Appearance
A contract machining shop wins a large order for 250 mm long, 18 mm diameter through-holes in 4140 steel. The shop owner has a gun drilling machine and orders a standard 18 mm gun drill. The job starts well but the cycle time is 12 minutes per hole and the customer wants 500 parts per week. That is 100 hours of cycle time per week — the shop has only one machine. The owner considers buying a second machine. A tooling engineer suggests a different solution: switch from gun drilling to BTA drilling. The same 18 mm hole with a BTA head runs at five times the feed rate. Cycle time drops to 2.5 minutes. No second machine needed. The BTA head costs 30% more than the gun drill but lasts three times longer between regrinds. This case illustrates the most important decision in deep hole drilling: selecting the right tool system for the application.
The Three Deep Hole Drilling Systems
Deep hole drilling tools are classified by how coolant is delivered and how chips are evacuated. There are three primary systems, each with distinct advantages.
| System | Chip Evacuation | Coolant Path | Diameter Range | Depth Capability |
|---|---|---|---|---|
| Gun drilling | External (V-flute) | Through the tool center | 1.5 – 40 mm | Up to 200:1 |
| BTA/STS | Internal (through tube bore) | Annular space outside tube | 12 – 1500 mm | Up to 100:1 |
| Ejector (DTS) | Internal (venturi suction) | Between concentric tubes | 18 – 200 mm | Up to 100:1 |
The fundamental difference is that gun drills evacuate chips outside the tool while BTA and ejector drills evacuate chips through the center of the tool. This distinction drives nearly every other difference in tool design, performance, and application.
Gun Drills: Design, Geometry, and Applications
Gun drills are the most widely used deep hole drilling tool for diameters under 20 mm. A gun drill consists of a solid carbide tip — with one cutting edge and two guide pads — brazed to a steel shank with a V-shaped flute running the full length.
How They Work
High-pressure coolant is pumped through an internal passage in the drill shank, exits at the tip through a coolant hole positioned just behind the cutting edge, and flows back along the V-flute carrying chips out of the hole. The chips never cross the cutting edge, so the tool cuts continuously without pecking.
Key Geometric Parameters
- Nose angle: Typically 30–40° depending on material. Smaller angles reduce cutting forces but increase drill walking tendency. Larger angles improve centering but increase thrust load.
- Outer angle: 30–36° from the drill axis. Larger angles improve surface finish at the expense of tool life.
- Inner angle: 20–25°. Controls chip formation direction and chip width.
- Back taper: 0.012–0.020 mm per 100 mm of shank length. Prevents the drill body from rubbing against the hole wall.
- Guide pad clearance: The difference between the drill OD across the guide pads and the cutting edge diameter. Typically 0.003–0.008 mm.
Applications
Gun drills excel in applications requiring extreme depth-to-diameter ratios (over 100:1) and excellent surface finish (0.4–3.2 µm Ra). Typical applications include:
- Fuel injector holes in diesel engines
- Medical device guide pins and surgical instruments
- Mold cooling channels
- Hydraulic valve orifices
- Gun barrels (the original application that gave the tool its name)
Limitations
Gun drills have relatively low feed rates compared to BTA tools — typically 0.005–0.030 mm/rev. At 18 mm diameter, a gun drill feeds at approximately 30 mm/min while a BTA head feeds at 150–200 mm/min. The V-flute also reduces the torsional rigidity of the shank compared to a solid tube.
BTA/STS Drill Heads: Brazed vs Indexable
BTA (Boring and Trepanning Association) drilling — also called STS (Single Tube System) — uses a cutting head attached to a hollow tube. Coolant flows between the tube OD and the hole wall, and chips are carried back through the center of the tube.
Brazed Carbide Heads
Brazed BTA heads have carbide cutting tips and guide pads brazed onto a steel body. The entire head is ground to the final diameter after brazing, achieving excellent concentricity.
| Advantage | Disadvantage |
|---|---|
| Lower initial cost | Discarded when dull — cannot replace individual inserts |
| Excellent concentricity | Limited to specific diameter |
| Good for smaller diameters (12–25 mm) | Fixed geometry — cannot adjust for different materials |
| Proven reliability | Longer changeover when retipping |
Brazed heads are preferred for smaller diameters and applications where the hole tolerance is critical (IT7–IT8). They are also common in high-volume production where the head is consumed fully before replacement.
