CNC-bewerking Parts: Complete Guide to Materiaals, Toleranties & Ordering

Belangrijkste conclusies

• CNC machined parts are components produced door computer-controlled cutting tools — milling, turning, drilling, and Slijpen — from solid metal or plastic stock. They serve every industry from aerospace to medical devices.
• The six factors that determine part cost: material, complexity (number of setups), tolerance requirements, surface finish, order quantity, and — for US buyers — tariff classification (28–78% on Chinese-made metal parts).
• Materiaal selection drives everything. 6061-T6 aluminium is the 80% workhorse. 316 stainless for corrosion. 7075 for strength. PEEK and Delrin for high-performance plastics. Each choice affects machinability, cost, and tariff rate differently.
• Toleranties follow a clear cost curve: ±0.1 mm is standard and cheap. ±0.05 mm costs ~20% more. ±0.01 mm requires a second operation and costs 2–3×. Only specify tight Toleranties where functionally necessary.
• US buyers face 28–78% tariffs on Chinese CNC parts depending on material (Section 301 + Section 232). Plastic and composite parts carry the lowest tariff burden. Always compare landed cost — part price + tariff + freight — not just part price.

Introduction: What Exactly Is a CNC Machined Part?

A CNC machined part starts as a solid block of metal or plastic — called a billet or blank — and is shaped door computer-controlled cutting tools that remove material layer door layer. Unlike casting (pouring molten metal into a mold) or Stansen (pressing Plaatwerk into shape), CNC-bewerking is a subtractive process: it cuts away everything that is not the finished part. This gives it two fundamental Voordelen precision (Toleranties down to ±0,005 mm) and material flexibility (any machinable metal, plastic, or composite, in any quantity from 1 to 100,000).

This guide covers everything a procurement engineer or hardware buyer needs to know about ordering CNC-bewerking parts — from understanding the core processes and material options to specifying Toleranties, choosing finishes, estimating costs, and navigating US import tariffs. Whether you are ordering 10 prototype brackets or 10,000 Productie housings, the rules are the same. Only the economics change.

Part Types: What Can a CNC Machine Actually Make?

Not every geometry is CNC-friendly. Understanding which parts suit which process is the first step to a manufacturable design.

Turned Parts (CNC Lathe)

The workpiece rotates while a stationary cutting tool removes material. Best voor cylindrical parts: shafts, pins, bushings, spacers, threaded rods, Klephuizen, and any part with rotational symmetry. CNC lathes produce excellent surface finishes (Ra 0.8 µm standard) and hold tight diametral Toleranties (±0.01 mm). Live tooling on modern lathes can add milled features — flats, cross-holes, keyways — without a second setup.

Milled Parts (3-Axis CNC Mill)

The cutting tool rotates and moves in X, Y, and Z while the workpiece is fixed. Best voor prismatic parts with flat faces, pockets, slots, and holes: brackets, enclosures, manifolds, heat sinks, mounting plates. 3-axis is the most common and cost-effective CNC process. Parts with features on multiple faces require re-fixturing — each setup adds cost.

5-Axis Complex Parts

The cutting tool approaches the workpiece from any angle, machining multiple faces in a single setup. Best voor complex geometry: impellers, turbine blades, orthopedic implants, aerospace structuraL-beugels. 5-axis eliminates fixturing errors between setups and can produce geometries impossible on 3-axis — undercuts, compound angles, sculpted surfaces. The trade-off: higher machine rate ($80–150/hr vs $40–80/hr for 3-axis).

Swiss-Type Parts (Sliding Headstock Lathe)

Ontwerped for small, long, slender parts: medical screws, watch components, connector pins, electronic contacts. The bar stock slides through a guide bushing while tools cut near the bushing — minimizing deflection. Swiss machines hold ±0,005 mm on diameters under 3 mm and can mill, drill, and thread in a single cycle. Ideal for parts under 32 mm diameter with length-to-diameter ratios up to 5:1.

Materiaal Selection: The Foundation of Every CNC Part

Materiaal choice is the single biggest lever on part performance and cost. A bracket made from 7075-T6 aluminum costs 40% more than 6061-T6 but is twice as strong. A medical implant in 316L stainless costs 3× more than 304 but resists body fluid corrosion. Understand the trade-offs before you specify.

Best voor cost-Efficiëntie 6061-T6 aluminium or Delrin (POM) — both machine fast, widely available, and have the lowest raw material cost.
Avoid when: Specifying titanium for non-aerospace Toepassings — the material cost is 10× aluminum and machining time is 4–5× longer. Unless weight or biocompatibility is the primary requirement, there is almost always a cheaper alternative.

Toleranties: How Tight Is Tight Enough?

Every dimension on a drawing that niet have a specific tolerance callout falls under the general tolerance standard — typically ISO 2768-mK or ASME Y14.5. Dimensions with explicit Toleranties override the general standard. The art of cost-effective CNC design is putting tight Toleranties only where they matter.

The tolerance rule of thumb: Before you write ±0.01 mm on a dimension, ask: “What happens if this feature is ±0.05 mm instead?” If the answer is “the assembly still works,” save the money. A drawing with 20 dimensions all toleranced at ±0.01 mm is a drawing that costs 3× more than it needs to — and the shop will quote accordingly.

