The fixture is the interface between the machine and the workpiece. In deep hole drilling, the fixture must resist high axial forces, maintain alignment over the full part length, and provide consistent location for every part. A poorly designed or misaligned fixture guarantees poor holes — regardless of machine quality, tool condition, or parameters.
Table Types
Machine Table Configurations
| Table Type | Workpiece Mounting | Flexibility | Rigidity | Best For |
|---|
| T-slot table | T-nuts and bolts | High | High | General purpose, varied parts |
| Modular fixturing system | Dowel pins and clamps | Very high | Moderate | Prototype, short runs |
| Grid plate (tooling plate) | Dowel holes + threaded inserts | High | High | Medium production, repeatable setups |
| Sub-plate (dedicated) | Custom pattern | Low (dedicated) | Very high | High production, one part type |
| Hydraulic clamping plate | Hydraulic clamps | Moderate | High | Long runs, automated clamping |
Table Selection Criteria
| Criteria | Consideration | Recommendation |
|---|
| Part length variability | Frequent changes in part length | Modular or T-slot table |
| Production volume | > 1,000 parts per year | Dedicated sub-plate |
| Changeover time target | < 15 minutes | Hydraulic or quick-change system |
| Part weight | > 500 kg | T-slot or grid plate with heavy-duty clamps |
| Accuracy required | IT6–IT7 holes | Grid plate with precision alignment features |
Fixture Design Principles
Key Design Requirements
| Requirement | Why It Matters | Design Guideline |
|---|
| Axial force resistance | Deep hole drilling generates 500–5,000 N axial force | Clamps must resist axial force without part movement |
| Torque resistance | Spindle torque tries to rotate the part | Anti-rotation feature or multiple clamp points |
| Alignment repeatability | Every part must be in the same position relative to spindle | Positive stops or location pins |
| Chip clearance | Chips must not pack around fixture | Open fixture design, angled surfaces |
| Coolant drainage | Coolant must not pool in fixture | Drain holes, sloped surfaces |
| Bushing support | Bushing must be rigidly supported near the part | Bushing plate integrated into fixture |
Fixture Body Construction
| Material | Stiffness | Weight | Vibration Damping | Cost | Best For |
|---|
| Steel (welded or cast) | Excellent | Heavy | Good | Medium | Large parts, high force |
| Aluminum (plate) | Good | Light | Moderate | Low to medium | Small parts, low force |
| Cast iron | Excellent | Heavy | Excellent | Medium | Precision, production |
| Composite (carbon fiber) | Very good | Very light | Excellent | High | Special applications |
Clamping Methods
Clamping by Workpiece Shape
| Workpiece Shape | Recommended Clamping Method | Number of Clamps | Clamping Force |
|---|
| Round bar (horizontal) | V-blocks + hold-down clamps | 2–3 along length | Moderate — enough to prevent rotation |
| Round bar (vertical) | Three-jaw chuck or collet + tailstock | 1 (chuck) + 1 (tailstock) | High at chuck, light at tailstock |
| Square/rectangular | Step clamps or toe clamps | 2–4 along length | High |
| Irregular castings | Custom contour blocks + strap clamps | 3–6 | As needed for rigidity |
| Thin-wall tube | Expanding mandrel or split collet | Internal support full length | Light — prevent distortion |
Hydraulic vs Manual Clamping
| Aspect | Manual Clamping | Hydraulic Clamping |
|---|
| Setup time | 5–20 minutes | 1–3 minutes |
| Clamping force consistency | Operator-dependent | Precise and repeatable |
| Cost | Low | High ($2,000–$10,000 per fixture) |
| Maintenance | Low | Moderate (seals, valves) |
| Best for | Low volume, prototype | High production, automated cycles |
| Clamp force monitoring | Not possible | Available with pressure transducers |
Clamp Placement Guidelines
| Guideline | Reason |
|---|
| Place clamps as close to the hole axis as possible | Minimizes leverage that can tilt the part |
| Distribute clamping force evenly along part length | Prevents bowing of long parts |
| Use at least 3 clamp points for round parts | Three points define a circle without wobble |
| Apply axial stop (against feed direction) | Prevents part from being pushed by drill |
| Allow chip clearance at all clamp locations | Chips trapped under clamps cause misalignment |
| Use copper or plastic pads on clamps for finished parts | Prevents surface damage |
Fixture Alignment Procedure
Initial Fixture Setup
| Step | Action | Detail |
|---|
| 1 | Clean table surface and fixture base | Remove all chips, debris |
| 2 | Position fixture on table | Rough alignment by eye |
