Deep Hole Machining Technology: Gun Drilling and Its Applications
2.1
Gun Drilling and Its Applications
Gun drilling is a relatively old deep hole machining process, a typical external chip removal deep hole machining method. Originally used for drilling gun barrels, it is called gun drilling. Gun drilling is a representative form of external chip removal deep hole drilling and a common method for machining small diameter (less than 10 mm). Gun drilling can achieve excellent machining results, achieving excellent accuracy and surface finish in a single pass.
2.1.1
Gun Drill Structure
A standard gun drill is made of high-speed steel or carbide, with the head brazed to the drill stem (Figure 2.1). It primarily consists of three components: the head, the drill stem, and the transmission section (shank).

For gun drills with diameters greater than 6 mm, alternative constructions include insert welding (Figure 2.2) and machine-clamped carbide (Figure 2.3). The cutting teeth, guide pads, and cutter body are welded or mechanically connected. Compared to brazed gun drills, the cutting section features a chip-splitting cutting edge design, resulting in lower cutting forces and more uniform, finer chips, ensuring smoother and more stable chip removal. Furthermore, the coated blades and guide strips enable high-speed cutting, a feat not possible with traditional brazed gun drills.

1. Gun Drill Head
The gun drill head is a critical component of the entire gun drill. Its unique and scientifically designed structure ensures complete cutting while also providing self-guiding functionality. The gun drill head features two basic angles, allowing the optimal combination to be selected based on the material and shape of the workpiece being cut. This ensures a balanced cutting force and chip breaking, while also transmitting cutting force to the guide pads, ensuring excellent straightness and coaxiality. The gun drill head has a slight back taper and a diameter slightly larger than the drill rod, ensuring free rotation of the drill rod within the cut hole without rubbing against the machined surface.
The gun drill head has an oil outlet on its end face. The shape and size of the outlet are determined primarily by the flow characteristics and flow requirements of the cutting fluid. Common oil hole shapes include single circular, half-moon, and double circular, as shown in Figure 2.4. Double circular and half-moon oil hole shapes have larger cross-sectional areas and are commonly used in large-diameter gun drills or double-edged gun drills. Small-diameter gun drills or single-edged gun drills often use single circular and half-moon cross-sectional shapes. To ensure hole accuracy, the gun drill head has two guide blocks, which form a three-point circle with the secondary cutting edge and provide self-guiding functionality. The form of the guide blocks varies depending on the gun drill head structure and diameter, as shown in Figure 2.5. A machine-clamped head uses mechanical clamping to hold two block-shaped guide blocks on the cutter body (Figure 2.5(a)). A welded head uses two strip-shaped guide blocks welded to the cutter body (Figure 2.5(b)). For larger diameter gun drills, the guide blocks are directly integrated as protruding blocks (Figure 2.5(c)). For smaller diameter gun drills, a flat surface can be ground onto the cylindrical surface to form an integrated cylindrical guide block (Figure 2.5(d)).

2. Drill Rod

Gun drill rods must be strong enough to provide the required cutting torque with minimal torsional deformation. At the same time, the shank must also be sufficiently tough to absorb the vibrations generated by the shank’s high-speed rotation. They are generally pressed from high-strength alloy steel pipe. Different drill rod types are available depending on the desired hole depth and drill bit configuration, as shown in Figure 2.6.

Typically, D-shaped drill rods are suitable for Speedbit gun drills; U-shaped drill rods are used for special-structure gun drills; V-shaped and round drill rods are suitable for standard (standard) gun drills. V-shaped drill rods are used for deeper holes, while round drill rods are used for shallower holes. Center-grooved drill rods are suitable for pin-type gun drills; and twisted drill rods are used for vertical drilling.

The drill rod diameter must be slightly smaller than the drill bit diameter, but not too small, as this will cause chips to leak outside the V-groove and scratch the machined surface.

3. Shank

The shank connects the drill bit to the machine tool. The shank and the connecting hole in the machine tool must be coaxial and secure to effectively transmit force and torque. Gun drill shanks come in a variety of styles. A common shank structure is shown in Figure 2.8.

2.1.2 Gun Drill Geometric Parameters
The elements of the gun drill cutting portion are shown in Figure 2.9. The gun drill has two main cutting edges and one secondary cutting edge. The main cutting edge closest to the drill center is called the inner cutting edge, and the other main cutting edge is called the outer cutting edge. The intersection of these two edges is called the drill tip.

