Chapter 22 Cutting-Tool Materials and Cutting Fluids
Titanium Carbide
A type of COATING material. This Carbide consist of a Nickel-Molybdenum structure. Has a higher wear resistance than Tungsten but is not as tough. Typically used in machining hard materials such as steel and cast irons, and hence it has a higher wear resistance, it can machine at a higher optimal temperature than Tungsten. Has a high flank-wear resistance in machining abrasive materials.
Titanium Nitride
A type of COATING material. This coating has a low friction coefficient, high hardness, resistance to high temperature, and good adhesion to the substrate. They greatly improve the life of high speed steels tools as well as carbide tools. This coating is gold in color. It performs well at high cutting speeds and feeds. Provides the tool low flank wear compared to uncoated tools. Negative side is its limitation of operating at low speeds.
Tungsten Carbide
A type of Carbide cutting material, consists of tungsten and carbide particles bonded together in a cobalt structure. Made using powder metallurgy techniques. Cobalt particles surround Tungsten-carbide particles and then are pressed. MORE COBALT MEANS THE MATERIAL STRENGTH, HARDNESS, AND WEAR RESISTANCE DECREASES, WHILE THE TOUGHNESS INCREASES. Are usually used for cutting steels, cat irons, and abrasive nonferrous materials. NOTE: Have replaced High Speed Steels due to their better performance. PRICE: 0.5-inch Insert $10-25.
Micrograin Carbides
A type of Carbide cutting material. Made of submicron and ultra-fine grained carbides. The grain size is very small (.2-.8 um). Sense its molecular compression compared to the other Carbide, this material is a lot stronger, harder, and wear resistant; thus improving productivity. Are usually used in micro-drills that fabricate circuit boards.
Sialon
A type of Silicon-Nitride-Based Ceramics. Is composed of Silicon, Aluminum, Oxygen, and Nitrogen. Has higher thermal shock resistance than silicon nitride. Used for casting nickel-based superalloys. ISSUE: Have poor chemical stability at high temperatures, thus cannot machine steels.
Cermets
A type of cutting material. A mix of CERamic and MEtal, consist of ceramic particles in a metallic structure. Also known as HOT-PRESSED CERAMICS. Made of 70% Aluminum Oxide and 30% Titanium Carbide. ISSUE: They are brittle and NOT cost effective. Also little improvement is achieved if this material is coated. USE: Light roughing cuts and high-speed finishing cuts.
Stellite
Another name given to Cast-cobalt Alloys. Begins with S.
Cutting Fluids
Fluids used in machining operations. They bring extensive advantages, including: Reduction of friction thus wear which improves tool life and surface finish of workpiece, it cools the cutting zones which also improves tool life by reducing thermal distortion of workpiece, it reduces forces and energy consumption, flushes away chips that are formed, and it protects the machined surface from environmental corrosion. Should be used depending on the severity of the particular machining operation, such as the temperatures and forces that will be present or how effectively the fluid can be supplied to the proper region at the tool-chip interface. ISSUE: Can cause chip to become more curly, thus concentrating the heat closer to the tool tip reducing tool life. USE: (From less->more severe) Sawing, turning, milling, drilling, gear cutting. thread cutting, tapping, and internal broaching. Includes: Oils, Emulsions, Semisynthetics, Sysnthetics.
Uncoated Carbides
Known as the plain carbide. Unlike High-speed Steels, this cutting material is tough throughout a wide range of temperatures. Its hot hardness is wide, thus its wear resistance and toughness has little variability throughout temperatures. ISSUE: Cannot be used at low speeds. PRICE: 0.5-inch Insert $10-15.
Dry Machining
Machining that involves no cutting fluids. Has been shown effective in various machining operations, especially turning, milling, and gear cutting, on steels, steel alloys, and cast iron.
Nanocrystalline
Materials formed at a nano scope. The properties of producing a material at a nano scope is a product that is far more superior in strength than in it's standard form.
