A deep hole is only as straight as its start. The first few millimeters of engagement determine whether the drill will follow a straight path or wander off-axis. This is why pilot holes and starting geometry are critical — they provide the initial guidance that sets the trajectory for the entire hole.
For dedicated deep hole drilling machines, this guidance comes from a guide bushing. For CNC machining centers (which lack a bushing), the pilot hole itself must serve this function.
1. Why a Pilot Hole Is Needed
Without a pilot hole, a deep hole drill — especially an asymmetrical single-lip gun drill — will “walk” across the workpiece surface before engaging, producing a bell-mouth entry and an off-center hole. The self-piloting mechanism requires the guide pads to be in contact with the bore wall before full cutting forces are applied. A pilot hole provides this initial engagement.
When a Pilot Hole Is Required
| Condition | Recommendation |
|---|---|
| L/D > 5:1 (twist drill) | Pilot hole recommended |
| L/D > 12:1 (twist drill) | Pilot hole required (MSC Industrial Supply) |
| Any gun drilling (CNC machine) | Pilot hole required (no guide bushing) |
| Any gun drilling (dedicated machine) | Guide bushing recommended instead |
| BTA drilling | Pilot hole or starting bush required due to pressure head seal |
| Ejector drilling | Pilot hole or starting bush required |
For peck drilling strategies in deeper holes, see the Peck Drilling Optimization Guide.
2. Pilot Hole Specifications
Diameter
The pilot hole should be the same diameter as the deep hole drill, with a slight clearance:
| Source | Recommendation | Tolerance |
|---|---|---|
| ISCAR | 0.03–0.05 mm larger than the drill | — |
| Guhring | G8 tolerance grade, drill at Ø m7 | Matched fit |
| General practice | +0.013–0.025 mm clearance | +0.0005–0.001 in |
For gun drilling, the pilot hole clearance should be tight enough that the drill’s guide pads engage the wall immediately but loose enough to prevent binding.
Depth
The pilot hole must be deep enough to fully engage the guide pads of the deep hole drill before cutting begins.
| Final Hole Depth (L/D) | Recommended Pilot Depth |
|---|---|
| Up to 20×D | 2.5–3.0×D |
| Up to 30×D | 2.0–3.0×D |
| Up to 40×D | 2.5–4.0×D |
| General (CNC, no bushing) | Minimum 2×D |
| Deepest holes (> 40×D) | Intermediate pilot at depth may be required |
Source: Guhring deep hole drilling technical guide; Allied Machine deep hole drilling guidelines.
Minimum for any deep hole operation: 1.5–2×D. The drill’s guide pads must be fully supported before the tool advances past the pilot depth.
Bottom Shape
A conical bottom (from a standard twist drill) is preferred over a flat bottom (ScienceDirect, “The effects of pilot hole geometry on tool-work engagement efficacy”). The conical surface:
- Provides smoother engagement for the deep hole drill’s cutting edge
- Reduces impact loading on the drill point
- For Inconel 718, research defines an Engagement Ratio (ER) where ER > 0.6 is recommended to avoid edge chipping
3. Point Angle Selection
The pilot drill should have a larger or equal included point angle compared to the deep hole drill.
| Tool | Recommended Point Angle | Source |
|---|---|---|
| Pilot drill | 140° or larger | ISCAR, Seco Tools |
| Deep hole drill (standard) | 136° | Seco Tools |
| Deep hole drill (gun drill, typical) | 30°–15° or 25°–20° split | UNISIG, VDI 3208 |
Why the angle must differ: When the pilot drill has a larger point angle than the deep hole drill, the deep hole drill’s center contacts the workpiece first. This allows the drill to center itself and seat its cutting edges gradually before the full diameter engages. If the angles are equal or reversed, the corners of the deep hole drill engage first, causing chatter and deflection.
4. Entry Chamfer
An entry chamfer on the pilot hole provides additional guidance:
- Helps center the drill during initial contact
- Reduces the risk of edge chipping
- Particularly important when positional tolerance is tight (Guhring)
For flat or uneven workpiece surfaces, a 90° spot face or chamfer should be machined before drilling (Walter USA). This is especially critical for inclined entry surfaces or castings.