Indexable Insert Heads
Indexable BTA heads use replaceable carbide inserts mounted in pockets or cartridges on the head body. The inserts can be indexed (rotated to a fresh cutting edge) or replaced without removing the head from the tube.
| Advantage | Disadvantage |
|---|---|
| Replaceable cutting edges | Higher initial head cost |
| Multiple carbide grades and geometries available | More complex body design |
| Cartridge systems allow diameter adjustment | Requires inventory of inserts |
| Lower cost per edge than brazed heads | Concentricity depends on insert seating |
Indexable heads dominate above 25 mm diameter. They offer lower operating cost per hole and greater flexibility. The BT-A drill from Allied Machine uses a single large T-A insert and achieves up to 2× the penetration rate of traditional BTA heads by directing coolant flow through large ports in the head sides.
Guide Pads
Both brazed and indexable BTA heads use carbide guide pads to support and stabilize the head in the hole. The guide pads are the primary wear surface on a BTA head and typically determine the regrind interval. Pad wear is monitored by measuring the head diameter across the pads — a reduction of 0.05–0.10 mm from the nominal diameter signals the need for replacement or regrinding.
Ejector Drill Systems
Ejector drilling — also called DTS (Double Tube System) — uses two concentric tubes to create a venturi effect that assists chip evacuation. Coolant is pumped between the outer and inner tubes. At the drill head, most of the coolant passes through the head for cutting edge cooling and chip transport, while a portion is redirected through nozzles that create suction in the inner tube.
Advantages Over BTA/STS
The key advantage of the ejector system is that it does not require a pressure head seal at the workpiece entry. This makes it suitable for use on conventional machine tools — lathes, boring mills, and machining centers — without the dedicated sealing hardware that BTA/STS requires.
| Condition | BTA/STS | Ejector |
|---|---|---|
| Pressure head needed? | Yes | No |
| Suitable for standard CNC lathe? | No (needs sealing) | Yes |
| Coolant pressure requirement | Higher (30–100 bar) | Lower (15–40 bar) |
| Depth capability | Up to 100:1 | Up to 80:1 |
Limitations
The ejector system is less efficient at chip evacuation than BTA/STS because the suction effect decreases as hole depth increases. For depths exceeding 60:1 depth-to-diameter ratio, BTA/STS is the preferred internal chip evacuation system.
Trepanning Heads
Trepanning is a variant of BTA drilling used when the center of the hole must be preserved as a solid core. Instead of cutting the full hole diameter, the trepanning head cuts an annular groove, and the central core is extracted after drilling.
Trepanning heads are typically indexable insert designs for diameters from 25 mm to 300 mm. The head has cutting teeth arranged in a ring pattern around the core diameter. The number of cutting teeth — typically 3–8 — determines the chip load per tooth and the surface finish.
Trepanning is used when:
- The core material has value (precious metals, high-cost alloys)
- The core will be used as a separate component (hollow shaft with a solid core extracted for another part)
- The hole diameter is too large for solid drilling with available machine power
Carbide Grades and Coatings
The carbide grade and coating selection for deep hole drilling tools directly affects tool life, cutting speed, and hole quality.
Substrate Selection
Solid carbide is the dominant substrate for all deep hole drilling tools. Its high stiffness (Young's modulus approximately 700 GPa) resists the deflection that causes hole straightness errors. The hardness range of 1500–2100 HV provides wear resistance at the cutting edge.
| Material Group | Recommended Grade Characteristics |
|---|---|
| Steel (P) | Medium grain size, 8–12% cobalt, TiAlN coating |
| Stainless steel (M) | Fine grain, 10–12% cobalt, AlTiN or TiAlN coating |
| Cast iron (K) | Coarse grain, 6–8% cobalt, uncoated or TiAlN |
| Aluminum (N) | Medium grain, 8–10% cobalt, polished or TiN coated |
| Titanium/superalloys (S) | Ultra-fine grain, 10–12% cobalt, AlTiN coating |
Coatings
Coatings extend tool life by reducing abrasive wear and thermal degradation at the cutting edge.