Multiphase Coatings
The application of multiple coatings on to a cutting tool. The result is multiple layers with different coatings that provide advantages. The way the layers are placed result in different advantages. Ex. High-speed, continuous cutting: TiC/Al2O3/TiN.
Whisker-Reinforced Tools
The application of reinforcing-fibers in composite cutting-tool materials.
Diamonds Coatings
The coating that includes Polycrystalline Diamond.
Finishing Cuts
The cut that is mostly left for last in order to give a good surface finish. The cut is typically performed at a lower feed and a smaller depth of cut.
Near-Dry Machining
The trend of using less cutting fluids when machining. Alleviates environmental impact and reduces machining costs (7-17% of machining costs comes from cutting fluids). Rather than flooding, a light mist of a very small amount of cutting fluid is used.
Inserts
To save money and time (rather than taking the whole cutting tool out to replace it), a cutting tool with several cutting points were made.
Tool Costs
Tool material, size, shape, chip-breaker features, and quality all determine the ------ of the cutting tool.
Hot Hardness
A PROPERT THE CUTTING-TOOL MATERIAL CAN POSSESS. The ability to maintain strength, hardness, and wear resistance of the tool as temperature is contacted.
Chemical Stability
A PROPERT THE CUTTING-TOOL MATERIAL CAN POSSESS. The tools ability to withstand chemical reactions (wear, adhesions, diffusion) that it might experience when in contact with such chemicals.
Thermal Shock Resistance
A PROPERTY THE CUTTING-TOOL MATERIAL CAN POSSESS. The ability to resist sudden temperature change.
Wear Resistance
A PROPERTY THE CUTTING-TOOL MATERIAL CAN POSSESS. The permissible wear the tool can have before having to replace it.
Toughness
A PROPERTY THE CUTTING-TOOL MATERIAL CAN POSSESS. The repeated (ex. vibration) impact that the tool receives from various forces that limits fracture or chip of the tool.
Flooding
A TYPE OF CUTTING FLUID APPLICATION. A continuous flow of cutting fluid that goes directly at the cutting zone. Flow rates can be 3 gal/minute and up to 60 gal/min. Fluid can also reach high pressures, up to 2000psi!
Mist
A TYPE OF CUTTING FLUID APPLICATION. This type supplies fluid to inaccessible areas. Since there is no stream, you can see the workpiece being machined. Requires ventilation in working zone however so it doesn't effect workers.
Lubricants
A cutting fluid that reduces friction.
Coolants
A cutting fluid that reduces high temperatures when machining. Ex. Water.
Coated Tools
A material placed on cutting-tools that give it the advantageous properties of having LOWER FRICTION, HIGHER RESISTANCE TO WEAR AND CRACKING, HIGHER HOT HARDNESS AND IMPACT RESISTANCE, ACTING AS A DIFFUSION BARRIER BETWEEN THE TOOL AND THE CHIP. These tools also last 10 times more, meaning it can be put in higher stresses such as high cutting speeds. Particularly useful in milling, turning, and drilling. OTHER ADVANTAGES ARE HIGHER HARDNESS AT ELEVATED TEMPERATURES, CHEMICAL STABILITY, LOW THERMAL CONDUCTIVITY, COMPATIBILITY, AND LITTLE TO NO POROSITY.
Cryogenic Machining
A new recent development in machining. Uses nitrogen or carbon dioxide at its liquid form which is sprayed into the cutting zone. The temperature of the liquid reaches -320F (-200C). This is not harmful to the environment, as the nitrogen simply evaporates.
Tool Reconditioning
A process that resharpens cutting-tools that have been worn out. Resharpening can be by hand or using a computer-controlled tool.
Polycrystalline Diamond
A type of diamond coating. Is used widely as a coating for cutting tools, particularly on tungsten-carbide and silicon-nitride inserts. Are effective in machining nonferrous (non Iron) metals, abrasive materials like aluminum alloys containing silicon, and graphite. Almost 10 folds of life can be gained compared to other coating! PRICE: 0.5-inch Insert $50-90.
