Chapter 4 CUTTING TOOLS: Material and Geometry Prof. Dr. S. Engin KILIÇ

Cutting Tools Tools must be so selected that they can cut properly and efficiently under the selected cutting conditions which may lead to a harsh cutting environment due to high cutting temperatures and high cutting pressures. In selection two principal aspects must be considered: a) tool geometry b) tool material. The geometry  the optimum performance for the given tool material and the operation. The tool material  highest possible strength, resistance and durability against forces, temperatures and wearing action during machining

Cutting Tools - Geometry The cutting tool geometry is of prime importance because it directly affects: 1. Chip control: The tool geometry defines the direction of chip flow. This direction is important to control chip breakage and evacuation. 2. Productivity of machining: The cutting feed per revolution is considered the major resource in increasing productivity. This feed can be significantly increased by adjusting the tool cutting edge angle. For example, the most common use of this feature is found in milling, where increasing the lead angle to 45° allows the feed rate to be increased 1.4-fold. As such, a wiper insert is introduced to reduce the feed marks left on the machined surface due to the increased feed. 3. Tool life: The geometry of the cutting tool directly affects tool life as this geometry defines the magnitude and direction of the cutting force and its components, the sliding velocity at the tool–chip interface, the distribution of the thermal energy released in machining, the temperature distribution in the cutting wedge etc. 3

Cutting Tools - Geometry 4. The direction and magnitude of the cutting force and thus its components: Four components of the cutting tool geometry, namely, the rake angle, the tool cutting edge angle, the tool minor cutting edge angle and the inclination angle, define the magnitudes of the orthogonal components of the cutting force. 5. Quality (surface integrity and machining residual stress) of machining: The correlation between tool geometry and the theoretical topography of the machined surface is common knowledge. The influence of the cutting geometry on the machining residual stress is easily realized if one recalls that this geometry defines to a great extent the state of stress in the deformation zone, i.e., around the tool. 4

Cutting Tools - Geometry (a) Schematic illustration of a right-hand cutting tool for turning. Although these tools have traditionally been produced from solid tool-steel bars, they are now replaced by inserts of carbide or other tool materials of various shapes and sizes, as shown in (b). 5

Cutting Tools - Geometry

Cutting Tools - Geometry Side Rake Angle (gs): great influence on chip formation less deformation of removed layer of w.p. as increased  less resistance to chip formation lower cutting forces lower power consumption less heat transfer area on rake substantial increase  weakening of cutting edge catastrophic tool failure

Cutting Tool Geometry Effect of Side Rake Angle on Tool Performance - Cutting force + - Side rake angle Tool life H3 H2 H1 H1>H2>H3 g1<g2<g3 Increased w.p hardness g1 g2 g3 Effect of Side Rake Angle on Tool Performance Add picture here!

Side Clearance Angle less rubbing between flank and w.p. as increased  reduced heat generation at tertiary def. zone reduced flank wear excessive clearance  reduction in strength of wedge larger for soft and ductile w.p., smaller for hard and brittle w.p. Tool life Side clearance angle a1 a2 a3 f1 f2 f3 f1> f2>f3 a1<a2<a3

Side Cutting Edge Angle Determines thickness and width of uncut chip layer: ac= f cos k aw= ap/cos k Hence, for the given feed and depth of cut, an increase in k causes a decrease in chip thickness and an increase in chip width. As increased: i) interface temp.  decreases since ii) distribution of heat generated over a longer cutting edge, hence longer tool life and higher permissible feed

Side Cutting Edge Angle (Cont’d) iii) increase in radial force component  high lateral deflections, poor dim. accuracy, severe vibration and chatter especially in long and slender w.p. iv) better surface finish since Rmax= f/(tank+cotke) where Rmax= max. surf. rough., f = feed rate k = side cut. edge angle, ke= end cut. edge angle Amplitude of chatter k Cutting speed for a fixed tool life 300 Cemented carbide HSS

End Cutting Edge Angle ke as increased: i) less cutting at the end cutting edge, hence less friction between end flank and finished w.p. surface and higher tool life ii) lower surface quality substantial increase  decrease in tool included angle  poorer heat transfer from the nose  shorter tool life Double check! 50-100 ke Cutting speed for a fixed tool life

Back Rake Angle b=0 b=(+) b=(-) Back Rake Angle basically affects chip flow.

