Appearance
In deep hole drilling, the fixture is not just holding the part — it is part of the cutting system. The guide bushing aligns the drill, the steady rest supports the tube, and the sealing system contains the high-pressure coolant. A fixture that fails at any of these functions will produce scrap, break tools, or create a safety hazard. Designing the fixture and the drilling process as an integrated system is the difference between reliable production and constant troubleshooting.
The Role of Fixtures in Deep Hole Drilling
Functions of a Deep Hole Drilling Fixture
| Function | Purpose | Failure Consequence |
|---|---|---|
| Drill guidance | Support the drill at entry point through a guide bushing | Hole position error, drill wandering |
| Coolant sealing | Contain high-pressure coolant around the drill rod | Pressure loss, chip evacuation failure |
| Drill tube support | Support the long drill tube along its length | Vibration, chatter, bell-mouth holes |
| Chip containment | Direct chips and coolant away from the cutting zone | Chip packing, operator hazard |
| Workpiece location | Position the bore axis concentric to the spindle | Oversize or non-concentric holes |
| Workpiece clamping | Hold the part rigidly against cutting forces | Part movement, tool breakage |
Key Differences from Conventional Fixtures
| Aspect | Standard Fixture | Deep Hole Drilling Fixture |
|---|---|---|
| Primary loads | Cutting forces (moderate) | Cutting forces + coolant pressure (high) |
| Chip management | Chip fall or air blast | High-pressure coolant + sealed chip flow |
| Tool guidance | Jig bushings optional | Precision guide bushing mandatory |
| Part support | Clamping points | Steady rests along workpiece length |
| Coolant handling | Not a design factor | Sealing is critical |
| Setup alignment | Edge finder or probe | Bushing-to-spindle concentricity ≤ 0.01 mm |
Guide Bushings and Holders
Guide Bushing Design
The guide bushing is the most critical element of a deep hole drilling fixture. It supports the drill at the entry point, provides the coolant seal, and establishes the hole position.
| Parameter | Gun Drilling | BTA Drilling |
|---|---|---|
| Bushing length | 2–4× drill diameter | 1.5–3× drill diameter |
| Clearance (ID minus drill OD) | 0.005–0.015 mm | 0.020–0.050 mm |
| ID concentricity to mounting bore | ≤ 0.005 mm TIR | ≤ 0.010 mm TIR |
| Entry chamfer | 0.5–1.0 mm × 30° | 1.0–2.0 mm × 45° |
| Material | Carbide (K10–K20) or hardened steel | Carbide or hardened steel |
Bushing Holder Design
The bushing holder must position the bushing rigidly relative to the spindle centreline and allow for alignment adjustment.
| Feature | Requirement |
|---|---|
| Mounting interface | Bolted to machine base or spindle housing |
| Adjustment | X-Y adjustment screws with ±2 mm range |
| Locking | Locking once aligned (screw + dowel) |
| Coolant connection | O-ring sealed port matching bushing coolant hole |
| Material | Steel or cast iron, stress-relieved |
Bushing Cooling and Lubrication
Bushings generate heat from friction with the rotating drill rod. For production drilling, bushing cooling should be considered:
| Cooling Method | Application | Effectiveness |
|---|---|---|
| Coolant flow through bushing | Standard — coolant passes through bushing before entering hole | Good |
| External lubricant drip | Low-speed or intermittent drilling | Moderate |
| Flood coolant | BTA — coolant flows around the drill tube | Good |
TIP
In high-production gun drilling (≥ 1,000 holes per day), a carbide bushing with dedicated coolant passages through the bushing wall significantly extends bushing life. The coolant flow removes frictional heat and flushes abrasive particles that would otherwise accelerate ID wear.
Steady Rests and Drill Tube Supports
Purpose
Steady rests (also called whip guides or support sleeves) support the long drill tube between the bushing and the machine spindle. Without them, the drill tube vibrates under cutting forces, causing chatter marks, oversize holes, and accelerated guide pad wear.
