Metal Cutting Principles: Forces, Tool Wear, and Operations

Posted on Feb 13, 2025 in Electromechanical Engineering

Cutting Forces and Temperatures

The chip consists of a plastically deformed workpiece. The plastic deformation of the cut layer occurs under the action of a force which must be greater than the resistance of the metal to deformation and destruction.

Temperature: Heat is released during the cutting process as the cut layer is deformed and frictional processes take place in the cutting area. Almost all mechanical work (about 99%) to cut the chip is converted into heat, so the temperature during cutting is very high.

Heat sources in the cutting area:

• Internal friction between the structural elements of the cut layer.
• Plastic deformation of the chip and friction between the chip and the front surface of the tool.
• Friction between the cutting and machined surfaces and the side surface of the tool.

Factors affecting temperature:

• Cutting speed
• Feed rate
• Depth of cut
• Side-rake angle
• Properties of cutting material
• Nose radius

Influence on details:

• Changing dimensions
• Larger form deviations
• Heated surface

Decreasing temperature:

• Use coolant
• Choose tools with good angles
• Try to cut evenly, without blows

Tool Wear and Life

Tool wear: Cutting usually takes place in difficult conditions. The contact surfaces of the tool and the workpiece are subjected to high contact pressures, high temperatures, chemically clean surfaces, so the wear intensity of cutting tools is significantly higher than the wear intensity of rubbing machine parts.

Why it is bad:

• The shape of the cutting edge changes.
• Cutting forces increase.
• Decreases dimensional accuracy.
• The roughness of the machined surface increases.
• A worn tool can break.

Cutting tool wear types:

• Flank wear
• Crater wear
• Notch wear
• Nose radius wear
• Comb (thermal) cracks
• Parallel (mechanical) cracks
• Built-up edge
• Gross plastic deformations
• Edge chipping or frittering
• Chip hammering
• Gross fracture

Abrasive wear: Occurs when hard inserts scratch the surface and remove material from it. Chips, sand particles, solid steel phases, built-up edge, solid particles in soft alloys can be drawn.

Adhesive wear: This wear occurs when the finest particles of the tool material are torn off due to the forces of adhesion. The rake surface of the tool suffers the most from adhesive wear.

Diffuse wear: Diffuse wear occurs when the temperature in the cutting area rises (more than 800°C) and the workpiece softens. At high temperatures, tool and workpiece particles diffuse from one material to another and form solid solutions in these materials.

Oxidative wear: Oxidative wear occurs when tool material components react with atmospheric air. The result is the formation of a notch, as indicated by the fading of the tool color near the notch.

Chemical and electroerosive wear: Chemical wear or corrosion occurs during chemical reactions between the workpiece and the tool material or the coolant and the tool material. Electroerosion wear occurs when an electric current is generated under the action of a thermoelectric force.

Durability of tool: The durability or durability period is the duration of cutting a new or newly sharpened tool before its allowable wear. Durability is expressed in minutes, but can also be measured in units of the workpiece surface of the tool, in units of surface area treated, in units of volume of material removed, and so on. The durability of the tool can be determined from the wear curves.

Cooling

Cooling and lubricating: When cutting metals, the amount of cutting force, the wear intensity and durability of the cutting tools, the quality of the machined surface and dimensional stability also depend on the properties of the environment in which the cutting process takes place. Targeted improvement of the properties of these media is one way to control the cutting process and reduce tool wear.

Liquid Medium: Lubricating-cooling fluids perform lubricating, cooling and washing functions in the cutting area. Lubricating-cooling fluids come in three types: neat oils, water-based and cryogenic fluids.

Gaseous Medium: When undesirable lubricating-cooling fluid residues (in the manufacture of medical, aerospace equipment) such media as air, helium, carbon dioxide, argon and nitrogen are used. Due to the low price, the most common is air.

Solid Medium: Solid media such as talc, graphite, molybdenum disulfide, wax, paraffin, and others are used more as lubricant-coolant fillers rather than alone. If it is not possible to use a lubricant-coolant, it is sometimes used as well.

Plastic Medium: Plastic medium, or consistent lubricants, occupy an intermediate space between lubricating lubricating-cooling fluids and solid media. They consist of two components: a liquid base and a thickener.

Lubricating-cooling fluid fog: Lubricating-cooling fog consists of small droplets of lubricating-cooling fluid mixed with air. This reduces chip contamination.

Minimum Quantity Lubrication (MQL): MQL is a near-dry machining method in which a fine aerosol mist of neat oil is substituted for water-based coolant.

Coolant supplying types:

• through capillary grids between the workpiece and tool surface.
• through cavities resulting from periodic detachment of the top of built-up edge.
• when there is damage between the workpiece and the tool due to vibrations.
• by diffusion through cracks and defects occurring in the cut layer by plastic deformation.

