Appearance
A gundrill cutting at 6,000 RPM, advancing at 0.02 mm per revolution, covers the distance from the guide bushing to the bottom of the pilot hole in about 0.1 seconds. In that tenth of a second, the cutting edges engage the workpiece material for the first time. If the pilot hole is misaligned by 0.05 mm, the runout at the tool tip exceeds 0.03 mm, or the guide bushing bore is 0.01 mm oversize, the bore deviation that originates in that moment will propagate for the entire depth of the hole. The first 2 mm of engagement determines the straightness of the next 2,000 mm.
Why Centering and Proofing Matter
Deep hole drilling tools — whether single-flute gun drills or BTA drill heads — cannot self-start. The asymmetric cutting geometry of a gun drill (with the cutting edge offset at approximately 25% of the diameter) and the multi-tooth layout of a BTA head both require initial guidance from either a pilot hole or a guide bushing.
| Tool Type | Self-Starting Capability | Required Guidance |
|---|---|---|
| Conventional twist drill | Yes (with split point) | None for short holes |
| Single-flute gun drill | No | Pilot hole or guide bushing |
| BTA drill head | No | Guide bushing (or pilot bore) |
| Ejector drill head | No | Guide bushing (or pilot bore) |
The centering and proofing procedure establishes three conditions before the cutting edges engage:
- The workpiece entry surface is square to the tool axis
- The tool is guided accurately to the entry point
- The initial chip formation is balanced and controlled
Pilot Hole Preparation
When a Pilot Hole Is Required
| Setup | Pilot Hole Required? | Reason |
|---|---|---|
| Dedicated gun drilling machine with guide bushing | No | Bushing provides initial guidance |
| CNC machining centre with gun drill | Yes | No built-in guide bushing |
| CNC lathe with gun drill through tailstock | Yes | No rotating bushing at the workpiece |
| BTA drilling on dedicated machine | No | Bushing integrated in the chip head |
| Deep hole drilling with twist drill | Recommended | Reduces wander at entry |
Pilot Hole Geometry
| Parameter | Recommendation | Source |
|---|---|---|
| Depth | 1–2× drill diameter (minimum engagement for guide pads) | Industry standard |
| Diameter tolerance | +0.0008" / -0.0000" relative to nominal drill diameter | Sandvik |
| Diameter tolerance (metric) | m7 | Brecht |
| Bottom form | Flat-bottom preferred | DME Tool |
| Point angle (if not flat) | Must match the outside angle of the gun drill | Star SU |
| Surface finish | Ra ≤ 1.6 μm | Industry practice |
The pilot hole must be deep enough to fully engage the guide pads on the gun drill tip, not just the cutting edge. For a gun drill with a 10 mm diameter and a typical tip length of 6–8 mm, the pilot hole should be at least 12–15 mm deep.
Pilot Drill Selection
| Tool Type | Recommended For | Key Consideration |
|---|---|---|
| Stub-length carbide drill | Most materials | Highest rigidity, minimal wander |
| Carbide spot drill + reamer | Tight-tolerance pilot holes | Best diameter control |
| Solid carbide drill with 140° point angle | Hard or sloped entry surfaces | Reduces cutting force imbalance at entry |
Warning: A standard center drill (combined drill and countersink with 60° or 90° included angle) should NOT be used as a pilot for gun drilling. The point angle mismatch between the center drill and the gun drill's outside angle creates unbalanced cutting forces at the transition, causing the gun drill to deflect on entry. Use a flat-bottom pilot or a pilot with the same angle as the gun drill.
Pilot Hole Positioning Tolerance
| Application | Position Tolerance (from true position) |
|---|---|
| General deep hole drilling | ±0.05 mm |
| Precision deep hole drilling | ±0.02 mm |
| Micro-deep hole drilling (< 3 mm) | ±0.01 mm |
Guide Bushing Setup
Bushing Types
| Type | Standard | Application |
|---|---|---|
| Hardened steel bushing | DIN 179A (medium) | Dedicated gun drilling machines |
| Tungsten carbide bushing | Custom | High-production, abrasive materials |
| Split bushing (adjustable) | Custom | Machines with adjustable spindle alignment |
| Rotary bushing (ball-bearing mounted) | Custom | Lathe tailstock applications |
Bushing Specifications
| Parameter | Specification |
|---|---|
| Bore tolerance | h6 or g6 relative to drill diameter |
| Hardness | 58–62 HRC (steel), 90+ HRA (carbide) |
| Length | 2–3× drill diameter |
| Clearance on drill diameter | 0.005–0.015 mm (steel bushings) |
| Clearance on drill diameter | 0.010–0.020 mm (carbide bushings) |
Bushing Alignment Procedure
The alignment between the guide bushing and the spindle axis is the single most critical geometric relationship in deep hole drilling:
- Mount a test bar in the spindle (ground to within 0.002 mm T.I.R.)
