Appearance
A dedicated deep hole drilling machine is the right tool for a shop that drills deep holes every day. For a shop that needs deep holes once a week — or needs to offer the capability to win a contract — a $200,000 machine that sits idle 80% of the time is a bad investment. Retrofitting an existing CNC lathe or machining centre for high-pressure coolant and a guide bushing system can deliver 80% of the capability at 20% of the cost. The key is understanding what modifications are actually required, which machines are suitable candidates, and where the retrofit stops making economic sense.
When Retrofitting Makes Sense
Feasibility Assessment
| Factor | Good Candidate | Poor Candidate |
|---|---|---|
| Machine utilisation | < 50% on existing work | > 80% (no spare capacity) |
| Spindle condition | Good bearings, low hours | Worn bearings, high hours |
| Enclosure | Full enclosure exists | Open machine, no chip guards |
| Coolant system | Flood coolant equipped | No existing coolant system |
| Spindle type | Belt or gear-driven with accessible top | Built-in motor spindle without through-tool path |
| Control capability | M-code programmable coolant, rigid tapping | Basic control, limited M-codes |
| Required L/D ratio | < 100:1 gun drilling | > 200:1 requires dedicated machine |
When NOT to Retrofit
- Production volumes above 1,000 holes/month — dedicated machine ROI is better
- Hole depths exceeding 2 metres — standard machine beds are too short
- Diameter below 1 mm — requires specialised spindles and coolant control
- Extreme tolerances (IT6 or better) — dedicated machine rigidity required
- BTA drilling above Ø50 mm — coolant flow requirements exceed typical machine capacity
Retrofit Approaches
Approach Comparison
| Approach | Description | Cost Range | Capability | Complexity |
|---|---|---|---|---|
| External coolant inducer | High-pressure coolant delivered through a stationary seal at the workpiece entry | $5,000–$15,000 | Gun drilling up to 150:1 | Low — no spindle modification |
| Through-spindle coolant (TSC) | Coolant routed through spindle and rotary union | $10,000–$25,000 | Gun drilling with ATC, automated | Medium — drawbar modification |
| Full deep hole unit | External attachment unit with independent feed and coolant | $15,000–$50,000 | Gun drilling or BTA up to 250:1 | High — separate control integration |
| Purpose-built module | Complete sub-system (pressure head, chip box, tube supports) mounted on machine bed | $30,000–$80,000 | BTA drilling, large diameters | Very high — significant engineering |
External Coolant Inducer (Simplest Approach)
The external coolant inducer is a stationary assembly that seals around the drill rod at the workpiece entry, delivering high-pressure coolant without any spindle modification:
| Component | Function |
|---|---|
| Coolant inlet body | Connects to high-pressure pump hose |
| Seal assembly | Lip or labyrinth seal around rotating drill rod |
| Guide bushing | Supports drill at entry (replaceable carbide bushing) |
| Mounting bracket | Attaches to machine table or fixture sub-plate |
Limitations:
- No automatic tool changes — the inducer is tool-specific
- Maximum spindle speed limited by seal wear (typically < 5,000 RPM)
- Only one hole size per setup
High-Pressure Coolant System
Pressure Requirements by Method
| Drilling Method | Minimum Pressure | Recommended Pressure | Typical Flow |
|---|---|---|---|
| Gun drilling, Ø1–3 mm | 80 bar | 100–200 bar | 10–30 L/min |
| Gun drilling, Ø3–10 mm | 50 bar | 70–150 bar | 30–80 L/min |
| Gun drilling, Ø10–25 mm | 30 bar | 50–100 bar | 80–200 L/min |
| BTA drilling, Ø20–60 mm | 20 bar | 30–70 bar | 100–400 L/min |
Pump Types
| Pump Type | Max Pressure | Flow | Cost | Maintenance |
