Is CNC drilling the same as milling?
1. Straight Answer: No—CNC Drilling and Milling Are Distinct but Complementary
CNC drilling and milling are both subtractive manufacturing processes (removing material to shape parts), but they differ fundamentally in motion logic, tool design, and processing purposes. While drilling focuses on creating cylindrical holes (the most common hole-making method), milling handles complex contours (e.g., slots, pockets, 3D surfaces).
Industry data highlights their unique roles:
  • CNC drilling accounts for ~35% of CNC machining operations (dominant in hole-making; Source: 2025 Global CNC Machining Application Report).
  • CNC milling accounts for ~45% (dominant in contour machining; Source: CNC Milling Market Analysis 2025).
  • 60% of mechanical parts (e.g., brackets with counterbored holes) require both processes to finish—drilling creates the base hole, milling adds the stepped contour.

2. Core Differences: Drilling vs. Milling (Side-by-Side Comparison)

Understanding the differences between the two is the key to making the right selection. Specifically, they can be distinguished from seven dimensions:
Comparison Dimension
CNC Drilling
CNC Milling
Core Purpose
Create cylindrical holes (through/blind, standard sizes)
Shape complex contours (slots, pockets, arcs, 3D surfaces) + refine holes
Motion Logic
Tool rotates (spindle-driven); workpiece/tool moves linearly (X/Y/Z) to feed
Tool rotates (spindle-driven); tool moves in 2-5 axes (X/Y/Z/A/B) for contour
Tool Design
Single-point/dual-flute (e.g., twist drill: 2 flutes for chip evacuation)
Multi-flute (e.g., end mill: 2-10 flutes for smooth cutting)
Material Removal Method
Axial cutting (material removed along tool centerline)
Radial/axial cutting (material removed from tool side/end)
Precision Range
Position accuracy: ±0.005-0.01mm; Hole diameter tolerance: H7-H8
Position accuracy: ±0.002-0.005mm; Contour tolerance: ±0.001-0.003mm
Efficiency (Hole-Making)
Fast for standard holes (e.g., φ8mm hole: 2s/piece in aluminum)
Slow for hole-making (e.g., φ8mm hole: 15s/piece—needs circular interpolation)
Key G-Codes (FANUC)
Hole cycles: G81 (spot drilling), G83 (peck drilling), G82 (counterboring)
Contour codes: G01 (linear), G02/G03 (arc), G73 (peck milling)
Critical Example: To make a φ8mm through hole in an aluminum bracket—drilling takes 2 seconds, while milling (using a φ8mm end mill) takes 15 seconds. But to add a φ12mm×3mm counterbore on top of the hole—only milling can achieve this (via G02 circular interpolation).

3. Application Scenarios: When to Choose Drilling vs. Milling

There are clear boundaries for the applicable scenarios of the two, but there is also room for synergy. The selection should be made according to the characteristics of the parts:

A. Choose CNC Drilling When:

  • You need standard cylindrical holes (diameter 0.1-100mm) in any material (steel, aluminum, PCB).
  • Efficiency is critical (high-volume hole-making, e.g., 1000×φ5mm holes in automotive engine blocks).
  • Hole depth is ≤5×diameter (standard twist drills) or ≤30×diameter (deep-hole drills with internal coolant).
Typical Applications:
  • Drilling oil holes in transmission shafts (φ3mm×15mm blind holes).
  • Drilling mounting holes in PCB boards (0.1mm micro-holes via dedicated drill centers).

B. Choose CNC Milling When:

  • You need complex contours (slots, pockets, angled surfaces) or non-circular holes (e.g., square holes, elliptical holes).
  • Hole precision requirements exceed drilling capabilities (e.g., hole position tolerance ±0.002mm for aerospace parts).
  • You need to refine holes (e.g., counterbores, chamfers, or enlarging irregular holes).
Typical Applications:
  • Milling a 6×6mm keyway in a φ25mm shaft (after drilling a φ5mm pilot hole).
  • Milling a 3D curved surface on a turbine blade (5-axis milling).
  • Milling a φ12mm counterbore on a φ8mm drilled hole (for bolt heads).

C. Synergistic Scenarios (Both Processes Required):

60% of mechanical parts need drilling first, milling second to finish complex features. Common combinations:
  1. Counterbored Holes: Drill φ8mm base hole → Mill φ12mm×3mm counterbore (for M8 bolts).
  1. Slotted Holes: Drill two φ5mm pilot holes → Mill a 5×20mm slot connecting them.
  1. Angled Holes: Drill a straight φ6mm hole → Mill the hole entrance to 45° (for pipe connections).
Industry Data: Automotive brackets with counterbored holes use this “drill-mill” combination—drilling handles 70% of the work, milling handles the remaining 30% (Source: Automotive CNC Machining Guide 2025).