5. Step-by-Step Entry Procedure (CNC Machining Centers)
For indexable insert or carbide deep hole drills on CNC machines (Allied Machine deep hole drilling guidelines):
Step 1: Pilot Hole
Drill the pilot hole to 2×D minimum depth at 100% speed and feed with coolant ON.
Step 2: Feed In
Reduce to 50 RPM maximum at 300 mm/min (12 IPM) and feed the deep hole drill to within 1.5 mm of the pilot hole bottom. Some sources recommend feeding with the spindle off to prevent the cutting edge from grabbing the pilot hole wall.
Step 3: Transition Drilling
Drill 1×D past the pilot hole bottom at 50% speed and 75% feed with coolant ON. Allow a 1-second dwell at the transition point for the spindle to reach full speed.
Step 4: Full Drilling
Continue at 100% recommended speed and feed for the main drilling operation. Peck cycles are not recommended for carbide insert drills (see the peck drilling guide for carbide-specific considerations).
Step 5: Breakout (Through Holes)
| Parameter | Reduction |
|---|---|
| Speed | Reduce by 50% |
| Feed | Reduce by 25% |
Apply this reduction 1–2 mm before breakthrough. Do not exceed 3 mm past full diameter.
Step 6: Retract
Reduce to 50 RPM maximum before retracting the drill from the hole.
Source: Allied Machine deep hole drilling guidelines.
6. Guide Bushing (Dedicated Machines)
On dedicated deep hole drilling machines, a guide bushing provides the starting guidance instead of a pilot hole:
| Parameter | Specification |
|---|---|
| Tolerance | G8 for hole, m7 for tool (Guhring) |
| Clearance | +0.0005–0.001 in typical |
| Alignment | Critical — misalignment causes drill breakage at entry (Tungaloy) |
| Wear limit | Replace when clearance exceeds specification |
Bushing vs Pilot Hole
| Feature | Guide Bushing | Pilot Hole |
|---|---|---|
| Machine type | Dedicated DHD machine | CNC machining center |
| Setup time | Faster (no drilling) | Slower (needs pilot operation) |
| Accuracy | Higher (precision ground) | Dependent on pilot drill |
| Cost | More expensive | Lower |
| Wear | Replaceable bushing | Rework next part |
7. Common Starting Defects and Causes
| Defect | Appearance | Cause |
|---|---|---|
| Bell mouth | Entry diameter larger than nominal | Worn or misaligned bushing; no pilot hole |
| Drill walk | Hole offset from intended position | Insufficient pilot depth; incorrect point angle |
| Edge chipping | Small fractures on cutting edge | Pilot depth too shallow; ER < 0.6 for superalloys |
| Chatter at entry | Vibration marks at hole start | Pilot angle mismatch; speed too high at entry |
| Oversize entry | First few diameters over tolerance | Excessive clearance in bushing or pilot hole |
For troubleshooting these and other issues, see the Troubleshooting Guide.
8. Summary
| Requirement | Specification | Why |
|---|---|---|
| Pilot depth | 2×D minimum | Must fully engage guide pads |
| Pilot diameter | Same as drill + 0.03 mm | Tight enough to guide, loose enough to run |
| Pilot point angle | ≥ 140° | Must exceed deep hole drill angle for self-centering |
| Entry chamfer | 90° spot face | Guides drill, prevents chipping |
| Entry speed | 50 RPM | Prevents drill walking at engagement |
| Transition | 50% speed, 75% feed for 1×D past pilot | Gradual loading of cutting edges |
| Breakout | −50% speed, −25% feed | Prevents exit burr and grab |
Key Sources
- Guhring, “Deep Hole Drilling Technical Guide” — pilot hole depth and tolerance specifications
- Allied Machine, “Deep Hole Drilling Guidelines” — step-by-step entry procedure
- ISCAR Drilling Handbook — pilot diameter and point angle recommendations
- Seco Tools — point angle selection guidance
- ScienceDirect, “The effects of pilot hole geometry on tool-work engagement efficacy in deep hole drilling” — engagement ratio research
- Tungaloy Catalog (p. 1046) — bushing alignment troubleshooting
- Walter USA, deep hole drilling recommendations — pilot hole requirements
- MSC Industrial Supply — pilot hole depth requirements for twist drills