Appearance
A twist drill for deep hole drilling bears little resemblance to a general-purpose jobber drill. The coolant holes must be positioned to survive hydrostatic pressures that would burst a poorly designed web, the margins must balance straightness against chip evacuation, and the entire geometry must be ground with runout measured in micrometres — because at 50× diameter, even 0.01 mm of runout at the shank becomes 0.5 mm of deviation at the cutting edge.
Deep Hole Drilling Tool Types
Three primary tool types are used in deep hole drilling, each with a distinct design philosophy:
| Tool Type | Diameter Range | L/D Ratio | Chip Evacuation | Feed Rate |
|---|---|---|---|---|
| BTA drill head | 20–200 mm | Up to 100:1 | Internal through drill tube | Highest |
| Gun drill | 1–20 mm | Up to 300:1 | External V-groove | Moderate |
| Carbide twist drill (through-coolant) | 1–20 mm | Up to 50:1 | Helical flutes | Lowest of the three |
While BTA and gun drilling are purpose-designed deep hole processes, through-coolant carbide twist drills are the bridge between conventional drilling and deep hole drilling — they use standard twist drill geometry but with the coolant delivery and margin design adaptations necessary for depth ratios beyond 5:1.
Tip: Through-coolant carbide twist drills are the most cost-effective solution when the required L/D ratio is between 5:1 and 30:1. Above 30:1, gun drilling or BTA becomes necessary because the flute design of a twist drill cannot evacuate chips reliably at extreme depths.
Through-Coolant Hole Design
Coolant Hole Placement
The most critical design feature of a deep hole twist drill is the position and size of the coolant holes within the web:
| Web Diameter | Coolant Hole Diameter | Hole Position (from centre) |
|---|---|---|
| 6 mm | 0.8–1.2 mm | 1.2–1.8 mm |
| 10 mm | 1.5–2.0 mm | 2.0–3.0 mm |
| 14 mm | 2.0–2.5 mm | 2.5–3.5 mm |
| 20 mm | 2.5–3.5 mm | 3.5–5.0 mm |
The holes must be positioned to:
- Maintain web integrity — sufficient material must remain between the coolant hole and the flute to prevent breakthrough during grinding or in-service pressure failure
- Deliver coolant to the cutting edge — the hole exit should be as close to the cutting edge as possible without weakening it
- Withstand hydrostatic pressure — deep hole drilling coolant pressures (50–150 bar) create significant hoop stress in the web
Coolant Hole Verification
Coolant hole breakage into the flutes is a manufacturing defect that renders the drill scrap. This occurs when the coolant hole is positioned too close to the flute surface and the grinding wheel breaks through during flute grinding.
ANCA's ToolRoom software addresses this with coolant hole verification at the design stage:
- The software models the coolant hole position relative to the finished flute geometry
- If the hole-to-flute wall thickness falls below a safety threshold, it alerts the designer
- This prevents scrap before grinding begins — eliminating the costly discovery of coolant hole breakage during final inspection
Pressure Capacity
| Coolant Hole Diameter | Max Recommended Pressure | Safety Factor |
|---|---|---|
| 1.0 mm | 150 bar | 2.5:1 |
| 1.5 mm | 120 bar | 2.5:1 |
| 2.0 mm | 100 bar | 2.5:1 |
| 3.0 mm | 80 bar | 2.5:1 |
The pressure capacity decreases with larger coolant holes because the wall thickness between the hole and the flute is reduced. For applications requiring both high coolant pressure and large coolant volume, the drill diameter must be increased to accommodate larger coolant holes with adequate wall thickness.