Steady Rest Types
| Type | Description | Best For |
|---|---|---|
| Fixed steady rest | Rigid support arm with adjustable pads | Straight, rigid workpieces |
| Travelling steady rest | Moves with the drill head | Long, flexible workpieces |
| Roller steady rest | Roller-contact support | High-speed drilling, low friction |
| Polyurethane whip guide | Flexible rubber/polyurethane sleeve | Vibration damping, coolant sealing |
| V-block support | V-shaped support for the drill tube | Quick setup, general purpose |
Steady Rest Spacing
The spacing between steady rests depends on the drill tube diameter and stiffness:
| Drill Tube Diameter | Maximum Spacing Between Supports |
|---|---|
| 10–20 mm | 300–500 mm |
| 20–40 mm | 500–800 mm |
| 40–60 mm | 800–1,200 mm |
| 60–80 mm | 1,200–1,800 mm |
A general rule: the unsupported length of the drill tube should not exceed 40–60× the tube diameter.
Whip Guide Design
Whip guides (polyurethane or rubber sleeves mounted on the drill tube) serve a dual function:
| Function | Benefit |
|---|---|
| Vibration damping | Reduces chatter at high spindle speeds |
| Coolant sealing | Prevents high-pressure coolant from escaping along the tube |
| Chip deflection | Directs returning chips away from the guide pad area |
| Tube support | Prevents the tube from whipping at critical speeds |
Whip guides should be positioned at the midpoint of the unsupported drill tube length. In long-tube applications, multiple whip guides may be used.
Clamping Systems
Clamping Principles for Deep Hole Drilling
| Principle | Application |
|---|---|
| Clamp near the cutting zone | Minimises workpiece deflection under cutting forces |
| Avoid clamping on unsupported thin walls | Prevents distortion and bore taper |
| Distribute clamping force evenly | Multiple low-force clamps better than one high-force clamp |
| Allow for thermal expansion | Drilling generates heat that expands the workpiece |
Round Workpiece Clamping
| Method | Best For | Notes |
|---|---|---|
| Self-centring chuck | Shafts, bars, tubes | 3-jaw or 4-jaw; 4-jaw preferred for concentricity |
| Clamping cone | Cylindrical parts, flange ends | Centres and clamps in one action; protects surface finish |
| Collet chuck | Small-diameter shafts (< 50 mm) | Excellent concentricity; limited diameter range |
| V-block + clamp | Irregular round parts | Quick setup; less concentric than chuck |
Prismatic Workpiece Clamping
| Method | Best For | Notes |
|---|---|---|
| Hydraulic vise | Blocks, rectangular parts | High clamping force; consistent |
| T-slot clamps | Large plates, mould bases | Flexible positioning; manual |
| Hydraulic/pneumatic cylinders | Production runs | Fast clamping; consistent force |
| Vacuum chuck | Thin plates | Evenly distributed low force |
Sealing Systems
Coolant Containment
Deep hole drilling operates at coolant pressures of 30–250 bar. Sealing the coolant path is critical for both process performance and operator safety.
| Seal Location | Purpose | Seal Type | Typical Pressure |
|---|---|---|---|
| Bushing-to-drill | Prevent coolant escape at entry | Close clearance + O-ring in bushing holder | Up to 250 bar |
| Bushing holder-to-machine | Seal coolant supply passage | O-ring or gasket | Up to 250 bar |
| Chip box | Contain chips and coolant at exit | Hinged cover with rubber seal | Up to 5 bar (non-pressurised) |
| Drill tube connection | Seal between tube sections | Threaded + O-ring | Up to 250 bar |
Chip Box Design
The chip box collects chips and coolant at the drill exit. For BTA drilling (where chips exit through the drill tube), the chip box must handle high-velocity chip and coolant flow.