|---|---|---|---|---|
| Air-driven piston pump | Up to 200 bar | Low (5–20 L/min) | $1,000–$3,000 | Moderate (seal wear) |
| Electric screw pump | Up to 100 bar | Moderate (20–100 L/min) | $3,000–$8,000 | Low |
| Electric piston pump | Up to 200 bar | Moderate (20–80 L/min) | $5,000–$15,000 | Moderate (valve wear) |
| Hydraulic intensifier | Up to 400 bar | High (50–200 L/min) | $10,000–$25,000 | High (hydraulic system) |
Filtration Requirements
| Particle Size | Filtration Level | Application |
|---|---|---|
| < 50 µm | Standard paper/bag filter | Rough gun drilling |
| < 25 µm | Fine bag filter | Standard gun drilling |
| < 10 µm | Cartridge or cyclonic filter | Precision gun drilling |
| < 5 µm | Magnetic + cartridge | BTA finishing |
Coolant filtration is often the most overlooked element of a retrofit. Inadequate filtration causes:
- Accelerated guide bushing wear (particles embed in bushing ID)
- Gun drill bearing pad scoring
- Coolant pump seal failure
- Nozzle blockage in small-diameter drills
Cooling Capacity
Deep hole drilling generates significant heat in the coolant:
| Drilling Power | Coolant Temperature Rise | Required Cooling |
|---|---|---|
| 5 kW (typical small gun drill) | 5–10°C above ambient | Passive tank dissipation |
| 15 kW (medium BTA) | 10–20°C above ambient | Heat exchanger recommended |
| 30 kW (large BTA) | 20–40°C above ambient | Chilled coolant required |
Through-Spindle Coolant Retrofitting
Spindle Compatibility
| Spindle Type | TSC Retrofit Feasibility |
|---|---|
| Belt-driven with hollow shaft | Good — rotary union mounts on shaft end |
| Gear-driven with hollow shaft | Good — access at top of shaft |
| Built-in motor spindle (HSK, BT) | Difficult — may not have through-bore path |
| Tapered roller bearing spindle | Check bearing cooling — TSC adds heat |
| Ceramic bearing spindle | Better — bearings tolerate less heat |
Rotary Union Selection
| Type | Max Pressure | Max RPM | Seal Type | Cost |
|---|---|---|---|---|
| O-ring rotary union | 20 bar | 3,000 RPM | O-ring contact | $200–$500 |
| Mechanical seal rotary union | 70 bar | 8,000 RPM | Carbide face seal | $500–$1,500 |
| Labyrinth seal rotary union | 200 bar | 15,000 RPM | Non-contact labyrinth | $1,500–$4,000 |
| Inductive rotary union | 350 bar | 20,000 RPM | Inductive coupling | $4,000–$10,000 |
Drawbar Modification
For machines with a solid drawbar, the modification involves:
- Removing the drawbar
- Boring a concentric through-hole (typically 6–12 mm diameter)
- Reaming and honing the bore for smooth coolant flow
- Tapping the top end for rotary union thread (typically 1/2" BSP or NPT)
- Reassembly with proper preload
WARNING
Drawbar modification is irreversible and affects spindle balance. It should only be performed by a qualified spindle rebuilder. The through-hole diameter is limited by the drawbar wall thickness — bore too large and the drawbar loses tensile strength, risking tool pull-out at high RPM.
Through-Spindle Tooling Requirements
| Component | TSC Requirement | Typical Cost |
|---|---|---|
| Pull stud | Hollow (centre hole for coolant) | $15–$40 |
| Tool holder | Through-bored with coolant channels | $100–$300 |
| Collet | Sealed ER collet (prevents side leakage) | $30–$60 |
| Collet nut | Bearing-type nut for sealing | $40–$80 |
| Gun drill | Through-bore for coolant passage | $80–$300 |
Guide Bushing and Seal Assembly
Bushing Holder Design for Retrofit
A retrofit guide bushing assembly must be independently adjustable relative to the machine spindle:
| Component | Specification |
|---|---|
| Bushing material | Carbide (K10–K20) for production |
| Bushing length | 2–4× drill diameter |
| Clearance (ID vs drill OD) | 0.005–0.015 mm |