Margin Design for Straightness
Single-Margin vs Double-Margin Drills
The margin is the cylindrical land on the drill that contacts the bore wall, providing guidance and burnishing. Deep hole margin design involves a fundamental trade-off:
| Feature | Single-Margin | Double-Margin |
|---|---|---|
| Number of margins per flute | 1 | 2 |
| Contact with bore wall | Single land per flute | Two lands per flute |
| Chip evacuation clearance | Maximum | Reduced |
| Hole straightness | Good | Excellent |
| Whirling vibration resistance | Moderate | High |
| Preferred material | Long-chipping (steel, stainless) | Short-chipping (cast iron, brass) |
Single-margin drills have one margin per flute with a larger relief area behind the margin. This provides maximum clearance for chip evacuation, making them the preferred choice for long-chipping materials where chip packing is the primary risk. The trade-off is reduced guidance, which can allow whirling vibrations to develop at high L/D ratios.
Double-margin drills have a second margin (often called a secondary land) behind the primary margin. This second contact point suppresses whirling vibrations by providing continuous contact with the bore wall on two points per flute. The result is significantly improved hole straightness and circularity — but the additional land reduces the space available for chip evacuation.
Floating Second Margin
A design compromise between single and double margins is the floating second margin:
- The primary margin is a full-height land at the drill diameter
- The second margin is reduced in height (typically 0.3–0.5 mm below the drill diameter)
- This provides the vibration-damping benefit of a second contact point while maintaining additional chip clearance compared to a full double-margin design
The floating second margin is effective in materials that are moderately long-chipping but require better straightness than a single-margin drill can provide.
Margin Width Selection
| Drill Diameter | Primary Margin Width | Secondary Margin Width (if applicable) |
|---|---|---|
| 6 mm | 0.5–0.8 mm | 0.3–0.5 mm |
| 10 mm | 0.8–1.2 mm | 0.5–0.8 mm |
| 14 mm | 1.0–1.5 mm | 0.6–1.0 mm |
| 20 mm | 1.2–2.0 mm | 0.8–1.2 mm |
Warning: Margin width that is too narrow for the application causes rapid margin wear, which reduces drill diameter and produces undersize holes. Margin width that is too wide increases friction and heat generation, which can cause workpiece surface hardening and built-up edge on the margins. The margin widths in the table above are starting points — adjust based on observed margin wear patterns.
Recent Research: Margin Geometry Optimisation
A 2023 study from TU Darmstadt (Krebs et al.) investigated margin geometry effects on whirling vibrations in deep hole drilling with twist drills. Key findings:
- Whirling vibrations were identified as the primary cause of hole diameter deviation and circularity errors in deep hole twist drilling
- A nonlinear physical model incorporating the contact area between margins and bore wall was developed
- Optimised margin geometries reduced radial vibrations significantly
- The improvement in hole diameter deviations and circularity was statistically significant across the tested parameter range
This research confirms that margin design is not a static specification — it can be optimised for specific material and parameter combinations to suppress vibration at its source.
Tool Runout Control
The Runout Challenge
Tool runout increases proportionally with drill length. A deep hole drill at 30× diameter that is ground with 0.005 mm runout at the grinding point will exhibit approximately 0.010–0.015 mm runout at the cutting edge due to the lever arm effect of the extended length.
| Drill Length (× diameter) | Runout at Shank | Runout at Cutting Edge |
|---|---|---|
| 10× | 0.005 mm | 0.008 mm |
| 20× | 0.005 mm | 0.012 mm |
| 30× | 0.005 mm | 0.018 mm |
| 50× | 0.005 mm | 0.030 mm |
Every 0.01 mm of runout at the cutting edge produces:
- Oversize hole diameter (approximately 1.5× the runout value)
- Asymmetric cutting load on the two cutting edges
- Reduced tool life (the loaded edge wears faster)
- Degraded surface finish on one side of the hole
ANCA P-Axis and Arobotech Support
The ANCA P-axis is a programmable steady rest system designed specifically for grinding long series tools. It uses an Arobotech steady rest support to stabilise the drill blank at the grind point:
| Feature | Specification |
|---|---|
| Support type | Hydraulic follow-down steady rest |
| Diameter range | 3–25 mm (interchangeable pad sets) |
| Pad set options | 3–10 mm, 10–16.5 mm, 16.5–25 mm |
| Surface finish achieved | Below Ra 0.2 |
| Setup | Single clamping — blank preparation, peel grinding, and geometry grinding in one setup |
How it works: The Arobotech steady rest supports the drill blank at the point of grinding, eliminating deflection caused by grinding forces. Without this support, a long slender blank would deflect away from the grinding wheel, producing taper and runout.