Coolant supplying methods: There are four ways to supply lubricant- coolant to the cutting area: cooling by watering (or low pressure), high pressure, through the tool and fog.

Dry processing: Lubricant-coolant liquid can be dispensed with in some operations. However, certain conditions must be met.

Cutting Tool Materials 1

The cutting tool materials is one of the most important elements of the machining system. Tool material and geometry must be carefully chosen in relation to the workpiece material to be machined, the kinematics and stability of the machine tool to be employed, the amount of material to be removed, and the required accuracy and finishing.

Tool material requirements:

• High penetration hardness
• High deformations resistance
• High fracture toughness
• Chemical inertness to the workpiece material
• High thermal conductivity
• High fatigue resistance
• High thermal shock resistance
• High stiffness
• Adequate lubricity (low friction)

Material types:

• High speed steel (HSS)
• Carbides
• Cermets
• Ceramics
• Superhard polycrystalline materials
• Coated tools

High speed steel: From a chemical point of view, high-speed steel is a steel composed of iron, carbon (about 1%) and a relatively large amount of alloying elements such as W, Mo, Co, V and Cr. This steel is characterized by high temperature resistance, impact resistance and fatigue resistance, tools made of it are easy to sharpen, they are cheap and easy to adapt to a variety of materials. Marking: HSS, HSS-T, HSS-M, HSS P / M.

Carbides: Carbides consist of high temperature sintered tungsten carbide (WC) and cobalt (Co) powders. Tungsten carbide powder can also be mixed with carbides of other metals (titanium (Ti), tantalum (Ta)). Carbide alloys are hard, resistant to fatigue and crushing, resistant to torsion, have good thermal conductivity. The main disadvantage is fragility.

Cermets: Cermets are non-tungsten carbides made of TiC, TiN and TiCN powders bonded with a metal binder. Cermets (compared to hard alloys) have lower diffuse wear, adhesion to the cutting material, lower coefficient of friction in pairs with steel, better chemical stability, high hardness, resistance to abrasive wear and temperature. They cannot be used for impact work due to their high fragility, as the plates break from the cermet due to the impact load.

Ceramics: Ceramics based on aluminium oxide (Al₂O₃) and silicon nitride (Si₃N₄) are used to make the tools. Interchangeable ceramic inserts are commonly used for high speed machining, especially suitable for smooth turning and milling operations. The main disadvantages are high brittleness, low resistance to thermal and mechanical shocks, tendency to decompose.

Superhard polycrystalline materials:

• Cubic boron nitride (CBN) is a very solid material that has a cubic lattice of centered walls made of boron and nitrogen atoms. This material is hard, has a high thermal conductivity and a low coefficient of thermal expansion. Individual crystals are almost never used for blade tools.
• Polycrystalline diamond (PCD) is the hardest of the tool materials, which has the best resistance to abrasion wear, low coefficient of friction during cutting and the best thermal conductivity of all tool materials. Disadvantages: diffusion of carbon atoms into iron at high temperatures, high brittleness, tendency to oxidize.

Cutting Tool Materials 2

Coatings: Most modern tools and their inserts, made of high-speed steel, traditional carbides, and ceramics, are coated with thin layers (from a few to several tens of micrometers) of various materials. Cutting or working surfaces of solid tools are usually coating.

Coating materials: The most commonly used coatings are: titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN), titanium and aluminum nitride (TiAlN), alumina (Al₂O₃), chromium nitride (CrN), hafnium nitride (HfN), titanium diboride (TiB₂), boron carbide (BC) and tungsten diamond type carbon coating (WC/C). The best known multilayer coating combinations are: Al₂O₃ on TiC or TiCN, TiN on TiC, TiN / TiC / TiN, TiN / TiCN / TiN, TiN / TiC / TiCN, TiN / Al₂O₃ / TiN, TiCN / Al₂O₃ / TiN, Al₂O₃ / TiC , TiN / TiC / Al₂O₃ / TiN.

Abrasive materials: An abrasive is any natural or artificial mineral whose grains are hard enough to cut (whack). Abrasives used in the manufacture of abrasive tools are divided into natural and artificial.

Corundum: Corundum is a mineral consisting of 70-95% alumina Al₂O₃ and iron oxide, mica, quartz and other impurities. Commonly used for machining various steels, as well as cast iron and titanium.

Carbides and nitrides: These materials are very hard, have sharp cutting edges, can work at high temperatures. Suitable for processing various hard and soft materials, especially in precision operations (lapping, polishing, honing).

