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CNC Machining Specifications

Complete reference for tolerances, materials, surface finishes, DFM guidelines, and cost optimization.

±0.0001"

Tightest Tolerance

1 Day

Fastest Lead

45+

Materials

Ra 0.4μm

Best Finish

UI overlay with a 'Process' dropdown showing 'CNC Machining' and an 'Apply configuration' button over a metal part background

Getting Requirements Right — Key Questions ▾
  • FUNCTION FIRST: What does this part do?
  • TOLERANCING: Which dimensions truly matter?
  • QUANTITY & TIMELINE: How many parts and when?
  • ASSEMBLY CONTEXT: What does it mate with?
  • MATERIAL RATIONALE: Why this material?
  • SURFACE FINISH PURPOSE: Aesthetic, functional, corrosion-resistant?
ISO 2768-1 Linear Dimension Tolerances (For Reference Only) ▾
Class 0.5–3mm 3–6mm 6–30mm 30–120mm
Fine (f) ±0.05 ±0.05 ±0.10 ±0.15
Medium (m) ±0.10 ±0.10 ±0.20 ±0.30
Coarse (c) ±0.20 ±0.30 ±0.50 ±0.80
Surface Roughness (Ra) Reference ▾
Ra (μm) Ra (μin) Description Process
0.1–0.4 4–16 Mirror polish Lapping
0.8–1.6 32–63 Fine machined Precision grinding
1.6–3.2 63–125 Standard machined Turning, milling
3.2–6.3 125–250 Coarse machined Rough milling

Surface Roughness (Ra) Reference

Common surface finish specifications and their applications

Ra Value (μm) Ra Value (μin) Finish Description Typical Process Applications
0.025 – 0.05 1 – 2 Super fine polish Lapping, superfinishing Optical surfaces, gage blocks
0.1 – 0.2 4 – 8 Fine polish Fine grinding, polishing Bearing surfaces, seals
0.4 – 0.8 16 – 32 Fine machined Precision grinding, fine turning Mating surfaces, hydraulic cylinders
1.6 – 3.2 63 – 125 Standard machined Turning, milling, boring General machined parts (most common)
6.3 – 12.5 250 – 500 Coarse machined Rough turning, drilling Non-critical surfaces, clearance
25 – 50 1000 – 2000 Very rough Sawing, flame cutting, as-cast Hidden surfaces, raw stock

Quick Reference:

Ra 3.2μm (125μin): Standard CNC finish, cosmetic acceptable

Ra 1.6μm (63μin): Fine finish, good for O-ring seals

Ra 0.8μm (32μin): Precision finish, adds cost

Ra 0.4μm (16μin): Ground finish, expensive

Cost Optimization Tips ▾
  • Use standard tolerances where possible (±0.005″)
  • Avoid deep pockets and cavities (max depth = 4x tool diameter)
  • Design parts that can be machined in 3 axes (avoid 5-axis)
  • Minimize the number of setups required
  • Use standard hole sizes to avoid custom tooling
  • Avoid thin walls and delicate features when possible
  • Consider material waste in design
  • Specify surface finish only where needed
  • Avoid small text and logos (min 0.020″ depth, 0.030″ width)
  • Use standard thread sizes (UNC/UNF, metric coarse/fine) – special pitches require custom taps ($30-150) and hand-tapping time
  • Consolidate features to one side when possible – reduces setups from 4-6 down to 2-3, saving 30-90 min per part
  • Avoid undercuts requiring T-slot cutters or lollipop mills – these add 15-45 min programming + specialized tooling costs

1. Material Selection

Softer materials like aluminum and brass machine faster with longer tool life, reducing cost. Harder materials like stainless steel and titanium require slower speeds, more frequent tool changes, and specialized tooling.

Impact: Can affect machining time by 2-5x and tool costs by 3-10x

2. Part Complexity & Geometry

Simple geometries with standard features (holes, pockets, flat surfaces) are fastest. Complex curves, undercuts, or intricate details require multiple setups, specialized tooling, or 5-axis machining.

Impact: Complex parts can take 3-10x longer and require 2-5x more setups

3. Tolerance Requirements

Standard tolerances (±0.005" / ±0.127mm) are achievable with normal machining. Tighter tolerances require additional operations, slower speeds, climate-controlled environments, and more frequent inspection.

Impact: Tight tolerances (±0.001") can double or triple machining time and cost

4. Surface Finish Specifications

Standard mill finishes (125 Ra) are included in typical machining. Finer finishes require additional operations like grinding, polishing, or honing with specialized equipment and skilled operators.

Impact: Mirror finishes (8-16 Ra) can add 50-200% to part cost

5. Feature Accessibility

Features accessible from one or two sides minimize setups and reduce cost. Deep pockets, internal features, or features requiring access from multiple angles need additional setups or 5-axis machining.

Impact: Each additional setup adds 15-30 minutes and increases error risk

Myths vs. Reality ▾
Myth Reality
Tighter Tolerance = Frequently Necessary Only where function requires.
CNC can make anything Almost anything but physical limits do exist.
5-axis always better 3-axis handles most parts.
Drawing is a formality If you don't need specific GD&T, fine. Otherwise, absolutely necessary
RFQ Best Practices ▾
Bad RFQ Good RFQ
"CNC part — need quote" L6061-T6 bracket, tolerances per attached dwg, QTY 50, Finish per attached dwg.
"Rush job" 50 pcs delivered by 4/25.
"See attached" See attached STEP File and PDF DWG. Quality Requirements per QMS 1.1 & 1.4 in QAUL-RFQ-001
Related Design Guides ▾

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