or over-aggressive edge rounding that breaks form or changes fits. Provide an edge-break requirement and call out no-burr zones (threads, precision bores, sealing lands).

GD&T callouts without a manufacturable datum strategy: Hole position and profile tolerances can be impossible or expensive if the datum features are not stable or not accessible for fixturing and inspection. If you’re using an AM preform, consider datum targets or adding machinable datum pads specifically for post-processing.

Assuming HIP “fixes” everything for AM parts: HIP can reduce porosity, but it does not automatically guarantee fatigue performance, dimensional accuracy, or surface integrity. Successful workflows typically follow a gated approach: (1) define powder/build controls and traceability, (2) perform stress relief and support removal, (3) HIP to densify, (4) perform any required heat treatment, (5) finish machine critical features, and (6) verify via CMM/NDE as required.

Mixing inspection intent (CMM vs gage vs scan) without stating acceptance criteria: Different methods answer different questions. If internal features matter, CT scanning may be necessary; if positional tolerance matters, CMM relative to datums is typical. RFQs should state what evidence of compliance must be provided (variable data, plots, CT slices, NDE reports).

Not aligning RFQ requirements with certification needs: Late discovery of ITAR handling, DFARS specialty metals, AS9102 FAI, or NADCAP special process requirements causes schedule slips and rework. A practical rule is to treat the RFQ as a mini “quality plan”: list required certifications, traceability, inspection deliverables, and record retention expectations upfront.

Model vs drawing authority: Some programs are model-based definition (MBD), others treat the 2D drawing as the controlling document. If a 3D model is provided “for reference only,” say so; if the model is authoritative for profiles, define it explicitly and specify the revision and file format to avoid misinterpretation.

Using consistent language in RFQs reduces supplier assumptions, accelerates quoting, and improves first-pass yield—especially when parts require a combination of AM, HIP/PM-HIP, precision CNC machining, special processes, and regulated documentation.