| 3 | Align fixture to spindle axis | Using test bar or indicator |
| 4 | Clamp fixture lightly | Enough to hold position |
| 5 | Verify alignment | Re-check after clamping |
| 6 | Tighten all mounting bolts | Cross-pattern, torque to specification |
| 7 | Final alignment verification | Check all critical axes |
| 8 | Set fixture datum in CNC | Record fixture offset values |
Alignment Tolerance
| Machine Axis | Tolerance | Method |
|---|
| Fixture axis to spindle axis (parallel) | < 0.01 mm per 300 mm | Dial indicator on test bar |
| Fixture perpendicular to spindle (transverse) | < 0.01 mm per 100 mm | Square + indicator |
| Fixture height (centerline to spindle centerline) | ±0.05 mm | Height gauge or laser |
| Bushing bore to spindle centerline | < 0.01 mm TIR | Test bar through bushing |
Quick-Change Fixturing
Systems for Production
| System | Changeover Time | Repeatability | Cost | Best For |
|---|
| Pallet system | 1–5 minutes | ±0.01 mm | High ($15,000–$50,000) | Multiple parts, high production |
| Zero-point mounting | 2–10 minutes | ±0.005 mm | Medium ($5,000–$15,000) | Medium to high production |
| Quick-change sub-plate | 5–15 minutes | ±0.02 mm | Low ($1,000–$5,000) | Low to medium production |
| Magnetic chuck | 1–5 minutes | ±0.02 mm | Medium ($3,000–$10,000) | Ferrous parts only |
Common Fixture Problems
| Problem | Symptom | Likely Cause | Corrective Action |
|---|
| Part moves during drilling | Oversize hole, bell mouth, tool breakage | Insufficient clamping force | Increase clamp force, add clamps |
| Inconsistent hole position | Position varies part to part | Fixture not repeatable | Check location pins, clean locating surfaces |
| Chatter / vibration | Surface finish degradation | Fixture not rigid | Add support, increase clamp force, use damping material |
| Part distortion after unclamping | Ovality, hole out-of-round | Over-clamping | Reduce clamp force, use distributed clamping |
| Chip packing at fixture | Chip evacuation problems | Poor chip clearance in fixture design | Modify fixture for chip flow |
FAQ
What is the most important consideration in fixture design for deep hole drilling?
Axial force resistance is the most important consideration. The drill feed force pushes the part away from the spindle with hundreds to thousands of newtons of force. The fixture must have a positive axial stop (against the feed direction) and sufficient clamping force to prevent the part from sliding. Without adequate axial force resistance, the part moves during drilling, causing oversize holes and tool breakage.
How do I align a fixture on a deep hole drilling machine table?
Clean the table and fixture base thoroughly. Place the fixture on the table and align it to the spindle axis using a test bar mounted in the spindle. Use a dial indicator to sweep the fixture's location features (V-blocks, bushing bore, or part location surfaces). Adjust until the alignment is within 0.01 mm per 300 mm. Tighten mounting bolts in a cross-pattern and re-verify alignment after tightening.
How many clamps do I need for a deep hole drilling fixture?
A minimum of two clamp points for short parts (under 300 mm), three for medium parts (300–1,000 mm), and one clamp per 300–500 mm of part length for longer parts. Round parts need V-blocks at each clamp point to prevent rotation. All clamps must be on the same side of the part (toward the tailstock) to resist the axial feed force without tilting the part.
When should I use hydraulic instead of manual clamping?
Use hydraulic clamping when: production volume exceeds 500 parts per year, changeover time reduction is critical, consistent clamping force is required for quality, automated or unattended operation is planned, or the part is delicate and precise clamping force control is needed. For low-volume production or prototype work, manual clamping is more economical and flexible.
What causes a part to move in the fixture during deep hole drilling?
Part movement is caused by: insufficient clamping force (most common), missing or inadequate axial stop, clamp placement too far from the cutting zone, smooth fixture surfaces (add serrations or high-friction pads), incorrect clamp sequence (should be applied facing the axial stop), or hydraulic system pressure drop (if using hydraulic clamps).
The fixture is the foundation of hole quality. A rigid, aligned, and properly clamped fixture eliminates most common drilling problems before they start. Invest time in fixture design and setup — it pays back in consistent hole quality and reduced tool breakage. This article reflects industry practice as of 2026.