Single clamping advantage: Because the blank is not re-clamped between operations (blank preparation → flute grinding → relief grinding → point grinding), concentricity errors from re-clamping are eliminated. This is the single largest contributor to improved runout in P-axis-ground drills.
Machine Platforms
| Machine | P-axis Configuration | Best For |
|---|---|---|
| MX7 ULTRA | Fixed Arobotech steady rest | High-volume deep hole drill production |
| TX7+ | Interchangeable P-axis (bush, Arobotech, or tailstock) | Mixed production — flexibility between long and short tools |
| TX7+Xchanger | Same as TX7+ with automatic wheel changer | Automated production, lights-out operation |
Drill Geometry and Point Design
Standard Point Geometry for Deep Hole Drilling
| Parameter | Conventional Twist Drill | Deep Hole Twist Drill |
|---|---|---|
| Point angle (standard steel) | 118° | 130–140° |
| Point angle (stainless) | 130° | 140–150° |
| Helix angle | 25–30° | 20–25° (lower for rigidity) |
| Web thickness (at point) | 8–12% of diameter | 12–18% of diameter (thicker for rigidity) |
| Web taper | Moderate | Increased — web thickens faster toward shank |
The deeper the hole, the more the drill geometry must be adapted:
- Wider point angle reduces the radial cutting force component, keeping the drill centred
- Lower helix angle increases torsional rigidity at the cost of chip evacuation speed
- Thicker web provides greater column strength against buckling — critical at L/D beyond 30:1
Chip Splitting
For deep hole twist drills above 10 mm diameter, chip splitting features are often ground into the point:
| Feature | Purpose | Typical Configuration |
|---|---|---|
| Notch/thinning | Splits wide chips into two narrower streams | Symmetrical notches behind each cutting edge |
| Split point | Reduces thrust force and assists centring | S-shaped or X-shaped point thinning |
| Chip breaker | Creates stress concentration for chip breakage | Ground groove behind the cutting edge |
Coolant Delivery Requirements
Pressure and Flow
| Drill Diameter | Minimum Coolant Pressure | Minimum Flow Rate |
|---|---|---|
| 6 mm | 80 bar | 15 L/min |
| 10 mm | 60 bar | 30 L/min |
| 14 mm | 50 bar | 50 L/min |
| 20 mm | 40 bar | 80 L/min |
Filtration
Through-coolant twist drills require filtration at 10 µm or finer — the same standard as gun drilling. Contaminated coolant will:
- Block coolant holes (reducing flow to the cutting edge)
- Accelerate margin wear (abrasive particles between margin and bore wall)
- Cause built-up edge (contaminated coolant reduces lubricity)
Material-Specific Considerations
| Material | Margin Type | Point Angle | Coolant Pressure | Notes |
|---|---|---|---|---|
| Low-carbon steel | Single-margin | 130° | 60–100 bar | Long chips — maximum chip clearance needed |
| Alloy steel (annealed) | Single-margin | 135° | 60–100 bar | Good chip breaking at optimal feed |
| Stainless steel (austenitic) | Single-margin | 140° | 80–120 bar | Work hardening risk — keep feed up |
| Cast iron (grey) | Double-margin | 120° | 40–60 bar | Short chips — second margin improves straightness |
| Cast iron (nodular) | Double-margin or floating second | 130° | 50–70 bar | More ductile than grey iron |
| Aluminium | Single-margin | 140° | 40–60 bar | Soft, gummy — polished flutes reduce adhesion |
| Titanium alloy | Single-margin | 135° | 80–150 bar | High coolant pressure essential for heat removal |
Tip: If switching between material groups, margin design is the specification most likely to need changing. A double-margin drill that produces excellent straightness in cast iron will create chip packing problems in stainless steel. A single-margin drill that evacuates chips perfectly in steel may produce wandering holes in cast iron.
