CNC Machining Process Overview

CNC machining remains the backbone of precision manufacturing for prismatic and cylindrical parts across automotive, aerospace, medical, industrial equipment, and electronics. At its core, computer numerical 

Materials and Typical Tolerance Capability

Metals: Aluminum, Stainless Steel, Titanium

• Standard CNC: ±0.005 in (±0.127 mm) is common across reputable providers.

• Precision CNC: ±0.001–0.002 in (±0.025–0.05 mm) achievable with rigid fixturing, premium tooling, and controlled environments. Titanium trends toward the wider end due to heat and tool wear.

• Surface finishes: Milling/turning typically 0.8–3.2 µm Ra, with grinding achieving 0.1–0.4 µm Ra.

Plastics: PEEK and Nylon

Plastics exhibit greater thermal expansion and, in the case of nylon, moisture absorption, which widen practical tolerance bands.

• PEEK: Low moisture absorption (<0.5%) and relatively low CTE make it dimensionally stable among engineering plastics. 

• Nylon: Hygroscopic; equilibrium moisture uptake can reach a few percent, changing dimensions. Pre‑drying and conditioning improve stability.

Typical tolerance guidance for plastics: standard ±0.005–0.010 in (±0.127–0.254 mm), precision ±0.002–0.005 in (±0.05–0.127 mm) with careful heat management and sharp tooling. These ranges align with aggregated service guidance and material data; measurement method and environment strongly influence outcomes.

Process Selection Factors 

Selecting the right process chain is about geometry access, functional requirements, material behavior, volume, and cost/time.

• Geometry and access

• Use multi‑axis to reach complex features in one setup and minimize tolerance stacking.

• For inaccessible internal corners or extremely hard materials, EDM provides precise profiles without mechanical cutting forces.

• Functional requirements

• Critical sealing or bearing surfaces often warrant grinding for both tolerance and finish.

• Precision holes generally require boring and/or reaming rather than drilling alone; for sub‑10 µm tolerances, consider grinding or EDM.

• Material machinability

• Titanium and stainless demand robust coolant strategies and conservative feeds to control heat and wear.

• Plastics need burr control and thermal management; nylon may require pre‑drying.

• Volume, lead time, and cost

• Tighter tolerances increase cycle time, tool wear, fixturing complexity, and inspection effort. Apply tight bands only where function demands; relax elsewhere using ISO 2768 general tolerances to reduce cost.

Escalation triggers for grinding/EDM include sub‑10 µm features, hardened materials, delicate edges requiring minimal burrs, and complex internal radii.

Design‑for‑Manufacturability (DFM) for Precision CNC

The following pragmatic rules help achieve tight features efficiently. They are not absolutes—they depend on machine, tooling, fixturing, and environment.

1) Tolerance Strategy

• Specify tight tolerances only for functional features; use ISO 2768 classes for general dimensions and GD&T for critical form/location.

• Plan measurement early: define CMM, gauges, and acceptance criteria; ensure measurement uncertainty is ≤ 10–20% of the tolerance.

2) Tooling, Holders, and Runout Control

• Use precision‑ground carbide tools; replace when dull to minimize burrs and deflection.

• Target toolholder runout below ~0.003 mm for finishing; hydraulic or shrink‑fit holders typically provide lower runout and better damping than standard collets.

• Keep tool length‑to‑diameter ratios as low as practical (ideally <4:1 for critical finishing) and use multi‑axis positioning to reduce overhang.

3) Toolpath Strategies

• Use adaptive clearing or trochoidal milling to maintain constant engagement and chip load, reducing heat and tool deflection.

• Reserve fine finishing passes with small stepovers (e.g., 5–10% of tool diameter) and shallow stepdowns (e.g., 0.1–0.5 mm) where surface finish and geometry matter.

• For stainless and titanium, prefer conservative feeds, high‑pressure coolant, and toolpaths that avoid dwelling.

Principles of high‑efficiency roughing and adaptive toolpaths are well documented across CAM resources; a practical overview can be found via industry CAM primers and HSM guidance.

4) Fixturing and Datum Strategy

• Design rigid, repeatable fixtures with hardened locators and pins; align datums to inspection strategy.

• Reduce setups: prioritize 5‑axis single‑setup machining when feasible to minimize tolerance stacking.

• Use in‑process probing to verify datums and compensate for minor thermal drift.

5) Cooling and Chip Evacuation

• Through‑tool and high‑pressure coolant improves deep pocket/slot performance; avoid recutting chips.