| Feature | Requirement |
|---|---|
| Material | Steel or stainless steel |
| Seal | Rubber gasket on hinged or removable cover |
| Drain | Large-drain port (≥ 2× tube diameter) |
| View window | Polycarbonate window for visual monitoring |
| Safety interlock | Machine stop if chip box is open |
Fixture Design Examples
Example 1: Shaft with Centre Hole
| Parameter | Design |
|---|---|
| Workpiece | Ø50 mm × 500 mm shaft, gun drilled through |
| Clamping | Self-centring chuck at drive end + tailstock centre |
| Guide bushing | Carbide bushing at entry, L = 100 mm (2× Ø50) |
| Steady rest | Fixed steady rest at midpoint (250 mm from entry) |
| Sealing | O-ring seal at bushing holder; chip box at exit |
| Result | Straightness 0.05 mm over 500 mm; setup time 15 min |
Example 2: Valve Body with Cross Holes
| Parameter | Design |
|---|---|
| Workpiece | 100 × 100 × 150 mm valve body, drilled from both sides |
| Clamping | Hydraulic vise on two prismatic inserts; 4 kN clamping force |
| Guide bushing | Steel bushing in replaceable bushing plate, L = 30 mm |
| Steady rest | Not required (short part) |
| Sealing | Coolant seal at bushing holder; open chip tray |
| Result | ±0.02 mm hole intersection; setup time 10 min |
Example 3: Thin-Walled Tube
| Parameter | Design |
|---|---|
| Workpiece | Ø80 mm × 1.5 mm wall × 600 mm tube, BTA drilled |
| Clamping | Internal expanding mandrel + external finger chuck |
| Guide bushing | Carbide bushing with reduced clearance (0.010 mm) |
| Steady rest | Three travelling steady rests on OD |
| Sealing | Full chip box enclosure; coolant pressure limited to 40 bar |
| Result | No distortion; wall thickness variation < 0.05 mm |
Fixture Design Checklist
| Item | Check |
|---|---|
| Bushing-to-spindle concentricity ≤ 0.01 mm | ☐ |
| Bushing length 2–4× drill diameter | ☐ |
| Bushing clearance appropriate for material and diameter | ☐ |
| Coolant supply sealed at all connections | ☐ |
| Chip box or chip deflector in place | ☐ |
| Steady rests positioned at correct spacing | ☐ |
| Whip guides installed on long drill tubes | ☐ |
| Clamping force sufficient for cutting torque | ☐ |
| Workpiece surface not damaged by clamping | ☐ |
| Coolant drain or return path clear | ☐ |
| Safety interlock functional (if chip box used) | ☐ |
| Spindle centreline height matches bushing centreline | ☐ |
FAQ
Q: What is the most important part of a deep hole drilling fixture? The guide bushing assembly. The bushing establishes hole position, supports the drill at entry, and seals the high-pressure coolant path. A worn or misaligned bushing causes more quality problems than any other fixture element.
Q: How is a guide bushing aligned to the spindle centreline? Using a test bar mounted in the spindle and a dial indicator measuring the bushing ID. Alignment to within 0.01 mm TIR is standard. Most production machines have X-Y adjustable bushing holders for this purpose.
Q: What is the purpose of a whip guide? A whip guide (support sleeve) dampens vibration in the long drill tube, prevents whipping at critical speeds, seals coolant along the tube, and deflects returning chips. It is typically made of polyurethane or rubber.
Q: How many steady rests are needed for deep hole drilling? For drill tubes up to 20 mm diameter, place steady rests at 300–500 mm intervals. For larger tubes, spacing can increase. The unsupported length should not exceed 40–60× the tube diameter.
Q: What material is used for guide bushings in production? Carbide (ISO K10–K20) for production runs. Hardened steel (HRC 58–62) for prototype or short runs. Bronze is used only when the drill rod must be protected from scoring.
Q: How is coolant sealed at the bushing? Through a close-clearance fit between the bushing ID and drill OD (0.005–0.015 mm) combined with an O-ring seal in the bushing holder assembly. In high-pressure applications, a separate seal ring is used.
Q: Can a standard vice be used for deep hole drilling? For short, rigid parts, a hydraulic or mechanical vise can work if properly aligned. For long parts or parts requiring high precision, a purpose-built fixture with guide bushing and steady rests is necessary.
Q: What is the chip box and why is it needed? The chip box encloses the drill exit point to contain high-velocity chips and coolant. It is mandatory for BTA drilling and recommended for deep gun drilling.
Q: How does fixture design affect hole straightness? Fixture alignment is the primary determinant of entry straightness. If the bushing centreline does not match the spindle centreline within 0.01 mm, the drill will enter at an angle, causing a curved hole.
Q: What is the typical setup time for a deep hole drilling fixture? 15–60 minutes for the initial setup including bushing alignment. Repeat setups on the same part with the same fixture typically take 5–15 minutes with quick-change bushing systems.