- Mount a dial indicator on the bushing housing or machine table
- Sweep the test bar at the bushing location — maximum permissible runout: 0.01 mm
- Adjust the bushing housing using shims or adjustment screws
- Recheck after tightening — bolting can shift alignment by 0.02–0.05 mm
- Align intermediate supports in sequence, using the test bar as reference
Runout Measurement and Correction
Runout at the tool tip is amplified by the length of the drill shaft. A small angular error at the spindle or bushing becomes a large positional error at the cutting edge.
| Runout Source | Typical Magnitude | Effect on Bore at 500 mm Depth |
|---|---|---|
| Spindle bore runout | 0.005–0.015 mm | 0.05–0.15 mm |
| Toolholder runout | 0.010–0.030 mm | 0.10–0.30 mm |
| Guide bushing misalignment | 0.010–0.050 mm | 0.10–0.50 mm |
| Intermediate support misalignment | 0.020–0.100 mm | 0.20–1.00 mm |
Permissible Runout Limits
| Measurement Point | Maximum Runout (T.I.R.) |
|---|---|
| Spindle taper bore | 0.005 mm |
| Toolholder at gage diameter | 0.010 mm |
| At guide bushing | 0.010 mm |
| At tool tip (with new drill) | 0.030 mm |
| At tool tip (rejected) | 0.050 mm |
Correcting Runout
| Cause | Correction |
|---|---|
| Dirty spindle taper | Clean with solvent and lint-free cloth |
| Damaged spindle taper | Regrind or replace spindle |
| Worn toolholder | Replace — toolholders are wear items |
| Debris between holder and spindle faces | Clean and inspect for burrs |
| Misaligned bushing | Shim or adjust bushing housing |
| Bent drill shaft | Replace — straightening is not reliable |
Centering on CNC Lathes
On a CNC lathe, the workpiece rotates and the tool is stationary or fed from the tailstock. The centering procedure is different from machining centre practice.
| Step | Action | Quality Check |
|---|---|---|
| 1 | Indicate workpiece true-running in chuck | Runout < 0.02 mm at the gripping diameter |
| 2 | Face the workpiece end square to the spindle axis | Square within 0.01 mm over the face diameter |
| 3 | Check tailstock alignment relative to spindle axis | Alignment within 0.05 mm over the working distance |
| 4 | Centre-drill or spot-face the entry point | Position within 0.05 mm of true centre |
| 5 | Drill pilot hole with stub drill | Depth 1.5× drill diameter, correct tolerance |
| 6 | Verify pilot hole position with test indicator | Concentric within 0.03 mm of spindle axis |
Tip: When re-centering a workpiece on a lathe (for example, when a previous centre hole is worn or untrue), never use a center drill or multi-edged tool for the re-centering operation. These tools will follow the existing offset centre. Use a turning tool to cut a new centre, then verify with a test indicator.