FAQ
What is a through-coolant twist drill?
A through-coolant twist drill has internal coolant holes running through the web of the drill, delivering high-pressure coolant directly to the cutting edge. Unlike conventional twist drills that rely on external coolant flood, through-coolant drills provide continuous cooling and chip flushing at the point of cut — essential for deep hole drilling where coolant cannot reach the cutting edge by flooding alone.
What is the difference between single-margin and double-margin drills?
Single-margin drills have one cylindrical land per flute that contacts the bore wall, providing maximum clearance for chip evacuation. Double-margin drills have a second land behind the primary margin, which improves hole straightness by suppressing whirling vibrations but reduces chip evacuation capacity. Single-margin is preferred for long-chipping materials; double-margin for short-chipping materials where straightness is critical.
What causes whirling vibrations in deep hole twist drilling?
Whirling vibrations are caused by asymmetric cutting forces acting on the long, slender drill body. The unsupported length of the drill acts as a vibrating beam, and the contact between the margins and the bore wall can either dampen or amplify these vibrations depending on margin geometry. The 2023 TU Darmstadt study demonstrated that optimised margin designs can significantly reduce whirling vibration amplitude.
How does the ANCA P-axis improve drill quality?
The ANCA P-axis uses an Arobotech hydraulic steady rest to support the drill blank at the grinding point, preventing deflection during grinding. This enables single-clamping manufacture (blank preparation through final geometry) and achieves surface finishes below Ra 0.2 with minimal runout on drills up to 50× diameter in length.
What coolant pressure is needed for through-coolant twist drills?
Minimum pressure ranges from 40 bar (for 20 mm drills in cast iron) to 150 bar (for small drills in titanium). The coolant holes must be designed to withstand these pressures without wall failure — verification at the design stage is essential.
When should I use a through-coolant twist drill instead of a gun drill?
Use through-coolant twist drills when the L/D ratio is below 30:1 and the hole diameter is under 20 mm. Above 30:1, gun drilling's external V-groove chip evacuation becomes more reliable. Below 5:1, a conventional twist drill with external coolant is sufficient.
What is the floating second margin design?
A floating second margin is a compromise between single and double margin designs. The primary margin is at the full drill diameter, while the second margin is reduced 0.3–0.5 mm below diameter. This provides vibration damping while maintaining more chip clearance than a full double-margin design.
How many times can a through-coolant twist drill be reground?
Typically 5–10 regrinds for carbide twist drills — fewer than gun drills (15–20) because the point geometry is more complex and the coolant hole exit position changes with each regrind. The drill must be inspected for coolant hole integrity after each regrind.
What filtration is required for through-coolant drilling?
10 µm or finer — the same standard as gun drilling. Through-coolant holes are small-diameter passages that clog easily with contaminated coolant. Blocked coolant holes cause immediate cutting edge failure.
What are the most common failure modes for through-coolant deep hole drills?
The three most common failures are: (1) margin wear causing undersize holes (incorrect margin width for the material), (2) coolant hole blockage leading to edge failure (inadequate filtration), and (3) flute clogging from chip packing (wrong margin type or insufficient coolant pressure).
Conclusion
Through-coolant twist drills occupy the overlap between conventional drilling and deep hole drilling — suited to L/D ratios from 5:1 to 30:1 where standard twist drills cannot reach and gun drilling may be unnecessarily complex. Their design centres on three interdependent features: coolant hole placement (must survive hydrostatic pressure without breaking into flutes), margin geometry (single-margin for chip clearance, double-margin for straightness, or floating second margin as a compromise), and runout control (grinding technology that maintains micrometre-level concentricity over extended lengths). Advances in grinding technology — specifically the ANCA P-axis with Arobotech steady rest support — have pushed the achievable length and precision of through-coolant twist drills beyond previous limits, enabling single-clamping manufacture of drills with surface finishes below Ra 0.2 and runout previously unattainable in long series tools.