Diamond: The structure, physical, mechanical, thermoelectric, and chemical properties of a very good quality synthetic diamond are almost indistinguishable from that of a natural diamond, and in some places it surpasses it. It is possible to process various materials, suitable for sharpening abrasive discs.

Turning Operations

Turning operations: Turning results in rotating body-shaped details that are symmetrical about the axis of rotation. Both internal and external surfaces are machined.

Basic turning operations: The external (turning) and internal (boring) surfaces are machined on a lathe. Depending on the direction of the feed rate relative to the workpiece axis, the surfaces can be formed by providing a longitudinal, transverse or combined feed rate. Depending on the intensity of the cutting process, there may be rough and smooth (finishing) turning.

Fixing of workpieces: If the workpiece is in the form of a rotating body, it is mounted on the most commonly used lathe mounting device, the three-cam gripper. Long workpieces are additionally supported by the center of the machine tailstock. The blanks can be turned and between the centers, square (or rectangular) cross-section rod blanks are fastened with a 4-cam gripper.

Drilling

Drilling machine produces internal cylindrical surfaces (holes) of moderate accuracy in terms of position, roundness, and straightness. Holes made by drilling are often used for mechanical fasteners such as bolts or rivets. Drilling is preliminary step for processes like tapping, boring, or reaming.

Drill types: The best known are three types of drills: sharpeners, feathers, and prefabricated with mechanically changeable inserts. Sharpeners are made of HSS (also powdered) or carbides (often coated), less often with a shank attached. They come in several types: twist drills, gun drills, stepped and with a guide.

Twist drills: Twist drills are sharpened, with soldered inserts or mechanically fastened carbide crowns. Twist drills come with a conical shank (CM), with a cylindrical shank and a shoulder, with cylindrical shank. Twist drills usually have two cutting edges, but there can be 1 to 4 of them.

Gun (deep holes) drills: Gun drills are used to drill deep holes (length equal to (5-100) d). Such drills are usually prefabricated with a soldered carbide tip or plate. Gun drills have a relatively high performance and a small deviation in the hole when used properly.

Feather drills: The feather drill consists of a bracket, a plate and a mounting screw. They are used to drill large diameter (10 to 150 mm) holes.

Stepped drills: Stepped drills are used to drill step holes in a single tool stroke. One of the most common stepped drills is centering drills used to make center holes at the shaft ends.

Spot drills: Spot drills are used to mark the centers of the holes before drilling. The spot drill is similar to a spiral, but its cutting part is short, making it stiffer.

Drills with interchangeable plates: Such drills are used instead of high-speed steel spiral drills, virtually all are made with internal lubrication-coolant channels. If the plate comes off, it can be rotated as well as the diameter can be adjusted.

BTA drills: Such drills are used in single or dual channel deep hole drilling systems that use a high pressure lubricating-cooling fluid stream. Applies to holes with a diameter of 6 to 300 mm.

Trepanning drills: Trepanning drills have cutting edges only at the periphery and cut only the circular gripper in the workpiece, leaving a solid rod in the center. It can later be used as a blank for other parts.

Taps: Taps are used to thread the inner cylindrical and conical thread. It is a tool that rotates about its axis, similar to a propeller. Threads can be used to thread threads of various profiles: rectangular, trapezoidal, round, support, triangular. Before making a thread with a tap, it is necessary to drill a hole whose diameter coincides with the size of the inner diameter of the thread.

Counterbore, Countersinking and Reaming

Core drills: Core drills are used when it is necessary to widen the beginning of the hole to hide the screw (conical) or bolt (cylindrical) head or to prepare a plane for the bolt head or washer.

Countersink: Core drills are usually made of high-speed steel, less often of carbides. The main characteristics of countersink are apex angle, small and large cone diameters. The number of cutting edges can range from 3 to 12.

Counterbore: The diameter of the counterbore can vary from 2 to 30 mm, and usually has 3 to 6 cutting edges. Their main indicators are the diameter of the guide pin, the diameter of the working part and the length of the working part.

Reaming: Reaming is the additional multi-edged machining of already drilled cylindrical and conical, through and blind holes (usually z = 6-14) to increase the accuracy of the hole diameter (up to IT6), shape accuracy and reduce surface roughness (up to Ra = 0.32 µm, sometimes lower).

Reamers types: Reamers can be single-edged and multi-edged, cylindrical and conical, push-in and insertable, solid and prefabricated, machine and manual, adjustable and non-adjustable, with screw and straight grooves, as well as stepped.

Conical reamers: Conical reamers are commonly used to machine precision taper holes for taper pins (1:50 taper), Morse and metric tapers, and base holes for plug reamers and core drills (1:30 taper).

