Feed Rate Selection for Deep Hole Drilling
Feed rate in deep hole drilling plays a dual role: it determines the material removal rate, and — critically — it controls chip thickness, which directly affects chip evacuation reliability. Too low a feed produces stringy chips that pack; too high a feed produces thick chips that can jam the flute or exceed machine torque capacity.
This guide covers feed rate selection principles, the chip evacuation relationship, and optimization methodology for all methods.
Feed Rate: The Chip Evacuation Connection
Unlike conventional drilling where feed rate primarily affects surface finish and material removal rate, in deep hole drilling feed rate is the primary control for chip shape.
| Feed Rate | Chip Shape | Evacuation Quality | Effect on Surface Finish |
|---|---|---|---|
| Too low | Long, stringy, thin | Poor — chips pack easily | Good (thin chips = fine finish) |
| Optimal | Short C-shaped | Reliable | Acceptable |
| Too high | Thick, heavy | Good (thick chips evacuate well) | Poor (heavy feed marks) |
The goal of feed rate selection in deep hole drilling: Find the lowest feed rate that still produces short, C-shaped chips. This gives the best surface finish while maintaining reliable chip evacuation.
Feed Rate by Method
Gun Drilling
Gun drills use a single cutting edge. Feed rate is typically 0.007–0.085 mm/rev depending on diameter.
| Diameter (mm) | Steel (mm/rev) | Cast Iron (mm/rev) | Aluminum (mm/rev) |
|---|---|---|---|
| 3–4 | 0.007–0.013 | 0.009–0.040 | 0.006–0.040 |
| 6–8 | 0.018–0.030 | 0.025–0.080 | 0.012–0.125 |
| 12–16 | 0.040–0.070 | 0.060–0.150 | 0.040–0.200 |
| 20–25 | 0.060–0.110 | 0.100–0.210 | 0.060–0.255 |
BTA Drilling
BTA heads use 2–4 cutting edges. The total feed rate is the sum of material removed by all edges.
| Diameter (mm) | Steel (mm/rev) | Cast Iron (mm/rev) | Aluminum (mm/rev) |
|---|---|---|---|
| 18–25 | 0.10–0.25 | 0.15–0.35 | 0.15–0.50 |
| 40–65 | 0.15–0.40 | 0.25–0.55 | 0.25–0.70 |
| 100–150 | 0.25–0.60 | 0.35–0.75 | 0.35–0.90 |
| 150–250 | 0.30–0.70 | 0.40–0.85 | 0.40–1.00 |
Ejector Drilling (DTS)
Ejector drilling feed rates are approximately 85–90% of BTA rates due to the slightly less efficient Venturi chip evacuation.
| Diameter (mm) | Steel (mm/rev) | Cast Iron (mm/rev) | Aluminum (mm/rev) |
|---|---|---|---|
| 18–25 | 0.08–0.20 | 0.12–0.30 | 0.12–0.40 |
| 40–65 | 0.12–0.30 | 0.20–0.40 | 0.20–0.60 |
| 100–150 | 0.20–0.40 | 0.30–0.50 | 0.30–0.70 |
Feed Rate per Edge
For multi-edge tools (BTA and DTS heads), the chip load per edge is more informative than the total feed rate:
Chip load per edge = Total feed rate (mm/rev) ÷ Number of cutting edges
| Number of Edges | Target Total Feed (mm/rev) | Chip Load per Edge (mm) |
|---|---|---|
| 2 | 0.20 | 0.10 |
| 3 | 0.30 | 0.10 |
| 4 | 0.40 | 0.10 |
For steel, target a chip load of 0.08–0.15 mm per edge.
Surface Finish vs. Feed Rate
Surface finish in deep hole drilling is determined primarily by:
- Feed rate — The most significant factor (higher feed = rougher finish)
- Guide pad condition — Worn pads degrade finish
- Tool nose radius — Larger radius = better finish
- Vibration — Any vibration produces chatter marks
Relationship:
Surface finish Ra ∝ Feed rate² / Tool nose radius
Rule of thumb: Halving the feed rate improves Ra by approximately 30%, but doubles cycle time.
Feed Optimization Process
- Start at the middle of the recommended feed range
- Check chip shape after the first few holes
- If chips are stringy: Increase feed 10–15%
- If chips are too thick or finish is poor: Reduce feed 10–15%
- If chip shape is not C-shaped at any feed: Check chip breaker geometry or insert type
Entry Feed
For the first 2–3 mm of cut, use 50% of the normal feed rate. This prevents the cutting edge from grabbing and chipping at the entry transition.
Breakthrough Feed (through-holes)
For the last 5–10 mm before breakthrough, reduce feed to 50%. This prevents uncontrolled breakout and edge damage.
Common Feed-Related Problems
| Problem | Likely Cause | Solution |
|---|---|---|
| Long, stringy chips | Feed too low | Increase feed 10–15% |
| Chip packing | Feed too low (stringy chips) or inadequate coolant | Increase feed; verify coolant |
| Poor surface finish | Feed too high | Reduce feed |
| Tool chipping at entry | Feed too high at entry | Reduce entry feed to 50% |
| Spindle overload | Feed too high for machine capacity | Reduce feed |
| Chatter | Feed too low | Increase feed slightly |
| Oversize hole | Feed too high causing tool deflection | Reduce feed |
Summary
Feed rate is the primary control for chip evacuation reliability in deep hole drilling. The optimal feed produces short, C-shaped chips while meeting surface finish requirements. Start at the middle of the recommended range and adjust based on chip shape. For multi-edge BTA and DTS tools, consider chip load per edge (0.08–0.15 mm for steel). Always use reduced feed at entry (50%) and breakthrough (50% for through-holes).
For method-specific feed tables, see gun drilling parameters, BTA parameters, or ejector parameters. For comprehensive process optimization, see process optimization guide. For a complete overview, visit the process parameters guide.