- TiAlN (Titanium Aluminum Nitride): The most common coating for deep hole drilling tools. Stable up to 800°C. Suitable for steel, stainless steel, and cast iron.
- AlTiN (Aluminum Titanium Nitride): Higher aluminum content than TiAlN for better oxidation resistance. Preferred for high-temperature alloys and titanium.
- TiN (Titanium Nitride): General-purpose coating. Lower temperature resistance but excellent lubricity. Used for aluminum and non-ferrous materials.
- TiCN (Titanium Carbonitride): Higher hardness than TiN. Used for abrasive materials and cast iron.
The coating thickness for deep hole drilling tools is typically 2–6 µm. Thicker coatings provide better wear protection but can round the cutting edge, increasing cutting forces.
TIP
When regrinding a coated tool, the coating must be completely removed from the ground surfaces before recoating. Regrinding without stripping the old coating — or recoating over a partially stripped surface — produces a tool that performs worse than a new uncoated tool. The residual coating creates an uneven substrate that reduces coating adhesion and promotes edge chipping.
Tool Life Management and Regrinding
Deep hole drilling tools are expensive relative to conventional drills. Proper tool life management — including regrinding at the correct interval — is essential for economical operation.
When to Regrind
The wear pattern on a deep hole drilling tool follows a predictable curve. The tool should be reground when:
- Surface finish degrades by more than 0.2 µm Ra compared to the baseline
- Spindle power increases by 15–20% above the baseline for a sharp tool
- The guide pad wear land exceeds 0.15 mm width
- The cutting edge shows visible chipping under 10× magnification
Regrind Interval Guidelines
| Tool Type | Typical Holes per Regrind |
|---|---|
| Gun drill, steel (4140) | 80 – 200 |
| Gun drill, aluminum | 500 – 2000 |
| BTA brazed head, steel | 50 – 150 |
| BTA indexable head (per edge) | 30 – 120 |
| Trepanning head (per edge) | 40 – 100 |
These ranges depend on material, cutting parameters, and coolant conditions. Establish site-specific baselines by tracking hole count per regrind and the reason for regrind.
Regrinding Quality
The quality of the regrind determines the performance of the next tool life cycle. Critical regrind parameters:
- Nose geometry: Must reproduce the original angles within ±1°
- Cutting edge condition: No burning or micro-chipping — grinding burn reduces tool life by 50% or more
- Coolant hole: Must be cleared of any debris or residual grinding swarf
- Guide pad OD: Must be restored to the nominal diameter within 0.005 mm
- Coating: Recoat after regrind with the same coating specification
A poorly reground tool will produce inconsistent hole quality from the first cycle. Many shops find that sending tools back to the original manufacturer for regrinding — despite the higher cost and longer lead time — produces more consistent results than in-house regrinding.
How to Select the Right Tool
The selection of a deep hole drilling tool system depends on five primary factors.
Factor 1: Hole Diameter
| Diameter | Recommended System |
|---|---|
| Under 6 mm | Gun drill only |
| 6 – 20 mm | Gun drill or BTA (BTA possible above 12 mm) |
| 20 – 40 mm | BTA (indexable) preferred for production; gun drill for finish |
| 40 – 150 mm | BTA or ejector |
| Over 150 mm | BTA or trepanning |
Factor 2: Depth-to-Diameter Ratio
| Ratio | System |
|---|---|
| Under 20:1 | Any system viable |
| 20:1 – 60:1 | BTA or ejector preferred for productivity |
| 60:1 – 100:1 | BTA or gun drill |
| Over 100:1 | Gun drill only |
Factor 3: Production Volume
| Volume | Recommendation |
|---|---|
| Low (under 100 holes) | Gun drill (lowest tool cost, no special machine hardware) |
| Medium (100 – 1000 holes) | BTA brazed head for smaller diameters; indexable for larger |
| High (over 1000 holes) | BTA indexable head (lowest cost per hole) |
Factor 4: Available Machine
| Machine Type | Compatible Systems |
|---|---|
| Dedicated gun drilling machine | Gun drill (primary), BTA with pressure head addition |
| Dedicated BTA machine | BTA/STS, trepanning |
| Conventional CNC lathe | Ejector (DTS) with adapter, gun drill with steady rest |
| Machining center | Gun drill with high-pressure coolant-through spindle |
Factor 5: Material
| Material | Consideration |
|---|---|
| Free-machining steel | Any system works well |
| Stainless steel | BTA preferred for chip evacuation (internal chip removal avoids flute clogging) |
| Aluminum | Gun drill or BTA — high feeds possible |
| Titanium | Gun drill preferred for small diameters; BTA for larger |
| Cast iron | BTA with uncoated or TiAlN-coated carbide |
FAQ
What is the difference between gun drilling and BTA drilling?
