Cutting Speed Optimization for Deep Hole Drilling
Cutting speed (surface speed at the cutting edge) is the most influential parameter on tool life in deep hole drilling. A 20% increase in cutting speed can reduce tool life by 50%. Selecting the optimal speed is the most important decision in parameter selection.
This guide covers cutting speed selection principles, optimization methodology, and the speed-tool life trade-off for all deep hole drilling methods.
Speed vs. Tool Life Relationship
The relationship between cutting speed and tool life follows the Taylor tool life equation:
VT^n = C
Where:
- V = cutting speed (m/min)
- T = tool life (minutes or holes)
- n = tool life exponent (typically 0.2–0.3 for carbide in deep hole drilling)
- C = constant (depends on tool and workpiece)
Practical meaning: A 20% increase in speed reduces tool life by approximately 50%. Conversely, a 20% reduction in speed can double tool life.
| Speed Change | Expected Tool Life Change (n=0.25) | Best For |
|---|---|---|
| -20% | +100–150% | Difficult materials, maximizing tool life |
| -10% | +40–60% | Conservative starting point |
| Baseline | Reference | Standard production |
| +10% | -25–40% | Soft materials, productivity priority |
| +20% | -50–60% | Aluminum, brass (if tool life still acceptable) |
Material Considerations
Effect of Material on Speed
| Material Group | Speed Range (m/min) | Why the Range |
|---|---|---|
| Low-carbon steel | 120–180 | Wide range — use higher for short holes, lower for deep |
| Alloy steel | 60–110 | Depends on hardness and alloy content |
| Stainless steel (austenitic) | 40–80 | Work hardening limits upper speed |
| Titanium | 15–30 | Low thermal conductivity — must keep speed low |
| Superalloys (Inconel) | 10–25 | Very low — heat management is critical |
| Aluminum | 80–200 | Wide range possible; limited by chip evacuation |
| Cast iron | 50–80 | Abrasive wear limits speed |
Effect of Hardness on Speed
For alloy and tool steels, hardness directly affects recommended cutting speed:
| Hardness (HB) | Speed (m/min) — Steel | Reduction vs. Soft |
|---|---|---|
| 150–200 HB | 100–130 | Baseline |
| 200–250 HB | 80–110 | -20% |
| 250–300 HB | 65–90 | -30% |
| 300–350 HB | 50–70 | -45% |
| 350–400 HB | 35–55 | -55% |
| > 400 HB (HRC 45+) | 20–35 | -75% |
Method-Specific Speed Considerations
Gun Drilling
In gun drilling, cutting speed is limited by:
- Heat concentration — The single-lip tool concentrates all heat at one cutting edge
- Coolant passage size — Small coolant holes limit flow; speed must be reduced for small diameters
- Tool rigidity — Long, slender tools are more sensitive to cutting speed vibration
| Diameter | Speed Adjustment vs. Table Value |
|---|---|
| < 3 mm | Reduce 30% (coolant flow limitation) |
| 3–6 mm | Reduce 15% |
| 6–12 mm | 100% (standard) |
| 12–25 mm | 100% |
| > 25 mm | Reduce 10% (larger diameter = more heat at edge) |
BTA Drilling
In BTA drilling, multiple cutting edges distribute heat, allowing higher speeds than gun drilling at the same diameter.
| Consideration | Effect on Speed |
|---|---|
| Multiple edges | Can run 10–15% higher than gun drilling at same diameter |
| Coolant volume | BTA has better cooling — supports higher speeds |
| Rigidity | Round tube is more rigid than gun drill shaft — supports higher speeds |
| Edge loading | Each insert sees full speed but shares feed load |
Ejector Drilling (DTS)
Ejector drilling speeds are similar to BTA, with slightly lower speeds recommended when retrofitted to CNC machines due to lower system rigidity.
Speed Optimization Process
Step-by-Step Methodology
- Start low — Use the lowest recommended speed for the material
- Run baseline — 50 holes at this speed; record tool wear and cycle time
- Increase speed 10% — Run 50 holes; compare tool wear
- Continue stepping — Increase until tool life drops below acceptable threshold
- Set operating speed — The highest speed that still meets your tool life target
Recording Template
| Run | Speed (m/min) | Holes | Tool Wear (mm) | Surface Finish (Ra) | Cycle Time |
|---|---|---|---|---|---|
| 1 | 80 | 50 | 0.15 | 0.6 | 120 sec |
| 2 | 88 | 50 | 0.20 | 0.6 | 109 sec |
| 3 | 97 | 50 | 0.28 | 0.7 | 99 sec |
| Optimal | 88 | — | 0.20 per 50 holes | 0.6 | 109 sec |
Common Speed-Related Problems
| Problem | Likely Cause | Solution |
|---|---|---|
| Excessive tool wear | Speed too high | Reduce speed 10–15% |
| Built-up edge (BUE) | Speed too low | Increase speed 10–15% |
| Chatter / vibration | Speed too high (resonance) | Reduce speed or increase feed |
| Burned chips | Speed too high or coolant insufficient | Reduce speed; increase coolant |
| Poor surface finish | Speed too high | Reduce speed |
| Edge chipping at entry | Speed OK — entry technique issue | Reduce entry feed |
Summary
Cutting speed is the primary determinant of tool life in deep hole drilling. Start at the lower end of the recommended speed range for the material, then increase incrementally while monitoring tool wear. Use the Taylor tool life relationship (20% speed increase → 50% tool life reduction) as a guide for estimating the impact of speed changes. Reduce speed for small diameters, hard materials, and high depth ratios.
For method-specific speed tables, see gun drilling parameters, BTA parameters, or ejector parameters. For complete process optimization, see process optimization guide. For a complete overview, visit the process parameters guide.