Deep Hole Drilling on CNC Swiss-Type Lathes

Deep Hole Drilling on CNC Swiss-Type Lathes

CNC Swiss-type lathes (also called Swiss screw machines or sliding headstock lathes) are uniquely suited for small-diameter deep hole drilling. The guide bushing design — a defining feature of Swiss-type machines — provides continuous support for both the workpiece and the drilling tool, making it possible to drill deep holes in small diameters that would be impossible on a conventional CNC lathe.

This guide covers Swiss lathe capabilities for deep hole drilling, tooling requirements, programming considerations, and limitations.

Why Swiss Lathes Excel at Deep Hole Drilling

The Guide Bushing Advantage

On a conventional CNC lathe, the workpiece is held in a chuck and the unsupported length increases as the tool moves away from the chuck. For deep hole drilling, this creates a problem: the drill must push against a slender, unsupported workpiece that can deflect or vibrate.

On a Swiss lathe, the workpiece is continuously supported by a guide bushing located just behind the cutting zone. The material feeds through the bushing as the tool cuts:

Conventional lathe:  [Chuck]=====Workpiece=====Drill→
                     Workpiece unsupported → vibration, deflection

Swiss lathe:  [Chuck]=====|Bushing|====Drill→
                           Workpiece supported at cutting zone

Result: Swiss lathes can drill deep holes in small diameters (1–12 mm) at depth ratios that would cause chatter or deflection on a conventional lathe.

Capability Comparison

Tooling for Swiss Lathe Deep Hole Drilling

Coolant-Through Toolholders

Standard Swiss lathe toolholders do not have coolant-through capability. For deep hole drilling, you need:

Gun Drilling on Swiss Lathes

Gun drilling is the preferred method for deep holes on Swiss lathes:

Gun drill installation requirements:

• Coolant-through toolholder with high-pressure seal
• Coolant pressure: 50–150 bar (depending on diameter)
• Guide bushing at the workpiece exit (or close to it)
• Pilot hole recommended (depth: 1.5–2×D)

Extended-Length Solid Carbide Drills

New extended-length solid carbide drills (CERATIZIT WTX-Deep UNI, Dormer Force DHD) are also suitable for Swiss lathe deep holes:

Micro Deep Hole Drills (Guhring, 2025)

For diameters below 1 mm:

• Sub-micron carbide substrate
• Ground facet point with concave cutting edge
• Internal coolant in 1 mm+ diameters
• Suitable for stainless, titanium, Inconel

Programming Considerations

Peck Cycle Selection

Example: G83 on Fanuc Control for Swiss Lathe

N100 T0101 (Gun drill, Ø2.5 mm)
N110 G97 S6000 M03 (Spindle speed)
N120 M08 (Coolant on — through-tool)
N130 G83 Z-100.0 Q3.0 R1.0 F0.015 (Deep hole peck)
N140 G80 (Cancel cycle)
N150 G28 U0 W0 (Return to reference)

Parameters for Swiss lathes:

• Q (peck depth): Start at 1×D, reduce at higher depth ratios. For Ø2.5 mm × 50 mm deep (20×D), Q = 2.5 mm initially, reduce to 1.0 mm at 20 mm depth.
• R (retract plane): R = 1.0 mm (above the previous peck depth). For Swiss lathes, keep R small to minimize cycle time.
• F (feed): Conservative start — 0.010–0.020 mm/rev for gun drilling in stainless or titanium.

Sub-Spindle Transfer for Deep Holes

For through-holes where the drill exits the part, Swiss lathes can transfer the part to the sub-spindle to complete drilling from the other side:

1. Part is machined on the main spindle
2. Sub-spindle picks up the part
3. Gun drill enters from the opposite face
4. Hole is completed through the full length

Advantage: Avoids the need for a long drill extending through the guide bushing.

Common Challenges and Solutions

Challenge 1: Coolant Pressure Drop at Small Diameters

Problem: Coolant pressure drops significantly through small-diameter gun drills (0.5–3 mm). At the cutting edge, pressure may be 50% of pump pressure.

Solution:

• Use a coolant pressure gauge at the tool holder (not the pump)
• Specify pump pressure 50% higher than the minimum required at the tool
• For 0.5–1.0 mm drills, consider 150–200 bar pump capacity

Challenge 2: Chip Evacuation in Small Flutes

Problem: In small-diameter gun drills (< 3 mm), the V-flute is tiny. Chips can pack easily.

Solution:

• Reduce peck depth (Q = 0.5–1×D)
• Increase feed rate (if chips are stringy)
• Verify coolant return flow — if no chips returning, stop immediately
• Consider through-tool coolant for all deep holes > 5×D

Challenge 3: Guide Bushing Clearance

Problem: If the guide bushing is too far from the drill entry point, the workpiece can deflect.

Solution:

• Set guide bushing position as close as possible to the drill entry
• For gun drilling, the bushing should be within 3 mm of the drill point at full extension
• Use a retractable guide bushing if machining both sides of the part

Application Examples

Example 1: Medical Bone Screw

Example 2: Fuel Injector Body

Limitation Summary

Summary

CNC Swiss-type lathes are well-suited for small-diameter deep hole drilling due to the guide bushing support that conventional lathes lack. Depth ratios of 20–50×D are achievable in diameters from 0.5 mm to 12 mm. Gun drilling is the preferred method for best straightness and tolerance, while new extended-length solid carbide drills offer higher throughput for less demanding applications. Key requirements include coolant-through toolholders, adequate coolant pressure (80–200 bar for small diameters), and appropriate peck cycle programming (G83 or custom macro). For G-code programming details, see CNC deep hole drilling G-code guide. For general CNC deep hole drilling, see deep hole drilling on standard CNC machines.