4. Key Technical Differences: Tools, Parameters & Cycles

A. Tool Selection: Drilling Tools vs. Milling Tools

Tool Category
CNC Drilling Tools
CNC Milling Tools
Common Types
Twist drill, spot drill, deep-hole drill, counterbore drill
End mill (flat/ball/nose), face mill, slot drill, chamfer mill
Flute Count
2 flutes (standard twist drill)
2-10 flutes (4 flutes for general milling, 10 for high-speed aluminum)
Material Focus
Carbide (for steel/aluminum), diamond-coated (for PCB micro-holes)
Carbide (most common), HSS (low-speed steel milling)
Key Feature
Pointed tip (centers itself in workpiece)
Flat/rounded tip (cuts side/end surfaces)
Example: Drilling a φ8mm hole uses a 2-flute carbide twist drill; milling a φ12mm counterbore uses a 4-flute flat end mill.

B. Cutting Parameter Optimization

Parameters differ due to tool design and material removal logic—misselection causes tool wear or poor quality:
Material
Process
Tool Type
Spindle Speed (S)
Feed Rate (F)
6061 Aluminum
Drilling
φ8mm Carbide Twist Drill
3000r/min
150mm/min
6061 Aluminum
Milling
φ8mm 4-Flute End Mill
4000r/min
200mm/min (linear)
45# Steel (HB200)
Drilling
φ8mm Carbide Twist Drill
1200r/min
50mm/min
45# Steel (HB200)
Milling
φ8mm 4-Flute End Mill
1500r/min
80mm/min (linear)
Key Logic: Milling uses higher spindle speed (more flutes = faster cutting) and linear feed rate (vs. drilling’s axial feed), while drilling prioritizes axial feed to push the tool through material.

C. Critical Cycles: Drilling Cycles vs. Milling Cycles

Process
Core Cycles (FANUC)
Purpose
Drilling
G81 (spot drilling): Fast shallow holes
Creates 2-5mm deep guide holes (prevents drill wandering)
G83 (peck drilling): Deep holes
Evacuates chips for holes >5×diameter (e.g., φ5mm×30mm hole)
G82 (counterboring): Stepped holes
Adds shallow counterbores (limited to small diameters ≤20mm)
Milling
G02/G03 (arc interpolation): Circular contours
Mills counterbores, arcs, or non-circular holes (e.g., φ12mm counterbore)
G73 (peck milling): Deep slots
Evacuates chips for deep slots (e.g., 5×20mm×10mm deep slot)
G17/G18/G19 (plane selection): Multi-axis milling
Defines cutting plane (XY for flat milling, XZ for cylindrical milling)

5. Practical Case: Drill-Mill Synergy for a Counterbored Bracket

Part Specs: 6061 aluminum bracket (100×50×10mm), 4×φ8mm through holes with φ12mm×3mm counterbores (positions: (20,20), (20,40), (80,20), (80,40)), surface roughness Ra≤1.6μm.

Step 1: Process Planning (Drill First, Mill Second)

  1. Spot Drilling: G81 creates 2mm-deep guide holes (avoids drill wandering).
  1. Final Drilling: G83 drills φ8mm×10mm through holes (peck drilling for chip evacuation).
  1. Counterbore Milling: G02 mills φ12mm×3mm counterbores (uses φ12mm 4-flute end mill).

Step 2: Equipment & Tools

  • Machine: FANUC 0i-MF 3-axis milling center (supports both drilling and milling cycles).
  • Tools: T01 (φ6mm spot drill), T02 (φ8mm carbide twist drill), T03 (φ12mm 4-flute flat end mill).

