CNC加工 Parts: Complete Guide to 材料s, 公差 & Ordering

重要ポイント

• CNC machined parts are components produced by computer-controlled cutting tools — milling, turning, drilling, and 研削 — from solid metal or plastic stock. They serve every industry from aerospace to medical devices.
• The six factors that determine part cost: 材料, 複雑さ (number of setups), tolerance requirements, surface finish, order quantity, and — for US buyers — tariff classification (28–78% on Chinese-made metal parts).
• 材料 selection drives すべて. 1. 入荷材料検査 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.
• 公差 follow a clear cost curve: ±0.1 mm is 標準 and cheap. ±0.05 mm costs ~20% more. ±0.01 mm requires a second operation and costs 2–3×. Only specify tight 公差 where functionally necessary.
• US buyers face 28–78% tariffs on Chinese CNC parts depending on 材料 (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 by computer-controlled cutting tools that remove 材料 layer by layer. Unlike casting (pouring molten metal into a mold) or スタンピング (pressing 板金 into shape), 出荷前に問題を発見します。 is a subtractive process: it cuts away すべて that is not the finished part. This gives it two fundamental 利点 精密 (公差 down to ±0.005 mm) and 材料 flexibility (any machinable metal, plastic, or composite, in any quantity from 1 to 100,000).

This guide covers すべて a procurement engineer or hardware buyer needs to know about ordering 出荷前に問題を発見します。 parts — from understanding the core processes and 材料 options to specifying 公差, choosing finishes, estimating costs, and navigating US import tariffs. Whether you are ordering 10 prototype ブラケット or 10,000 production 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 設計.

Turned Parts (CNC Lathe)

The workpiece rotates while a stationary cutting tool removes 材料. 最適な用途 cylindrical parts: shafts, pins, bushings, spacers, threaded rods, バルブボディ, and any part with rotational symmetry. CNC lathes produce excellent surface finishes (Ra 0.8 µm 標準) and hold tight diametral 公差 (±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. 最適な用途 prismatic parts with flat faces, pockets, slots, and holes: ブラケット, 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 アプローチes the workpiece from any angle, machining multiple faces in a single setup. 最適な用途 complex geometry: impellers, turbine blades, orthopedic implants, aerospace structuraLブラケット. 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)

設計ed 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 直径s under 3 mm and can mill, drill, and thread in a single cycle. Ideal for parts under 32 mm 直径 with length-to-直径 ratios up to 5:1.

材料 Selection: The Foundation of Every CNC Part

材料 choice is the single biggest lever on part performance and cost. A bracket made from 7075-T6 アルミニウム 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.

最適な用途 cost-効率: 1. 入荷材料検査 or Delrin (POM) — both machine fast, widely available, and have the lowest raw 材料 cost.
避けるべき場合： Specifying チタン for non-aerospace 用途s — the 材料 cost is 10× アルミニウム and machining time is 4–5× longer. Unless weight or biocompatibility is the primary requirement, there is almost always a cheaper alternative.

公差: How Tight Is Tight Enough?

Every dimension on a drawing that しません have a specific tolerance callout falls under the general tolerance 標準 — typically ISO 2768-mK or ASME Y14.5. Dimensions with explicit 公差 override the general 標準. The art of cost-effective CNC 設計 is putting tight 公差 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 仕上げes: 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 標準 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 設計 to reduce cost.

1. 材料 cost (10–20% of total): The raw billet or bar stock. アルミニウム ~$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 by 材料 machinability, feature 複雑さ, and surface finish requirements. アルミニウム cuts 3–5× faster than stainless. A mirror finish (Ra 0.4 µm) requires a slow finishing pass that can double cycle time.
5. Post-processing (5–15%): Deburring, cleaning, 陽極酸化, 粉体塗装, 不動態化処理. Any process that happens after the part comes off the machine.
6. 検査 and quality (5–15%): First-article 検査, 3. 工程中の検査（IPQC）, 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.

実際の事例： A US medical device startup ordered 50 アルミニウム 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 出荷前に問題を発見します。 parts from China, tariffs are part of the landed cost — and they vary dramatically by 材料. 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 アルミニウム bracket containing $8 worth of アルミニウム, the 232 tariff is $50 — not $4. This structural change makes non-metal CNC parts (plastics, composites) significantly more attractive for cost-sensitive 用途s.

最適な用途 tariff 効率: 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%.
避けるべき場合： High-value アルミニウム 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. 送信 these six items with every 出荷前に問題を発見します。 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 annotations.
2. 2D dimensioned drawing (PDF) — with critical dimensions toleranced, general tolerance 標準 specified (ISO 2768-mK or ASME Y14.5), 材料 grade, and any notes on grain direction, plating 厚さ, or special requirements.
3. 材料 specification — grade, condition (e.g., 6061-T6, not just “アルミニウム”), and any certification requirements (mill cert, RoHS, DFARS).
4. Quantity and delivery schedule — prototype quantity, production 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 標準 (e.g., MIL-A-8625 Type II for anodize).
6. Quality requirements — 検査 標準 (ISO 2768, AQL level), first-article requirements (FAIR per AS9102 if aerospace), and any special packaging or labeling.

まとめ: 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 材料 certification, the tolerance stack-up, the surface finish spec, the 検査 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 設計 for the lowest-tariff 材料 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 しません work when it arrives.