Back Rake Angle (Cont’d) It also affects specific cutting energy and chip-tool interface temperature. Note that for machining hard materials with cemented carbides, b  -200

Cutting Tool Materials Requirements: Hot hardness Wear resistance Toughness Low friction Favorable cost Classification: Carbon Tool Steels Medium Alloy Steels High Speed Steels Cast Cobalt Based Alloys Cemented Carbides Ceramics and Ultra-hard Materials

Cutting Tool Materials Cutting Tool Materials with Their Approximate Dates of Initial Use and Allowable Cutting Speeds ______________________________________________________________________________________ Year of Allowable Cutting Speed Tool Material Initial Use ft/min (m/min)a Plain carbon tool steel 1800s Non steel cutting Below 30 (10) Steel cutting Below 15 ( 5) High-speed steel 1900 Nonsteel cutting 75-200 (25-65) Steel cutting 50-100 (17-33) Cast cobalt alloys 1915 Nonsteel cutting 150-600 (50-200) Steel cutting 100-300 (33-100) Cemented carbides (WC) Nonsteel cutting 1930 1000-2000 (330-650) Steel cutting 1940 300-900 (100-300)

Cutting Tool Materials Cutting Tool Materials with Their Approximate Dates of Initial Use and Allowable Cutting Speeds (cont’d) ______________________________________________________________________________________ Year of Allowable Cutting Speed Tool Material Initial Use ft/min (m/min)a Cermets (TiC) 1950s Steel cutting 500-1200 (165-400) Ceramics (Al2O3 ) 1955 Steel cuttingb 1000-2000 (330-650) Synhetic diamonds 1954, 1973 Nonsteel cutting 1200-4000 (390-1300) Cubic boron nitride 1969 Steel cutting 1500-2500 (500-800) Coated carbides Steel cuttingc 1970 500-1200 (165-400) ____________________________________________________________________________________________ aAllowable cutting speeds are expressed as a range of values because of the variety of work materials and applications machined with these tools. The values are intended to represent typical and comparative speeds, not absolute limits. bCeramic tools are normally used at lower feeds and depths because of their brittleness. cCoated carbides are normally used as substitutes for steel cutting grades of cemented carbides.

Carbon Tool Steels Martensite based / tempered & hardened. Operate at low cutting speeds. Poor hot hardness (max. temp. 200oC) Typical composition: 0.8 - 4.3 % C 0.1 - 0.4 % Si 0.1 - 0.4 % Mn Hardness and wear resistance at room temperature increase with increased C% up to 0.8 - 1%. Generally used for cutting wood and plastics.

Medium Alloy Steels Improved hardenability due to small additions of Cr & Mo. Addition of up to 4% of W improves wear resistance. Poor hot hardness is NOT satisfactory for high speed turning or milling.

High Speed Steels (HSS) First introduced in 1900 by Taylor and White. Superior hot hardness and wear resistance (max. temp 600oC). Typical compositions of various High Speed Tool Steels: Designation Type W Cr V Mo C Fe T-1 W 18 4 1 - 0.7 Bal. M-1 Mo 1.5 4 1 8.5 0.8 Bal. M-2 W-Mo 6 4 2 5 0.8 Bal. American Institute for Steel Industry (AISI) classifies them based on composition: Tungsten based: T1 – T9, T15 Molybdenum based: M1 – M10 Molybdenum + Cobalt based: M30 – M46

HSS (Cont’d) Tungsten and Molybdenum behave in the same general way, however Mo is twice as effective as W to improve hot hardness. They tend to increase hot hardness by forming strong complex carbides. Addition of Co to the structure in 4, 8 or 12% further improves the hot hardness. E.g. T-4 has the same composition as T-1 but 4% Co is added. It increases hot hardness by going into the solution in ferrite matrix increasing the recrystallisation temperature.

HSS (Cont’d) Tungsten and Molybdenum behave in the same general way, however Mo is twice as effective as W to improve hot hardness. They tend to increase hot hardness by forming strong complex carbides. Addition of Co to the structure in 4, 8 or 12% further improves the hot hardness. E.g. T-4 has the same composition as T-1 but 4% Co is added. It increases hot hardness by going into the solution in ferrite matrix increasing the recrystallisation temperature.

