Mastering Cutting Force, Temperature, and Tool Materials in Production Engineering

Introduction to Cutting Mechanics in Production Engineering

In production engineering, understanding the mechanics of cutting operations is crucial for optimizing manufacturing processes. This tutorial covers key concepts from EML6324 Fundamentals of Production Engineering, including cutting forces, shear angles, chip thickness, temperature effects, and tool material selection. Whether you are preparing for an exam or brushing up on fundamentals, these insights will help you grasp the core principles.

Cutting Forces and Their Dependence on Process Parameters

One of the first questions in any cutting operation is how changes in depth of cut and rake angle affect cutting forces. Increasing the depth of cut generally increases the cutting force because more material is being removed. Conversely, decreasing the rake angle (making it more negative) increases shear forces, not decreases them. This is because a smaller rake angle increases chip deformation and friction.

For example, in orthogonal cutting of a steel rod, if the depth of cut is doubled, the cutting force may increase by nearly the same factor. Similarly, a negative rake angle tool increases the shear plane area and thus the shear force. Understanding these relationships helps in selecting optimal cutting parameters for efficiency and tool life.

The Role of Cutting Fluids

Cutting fluids serve three major roles: cooling, lubrication, and chip removal. They reduce temperature at the tool-chip interface, minimize friction, and flush chips away from the cutting zone. In high-speed machining, effective cooling can prevent thermal damage to the workpiece and tool.

"Cutting fluids are essential for maintaining tool life and surface finish in production engineering."

Orthogonal Cutting Calculations: Shear Angle, Chip Thickness, and Velocities

Consider an orthogonal cutting operation with a rake angle α = 0°, chip width b = 5 mm, shear-plane angle φ = 28°, cutting speed v = 3 m/s, undeformed chip thickness h1 = 0.25 mm, force F = 2200 N, and friction angle β = 20°. We can calculate several parameters:

Chip Thickness After Cut (h2)

The chip thickness ratio r = h1/h2 = sin φ / cos(φ - α). With α = 0°, r = sin φ / cos φ = tan φ = tan 28° ≈ 0.5317. Thus h2 = h1 / r = 0.25 / 0.5317 ≈ 0.470 mm.

Shearing Velocity (vs) and Chip Velocity (vc)

Shearing velocity vs = v * cos α / cos(φ - α) = 3 * cos 0° / cos 28° ≈ 3 / 0.8829 ≈ 3.398 m/s. Chip velocity vc = v * sin φ / cos(φ - α) = 3 * sin 28° / cos 28° ≈ 3 * 0.4695 / 0.8829 ≈ 1.595 m/s.

Shearing Force (Fs) and Power (Ps)

Fs = F * cos(φ + β - α) / cos(β - α). Here φ+β-α = 28+20-0 = 48°, β-α = 20°. So Fs = 2200 * cos 48° / cos 20° ≈ 2200 * 0.6691 / 0.9397 ≈ 1567 N. Shearing power Ps = Fs * vs ≈ 1567 * 3.398 ≈ 5325 W.

Friction Force (Ff) and Power (Pf)

Ff = F * sin β = 2200 * sin 20° ≈ 2200 * 0.3420 ≈ 752.4 N. Friction power Pf = Ff * vc ≈ 752.4 * 1.595 ≈ 1200 W.

Temperature Distribution in Cutting

The maximum temperature in cutting occurs approximately at the middle of the tool-chip interface due to two principal heat sources: the shear plane and the tool-chip interface. The shear plane generates heat from plastic deformation, while the interface generates heat from friction. The combined effect creates a temperature peak away from the cutting edge, typically at the center of the contact length. This is critical for tool wear and thermal damage.

Requirements for Cutting Tool Materials

Cutting tool materials must satisfy three basic requirements: hardness (to resist wear), toughness (to withstand impact and chipping), and chemical stability (to avoid reaction with the workpiece at high temperatures). For example, high-speed steel (HSS) is tough but less hard, while ceramics are hard but brittle.

Purpose of Chamfers on Cutting Tools

Chamfers on cutting tools serve to strengthen the cutting edge, reduce chipping, and improve heat dissipation. They are commonly used on ceramic and carbide inserts to prevent edge fracture during interrupted cuts.

Effect of Temperature on Cutting-Tool Performance

Temperature has a significant effect on cutting-tool performance. High temperatures can cause thermal softening of the tool material, reducing hardness and accelerating wear. Additionally, temperature gradients can induce thermal stresses leading to cracking. For instance, in high-speed machining of steels, the tool tip may reach 1000°C, necessitating heat-resistant materials like cubic boron nitride (CBN).

Diamond vs. Cubic Boron Nitride for Machining Steels

Cubic boron nitride (CBN) is more suitable for machining steels than diamond because diamond reacts chemically with iron at high temperatures, causing rapid tool wear. CBN, however, is chemically inert to iron and retains hardness at elevated temperatures, making it ideal for ferrous materials.

Complex Grades vs. Straight Grades for Cutting Steels

Complex grades of cutting tools (e.g., coated carbides) should be used instead of straight grades (e.g., uncoated WC-Co) to cut steels because they offer a balance of wear resistance and toughness. Coatings like TiN, TiCN, and Al2O3 reduce friction and heat, while the tough substrate resists fracture. Straight grades may lack sufficient hardness or thermal stability for steel cutting.

Consequences of Thermal Expansion Mismatch in Coatings

If a cutting tool coating has a different coefficient of thermal expansion than the substrate, thermal cycling during cutting can cause coating delamination or cracking. This reduces tool life and may lead to catastrophic failure. For example, a ceramic coating on a carbide substrate must have matched expansion to avoid stress at the interface.

Coating Processes for Carbide Inserts

The two best processes for providing thin coatings on coated carbide inserts are chemical vapor deposition (CVD) and physical vapor deposition (PVD). CVD uses chemical reactions to deposit a uniform layer, while PVD uses physical sputtering or evaporation. Both produce adherent, wear-resistant coatings.

Hardness Comparison of Tool Materials

Among aluminum oxide, cubic boron nitride, high-speed steel, titanium carbide, and tungsten carbide, cubic boron nitride has the highest hardness, followed by aluminum oxide and titanium carbide. Tungsten carbide and HSS are less hard but tougher.

Why Ceramic Tools Use Negative Rake Angles

Ceramic cutting tools are generally designed with negative rake angles because ceramics are brittle and negative rake angles increase the compressive stress on the cutting edge, reducing the risk of fracture. Negative rake also strengthens the edge and improves heat dissipation.

Why Carbide Tools Are Not Fully Replaced by Ceramics

Despite advantages like higher hardness and wear resistance, ceramic cutting tools are not used to a greater extent because they are more brittle, have lower toughness, and are more expensive than carbide tools. Carbide tools also offer better performance in interrupted cuts and with coolants.

Conclusion

Mastering the fundamentals of cutting forces, temperature distribution, and tool material selection is essential for production engineering. By applying these principles, engineers can optimize machining processes for efficiency, quality, and cost. For more practice, work through similar calculations and review the roles of cutting fluids and tool coatings.