Why Pilot Hole Geometry Matters
The pilot hole is the only thing guiding the gun drill during the first few millimeters of entry. If the pilot geometry is wrong, the drill starts off-center or at an angle, and the rest of the hole follows that bad start.
I have measured the effect of pilot geometry on hole straightness across dozens of jobs. A pilot hole that is 0.1mm oversized produces a starting hole that is 0.05-0.08mm oversized at entry and the error propagates as the drill goes deeper.
Pilot Diameter Selection
The pilot diameter must match the gun drill diameter within 0.05mm. This is the most critical dimension of the pilot hole.
| Condition | Effect on Hole | Result |
|---|---|---|
| Pilot 0.02-0.05mm smaller than drill | Good guidance | Straight hole |
| Pilot matches drill within 0.02mm | Ideal guidance | Best accuracy |
| Pilot 0.05-0.10mm larger than drill | Some wobble | Oversized entry, positional error |
| Pilot 0.10mm+ larger than drill | Significant wobble | Scrap part |
| Pilot 0.05mm+ smaller than drill | Edge chipping | Damaged drill |
I aim for the pilot to be 0.01-0.03mm smaller than the gun drill diameter. This gives a light interference fit that centers the drill tip without forcing the cutting edge to ream the pilot.
If I am using a reamer for the pilot hole, I select a reamer diameter that is 0.02mm under the gun drill diameter. The reamer produces a consistent, round hole with a good surface finish.
Pilot Depth Selection
The pilot depth depends on the gun drill diameter. I use the 3x diameter rule as a starting point and adjust based on the specific job.
| Gun Drill Diameter | Recommended Pilot Depth | Min Depth | Max Depth |
|---|---|---|---|
| Under 6mm | 12-18mm | 10mm | 20mm |
| 6-12mm | 18-36mm | 15mm | 40mm |
| 12-25mm | 36-75mm | 30mm | 80mm |
| 25-40mm | 50-100mm | 40mm | 120mm |
| Over 40mm | 75-150mm | 50mm | 180mm |
A pilot that is too shallow does not provide enough bearing surface to guide the drill straight. The drill tip wobbles in the first few millimeters of engagement and starts the hole oversized.
A pilot that is too deep wastes time and tool life. The pilot drill has to cut deeper than necessary, and the extra depth does not improve the gun drilling accuracy. The 3x rule gives enough guidance without wasting cycle time.
For blind holes, the pilot depth must leave enough material at the bottom of the pilot to support the gun drill tip. The minimum remaining material should be half of the gun drill diameter. For a 10mm drill with a 30mm pilot depth, the part needs at least 5mm of material below the pilot bottom.
Pilot Bottom Condition
The shape at the bottom of the pilot hole affects how the gun drill tip engages. The gun drill has a specific tip geometry with a center point and cutting edges. The pilot bottom should match that geometry.
| Pilot Bottom Shape | Effect on Gun Drill | Recommended |
|---|---|---|
| Flat bottom | Tip center engages first, edges follow | Good for most drills |
| 120-degree conical | Matches standard drill point | Best for standard gun drills |
| Ragged or stepped | Tip deflects on engagement | Avoid |
| Tapered wider at bottom | No support at depth | Avoid |
I program the pilot drill to leave a flat or slightly conical bottom. If the pilot drill has a standard 118-degree point, the bottom will be conical. That works fine for most gun drill geometries.
I avoid pilot drills with split points or chip-splitter geometries for the pilot hole. Those geometries leave an uneven bottom that does not support the gun drill tip evenly.
Concentricity Requirements
The pilot hole must be concentric with the spindle axis. A pilot that is off-center by 0.05mm will cause the gun drill to start drifting in the direction of the offset.
The best way to ensure concentricity is to drill the pilot in the same setup as the gun drilling operation. I do not remove the part between the pilot drilling and the gun drilling. If the part moves, the concentricity is lost.
| Pilot Off-center | Hole Error at 100mm Depth | Hole Error at 500mm Depth |
|---|---|---|
| 0.02mm | 0.03mm | 0.08mm |
| 0.05mm | 0.08mm | 0.20mm |
| 0.10mm | 0.15mm | 0.40mm |
| 0.20mm | 0.30mm | 0.80mm+ |
I indicate the pilot drill position before drilling the pilot. I use a dial indicator on the drill body and adjust the tool holder until the runout is under 0.02mm.
Pilot Drill Selection
I use short, rigid carbide drills for pilot holes. The pilot drill should be as short as possible to minimize deflection. A stub-length carbide drill is ideal.
| Pilot Drill Type | Cost | Best For | Avoid When |
|---|---|---|---|
| Carbide stub drill | $25-40 | General purpose, under 20mm | Deep pilots over 50mm |
| Carbide jobber drill | $15-25 | Deep pilots | Large diameters, over 20mm |
| Solid carbide drill | $30-60 | Precision work, tight tolerances | Low-volume jobs |
| Indexable drill | $50-100 | Large diameters over 20mm | Small diameters under 10mm |
The pilot drill diameter should match the target pilot diameter as closely as possible. I avoid drilling an undersized pilot and reaming it to size unless the concentricity tolerance is under 0.02mm.
Key Takeaways
Pilot hole geometry is the foundation of accurate gun drilling. I match the pilot diameter within 0.05mm of the gun drill, use a depth of 3x the drill diameter, and drill the pilot in the same setup as the gun drilling. The pilot bottom should be flat or conical to support the gun drill tip. A good pilot hole produces a straight hole. A bad pilot hole produces scrap.