Centering on Machining Centres
| Step | Action | Quality Check |
|---|---|---|
| 1 | Indicate workpiece on the machine table or in a vise | Position within 0.02 mm |
| 2 | Spot-face the entry surface (if not already flat) | Surface square to spindle within 0.01 mm |
| 3 | Drill pilot hole with stub-length carbide drill | Depth 1.5× drill diameter |
| 4 | Ream pilot hole (if tight tolerance required) | Diameter within m7 tolerance |
| 5 | Verify pilot hole position with edge finder or probe | Position within 0.03 mm of programmed location |
| 6 | Install gun drill in hydraulic or shrink-fit holder | Runout at drill tip < 0.03 mm |
Toolholding for Pilot Hole Drilling
| Holder Type | Runout | Recommended For |
|---|---|---|
| Hydraulic chuck | < 0.003 mm | Precision pilot holes |
| Shrink-fit holder | < 0.005 mm | High-speed pilot drilling |
| Milling chuck (ER collet) | < 0.010 mm | General pilot drilling |
| End mill holder (screw-driven) | 0.010–0.030 mm | Not recommended for deep hole drilling pilots |
The Entry Procedure
The entry sequence is the critical moment in deep hole drilling. The following procedure applies to gun drilling on a machining centre or lathe:
Standard Entry Sequence
| Phase | Action | Duration |
|---|---|---|
| 1 | Insert gun drill through guide bushing (if used) | — |
| 2 | Advance drill at low RPM (< 50 RPM or stationary) to the pilot hole | Rapid traverse |
| 3 | Stop approximately 2–3 mm before the pilot hole bottom | — |
| 4 | Turn on coolant to full pressure | 1–2 seconds |
| 5 | Ramp spindle to full RPM | 1–3 seconds |
| 6 | Engage feed at the programmed rate | Continuous |
| 7 | Tool enters the pilot hole bottom — full engagement | 0.1–0.3 seconds |
| 8 | Continue drilling without pecking to full depth | As required |
Coolant Coordination
| Coolant Type | Activation Point | Rationale |
|---|---|---|
| High-pressure liquid | Before spindle ramp-up (Step 4) | Ensures chip evacuation from the first revolution |
| Spray mist | Before spindle ramp-up | Lubricates the entry point |
| Through-tool coolant | Before spindle ramp-up | Prevents chip welding at the cutting edge |
Entry Feed Rate Adjustments
| Surface Condition | Feed Rate During Entry | Duration |
|---|---|---|
| Flat, square entry | 100% of programmed feed | — |
| Slightly sloped entry | 50–70% of programmed feed | First 1–2 mm |
| Rough surface (cast, forged) | 30–50% of programmed feed | First 2–3 mm |
| Interrupted cut (cross-hole at entry) | 25% of programmed feed | Until full engagement |
The Six Critical Control Factors for Bore Straightness
Taguchi methods applied to deep hole drilling research have identified six primary factors that determine bore straightness:
| Factor | Effect on Straightness | Control Strategy |
|---|---|---|
| 1. Tool diameter | Larger diameter = stiffer tool = straighter bore | Select largest practical diameter |
| 2. Feed rate | Higher feed increases radial cutting forces | Optimise within recommended range |
| 3. Shaft length | Longer shaft = more deflection | Minimise overhang, use intermediate supports |
| 4. Spindle-to-bushing distance | Longer distance amplifies misalignment | Minimise distance, align precisely |
| 5. Pilot bushing misalignment | Directly causes initial bore deviation | Align within 0.01 mm |
| 6. Intermediate support misalignment | Propagates bore deviation along depth | Align supports in sequence with test bar |
Proofing Procedure (Pre-Production Verification)
Before drilling production parts, a proofing cycle verifies that the centering and setup are correct:
| Proofing Step | Method | Acceptance Criterion |
|---|---|---|
| Check runout at tool | Dial indicator at drill tip, rotate spindle by hand | < 0.03 mm T.I.R. |
| Check coolant flow | Verify flow through tool before engagement | Steady, no interruption |
| Cut test hole | Drill a test workpiece of the same material | Measure bore straightness and position |
| Measure test hole straightness | Dial indicator or air gauge | Within specification for the application |
| Inspect tool condition | 10× magnification of cutting edges | No edge damage, no built-up edge |
| Verify chip form | Collect chips from test hole | C-shaped or short spiral — not powder or string |
Troubleshooting Centering and Proofing Problems
| Symptom | Likely Cause | Corrective Action |
|---|---|---|
| Bore deviates in a consistent direction | Guide bushing misaligned — tool pulled toward one side | Realign bushing, verify with test bar |
| Oversize bore at entry, correct at depth | Pilot hole too large for the drill diameter | Reduce pilot hole diameter tolerance |
| Tool breaks on entry | Pilot hole too shallow — guide pads not fully engaged | Increase pilot hole depth to 1.5× drill diameter |
| Chatter at the start of the hole | Pilot hole point angle does not match drill | Machine pilot hole flat-bottom or match drill angle |
| Bore position is offset from true position | Pilot hole drilled at incorrect position | Verify pilot hole location before drilling |
| Gradual spiral deviation | Intermediate support misalignment | Align supports sequentially with test bar |
| Poor surface finish at entry | Coolant not engaged before feed start | Verify coolant activation sequence |
| Tool squeals on entry | Insufficient runout control — cutting edges load unevenly | Check runout at tool tip, replace holder if worn |
FAQ
Why can't a gun drill self-start without a pilot hole or guide bushing?