Step 3: Programming Snippet (FANUC System)

O0004 (Counterbored Bracket Program: Drill + Mill)
G90 G54 G00 X0 Y0 Z10 // Absolute mode, G54 coordinate, safe position
M08 // Coolant on
// 1. Step 1: Spot Drilling (T01)
T0101 S3500 M03 // Spot drill: 3500r/min (aluminum)
G81 X20 Y20 Z-2 R2 F200 // Spot drill at (20,20), depth 2mm
X20 Y40
X80 Y20
X80 Y40
G80 // Cancel drilling cycle
// 2. Step 2: Final Drilling (T02)
T0202 S3000 M03 // Twist drill: 3000r/min
G83 X20 Y20 Z-12 R2 Q5 F150 // Peck drill: Q5=5mm peck, Z-12=through hole
X20 Y40
X80 Y20
X80 Y40
G80
// 3. Step 3: Counterbore Milling (T03)
T0303 S4000 M03 // End mill: 4000r/min
G00 X20 Y20 Z5 // Move to first counterbore position
G01 Z-3 F200 // Feed to counterbore depth (3mm)
G41 X20 Y26 D03 // Activate tool radius compensation (D03=6mm for φ12mm end mill)
G02 X20 Y20 I0 J-6 F150 // Mill φ12mm circle (I0 J-6: center at (20,20))
G40 X20 Y14 // Cancel radius compensation
G00 Z5 // Retract
// Repeat for other counterbores (X20 Y40, X80 Y20, X80 Y40)
X20 Y40 Z5
G01 Z-3 F200
G41 X20 Y46 D03
G02 X20 Y40 I0 J-6 F150
G40 X20 Y34
G00 Z5
// Program End
G00 X0 Y0 Z50
M05 M09
M30

Step 4: Quality Verification

  • Hole Size: φ8mm hole (pin gauge φ8H7 passes); φ12mm counterbore (calipers measure 12±0.005mm).
  • Counterbore Depth: 3±0.01mm (depth gauge).
  • Surface Roughness: Ra=1.2μm (bore scope inspection—meets requirement).

6. Common Misconceptions & Solutions

1. Misconception: “Milling can replace drilling for all holes.”

  • Reality: Milling is inefficient for standard cylindrical holes—drilling is 5-10x faster. For example, drilling 100×φ8mm holes takes 3 minutes; milling the same holes takes 25 minutes.
  • Solution: Use drilling for standard holes; reserve milling for non-circular holes or counterbores.

2. Misconception: “Drilling can create counterbores (no need for milling).”

  • Reality: Drilling’s G82 cycle only handles small counterbores (≤20mm diameter, ≤5mm depth) with poor surface quality (Ra≥3.2μm). Milling creates larger, smoother counterbores (Ra≤1.6μm).
  • Solution: For counterbores >φ20mm or depth >5mm, use milling (G02/G03) instead of G82.

3. Misconception: “Hole precision is the same for drilling and milling.”

  • Reality: Milling achieves higher position accuracy (±0.002mm vs. drilling’s ±0.005mm) because it uses multi-axis interpolation and tool radius compensation.
  • Solution: For precision holes (e.g., aerospace engine holes with ±0.003mm tolerance), mill the hole after drilling (or mill directly for small batches).

7. Q&A: High-Frequency Questions About Drilling vs. Milling

Q1: Can I use a milling end mill to drill holes (instead of a twist drill)?

  • Yes, but it’s inefficient: A φ8mm end mill drills a hole 5x slower than a twist drill (needs G02 circular interpolation to “carve” the hole). Use this only if:
    • The hole is non-circular (e.g., square) or has tight tolerance (±0.002mm).
    • You don’t have a suitable twist drill (emergency only).

Q2: What’s the cost difference between drilling and milling?

  • Drilling: Lower cost—twist drills ((5-)20 each) last 500-1000 holes; dedicated drill centers ((50k-)100k) have low maintenance.
  • Milling: Higher cost—end mills ((15-)50 each) last 300-500 contours; 3-axis milling centers ((80k-)200k) need more frequent servo calibration.
  • Synergy Tip: Use a milling center for small-batch “drill-mill” parts (avoids buying two machines); use dedicated drill centers + milling centers for high-volume production.

Q3: How to process a complex hole (e.g., φ8mm hole with a 45° angled slot)?

  • Combine drilling, milling, and 5-axis movement:
    1. Drill φ8mm through hole (G83).
    1. Tilt the workpiece to 45° (5-axis A-axis).
    1. Mill the 45° slot (G01 linear interpolation) using a φ3mm end mill.

Final Thought

CNC drilling and milling are not competitors—they are complementary tools in the machinist’s toolkit. Drilling excels at fast, standard hole-making, while milling handles the complex contours that drilling cannot. The key to efficient production is understanding their strengths (drilling for speed, milling for precision) and using them in synergy (e.g., drill the base hole, mill the counterbore).
Have you faced confusion about choosing drilling vs. milling for a part? Or need guidance on optimizing a “drill-mill” process (e.g., reducing cycle time)? Share your questions in the comments—I’ll provide targeted solutions!

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