High Speed Steels (Cont’d) Vanadium inhibits grain growth at high temperatures required in heat treatment and increases the wear resistance. Vanadium steels are very difficult to grind and used machining highly abrasive stock. Co & Mo have a tendency to promote decarburization. Such steels should be ground to a greater depth in finishing to remove decarburized layer.

High Speed Steels (Cont’d) Alloying Elements in High-Speed Steel and Their Effects on Properties and Processing _______________________________________________________________________________________ Alloying Element Functions in High Speed Steel Tungsten Increases hot hardness Improves abrasion resistance through formation of hard carbides in HSS Molybdenum Increases hot hardnesss Chromium Depth hardenability during heat treatment Corrosion resistance (minor effect) Vanadium Combines with carbon for wear resistance Retards grain growth for better thoughness Cobalt Increases hot hardness Carbon Principal hardening element in steel Provides available carbon to form carbides with other alloying elements for wear resistance.

High Speed Steels (Cont’d) HSS tools are manufactured in wide range of sizes and shapes but mostly in the form of solid tools. Solid tools are then ground to required geometry. There is tendency of clamping, brazing or welding HSS tool to a cheaper low alloy or carbon steel body. Recent developments are Powder metal HSS Coated HSS

Powder Metal HSS Powder Metal HSS has the following advantages: Superior structure. Free from segregation. Ensure good and nearly uniform properties in all directions. Lower incidence of premature failure. Ability to produce steels with higher alloy content.

Coated HSS Coated HSS has the following features: Tools can be coated with thin layers of refractory metal carbide or nitride using physical or chemical vapor deposition techniques (less than 10m thickness). Titanium nitrate (TiN) coating has a distinguishing gold color whereas titanium carbide (TiC) has a black color. Life may be as high as 300% or 400% of the life of uncoated tools Built-up edge formation is nearly eliminated. Regrinding must be followed by a careful polishing and recoating.

Cobalt Based Alloys Known as stellites, composed of a number of nonferrous alloys high in cobalt. Representative composition: Co: 40-50% Cr: 27-32% W: 14-29% C: 2-4% Cannot be heat treated and are used as cast. Ground to its final shape (single-edge tools or saw blades) If compared to HSS, stellites can retain hardness at a much higher temperature. They can be used at higher cutting speeds than HSS tools (25% ... 200%)

Cobalt Based Alloys (Cont’d) Not as hard as carbon tool steels at room temperature but retain hardness to much higher temperatures. Used when tools are required to be used for a wide range of cutting speeds, i.e. facing very large diameter parts at constant rotational speed. Not widely used. Expensive due to shortages on strategic materials (Co, W, Cr, etc.) Brittle, hence need proper support.

Cemented Carbides Became popular during WWII. Increased cutting speed by four- or fivefolds (if compared to HSS!). Produced by Powder Metallurgy Mixtures of transition metal carbides and metals in which the metal, usually the cobalt, binds carbides together. Contains carbides of tungsten, titanium and tantalum at least 80% by volume. Strongly metalic in character Good electrical and thermal conductivities Metalic appearance

Cemented Carbides (Cont’d) Characteristics of Cemented Carbide Tools: High hardness over a wide range of temperatures. Maintain hardness up to 1200oC. Very stiff (Young's modulus three times that of steel). Very brittle No plastic flow even at very high stresses (up to 3.5 GPa). Low thermal expansion. Relatively high thermal conductivity (especially K-Grade). Strong tendency to form pressure welds at low cutting speeds.

Cemented Carbides (Cont’d) Carbide grades are classified according to codes developed by various organizations: C1 – C2 grades (AISI) or K grade (ISO): used for machining cast iron (CI) C4 – C8 grades or P grade (ISO): used for machining steel. M-grade: general purpose C1-C3 grades contain only tungsten carbide and cobalt. Effective in machining CI and certain abrasive nonferrous alloys. Not suitable for steels due to their high affinity towards that alloy Addition of titanium carbide and/or tantalum carbide lowers that affinity, hence making steel cutting possible (steel P-grades).

