Technologie obrábění hlubokých děr: Obrábění řezáním

3. Chip Curl

In deep hole machining, it is desirable to produce small, regular C-shaped chips. These chips have a low chip volume coefficient, are easily discharged, and consume little power. The formation of C-shaped chips is achieved by the action of the chip curling shoulder, properly coordinated with the cutting speed v and feed rate f.

Chip curling is the curling of the chip. This refers to a difference in chip outflow velocity in a certain direction, resulting in uneven force on the chip, either vertically or horizontally, causing the chip to curl. Based on the direction the chip leaves the tool, it can generally be divided into upward curling and lateral curling.

Upward chip curling is caused by uneven force on the top and bottom surfaces of the chip, resulting in upward curling in the thickness direction. As shown in Figure 1.22, the outflow velocity Vht of the bottom layer of the chip is greater than the outflow velocity V of the upper layer, and its axis is approximately parallel to the separation line between the tool and the chip.

Lateral chip curling occurs when uneven forces are applied to the left and right sides of the chip, causing it to curl across its width. As shown in Figure 1.23, a velocity gradient exists within the chip’s base along its width, causing the chip to rotate at an angular velocity ωz about the normal to the chip’s base, resulting in lateral curling. The curling axis is generally perpendicular to the chip’s base.

Upward chip curling is a two-dimensional deformation of the chip, while lateral chip curling is a three-dimensional deformation. In actual machining, chip curling is influenced by factors such as cutting conditions, cutting parameters, and tool geometry. In deep-hole drilling, it is also affected by the tool’s rake angle λ, the radius of the secondary cutting edge and the tool nose, and variations in the shear angle along the cutting edge. Taking these factors into account, true lateral curling is not achieved during deep-hole drilling. In most cases, an oblique curling pattern, a combination of upward and lateral curling, is observed. This is because only this curling pattern can produce a spirally wrinkled, pyramid-shaped chip.

4. Chip Breaking

Chip breaking refers to the spontaneous breaking of chips at appropriate intervals. As the chip flows out from the rake face, the inclined (or arc-shaped) chip curling platform causes additional deformation, causing the chip material to lose some plasticity. The chip then pushes against the bottom of the hole, where it deforms further under the action of the bending moment. When this deformation reaches a sufficient degree, chip breakage occurs. During chip breaking, care should be taken to minimize power consumption and avoid excessive additional deformation of the chip. Figure 1.24 shows the evolution of an upwardly curled chip during a single breaking cycle.

Figure 1.24 shows that when the chip curl radius reaches its maximum, the chip is about to break. After the chip is cut, the curl radius is at its minimum, but not zero. This indicates that the chip does not break at the root, but instead retains a portion of its initial curl after leaving the rake face. The chip undergoes strain on the inner annular surface (i.e., the top surface of the chip), which increases as the chip continues to flow out, ultimately breaking the chip into a C-shaped chip. There are four common chip breakage patterns: workpiece obstruction, spiral, flank obstruction, and transverse curl, as shown in Figure 1.25.

The chips formed during the cutting process undergo significant plastic deformation, increasing their hardness while significantly reducing their plasticity and toughness. This phenomenon is known as work hardening. After work hardening, the chips become hard and brittle, easily breaking when subjected to alternating bending or impact loads. The greater the plastic deformation experienced by the chips, the more pronounced the hardness and brittleness, and the easier it is to break. When cutting high-strength, high-plasticity, and high-toughness materials that are difficult to break, efforts should be made to increase chip deformation to reduce their plasticity and toughness, thereby achieving chip breaking.

In deep-hole machining, periodic chip breaking facilitates chip handling. If the chips do not break periodically or break unnaturally, forced breakage must be achieved. Common methods include chip breaker grooves, changing tool geometry, and adjusting cutting parameters. Regardless of the method used, the fracture theory is based on the maximum strain theory. The strain generated within the chip is directly proportional to the chip thickness and inversely proportional to the chip curl radius. To achieve chip breaking, it is necessary to increase chip thickness, decrease chip curl radius, and reduce the chip breakage strain value. These three factors play equally important roles in chip breaking, and only by properly matching their values ​​can satisfactory results be achieved.

5. No Chip Breaking

For difficult-to-machine materials such as austenitic stainless steel 1Cr18Ni9Ti, precipitation-hardened stainless steel 0Cr17Ni4Cu4Nb, and titanium alloy TC4, chip breaking is difficult to achieve, even with the various machining methods mentioned above. Instead, thick, tough, spiral chips are formed, often leading to chip jamming. Practice has proven that chip breaking is not suitable for machining these materials. Instead, long, thin chips can be formed by controlling chip width and thickness to create narrow, thin, wrinkled, and long chips that can be discharged smoothly with the cutting fluid. This ensures smooth cutting during the drilling process, avoiding impacts caused by chip breaking, improving drill bit durability, and ensuring smooth deep-hole machining. In summary, chip management is a key technology in deep-hole machining, and chip breaking alone is not the only goal. For certain difficult-to-cut materials and small-diameter deep-hole machining, chip-free cutting is often a prerequisite for proper cutting.

