CNC Machining Guide: From Design to Finished Part

Start with the Design

Prepare the right 3D files and drawings.
CNC machining always starts with a good design. Typically, you need:
· 3D model files: Common formats include STEP (.step/.stp), IGES (.iges/.igs), Parasolid (.x_t) – the most widely accepted neutral formats for CNC machining.
· 2D engineering drawings: Used to specify dimensions, tolerances, surface finish, threads, and other key requirements. Common formats are PDF or DWG.
Tips:
① If you only have a hand sketch, you can ask a designer (or us) to convert it into a manufacturable 3D file.
② Consider tolerances and materials during the design stage to avoid costly redesigns.
③ Keep manufacturability in mind — avoid unnecessary deep blind holes or overly complex features.

Drawing Symbols and Annotations

Understanding drawing symbols is critical — they determine whether the finished part meets your requirements. Common symbols include:± – Dimensional tolerance range
⌀ – Diameter (e.g. ⌀10 = Ø10mm hole).
R – Radius (e.g. R5 = 5mm radius arc).
SR – Spherical radius.
⌒ – Arc length.
⌴ – Surface roughness (Ra1.6, Ra3.2) – lower value = smoother finish.
CHAMFER / C – Chamfer (e.g. 0.5 × 45°).
DEBURR – Remove sharp edges/burrs.
SECTION A-A – Section view.
SEE DETAIL B – Enlarged detail view.
Geometric tolerances – ∥ (parallelism), ⊥ (perpendicularity), ⌯ (concentricity).
Knowing these symbols ensures you communicate your design intent clearly and avoid misunderstandings with the manufacturer.

Thread Requirements & Standards

Threads are one of the most common CNC machining features. Major standards include:
· Metric threads (M series): e.g. M6×1, M10×1.5
· Imperial threads (UNC/UNF): e.g. 1/4-20 UNC
· Pipe threads (G, NPT): used for fluid connections
Always specify thread size, depth, and gauge requirement (Go/No-Go) on your drawing to ensure proper fit.

Material Selection

Material choice impacts part performance, durability, and cost. Common materials include:

· Aluminum alloys (6061, 7075): Lightweight, corrosion-resistant, cost-effective — popular for aerospace and automotive lightweight parts.

· Stainless steel (304, 316, 17-4PH): High strength, corrosion-resistant — ideal for medical devices and food equipment.

· Steel (45#, 4140, P20): High strength — for load-bearing parts or molds.

· Plastics (POM, PEEK, ABS): Lightweight, electrically insulating — used in medical and electronic components.
Tips:
①Choose common materials with standard tolerances for lower cost.
②Select high-strength or high-temperature materials for demanding applications.

CNC Programming

After the design is finalized, engineers use CAM software (MasterCAM, UG NX, Fusion 360) to generate toolpaths.

· G-code is created to tell the machine how to move the cutting tool, set spindle speeds, and depth of cut.

· The CNC controller executes the G-code, carrying out milling, turning, drilling, and other operations.
Programming is a highly specialized process and is the key to achieving precision machining.

CNC Processes Explained

Common CNC machining processes include:

· Turning: Best for cylindrical parts

· Milling: For flat surfaces, slots, and complex geometries

· 5-Axis Machining: Ideal for complex aerospace components and curved surfaces

· Drilling / Tapping: For holes and threads
Choosing the right process reduces costs and lead time.

Surface Finishing – More Than Just Looks

Surface finishing improves corrosion resistance, wear resistance, and appearance. Common finishes include:

· Anodizing: Forms an oxide layer on aluminum, improves corrosion resistance

· Plating (Nickel/Zinc/Chrome): Increases surface hardness, protects against rust

· Sandblasting: Creates a matte texture, hides machining marks

· Polishing: Improves surface shine
Note::
①Different materials require different surface treatments (e.g. carbon steel cannot be anodized).
②Surface finishing can slightly change dimensions — tolerance compensation is needed.

Quality Control

Quality control is the final step to ensure accuracy. In our factory, QC includes:

· First Article Inspection (FAI): Full dimension check after first part to confirm setup

· In-Process Inspection: Periodic checks during production to prevent defects

· Final Inspection: 100% check for functional parts, sampling for appearance parts, 100% visual check for all surfaces
Strict quality control ensures every part meets customer requirements.

Conclusion

CNC machining is a complete and systematic process. From design, drawings, material selection, programming, machining, surface finishing, to inspection — every step affects the final quality.
Choosing an experienced CNC supplier with standardized processes helps you reduce communication costs, minimize rework, and get your product to market faster.

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REKO is a high-quality CNC machining factory from Shenzhen, China. It was established in 2016. We provide CNC turning, milling, five-axis machining and other services. We have long-term cooperative suppliers to provide customers with a variety of surface treatment processes including anodizing, electroplating, sandblasting, heat treatment and other services. Need CNC machining services? Contact us today for high-precision custom parts!