Cubic Boron Nitride
Next to Diamond, this cutting material is the hardest material available. A layer of polycrystalline cubic boron nitride is added to carbide under high pressure and high temperature. This material provides shock resistance, very high wear resistance, cutting-edge strength, and thermochemical stability. It can work on extreme measure up to 2200F (1200C). Has high oxidation resistance thus good with machining ferrous metals. ISSUE: Can chip and crack, thus it's important to have stiffness in the machine tool to avoid vibration.
Roughing Cuts
The cut that's main task is to remove a ton of material at a quick rate. Surface finish is NOT important in this stage, only getting high feed rates and large cut depths.
Diamond Tools
The hardest material! This cutting material has very high hardness and toughness; it also has abrasive wear resistance. This material also maintains it's cutting edge. Particularly used when a good surface finish or accuracy are required. ISSUE: This material should be used at low strengths and has a low chemical stability at high temperatures. PRICE: 0.5-inch Insert $150-200.
Chip Breaker
The piece attached to the insert that controls how long a chip gets (by breaking it), controlling direction of chip, reducing heat generated, and reducing tendency for vibration.
Silicon-Nitride-Based Ceramics
This cutting material consists of Silicon Nitride along with aluminum oxide, yttrium oxide, and titanium carbide. Have high toughness and hot hardness plus good thermal-shock resistance. Includes Sialon.
Carbides
This cutting material has a high hardness over a wide range of temperature, high elastic modulus, high thermal conductivity, and low thermal expansion.AMONG THE MOST IMPORTANT, COST EFFECTIVE, AND COST-EFFECTVE TOOL AND DIE MATERIALS FOR A WIDE RANGE OF APPLICATIONS. ISSUE: Almost none! They are cost effective than steel and cobalt alloy. TYPES: Tungsten Carbide and Titanium Carbide.
Polycrystalline Cubic Boron Nitride
This cutting material has a high hot hardness range. Although good at maintaining it's strength and wear resistance, it has low chemical stability and a low overall strength. PRICE: 0.5-inch Insert $130-180.
Cast-Cobalt Alloys
This cutting material has high hardness (not as tough as steel though), 58-64 HRC, and has good wear resistance and can maintain their hardness at elevated temperatures. ISSUE: This material is sensitive to sudden impact forces, thus limited at cutting speeds. USES: Usually used for deep continuous roughing cuts at high feeds and speeds. TYPES: This alloy is made of 38-53% Cobalt, 30-33% Chromium, 10-20% Tungsten.
Alumina-Based Ceramics
This cutting material has very high abrasion resistance and hot hardness (consistency of wear and hardness through change of temperatures). THEY ARE MORE CHEMICALLY STABLE THAN HIGH-SPEED STEELS AND CARBIDES, thus adhere (stick) less in metals during machining. This also means it has lower tendency to build an built-up edge. ISSUE: They lack toughness, thus they tend to fracture resulting in premature tool wear. Use: Provides good surface finish in cast irons and steels, they are also effective in high-speed uninterrupted machining, this though goes along side with cutting fluids.
High-Speed Steels
This cutting material is tough to begin with, but does not resist temperature hot-hardness. HRA (Hardness) is around 85, but throughout application of temperature, this material can lose its wear resistance and hardness. They are relatively inexpensive. ISSUE: Low hot hardness. USES: high positive rake-angle tools, interrupted cuts, vibration and chatter machining, complex tool shapes. TYPES: Molybdenum (M-Series) and Tungsten (T-Series), M-Series being the most resistant and a lot cheaper thus more common (about 95%).
Ceramics
Unlike some, this cutting material hardness rises as it is exposed to heat. It also has high abrasive wear resistance. ISSUE: As material gets harder, its toughness lowers. This material is made of fine grained high-purity aluminum oxide. They are pressed into insert shapes in high pressure. PRICE: 0.5-inch $30-50.