A single-flute gun drill has an asymmetric cutting geometry with the cutting edge offset at approximately 25% of the diameter from the tool centreline. This offset means the tool cannot centre itself in the material — it will walk off-centre unless guided by a bushing or a pre-drilled pilot hole that engages the drill's guide pads.
How deep should a pilot hole be for gun drilling?
The pilot hole should be 1–2 times the drill diameter in depth, with 1.5× being the recommended minimum. The critical requirement is that the guide pads on the gun drill tip must be fully engaged before the cutting edge contacts the pilot hole bottom. For a typical gun drill, this requires approximately 6–15 mm of pilot depth depending on diameter.
What is the correct pilot hole tolerance for gun drilling?
The pilot hole diameter should be +0.0008" / -0.0000" relative to the nominal gun drill diameter (approximately m7 tolerance in metric). Too tight and the drill binds on entry; too loose and the drill has no initial guidance and will wander.
Should a centre drill be used for pilot hole preparation?
No. Standard centre drills have a 60° or 90° point angle that does not match the gun drill's geometry. The mismatch creates unbalanced cutting forces at the transition from pilot to full drilling, causing the gun drill to deflect. Use a flat-bottom pilot or a pilot with the same outside angle as the gun drill.
What is the maximum permissible runout at the tool tip?
The maximum permissible runout at the tool tip is 0.03 mm T.I.R. for general deep hole drilling. For precision applications, 0.015 mm T.I.R. is recommended. Runout above 0.05 mm T.I.R. will cause measurable bore deviation and accelerated tool wear.
How is guide bushing alignment verified?
Insert a precision test bar (ground to within 0.002 mm T.I.R.) through the bushing into the spindle, or into a holder in the spindle. Mount a dial indicator on the bushing housing and sweep the test bar. The maximum permissible runout at the bushing location is 0.01 mm.
What feed rate should be used during the entry phase?
For a flat, square entry surface, use 100% of the programmed feed rate. For sloped or rough surfaces, reduce feed to 50–70% for the first 1–2 mm of engagement. For interrupted cuts (cross-holes at the entry surface), reduce to 25% until full engagement.
Why should coolant be turned on before the spindle ramps up?
Turning on coolant before spindle rotation ensures that the chip evacuation path is pressurised and flowing before any chips are generated. If the spindle reaches full speed before coolant flow is established, the first chips can weld to the cutting edge, causing built-up edge and immediate bore deviation.
What are the six critical factors for bore straightness?
The six factors identified by Taguchi methods are: tool diameter, feed rate, shaft length, spindle-to-bushing distance, pilot bushing misalignment, and intermediate support misalignment. Of these, bushing misalignment and support misalignment are the factors most frequently controlled incorrectly in production.
How is a proofing cycle conducted before production drilling?
A proof cycle consists of: checking runout at the tool tip (< 0.03 mm T.I.R.), verifying coolant flow, drilling a test hole in a workpiece of the same material, measuring the test hole straightness and position, inspecting the tool condition under 10× magnification, and verifying chip form.
Conclusion
Workpiece centering and proofing is not a preliminary step that can be rushed through on the way to production drilling. The geometric errors established at this stage — pilot hole position, bushing alignment, runout, and entry technique — are not corrected as the drill advances; they are amplified. The three critical measurements are runout at the tool tip (maximum 0.03 mm T.I.R.), bushing-to-spindle alignment (maximum 0.01 mm at the bushing location), and pilot hole depth (minimum 1.5× the drill diameter with the correct diameter tolerance). And the single most important procedural rule: coolant must flow before the spindle turns, and the spindle must turn before the tool feeds. Violate that sequence, and the hole is compromised before it has begun.