Reference

CNC Machining Glossary

Plain-English definitions of the 31+ terms engineers and buyers meet when sourcing precision machined parts — processes, tolerances, materials, and sourcing concepts. Written by the Ginwate engineering team.

Processes Tolerances & Quality Materials Design & Sourcing

Processes

CNC Machining: Computer Numerical Control machining is a subtractive manufacturing process in which pre-programmed software directs cutting tools to remove material from a solid block (the workpiece) to produce a finished part. It covers milling, turning, drilling, and grinding.
CNC Milling: A machining process where a rotating multi-point cutting tool moves across a stationary workpiece to remove material. Used to produce prismatic parts, pockets, slots, and complex 3D surfaces. Available in 3-axis, 4-axis, and 5-axis configurations.
CNC Turning: A machining process in which the workpiece rotates against a stationary single-point cutting tool. Used to produce cylindrical parts such as shafts, bushings, and connectors on a lathe.
5-Axis Machining: Milling in which the cutting tool moves across five axes simultaneously (three linear plus two rotational). It machines complex geometry in a single setup, improving accuracy and surface finish on contoured surfaces such as turbine blades and impellers.
EDM (Electrical Discharge Machining): A non-contact process that removes material using controlled electrical sparks between an electrode and the workpiece. Wire EDM and sinker EDM cut hardened metals and intricate shapes that conventional cutting cannot.
Injection Molding: A manufacturing process for producing plastic parts at volume by injecting molten material into a mold cavity. Economical above a few hundred to thousands of units; requires upfront tooling investment.
Rapid Prototyping: The fast fabrication of a physical part from a CAD model, typically by CNC machining or 3D printing, to validate design, fit, and function before committing to production tooling.
Surface Finishing: Secondary processes applied after machining to alter a part's surface — for appearance, corrosion resistance, hardness, or wear. Examples include anodizing, powder coating, electropolishing, passivation, and bead blasting.

Tolerances & Quality

Tolerance: The permissible amount of variation in a part dimension. For example, 25 mm ±0.02 mm means any value between 24.98 mm and 25.02 mm is acceptable. Tighter tolerances cost more to achieve.
ISO 2768: An international standard defining general (default) tolerances for linear and angular dimensions on a drawing, so each dimension need not be toleranced individually. Common grades are fine (f), medium (m), and coarse (c).
GD&T (Geometric Dimensioning & Tolerancing): A symbolic engineering language (ASME Y14.5 / ISO 1101) that defines allowable variation in a part's geometry — form, orientation, location, and runout — relative to datum references. It communicates design intent more precisely than +/- dimensions alone.
True Position: A GD&T control that specifies how far a feature (such as a hole) may deviate from its theoretically exact location, defined as a cylindrical tolerance zone around the ideal point.
CMM (Coordinate Measuring Machine): A precision instrument that measures the geometry of a physical part by probing points on its surface with a touch probe, producing a traceable dimensional inspection report. Used for first article inspection and quality verification.
FAI (First Article Inspection): A documented verification, often per AS9102 in aerospace, that a production process can consistently produce a part conforming to its drawing. Every drawing characteristic is measured and recorded on the first production piece.
Surface Roughness (Ra): The arithmetic average of a surface's micro-irregularities, measured in micrometers (µm). Lower Ra means a smoother surface. A standard machined finish is about Ra 3.2 µm; a fine finish is Ra 0.8 µm or below.
Tolerance Stack-Up: The cumulative effect of individual part tolerances on the fit and function of an assembly. Analyzed (worst-case or statistically) to ensure mating parts still work when each is at its tolerance extreme.

Materials

Aluminum 6061: A versatile, weldable magnesium-silicon aluminum alloy with good strength, excellent machinability, and corrosion resistance. The default choice for most general-purpose machined parts, brackets, and enclosures.
Aluminum 7075: A high-strength zinc-based aluminum alloy with strength approaching that of some steels at one-third the weight. Used for aerospace and high-load structural parts; poor weldability and lower corrosion resistance than 6061.
Stainless Steel 316L: A low-carbon austenitic stainless steel with molybdenum for superior corrosion resistance, especially against chlorides. Common in marine, medical, food, and chemical-processing parts.
Titanium Ti-6Al-4V (Grade 5): The most widely used titanium alloy, combining high strength-to-weight ratio, excellent corrosion resistance, and biocompatibility. Standard for aerospace structures and medical implants; difficult to machine.
PEEK: Polyether ether ketone — a high-performance engineering thermoplastic with excellent chemical resistance, high-temperature stability, and biocompatibility. Used as a lightweight metal replacement in medical, aerospace, and semiconductor parts.
POM (Delrin): Polyoxymethylene, an engineering thermoplastic with high stiffness, low friction, and good dimensional stability. Ideal for gears, bushings, and precision mechanical components.
Inconel: A family of nickel-chromium superalloys that retain strength and resist oxidation at extreme temperatures. Used for turbine, exhaust, and high-heat components; work-hardens rapidly and is challenging to machine.

Design & Sourcing

DFM (Design for Manufacturability): The practice of designing a part so it is easy and economical to manufacture — for example, adding internal corner radii, avoiding deep narrow pockets, and applying tight tolerances only where functionally required. A DFM review flags cost drivers before production.
MOQ (Minimum Order Quantity): The smallest order a supplier will accept, set so fixed setup costs can be amortized across enough parts. A no-MOQ supplier accepts single-piece orders, useful for prototyping and low-volume production.
Lead Time: The elapsed time between order confirmation and shipment. Prototype CNC parts typically take 3–7 days; production runs 2–4 weeks, depending on quantity, process, and finishing.
STEP File: A neutral 3D CAD file format (.step / .stp) used to exchange part geometry between different CAD systems. The preferred format for requesting a machining quote because it preserves exact solid geometry.
Material Certificate (EN 10204 3.1): A mill document certifying a material's chemical composition and mechanical properties, traceable to a specific heat lot. Required for aerospace and medical work to prove the metal matches specification.
Setup: One fixturing and orientation of a workpiece on a machine. Each additional setup adds cost and a potential source of positional error, so parts are designed to minimize the number of setups required.
Undercut: A recessed feature that a standard straight tool cannot reach from the part's primary access direction, such as an internal groove. Undercuts require specialty tooling or multi-axis machining, increasing cost.
Anodizing: An electrochemical finishing process for aluminum that grows a hard, corrosion-resistant oxide layer on the surface, which can also be dyed for color. Type II is decorative/protective; Type III (hard anodize) maximizes wear resistance.

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