Gun drilling evacuates chips externally through a V-flute on the drill shank. BTA drilling evacuates chips internally through a hollow tube. BTA drilling achieves 5–7 times higher feed rates but requires a dedicated machine with a pressure head seal at the workpiece entry.
Can I use BTA tools on a gun drilling machine?
Not without modification. BTA requires a pressure head that seals against the workpiece face to direct coolant flow. Gun drilling machines do not have this feature. Some dual-purpose machines are available that support both systems.
How many times can a BTA head be reground?
A brazed BTA head can typically be reground 5–15 times before the carbide insert is consumed. An indexable head is reground only for the head body — the inserts are replaced. The body regrind is needed every 20–50 insert changes to restore guide pad clearance.
What coolant pressure is needed for BTA drilling?
Typically 30–100 bar (435–1450 psi) depending on diameter. Smaller diameters require higher pressure. The coolant velocity should be 15–18 m/s for effective chip transport through the tube.
Why would I choose an ejector drill over a BTA drill?
Ejector drilling does not require a pressure head seal, making it compatible with conventional CNC machine tools. It typically operates at lower coolant pressures. The trade-off is slightly lower depth capability and chip evacuation efficiency.
What carbide grade is best for gun drilling stainless steel?
Fine-grain carbide with 10–12% cobalt content and an AlTiN or TiAlN coating. The fine grain provides edge sharpness for cutting the work-hardened layer, while the higher cobalt content provides toughness against chipping.
How do I know when a gun drill needs regrinding?
Monitor spindle power trend. When power increases 15–20% above baseline, plan for regrind within the next 5–10 cycles. Also check surface finish — an increase of 0.2 µm Ra above baseline signals edge degradation.
What is the typical tool life of a BTA indexable insert?
30–120 holes per cutting edge depending on material and parameters. Each insert typically has 3–4 indexable edges. Total tool cost per hole is the sum of insert cost per edge plus head body amortization.
Is trepanning faster than solid BTA drilling?
No. Trepanning removes only the annular ring of material, so the cutting volume per hole is less. However, the feed rate is also lower because the trepanning head has multiple cutting teeth sharing the chip load. The cycle time comparison depends on the specific diameter and depth.
Can I drill blind holes with BTA tools?
Yes, but with limitations. BTA internal chip evacuation requires the chips to flow freely through the tube. In blind holes, chip evacuation becomes less efficient as the hole bottom approaches. BTA blind hole drilling is practical for depth-to-diameter ratios up to 30:1. Gun drilling is preferred for deep blind holes.
Summary
Selecting the right deep hole drilling tool is the most consequential decision in the process design. The three main systems — gun drilling, BTA/STS, and ejector drilling — each have distinct capabilities that match specific application requirements.
The selection decision in order of priority:
Diameter and depth ratio eliminate systems immediately. Below 6 mm requires gun drilling. Above 150 mm requires BTA or trepanning. Over 100:1 depth ratio requires gun drilling.
Production volume determines the economic justification. BTA's higher feed rates (5–7× gun drilling) justify the machine investment when volumes exceed approximately 1000 holes per year.
Machine compatibility often dictates the practical choice. A shop with a gun drilling machine will use gun drills. A shop adding deep hole drilling capability should evaluate all three systems.
Material and quality requirements refine the selection within the compatible systems. Stainless steel favors BTA for chip control. Extreme surface finish requirements favor gun drilling.
Within the selected system, the choice of brazed vs indexable, carbide grade, coating, and regrind interval determines the operating cost and quality consistency. Track tool life data systematically, regrind at the correct interval, and use the manufacturer's recommended regrind specifications to maximize the economic return on the tool investment.