| X-Y adjustment range | ±2 mm minimum |
| Locking | Screw + dowel after alignment |
Alignment Procedure
- Mount a test bar in the spindle
- Indicate the test bar OD near the bushing location
- Adjust bushing holder X-Y until TIR ≤ 0.01 mm
- Lock adjustment screws and re-check
- Install bushing and indicate ID — verify ≤ 0.01 mm concentricity
Coolant Inducer Design
For external coolant delivery (non-TSC retrofit):
| Seal Type | Max Pressure | Application |
|---|---|---|
| Lip seal (nitrile) | 30 bar | Low-pressure, low RPM |
| Lip seal (polyurethane) | 70 bar | Moderate pressure, gun drilling |
| Labyrinth + purge seal | 150 bar | High-pressure, continuous operation |
| Mechanical face seal | 200+ bar | Maximum pressure, production |
Chip Box
A chip box at the drill exit point contains chips and coolant:
| Feature | Requirement |
|---|---|
| Material | Steel or stainless steel, with replaceable wear liner |
| Seal | Rubber gasket on hinged or sliding cover |
| Drain | Minimum 2× drill tube diameter |
| View window | Polycarbonate for visual monitoring |
| Safety interlock | Machine stop if cover is opened |
Steady Rests and Tube Supports
Support Requirements by L/D Ratio
| L/D Ratio | Support Requirement |
|---|---|
| < 20:1 | None — drill is rigid enough |
| 20:1–50:1 | Single steady rest near workpiece entry |
| 50:1–100:1 | Two steady rests along drill path |
| > 100:1 | Multiple steady rests at 300–500 mm spacing |
Steady Rest Types for Retrofit
| Type | Clamping | Best For |
|---|---|---|
| Fixed V-block | Manual | Simple setup, short runs |
| Adjustable roller steady rest | Manual set-screw | Production, multiple diameters |
| Hydraulic steady rest | Hydraulic pressure | High-volume, automated |
| Polyurethane whip guide | Friction-fit on drill tube | Vibration damping |
CNC Control Integration
Coolant Control
| Function | M-Code (Typical) | Notes |
|---|---|---|
| Coolant on (flood) | M08 | Standard — may need M08 for low-pressure pump |
| Coolant off | M09 | Standard |
| High-pressure coolant on | M40 or M41 | Machine-dependent; may need custom M-code |
| High-pressure coolant off | M41 or M42 | Machine-dependent |
| Variable pressure | S-code or analogue output | Requires pressure control valve interface |
Feed and Speed Considerations
| Parameter | Retrofit Machine | Dedicated Machine |
|---|---|---|
| Feed rate override | Full range | Full range |
| Spindle speed override | Full range | Full range |
| Peck cycle support | Standard G73/G83 | Standard |
| Depth-dependent feed | Requires macro programming | Often built-in |
| Torque monitoring | Spindle load meter | Coolant pressure feedback |
Macro Programming for Deep Holes
For retrofitted machines without built-in deep hole cycles, custom macros can provide:
- Variable feed rate by depth — reduce feed at entry and exit, increase at mid-depth
- Conditional peck cycles — peck based on drilling torque rather than fixed depth
- Breakthrough detection — feed reduction when tool approaches exit
- Tool wear compensation — adjust feed based on cumulative drilling time
Machine Limitations and Risks
Spindle Bearing Loads
High-pressure coolant exerts axial force on the spindle when using TSC:
| Coolant Pressure | Axial Force on Spindle Bearings (Ø10 mm drill) |
|---|---|
| 50 bar | ~390 N (acceptable for most spindles) |
| 100 bar | ~785 N (check bearing specifications) |
| 200 bar | ~1,570 N (may exceed spindle bearing capacity) |
WARNING
The axial force from high-pressure coolant adds to the cutting force load on spindle bearings. For older machines or machines with marginal bearings, this combined load can cause premature bearing failure. Always calculate the total axial load and compare with the machine manufacturer's specifications.