Milling Operations

The process involves a prismatic workpiece or workpiece with sides. A rotating cutter with multiple cutting edges removes material. Either the cutter or workpiece can move or feed.

Milling configurations: There are two possible end milling configuration depending on the tool velocity relative to the workpiece. In down or climb milling, the feed motion is given in the direction of cutter rotation while in conventional. In up milling, the workpiece is fed in a direction opposite to the cutter motion.

Basic milling operations: Depending on the resulting surface, tool type and shape, and process kinematics, the milling processes are divided as follows:

• Plane milling
• Round milling
• Spiral milling
• Gear milling
• Shaped milling

Face milling: Face milling is an operation in which the auxiliary cutting edges of the face mill generate a flat surface that is perpendicular to the axis of the mill. Face milling can be rough and smooth. The best working conditions are when 3/4 of the face mill diameter is involved in the cutting process.

Peripheral milling: During peripheral milling, the material is removed and the flat surfaces are formed with the main cutting edges. Cylindrical mills with helical teeth are used to reduce loads. Peripheral milling is only suitable for conventional horizontal milling machines, where the cutter is mounted on a slot, one end of which is mounted on a spindle and the other is held in a holder.

Contour milling: It is an operation in which a end mill removes material from the peripheral surface of a workpiece or its machined element with more cutting edges on the side surface and less with end edges. In this way, the external walls of parts, the walls and bottom of pockets, large-diameter holes, steps, grooves are machined. The most productive work takes place when 70-90% of the mill diameter is used during cutting.

3D milling: Milling 3D surfaces is possible with CNC milling machines and machining centers, where it is possible to control tool movements simultaneously along at least three coordinate axes. For this purpose, special end mills are used, the rear cutting edges of which are rounded to a certain radius.

Groove milling: The grooves can be cut with disc mills, then three groove surfaces are machined at once. The grooves can be of various shapes.

Thread (spiral) milling: Such milling is only possible with CNC machines to control the tool feed along three axes. This method is much superior to threading. Milling is performed with special mills, only the outer (or inner) surfaces must be prepared earlier.

Text engraving: Engraving is required when the serial number or other information needs to be marked on the surface of the part. The operation is performed in CNC machining centers, using a small- diameter end mill.

Types of mills: Mills can be classified according to:

• Tooth shape and sharpening method
• The shape and arrangement of the cutting edges with respect to the axis of rotation of the mill
• The inclination of the teeth with respect to the axis of the mill
• Mounting on the machine
• Construction

Grinding Operations

Grinding machines use abrasive wheels that rotate at high speed to either remove a layer of material (in rough grinding) or finish the part surface (in finishing grinding). Differences between grinding and cutting are the scale of the chips produced and the amount of force/ energy required.

Types of grinding: It is possible to sand various surfaces: flat, round, special profile.

Types of abrasive wheels: Abrasive wheels (heads, segments) are produced in various shapes and dimensions. They are selected according to the configuration and dimensions of the workpiece, the requirements for the machined surface, the nature of the technological operation, the type and dimensions of the equipment.

Requirements for grinding:

• Work safety
• Sharpening discs
• Disc balancing
• Disc checking
• Cooling

Honing: Finishing of holes with a rotary tool hon, which is slidable along the axis. This is the final operation, which reduces the surface roughness and makes a special relief for the grease.

Polishing: Polishing is the final operation for extremely smooth surfaces. Soft tools are used for this, and the abrasive particles are fed to the treatment area in the form of a paste or suspension. Various surfaces and materials can be polished.

Other Cutting Operations

Electric discharge machining (EDM): Is a metal fabrication process whereby a desired shape is obtained by using electrical discharges (sparks). Material is removed from the work piece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectricliquid and subject to an electric voltage. Types of EDM: It is two main types of EDM: wire and sink.

Laser cutting: Is a technology that uses a laser to slice materials. The focused laser beam is directed at the material, which then either melts, burns, vaporizes away, or is blown away by a jet of gas, leaving an edge with a high-quality surface finish. Materials for laser cutting: Laser cutting is suitable for a lot of different materials. Among them are different metals, acrylic, MDF, wood, paper, etc.

Waterjet cutting: A water jet cutter is an industrial tool capable of cutting a wide variety of materials using an extremely high-pressure jet of water, or a mixture of water and an abrasive substance. Waterjet cutting is often used during fabrication of machine parts. Materials for waterjet cutting: Waterjet cutting is suitable for: aluminum, brass, carbon steel, copper, stainless steel, titanium, tool steel, ceramic tile, glass, granite, leather, marble, wood, carbon fiber, fiberglass, kevlar, phenolic, acrylic, linoleum tile, polycarbonate, rubber and others.