1.3.3

Proper Guidance

Due to the large aspect ratio of deep holes, the drill rod is thin and long, resulting in low rigidity. This can easily cause vibration and lead to hole deviation, compromising machining accuracy and production efficiency. Therefore, guidance issues in deep-hole machining require a thorough solution. Deep-hole cutting tools typically utilize a three-point self-guiding system: the secondary cutting edge and two guide blocks. Proper arrangement of these three points is one of the key issues required for proper deep-hole machining. The following two principles are generally used to determine the optimal distribution of guide blocks for deep-hole tools.

1. Determine the optimal distribution of guide blocks based on the tool’s stability.

1) Force Conditions of Deep-Hole Tools: The force conditions of deep-hole tools are shown in Figure 1.26. The forces acting on deep hole cutters can be divided into the following three categories. (1) Cutting force: It can be decomposed into mutually perpendicular tangential force Fzi, radial force F and axial force Fu. (2) Friction force: When the guide block rotates relative to the hole wall, friction forces Fn and Fa are generated; when the guide block moves axially, axial friction forces Fa1 and Fa2 are generated between the guide block and the hole wall. Similarly, the friction forces between the secondary cutting edge and the hole wall are Fa and Ft3o. (3) Extrusion force of the guide block: The extrusion forces between the guide block and the secondary cutting edge and the hole wall are N, Ns and Ns. 2) Stability of deep hole cutters For deep hole cutters to work properly in the workpiece, the guide block must always maintain contact with the machined hole wall and a certain pressure exists to ensure the stability of the machining process. Based on this, the concept of “stability” in statics is introduced as the theoretical basis for the reasonable arrangement of the cutting edge and guide block position. Stability here refers to a certain guide block. The stabilizing torque refers to the torque that presses the non-examined guide block against the hole surface with the guide block to be examined as the fulcrum. In contrast, the overturning moment refers to the moment that causes the non-examined guide block to separate from the hole wall. Therefore, a deep-hole tool has two stabilities: the stability Si of guide block 1 and the stability S of guide block 2. For the stability of the entire deep-hole tool, the smaller of the two should be considered the tool’s stability S:

When S1>S2, S=S2;

When Si<S2, S=S1;

When S>1, the tool is stable; when S=1, it is critical; and when S<1, it is unstable.

Stability S can be used as a basis for determining the position of the guide blocks. To calculate the stability of a deep-hole tool, based on four possible guide block placements, as shown in Figure 127, the formulas for calculating the stabilizing moment Mw and the overturning moment Mq for each of the four scenarios are given (see Equations (1.4) to (1.7). In the formula, Fver is the vertical (Z-direction) resultant force, N; Fhor is the horizontal (Y-direction) resultant force, N; Ms is the cutting torque, N·m; Mb is the drill bit support torque, N·m; R is the drill bit radius, m; and 8 and 8 are the position angles of the two guide pads.

According to the calculation formula for the stabilizing moment and overturning moment:

S=f(F, Fo, M, M, R, 8, 8)

After the cutting forces are calculated, Fver, Fhor, Ms, and M are constants, and R is also a constant. In this case, the stability S is a function of the position angles 8 and 8, that is,

S=f(8, 8)

Taking the range of possible variations of 8 and 8, and taking appropriate increments (generally 1° to 5°), calculate the drill bit stability for any combination of 8 and 8. The 8 and 8 that maximize stability (Si=Sz) are taken as the position angles for the guide pads. In practical applications, to reduce computational complexity and save computer runtime, & and & can be appropriately matched (generally, & – & = 90°). This makes S a single-variable function. Through single-variable recursive calculations, the position angle at which & and & achieve maximum stability under a certain matching can be obtained.

2. Determine the appropriate distribution of guide blocks based on the forces acting on the guide blocks.

The layout of deep-hole tool guide blocks can also be determined based on the principle that the normal forces N and N acting on the two guide blocks are equal. When the forces are equal, the two guide blocks wear evenly and meet the blunting standard. This avoids uneven forces and premature tool failure caused by excessive wear of one guide block, thus saving material. Within the possible range of 8 and &, appropriate increments (generally 1° to 5°) are used to calculate the values ​​of & and & that achieve N = Nz. This value is the position angle of the two guide blocks when the forces are equal.

The guide block position angles calculated based on the principle of maximizing stability S and the principle of equal normal forces M and N acting on the two guide blocks are different. Considering deep-hole machining stability, machining accuracy, and tool durability, it’s more reasonable to determine the guide block position angles and θ according to the principle of maximum stability. The resulting unequal forces and uneven wear on the two guide blocks can be compensated by increasing the width of the guide block bearing the greater force, thereby increasing the load-bearing area.

Thus, deep-hole machining technology can be understood as employing a cooling and lubricating fluid with a constant pressure, a chip removal system, and well-guided deep-hole tools, machine tools, and auxiliary equipment to achieve efficient and high-precision deep-hole machining.