Common Retrofit Problems
| Problem | Cause | Solution |
|---|---|---|
| Coolant leaks at rotary union | Worn seal, misalignment | Replace seal, realign union |
| Gun drill breakage at entry | Bushing misaligned to spindle | Re-align bushing to ≤ 0.01 mm TIR |
| Poor surface finish | Coolant pressure too low | Increase pressure, check for blockages |
| Spindle bearing noise after retrofit | Bearing preload affected by drawbar modification | Return to spindle rebuilder |
| Coolant overheating | Insufficient tank capacity | Add tank volume or heat exchanger |
| Chip packing in drill flute | Inadequate flow rate | Increase flow, check coolant hole size |
Cost Breakdown and ROI
Retrofit Cost Scenarios
| Scenario | Components | Estimated Cost |
|---|---|---|
| Basic external coolant (lathe, gun drilling up to Ø10 mm) | Air-driven pump, bushing holder, guide bushing, chip box, hoses | $5,000–$10,000 |
| TSC retrofit (VMC, gun drilling up to Ø10 mm) | Rotary union, drawbar modification, TSC tooling, electric pump, filtration | $12,000–$25,000 |
| Full external unit (lathe, gun drilling up to Ø25 mm, 150:1 L/D) | Electric piston pump (150 bar), bushing assembly, steady rests, chip box, coolant tank, filtration | $20,000–$40,000 |
| BTA retrofit (lathe, Ø20–60 mm) | Hydraulic pump (70 bar, 200 L/min), pressure head, drill tube supports, chip box, filtration, control integration | $40,000–$80,000 |
Specialised Tooling Costs
| Item | Cost | Notes |
|---|---|---|
| Gun drill (Ø5 mm, 500 mm long) | $100–$200 | Per drill, per size |
| Gun drill (Ø10 mm, 1,000 mm long) | $200–$400 | Per drill, per size |
| BTA head (Ø25 mm, indexable) | $150–$400 | Head only, inserts separate |
| Guide bushing (carbide) | $100–$300 | Per size |
| Tool holders (TSC, ER series) | $150–$350 | Each, depending on size |
ROI Calculation Example
Scenario: Machine shop retrofits a CNC lathe ($15,000 total) to gun-drill Ø10 mm × 400 mm holes in 4140 steel.
Before retrofit: Send out — $25/hole, 3-week lead time After retrofit: In-house — $12/hole, 2-day lead time
| Volume (holes/year) | Outsourced Cost | In-House Cost | Annual Savings | Payback Period |
|---|---|---|---|---|
| 500 | $12,500 | $6,000 | $6,500 | 2.3 years |
| 1,000 | $25,000 | $12,000 | $13,000 | 1.2 years |
| 2,000 | $50,000 | $24,000 | $26,000 | 0.6 years |
At 1,000 holes per year, the retrofit pays for itself in just over one year. Additional benefits include lead time reduction and the ability to win new business requiring deep holes.
FAQ
Q: What is the cheapest way to add deep hole drilling capability to a CNC machine? An external coolant inducer with an air-driven piston pump ($5,000–$10,000) delivers high-pressure coolant to the drill without any spindle modification. The inducer seals around the drill rod at the workpiece entry.
Q: Can any CNC machine be retrofitted for through-spindle coolant? No. The spindle must have a hollow shaft or a drawbar that can be bored. Built-in motor spindles often lack a through-bore path. The machine must also have been ordered with a "TSC-ready" option or the spindle must be disassembled for modification.
Q: What coolant pressure is needed for gun drilling? Gun drilling requires 50–200 bar depending on diameter. Small diameters (1–3 mm) need the highest pressure (100–200 bar). Larger diameters (10–25 mm) need 30–100 bar.
Q: How much does a deep hole drilling retrofit cost? $5,000–$80,000 depending on complexity. A basic external coolant system for gun drilling on a lathe costs $5,000–$15,000. A full BTA retrofit with hydraulic pump, pressure head, and control integration costs $40,000–$80,000.
Q: Does retrofitting void the machine warranty? Modifications to the spindle (drawbar boring, rotary union installation) will void the spindle warranty. External additions (coolant pump, bushing holder) that do not alter the machine structure typically do not affect warranty on other components.
Q: What filtration is needed for retrofitted deep hole drilling? Minimum 25 µm filtration for gun drilling. For precision work or small diameters, 10 µm or finer is recommended. Cyclonic filtration reduces maintenance compared to bag filters.
Q: Can a CNC lathe be retrofitted for BTA drilling? Yes, but the retrofit is more complex than gun drilling. It requires a pressure head (coolant inducer on the workpiece), drill tube supports along the bed, and a high-flow coolant pump (100–400 L/min).
Q: What is the maximum L/D ratio achievable with a retrofit system? With proper guide bushing alignment and steady rest support, a retrofitted lathe can achieve 100:1–150:1 L/D for gun drilling. Beyond this, the machine bed length typically becomes the limiting factor.
Q: How is the guide bushing aligned to the spindle? Using a test bar in the spindle and a dial indicator on the bushing ID. Alignment to within 0.01 mm TIR is standard. Most bushing holders have X-Y adjustment screws for this purpose.
Q: What are the main risks of a DIY retrofit? The most common failures are: drawbar weakening from excessive boring, rotary union misalignment causing premature seal failure, inadequate coolant filtration damaging the pump, and bushing misalignment causing drill breakage.