Factors affecting selection of Tool Materials Hardness and condition of the workpiece material Operations to be performed Amount of stock to be removed Accuracy and finish requirements Type, capability, and condition of the machine tool to be used Rigidity of the tool and workpiece SAM, VJTI
Factors affecting selection of Tool Materials Production requirements influencing the speeds and feeds selected Operating conditions such as cutting forces and temperatures Tool cost per part machined, including initial tool cost, grinding cost, tool life, frequency of regrinding or replacement, and labor cost—the most economical tool is not necessarily the one providing the longest life, or the one having the lowest initial cost No Tool Material Satisfies All These Criterion SAM, VJTI
High Speed Steel High alloy steel They are either molybdenum or tungsten based but necessarily contains 4% chromium M = Molybdenum T = Tungsten M >40 = Super HSS materials; capable of treating to high hardness SAM, VJTI
Cemented Tungsten Carbide Tungsten carbide is extremely hard and offers the excellent resistance to abrasion wear The most significant benefit of TiC is a reduction in the tendency of the tool to fail by cratering. The most significant contribution of TaC is that it increases the hot hardness of the tool, which in turn reduces thermal deformation Effect of Co as a binder Co is more sensitive to heat, abrasion and welding The more cobalt present, the softer the tool, making it more sensitive to thermal deformation, abrasive wear and chip welding Cobalt is stronger than carbide. Therefore, more cobalt improves the tool strength and resistance to shock
Cemented Tungsten Carbide Classification system ISO classification number ranges from 05 to 50 : e.g. P20, K35, M40; 05 is most wear resistance whereas 50 is most fracture resistance Coated carbide tools is the most significant advance in cutting tool materials since the development of WC tooling Various single and multiple coatings of carbides and nitrides of titanium, hafnium, and zirconium and coatings of oxides of aluminum and zirconium, as well as improved substrates better suited for coating, have been developed to increase the range of applications for coated carbide inserts. C- Classification C1 to C4 for Cast iron C5 to C8 for Steel ISO- Classification P = Stainless Steel M = Steel K = Cast Iron
Ceramics Ceramics are primarily aluminum oxides Inconsistent and unsatisfactory results during initial periods of development Improvements :better control of microstructure (primarily in grain size refinement) and density, improved processing, the use of additives, the development of composite materials, and better grinding and edge preparation methods. Tools made from these materials are now stronger, more uniform, and higher in quality Types of ceramics Plain ceramics, which are highly pure (99 percent or more) and contain only minor amounts of secondary oxides (produced by powder metallurgy)
Ceramics Types of ceramics Plain ceramics, which are highly pure (99 percent or more) and contain only minor amounts of secondary oxides (produced by powder metallurgy) Composite ceramics : are Al203-based materials containing 15–30 percent or more titanium carbide (TiC) and/or other alloying ingredients
Ceramics Advantages Increased productivity: Ceramic cutting tools are operated at higher cutting speeds than tungsten carbide tools Good hot hardness, low coefficient of friction, high wear resistance, chemical inertness, and low coefficient of thermal conductivity Most of the heat generated during cutting is carried away in the chips, resulting in less heat buildup in the workpiece, insert and toolholder Better size control by less tool wear Machining of many hard materials
Ceramics Limitations Brittle than carbides Less mechanical and thermal shock resistance Less interchangeability with the carbide tool holders Applications High speed machining of steel and cast iron requiring continuous machining Most suitable for machining of chemically active materials Face milling and turning applications
Single crystal and polycrystalline diamonds Best suited for precision machining with very high surface finish and to increase productivity by reducing downtimes Diamond is the cubic crystalline form of carbon that is produced in various sizes under high heat and pressure. Natural, mined single-crystal stones of the industrial type used for cutting tools are cut (sawed, cleaved, or lapped) to produce the cutting-edge geometry required for the application. Advantages Hardest material known. Indentation hardness is five times than carbide. Extreme hardness and abrasion resistance can result retaining their cutting edges virtually unchanged throughout most of their useful lives Because of the diamond’s chemical inertness, low coefficient of friction, and smoothness, chips do not adhere to its surface or form built-up edges when nonferrous and nonmetallic materials are machined.
Cubic Boron Nitride Super abrasive crystal that is second in hardness and abrasion resistance only to diamond CBN crystals are used most commonly in super abrasive wheels for precision grinding of steels and super alloys Advantages Greater heat resistance than diamond tools High level of chemical inertness Compacted CBN tools are suitable, unlike diamond tools, for the high speed machining of tool and alloy steels with hardness to Rc70, steel forgings and Ni-hard or chilled cast irons with hardness from Rc45–68, surface-hardened parts, and nickel or cobalt-based super alloys They have also been used successfully for machining powdered metals, plastics, and graphite.
Abrasive Wear (Abrasion) Abrasive wear occurs as a result of the interaction between the workpiece and the cutting edge. The width of the wear land is determined by the amount of contact between the cutting edge and the workpiece. Rapid failure Constant Period Break In Period
Heat Related Tool Failure Mechanisms Cratering(Chemical Wear) The chemical properties of the tool-material and the affinity of the tool-material to the workpiece material determine the development of the crater wear mechanism Hardness of the tool-material does not have much affect on the process. The metallurgical relationship between the materials determines the amount of crater wear. Tungsten carbide and steel have an affinity to each other The mechanism is very temperature-dependent, making it greatest at high cutting speeds. Atomic interchange takes place with a two-way transfer of ferrite from the steel into the tool. Carbon also diffuses into the chip.
Heat Related Tool Failure Mechanisms Built-up Edge (Adhesion) It occurs mainly at low machining temperatures on the chip face of the tool. It can take place with long chipping and short-chipping workpiece materials—steel and aluminum. This mechanism often leads to the formation of a built-up edge between the chip and edge. It is common for the build-up edge to shear off and then to reform. At certain temperature ranges, affinity between tool and workpiece material and the load from cutting forces combine to create the adhesion wear mechanism. Machining work-hardening materials, such as austenitic stainless steel, this wear mechanism can lead to rapid build-up at the depth of cut line resulting in notching as the failure mode.
Heat Related Tool Failure Mechanisms Built-up Edge (Adhesion) Increased surface speeds, proper application of coolant, and tool coatings are effective control actions for built-up edge Thermal Cracking (Fatigue wear) Thermal cracking is a result of thermo mechanical actions Temperature fluctuations plus the loading and unloading of cutting forces lead to cracking and breaking of the cutting edge Carbide and ceramics are relatively poor conductors of heat which leads to fatigue wear Thermal Deformation As the cutting edge loses its hot hardness the forces created by the feed rate cause the cutting edge to deform
Mechanical Failure Mechanisms Chipping (Mechanical) Small chipping of tool material Cutting force should be less than shearing force. Chipping is larger on flank surface than on a face Rake Surface Flank Surface
Mechanical Failure Mechanisms Insert Fracture When the edge strength of an insert is exceeded by the forces of the cutting process the inevitable result is the catastrophic failure called fracture. Excessive flank wear land development, shock loading due to interrupted cutting, improper grade selection or improper insert size selection are the most frequently encountered causes of insert fracture