Carbide Inserts

TiC Based Tools Commonly used in Automotive Industry Employed as throw-away tool tips. Difficult to braze. Bonding metal is Ni instead of Co (in 10-20%). Resistant to diffusion wear in steel cutting. Cutting speed is 2 ... 5 times that of HSS. Can be used at higher cutting speeds than the conventional WC based tools. Low toughness. Lack reliability and consistency of performance

Coated Tools About 10 m thick coatings of TiC, Al203, TiN can be coated on the surface of a tough WC substrate using physical or chemical vapour deposition technique. First layer: TiC (Strength and wear resistance) Second layer: Al203 (Chemical stability at high temp and resistance to abrasive wear) Final layer: TiN (Low coefficient of friction) Reduce tool wear. Increase in cutting speed is possible.

Ceramics Basic material is Alumina (Al203) and may contain Mg0, TiO and other additions to promote densification and grain size stability. Major advantages are Retention of hardness and compressive strength to higher temperatures than with carbides. Practically inert to steel up to melting point. Lower toughness and tensile strength than that of carbides. Non-metallic in character, hence electrical insulator with poor thermal conductivity.

Ceramics (Cont’d) Can be used to cut steels at much higher speeds than possible with carbides (a cutting speed of 600-750 m/min at a feed of 0.25 mm without excessive wear when cutting CI and steels). Negative rake throw-away tools are used. Main usage in cutting grey CI with very good surface finish to eliminate subsequent grinding operation (e.g. clutch facings and brakes). Alumina containing up to 30% TiC is suitable for turning and milling operations on CI and continuous machining of steel. Not suitable for machining Al-alloys.

Ceramics (Cont’d) Edge Chamfer, commonly used on ceramic and carbide inserts: C~0.08-0.5mm; b~10-300

Sialons Si-Al-0-N's are silicon nitrade-based materials with aluminum and oxygen additions. Tougher than alumina. In interrupted cutting, higher feeds and speeds without fracture are possible than those attainable with alumina ceramics. In machining aerospace alloys, Ni-based gas turbine discs are faced with sialon tips at 180-300 m/min at a feed of 0.2 mm/rev, whereas carbide tools can be used at only 60 m/min. Higher thermal conductivity. Lower coefficient of expansion. Increased resistance to shock and thermal fatigue compared to alumina.

Cubic Boron Nitrade Next hardest substance to diamonds. Two commercially available products: BZN (GEC) and Amborite (DeBeers) different in character. BZN-laminated tool tip, consolidated CBN (0.5 mm thick) on cemented WC-Co substrate. Amborite-entirely of consolidated CBN. 20-23 times as costly as cemented carbide tools. Advantage over diamonds, its stability at high temperatures (over 1000oC) in air or in contact with iron and other metals.

Cubic Boron Nitrade (Cont’d) CBN can be most economically used in machining hardened steel (60-68 Rc) and chilled CI at speeds (45-60 m/min) and feeds of 0.2-0.4 mm/rev. Long tool life so that rolls may be machined to a dimensional tolerance and surface finish which eliminate grinding operation. High hot hardness value. Excellent abrasive resistance and resistance to react with ferrous materials. Good toughness when used with negative rake and chamfers can be used for interrupted cutting of hardened steel.

Diamonds Hardest of all materials. Used in operations where other tools cannot perform effectively. Have much lower wear rate and longer tool life than carbides and ceramics where abrasion is dominant wear mechanism. Single crystal natural diamonds are used to produce surfaces of extremely high accuracy and finish. (e.g. optical instruments and gold jewellery). Deficient in toughness, easy chipping of cutting edges. Polycrystalline diamond tools are made with a layer of consolidated synthetic diamonds (0.5 - 1 mm thick) bonded on cemented carbide substrates (2 - 2.5 mm thick).

Diamonds (Cont’d) Cost 20-30 times the equivalent carbide tool. Edges less sensitive to accidental damage. Maintain exceptional wear resistance. Recommended for machining aluminium alloys (speeds can be over 500 m/min with long life). Also used in machining copper and copper alloys and cemented carbides in pre-sintered condition. Not used for high speed machining of steel and nickel because of excessive wear. Diamond does not revert to graphitic form in the absence of air at temperatures below 1500oC. In contact with iron, graphitization begins just over 730oC.

Hardness of Tool Materials

Hardness of Tool Materials Viktor P. Astakhov and J. Paulo Davim