Appearance
The hole may be straight, round, and on size — but if the fixture allows the workpiece to shift by 0.1 mm under cutting forces, every hole is scrap before the first chip forms.
Overview
Workholding in deep hole drilling serves three distinct functions that do not exist in conventional machining:
- Tool guidance — guide bushings align the drill at the entry point and maintain its path
- Drill tube support — steady rests prevent the long, slender drill tube from whipping or buckling
- Workpiece restraint — clamping must resist high cutting torque without distorting the bore
Each function has specific design requirements that depend on the drilling method (gun drilling vs BTA), workpiece geometry, and production volume.
Guide Bushings
The guide bushing is the most critical workholding element in deep hole drilling. It provides the starting alignment for the drill and maintains tool position throughout the cut.
Function
The guide bushing:
- Aligns the drill with the spindle axis at the entry point
- Prevents drill wander during the first few diameters of cut
- Delivers coolant to the cutting zone (coolant-through bushings)
- Supports the drill against radial cutting forces
Alignment Tolerance
The concentricity between the guide bushing and the spindle must be maintained within 0.02 mm (0.0008 in) for both STS (stationary tool) and DTS (rotating tool) systems. This tolerance is the foundation of all deep hole drilling accuracy.
| System | Guide Bush Type | Alignment Criticality |
|---|---|---|
| STS (workpiece rotates) | Fixed guide bush | Spindle to bush: ≤ 0.02 mm |
| DTS (tool rotates) | Rotating guide bush | Bush to workpiece: ≤ 0.02 mm |
| Gun drilling | Guide bush holder | Tool to bush: ≤ 0.02 mm |
Guide Bush Tolerances
Guide bush bore tolerance is specified as G6 per ISO standards:
| Diameter Range (mm) | G6 Tolerance (mm) |
|---|---|
| 16 – 18 | +0.006 to +0.017 |
| 18 – 30 | +0.007 to +0.020 |
| 30 – 50 | +0.009 to +0.025 |
| 50 – 80 | +0.010 to +0.029 |
| 80 – 120 | +0.012 to +0.034 |
The clearance between the drill shank and the guide bush bore must be 0.003–0.008 mm for gun drilling and slightly larger for BTA drilling.
Guide Bush Materials
| Material | Hardness | Application | Life |
|---|---|---|---|
| Bearing steel (GCr15) | HRC 58–62 | General gun drilling | Good |
| High-speed steel (HSS) | HRC 62–64 | High-wear applications | Better |
| Tungsten carbide | HRA 87–89 | Production BTA, workpiece rotating | Best |
| Powder metal steel | HRC 60–64 | Compromise between steel and carbide | Very good |
For workpiece-rotating systems (STS), carbide guide bushes are recommended because the rotating workpiece creates higher sliding speeds at the bush interface.
Guide Bush Types
| Type | Features | Best For |
|---|---|---|
| Standard cylindrical | Simple, replaceable | General gun drilling |
| Coolant-through | Internal coolant passages | Gun drilling, BTA |
| Gun drill bushings | One or two-piece, precision ground | Deep hole drilling |
| Split bush | Opens for tool change | High production |
| Oil-groove | Internal lubrication channels | High-speed applications |
Steady Rests
Steady rests support the drill tube along its length to prevent whipping, buckling, and vibration.
Why Steady Rests Are Needed
A gun drill or BTA drill tube has a length-to-diameter ratio that can exceed 100:1. Without support, the tube acts as a slender column under compression (from feed force) and torsion (from cutting torque). At a critical buckling length, the tube whips, causing:
- Hole straightness deviation
- Oversized diameter at the whip node
- Premature guide pad wear
- Catastrophic tool failure
Steady Rest Spacing
| Drill Tube Diameter (mm) | Maximum Unsupported Length (m) | Recommended Spacing (m) |
|---|---|---|
| 10 – 20 | 0.5 – 1.0 | 0.3 – 0.5 |
| 20 – 40 | 1.0 – 2.0 | 0.5 – 1.0 |
| 40 – 65 | 2.0 – 3.0 | 1.0 – 1.5 |
| 65 – 100 | 3.0 – 4.5 | 1.5 – 2.5 |
Steady Rest Design
| Component | Function | Material |
|---|---|---|
| Base body | Mounts to machine bed | Cast iron or steel |
| Support pads | Contact the rotating drill tube | Polyurethane, bronze, or carbide |
| Adjustment mechanism | Positions pads to tube diameter | Screw or hydraulic |
| Coolant seal | Prevents coolant leakage at support point | Elastomeric seal |
The support pads should be adjustable to accommodate different drill tube diameters and to compensate for wear. Polyurethane pads are common for general use; bronze pads for high-speed applications; carbide pads for abrasive conditions.