Surface Afwerkinges: Protection, Appearance, and Function

The as-machined surface from a CNC tool is typically Ra 1.6–3.2 µm — functional for most internal surfaces but visibly marked with tool paths. For external surfaces, cosmetic requirements, or functional coatings, choose from these standard options:

How CNC Parts Are Priced: The Quote Decoder

When a machine shop quotes your part, they are not pulling a number from the air. The price is built from six components. Understanding them lets you identify overpriced quotes — and optimize your design to reduce cost.

1. Materiaal cost (10–20% of total): The raw billet or bar stock. Aluminium ~$4–6/kg; 316 SS ~$8–12/kg; PEEK ~$80–120/kg.
2. Programming time (5–10%): CAM programming — creating the toolpaths. One-time cost amortized over the order quantity. For 1–5 prototype parts, this is a significant share. For 1,000 parts, it rounds to zero.
3. Setup time (10–25%): Fixturing the workpiece, loading tools, setting work offsets. Each additional setup (flipping the part to machine another face) adds setup cost. A 3-axis part with 4 setups costs more than a 5-axis part machined in 1 setup — even though the 5-axis machine rate is higher.
4. Cycle time (30–50%): The actual cutting time. Driven door material machinability, feature complexity, and surface finish requirements. Aluminium cuts 3–5× faster than stainless. A mirror finish (Ra 0.4 µm) requires a slow finishing pass that can double cycle time.
5. Nabewerking (5–15%): Deburring, cleaning, anodiseren, poedercoaten, Passivering. Any process that happens after the part comes off the machine.
6. Inspectie and quality (5–15%): First-article Inspectie, tussentijdse controles, final dimensional reports. Specifying AS9102 (aerospace FAIR) or requiring a CMM report on every dimension adds cost but catches errors before they reach your assembly line.

Praktijkvoorbeeld: A US medical device startup ordered 50 aluminum housings. Quote 1: $185/part from a US shop (3 weeks). Quote 2: $62/part from a Chinese supplier + 78% tariff ($48) + air freight ($5) = $115 landed (4 weeks). The 38% savings on a $5,750 order bought the engineering team an extra week of development time. They now budget the tariff into their BOM cost from day one.

The US Buyer’s Tariff Reality

If you are sourcing CNC-bewerking parts from China, tariffs are part of the landed cost — and they vary dramatically door material. This is not a footnote. It is a line item that can equal or exceed the part price itself.

The key variable for US buyers: Section 232 tariffs on metal parts apply to the full part value, not just the raw metal content (as of April 2026). On a $100 aluminum bracket containing $8 worth of aluminum, the 232 tariff is $50 — not $4. This structural change makes non-metal CNC parts (plastics, composites) significantly more attractive for cost-sensitive Toepassings.

Best voor tariff Efficiëntie Plastic CNC parts (PEEK, Delrin, nylon) — 28% total tariff vs 78% for metals. If plastic meets your functional requirements, the landed cost advantage over US domestic is 40–60%.
Avoid when: High-value aluminum parts where the tariff alone ($50 on $100) erases the China cost advantage. For these, compare with US domestic or Mexico alternatives.

Ordering CNC Parts: The RFQ Checklist

A complete RFQ package eliminates the back-and-forth that adds days to your lead time. Verstuur these six items with every CNC-bewerking quote request:

1. 3D CAD model — STEP (.stp) or IGES (.igs) format. This is the master geometry reference. Include all features, threads, and surface finish callouts in the model Annotaties.
2. 2D dimensioned drawing (PDF) — with critical dimensions toleranced, general tolerance standard specified (ISO 2768-mK or ASME Y14.5), material grade, and any notes on grain direction, plating thickness, or special requirements.
3. Materiaal specification — grade, condition (e.g., 6061-T6, not just “aluminum”), and any certification requirements (mill cert, RoHS, DFARS).
4. Quantity and delivery schedule — prototype quantity, Productie quantity, and whether this is a one-time order or recurring. Suppliers quote recurring orders more aggressively.
5. Surface finish requirements — as-machined, anodized, powder coated, passivated. Specify the standard (e.g., MIL-A-8625 Type II for anodize).
6. Quality requirements — Inspectie standard (ISO 2768, AQL level), first-article requirements (FAIR per AS9102 if aerospace), and any special packaging or labeling.

Conclusie: CNC Parts Are a Commodity — Good CNC Parts Are Not

Any shop with a CNC mill can cut metal. The difference between a part that fits and a part that fails is in the details: the material certification, the tolerance stack-up, the surface finish spec, the Inspectie report. A good CNC machined part starts with a clear drawing and ends with a supplier who reads it — not one who assumes. For US buyers, the additional layer of tariff management makes supplier selection even more critical. The right supplier helps you navigate HS codes, optimizes your design for the lowest-tariff material that meets functional requirements, and delivers consistent quality that makes the 28–78% tariff worth paying. Because the most expensive CNC part is not the one with the highest tariff — it is the one that niet work when it arrives.