1. Cutting Angles
The geometric angles of the gun drill cutting portion are shown in Figure 2.10.

1) Rake and Relief Angles
The rake angle yo of both the inner and outer cutting edges of a gun drill is generally set at 0°. This flat rake face facilitates manufacturing and resharpening. The outer cutting edge relief angle is usually ground to a double relief angle, with a value of o = 8° to 15° (8° to 12° for cutting steel, with the lower value being used for higher hardness; 15° for drilling aluminum, magnesium, and their alloys). The outer cutting edge’s secondary relief angle, a2 = 15° to 25°, prevents chip accumulation and allows cutting fluid to reach the cutting edge. The internal cutting edge clearance angle is typically set at 10° to 15°. Since the actual cutting clearance angle decreases significantly as the drill approaches the center, the upper limit should be used.
The secondary cutting edge (also known as a cylindrical edge) clearance angle is often set at α = 8°, with a margin width ba remaining. Generally, the margin width bu is 0.4 to 0.6 mm. A margin width that is too narrow can easily break the oil film, while a width that is too wide can increase friction and cause the drill to stick.
2) Deflection Angle and Drill Tip Position
The main deflection angle of the internal and external cutting edges of a gun drill significantly affects the cutting edge stress state, tool tip strength, chip breaking, and chip evacuation. It is customary to indicate the residual deflection angle rather than the main deflection angle. The residual deflection angles of the external and internal cutting edges are α and w, respectively. The residual deflection angles of the internal and external cutting edges of a gun drill are called the internal angle and external angle, respectively. The distance from the drill tip to the drill axis is called the drill tip eccentricity e. The internal and external angles and drill tip eccentricity directly affect the cutting edge stress state, as shown in Figure 2.11.

In Figure 2.11(a), the outer edge is larger than the inner edge. Excessive radial force on the outer edge will increase the extrusion pressure on the guide block, increasing friction, deteriorating the machined surface, and increasing heat.

In Figure 2.11(b), the outer edge is smaller than the inner edge. Excessive radial force on the inner edge will cause the resultant force to act on the secondary cutting edge, resulting in large straightness errors, excessive hole diameter, increased surface roughness, and premature wear of the cutting edge.

The optimal situation is when the radial forces on the inner and outer edges are equal, or when the radial force on the outer edge is slightly greater than the radial force on the inner edge, as shown in Figure 2.11(c). This allows the resultant force to act on the guide block, preventing the drill from deviating. e = do/4 is typically used. Considering the lower cutting speed of the inner edge and the more complex cutting conditions in the core, to ensure that the radial force on the outer edge is equal to or slightly greater than the radial force on the inner edge, w > r must be used. When machining general materials, e = do/4, r = 30°-40°, and yt = 20°-25°. For soft materials, the smaller value is used. When gun drilling overlapping steel plates, to prevent sticking, the taper created by the inner cutting edge must be very small. In this case, the values ​​are θ = 30° to 45°, r = 2° to 10°, and e = do/6.
3) Flute Angle
The flute angle θ refers to the angle of the V-shaped groove that allows for chip removal, as shown in Figure 2.10. A flute angle of θ = 110° to 130° is suitable. Too small a flute angle reduces the chip removal space, hindering chip evacuation. Too large a flute angle weakens the drill stem rigidity. A typical value is θ = 110°, resulting in a torsional stiffness of 35% of that of the pre-rolled round steel pipe.
2. Other Geometric Parameters
1) Zero-Point Core Rod
The main cutting edge of the gun drill must pass through or slightly below the drill bit center; it must not exceed the center to prevent chipping. Because the inner cutting edge is below the center angle, a cylindrical core rod is formed during cutting. This core rod is called the zero-point core rod, as shown in Figure 2.12. The core rod reduces axial force and minimizes deviation from the hole axis, providing additional guidance for the gun drill and facilitating drilling. The core rod diameter should not be too large to facilitate automatic breaking and removal with the chips. The zero-position core rod diameter, d, is typically set at 2h = (0.03-0.05)d0, generally no larger than 0.4mm. To avoid friction between the core rod and the drill body, the inner cutting edge generally extends slightly beyond the drill center. This excess is represented by b. The b. value must be greater than the core rod radius, typically taking be = (1-1.5)d.

2) Cutting Section Back Taper
Like other drills, the cutting section of a gun drill requires back taper grinding. This means the diameter of the drill’s cutting section gradually decreases from the tip to the tail. This primarily reduces friction between the drill and the workpiece hole wall and prevents damage to the drill guide and cutting edge surfaces.
The back taper value directly affects the durability and service life of the gun drill. The principle for determining the back taper value is to consider both the precision requirements of the workpiece being machined and the maximum possible service life of the gun drill. The relationship between the gun drill cutting section length, hole accuracy, and back taper angle is:
1, = (dmax – dmin) / (2tana)
(2.4) where 1 is the gun drill cutting section length , mm; dmax is the maximum hole diameter, mm; dmin is the minimum hole diameter, mm; α is the inverted taper angle, (°). Standard gun drills are generally marked by the diameter reduction per 100 mm of length. This reduction, K, can be calculated using formula (2.5): (2.5)K = 0.02d0.
3) Guide Pad Lag
To prevent the front end of the guide pad from cutting ahead of the cutting edge, the front end of the guide pad must axially lag behind the outer cutting edge by a certain distance, known as the guide pad lag. Lag leads to unstable cutting. Typically, the lag is 0.5-1.2 mm, or (2-4)f, where f is the feed rate.

2.1.3
Using a Gun Drill
To achieve satisfactory results when drilling deep holes with a gun drill, it is necessary to rationally select cutting parameters, tool geometry, tool material, and coolant to achieve excellent machining performance and satisfactory machining quality.
1. Selecting Cutting Parameters
The selection of cutting parameters is related to the cutting process and chip formation, as well as the material of the workpiece, the required precision, and the machine tool characteristics.
1) Cutting Speed
The cutting speed is primarily determined by the tool material and is limited by the drill bit’s durability and the machine tool’s rpm. High-speed steel gun drills generally use a cutting speed of v = 35-70 m/min; for carbide gun drills, refer to Table 2.1 for selected cutting speeds.