Three specialized deep hole methods (L/D > 10:1). Choose by diameter, depth, and production volume.
Quick recommendations based on your dominant requirement. Per VDI 3209 Blatt 1 (2024).
| Characteristic | Gundrilling | BTA (STS) | Ejector (DTS) |
|---|---|---|---|
| Diameter (optimal) | 0.5–50mm (1–25mm) | 6–2000mm (20–250mm) | 18–250mm (20–150mm) |
| Max L/D | 300:1 (extreme 400:1) | 200:1 | 100:1 |
| Chip removal | External (V-groove) | Internal (center hole) | Internal (Venturi) |
| Chip-clearance area | 22–26% of hole | >60% of hole | 35–40% of hole |
| Coolant pressure | 50–150+ bar | 15–100 bar | 10–50 bar |
| Coolant flow rate | 10–60 L/min | 50–500+ L/min | 25–200+ L/min |
| Surface finish Ra | 0.4–6.3 μm | 0.4–3.2 μm | 0.8–3.2 μm |
| Tolerance | IT6–IT11 (opt. IT5–IT7) | IT8–IT10 | IT9–IT11 |
| Straightness | 0.001 in/in | 0.001 in/ft | 0.002 in/ft |
| Productivity | 1× baseline | 5–10× | 3–5× |
| Face seal needed? | Yes | Yes (flat face) | No |
1. Check diameter: D < 15mm → Gundrill only · D 15–30mm → Gundrill or BTA (evaluate precision vs. productivity) · D > 30mm → BTA or Ejector
2. Check L/D: L/D > 100:1 → Gundrill only · L/D 50–100:1 → Gundrill or BTA · L/D < 50:1 → All three viable
3. Check batch size: High (>1000/mo) → BTA (lowest per-hole cost) · Medium (100-1000/mo) → BTA or Ejector · Low (<100/mo) → Gundrill
Beyond the three core methods, these specialized processes serve specific applications.
| Method | Description | Best For |
|---|---|---|
| Trepanning | Annular cut preserves center core slug. Reduces chip volume ~60% and power requirements. Typical 90–600mm diameter. | Large valve bodies, heavy wall cylinders, expensive alloys (core is reusable) |
| Skive & Roller Burnishing | Cutting + cold pressing in one pass. Ra <0.2μm, IT7–IT8. Per VDI 3209 Blatt 2. Cutting speed up to 200 m/min, feed 1–5 mm/rev. | Hydraulic cylinders, mirror-finish bores, eliminating honing operations |
| Pull Boring | Finishing process. Tool is pulled (not pushed) through bore, placing bar in tension. Straightness 0.0005–0.001 in/ft (0.04–0.08 mm/m). | Aerospace components requiring superior concentricity, hydraulic components |
| Factor | Gundrilling | BTA | Ejector |
|---|---|---|---|
| Machine investment | $50k–$200k | $200k–$800k+ | $30k–$100k retrofit |
| Tool cost per edge | Low–Moderate (regrindable) | Higher (indexable inserts) | Moderate |
| Setup time per job | Short | Longer (pressure head setup) | Moderate |
| Cost/hole (low volume) | Lowest | Highest | Moderate |
| Cost/hole (high volume) | Highest | Lowest | Moderate |
| Break-even batch | Baseline | >500–1000/mo | >200/mo |
Hydraulic cylinder barrel: 50mm × 2000mm (L/D=40:1), 4140 steel, batch 200/month.
| L/D Ratio | Classification | Typical Process |
|---|---|---|
| ≤ 5:1 | Standard hole | Conventional twist drill |
| 5:1–10:1 | Semi-deep | Peck drilling, through-coolant drill |
| 10:1–20:1 | Deep hole | Gundrilling or short BTA |
| 20:1–50:1 | Very deep | Gundrilling or BTA with steady rests |
| 50:1–100:1 | Ultra-deep | Specialized gundrilling or BTA |
| > 100:1 | Extreme | Gundrilling with whip guides |