• For plastics, use air blast or compatible coolants and lighter passes to limit heat buildup and dimensional drift.

6) Thermal Control and Inspection Planning

• Machine and measure in controlled temperatures; allow parts to equilibrate before final inspection.

• Plan SPC sampling and capability targets (Cp/Cpk) for production; select measurement methods that suit the tolerance (CMM for complex GD&T, gauges for simple size control).

Industry Application Examples (Process–Material–Tolerance Map)

Automotive

• Typical parts: Shafts, valve bodies, engine components.

• Processes: Milling/turning for general features; grinding for bearing journals; EDM for intricate internal passages.

• Tolerances & finish: ±0.01 mm on journals/bores; 0.8–1.6 µm Ra on functional surfaces.

Aerospace

• Typical parts: Sensor housings, structural brackets, turbine components.

• Processes: 5‑axis milling; grinding for precision fits; EDM for hard alloys and complex profiles.

• Tolerances & finish: ±0.01 mm or tighter on critical interfaces; ≤0.8 µm Ra on aerodynamic/fit surfaces. Extensive GD&T is standard.

Medical

• Typical parts: Implants and surgical instruments.

• Processes: Milling/turning for bulk geometry; grinding for articulation surfaces; EDM for intricate cavities with minimal burrs.

• Tolerances & finish: Down to ±0.005 mm on critical features; ~0.2–0.4 µm Ra where articulation or wear dictates.

Industrial Equipment

• Typical parts: Hydraulic valve blocks, pump bodies, gear components.

• Processes: Milling/turning; grinding for sealing/wear surfaces; EDM for hardened inserts.

• Tolerances & finish: ±0.01–0.05 mm depending on function; 1.6–3.2 µm Ra common for non‑critical surfaces.

Electronics

• Typical parts: Heat sinks, precision connectors, sensor housings.

• Processes: Precision milling; EDM for thin conductive features.

• Tolerances & finish: Micron‑level tolerances for fit/contacts; ≤0.4 µm Ra on mating surfaces.

Practical Heuristics: Choosing the Process Chain

• If the part’s tightest feature is ±0.01 mm and tolerances stack across multiple orientations, prefer 5‑axis single‑setup.

• If a precision hole must maintain roundness and straightness, design for boring/reaming; consider grinding for exceptional fits.

• If the material is hardened tool steel or exotic superalloys and the geometry is delicate or inaccessible, wire/sinker EDM becomes the reliable option.

• If the part includes thin walls or long tool reach, split roughing/finishing strategies and use stiffer tools enabled by multi‑axis access.

• If many surfaces are non‑critical, apply ISO 2768 general tolerances and reserve tight bands for functional interfaces.

CNC Milling and Turning Parameters: Notes for Metals vs Plastics

• Metals (Aluminum, Stainless, Titanium)

• Aluminum permits aggressive adaptive roughing and fine finishing with modest coolant demands.

• Stainless requires robust coolant and attention to work hardening; avoid dwelling; use sharp tools and stable feed.

• Titanium needs high‑pressure coolant, conservative engagement, and minimal tool overhang to limit heat and deflection.

• Plastics (PEEK, Nylon)

• Use sharp, polished tools; limit heat via air blast or compatible coolants.

• Expect wider tolerance bands; for nylon, pre‑dry and condition to stabilize dimensions.

Measurement and Validation Considerations

• GD&T: Use feature control frames to specify form (flatness, cylindricity), orientation (perpendicularity), and location (true position). Datum selection should mirror functional assembly.

• CMM: Acceptance and verification align to DIN EN ISO 10360; routine calibration and artifact checks are mandatory. Metrology guidance is summarized in the Zeiss ISO 10360 acceptance test overview.

• Surface Finish: Specify via Ra/Rz per ISO 21920; choose measurement method (profilometer or CMM surface probe) appropriate to the feature,

Conclusion 

CNC machining offers a spectrum of processes—milling, turning, drilling/boring/reaming, grinding, EDM, and multi‑axis—that can reliably achieve functional precision across metals and plastics when paired with the right DFM and metrology strategy. The pragmatic route to micron‑level tolerances is to minimize setups (favor multi‑axis), control tooling/runout, manage heat and chips, and escalate to grinding or EDM where geometry or material demands it.

Submit CAD files for a feasibility review to validate tolerances, process choices, and inspection plans before committing to production.