Whip guides prevent drill tube damage
A whip guide (also called a support sleeve) is mounted directly on the drill shaft and travels with the tool into the hole. Made from polyurethane or rubber with a V-shaped or round center hole, it prevents contact between the rotating drill tube and the bore wall. Whip guides are sacrificial — they wear instead of the drill tube or workpiece bore — and should be inspected regularly and replaced when worn.
Pressure Head Design (BTA)
For BTA (STS) drilling, the pressure head seals against the workpiece and delivers high-pressure coolant.
Pressure Head Components
| Component | Function |
|---|---|
| Seal ring | Seals against workpiece face |
| Coolant inlet | Delivers coolant to annular gap |
| Guide bush holder | Holds the guide bush |
| Clamping mechanism | Engages workpiece with hydraulic or mechanical force |
Sealing Requirements
The pressure head must seal against the workpiece face at coolant pressures of 20–80 bar. Two sealing approaches:
| Seal Type | Pressure Range | Application | Limitations |
|---|---|---|---|
| Elastomeric face seal | 20 – 40 bar | Standard BTA | Wears, requires replacement |
| Metal-to-metal seal | 40 – 80 bar | High-pressure BTA | Requires clean surfaces |
| Hydraulic expanding seal | 30 – 60 bar | Large diameters | Complex, expensive |
Pressure Head Alignment
The pressure head axis must be aligned with the spindle within 0.03 mm to prevent the drill from entering the workpiece at an angle. Misalignment here causes oversize holes at entry and accelerated guide bush wear.
Workpiece Clamping
Challenges with Thin-Walled Tubes
Hydraulic cylinder tubes and similar deep hole drilling workpieces are often thin-walled and easily distorted by clamping forces.
| Wall Thickness Ratio | Clamping Risk | Recommended Approach |
|---|---|---|
| t/D > 0.15 | Low | Standard clamping |
| t/D = 0.10 – 0.15 | Moderate | Split clamps, reduced force |
| t/D = 0.05 – 0.10 | High | Internal support mandrel |
| t/D < 0.05 | Extreme | Specialized clamping only |
Clamping Methods
| Method | Application | Advantages | Disadvantages |
|---|---|---|---|
| Hydraulic V-blocks | Tube support along length | Distributes force evenly | Higher cost |
| Split clamping rings | End clamping | Minimal distortion | Limited to ends |
| Internal expanding mandrel | Thin-wall tubes | Supports from inside | Requires bore access |
| Three-jaw chuck | Standard workpiece | Versatile | Can distort thin walls |
| Hydraulic steady rests | Shaft support | Self-centering | Additional setup |
| Clamping cones | Tube end gripping | No bore distortion | Tube end prep needed |
Torque Reaction
Deep hole drilling generates significant cutting torque. The clamping system must resist this torque without allowing workpiece rotation:
| Parameter | Gun Drilling | BTA Drilling |
|---|---|---|
| Typical torque (50 mm Ø in steel) | 15 – 30 Nm | 80 – 200 Nm |
| Clamping force required | Moderate | High |
| Anti-rotation feature | Key or flat | Drive pin or spline |
Fixture Design Principles
1. Rigid Machine Interface
The fixture must be rigidly mounted to the machine bed or table. A flexible fixture amplifies vibration and degrades hole quality. Use:
- Heavy base plate (cast iron or welded steel)
- Minimum overhang of fixture components
- Direct bolting to T-slots or tapped holes
2. Coolant Management
All deep hole drilling fixtures must accommodate high-pressure coolant:
- Sealed coolant collection at the tool exit
- Chip trough or conveyor integration
- Coolant drainage at fixture low points
- Splash guards for operator safety
3. Quick Changeover
For production operations, fixture changeover time directly affects throughput:
| Production Volume | Fixture Type | Changeover Time |
|---|---|---|
| Low (1 – 10 parts) | Manual adjustable | 15 – 30 min |
| Medium (10 – 100 parts) | Quick-change vice | 5 – 15 min |
| High (> 100 parts) | Dedicated fixture | 1 – 5 min |
4. Chip Evacuation Path
The fixture must not obstruct chip flow:
- Clear path for chips at the tool exit
- No sharp edges that could catch chips
- Sloped surfaces to prevent chip accumulation
Summary
| Fixture Element | Gun Drilling | BTA Drilling | Ejector Drilling |
|---|---|---|---|
| Guide bush material | Bearing steel or HSS | Carbide preferred | Hardened steel |
| Guide bush tolerance | G6 | G6 | G6 |
| Alignment tolerance (mm) | ≤ 0.02 | ≤ 0.02 | ≤ 0.02 |
| Pressure head | Not required | Required | Not required |
| Steady rest spacing | 0.3 – 1.5 m | 0.5 – 2.5 m | 0.5 – 2.5 m |
| Clamping method | Chuck or V-block | Chuck + steady rest | Chuck or fixture |
| Coolant collection | Splash guard | Sealed system | Splash guard |
FAQ
Why is guide bush alignment critical in deep hole drilling?
The guide bush establishes the drill's entry path. If the bush is misaligned by more than 0.02 mm relative to the spindle axis, the drill enters the workpiece at an angle. This causes oversize holes at entry (the drill "walks" to find its own axis), accelerated guide bush wear on one side, and reduced straightness along the entire hole. Alignment must be checked after any machine maintenance or guide bush replacement.
What is the difference between a guide bush and a pressure head?
A guide bush aligns and supports the drill at the entry point. A pressure head seals against the workpiece to deliver high-pressure coolant into the annular gap around the drill tube. In BTA drilling, the pressure head contains the guide bush within its assembly. In gun drilling, only a guide bush is used — there is no pressure head because coolant flows through the drill's internal passage, not through an external annular gap.
How do I prevent thin-walled tubes from collapsing during clamping?
Use split clamping rings or V-blocks that distribute clamping force around the full circumference. For very thin walls (t/D < 0.10), use an internal expanding mandrel that supports the bore from inside while external clamps apply minimal force. Alternatively, clamping cones that grip only the tube ends avoid any force on the bore surface.
How often should guide bushes be replaced?
Guide bush replacement frequency depends on production volume and material. For steel drilling, a carbide guide bush typically lasts 5,000–20,000 holes before the bore wear exceeds tolerance. Steel bushes last 500–3,000 holes. Inspect the bush bore with a plug gauge every 500 holes (steel) or 2,000 holes (carbide). Replace when the gauge indicates wear beyond the specified clearance.
What is the proper spacing for steady rests?
Steady rests should be spaced at intervals no greater than 50–75× the drill tube diameter. For a 30 mm drill tube, steady rests every 1.5–2.3 m. The first steady rest should be within 300 mm of the guide bush. Additional rests are added as needed for longer drill tubes. Proper spacing prevents the drill tube from reaching its critical buckling length.
Can I use the same fixture for gun drilling and BTA drilling?
Rarely. Gun drilling fixtures are simpler (guide bush + basic clamping) because coolant pressure is delivered through the drill, not through the fixture. BTA fixtures must include a pressure head with a high-pressure coolant seal, which is fundamentally different. Combination machines may use interchangeable fixture platens to switch between methods, but the workpiece interface is different for each process.
Workholding design depends on machine configuration, workpiece geometry, material, and production volume. The values in this article are typical ranges. Consult fixture designers and machine builders for application-specific recommendations. This article reflects industry knowledge as of 2026.