Cutting tools -NAM Two principal aspects: 1. Tool geometry 2. Tool material According to the number of major cutting edges (points) involved as follows: • • • Single point: e.g., turning tools, shaping, planning and slotting tools and boring tools Double (two) point: e.g., drills Multipoint (more than two): e.g., milling cutters, broaching tools, hobs, gear shaping cutters etc. and other very hard tool materials Three ways of holding and presenting the cutting edge for a single-point tool: (a) solid tool, typical of HSS; (b) brazed insert, one way of holding a cemented carbide insert; and (c) mechanically clamped insert, used for cemented carbides, ceramics, Common insert shapes: (a) round, (b) square, (c) rhombus with two 80 point angles, (d) hexagon with three 80 point angles, (e) triangle (equilateral), (f) rhombus with two 55 point angles, (g) rhombus with two 35 point angles. Also shown are typical features of the geometry. What is tool signature ? And what are the different systems of specifying tool geometry? In simple words The numerical code that describes all the key angles of a given cutting tool is called tool signature Convenient way to specify tool angles by use of standardized abbreviated system is known as tool signature or tool nomenclature. The tool signature comprises of seven elements and is specified in different systems . Tool-in-Hand System Machine Reference System - ASA system Tool Reference Systems 1. 2. 3. ◦ ◦ 4. Orthogonal Rake System - ORS Normal Rake System - NRS Work Reference System - WRS There is no quantitative information, i.e., value of the angles. seven element defining the tool signature ASA ( American Standards Association) system πR = Reference plane; plane perpendicular to the velocity vector πX = Machine longitudinal plane; plane perpendicular to πR and taken in the direction of assumed longitudinal feed πY = Machine Transverse plane; plane perpendicular to both πR and πX [This plane is taken in the direction of assumed cross feed] The axes Xm, Ym and Zm are in the direction of longitudinal feed, cross feed and cutting velocity (vector) respectively. • Rake angles γx = side (axial rake: angle of inclination of the rake surface from the reference plane (πR) and measured on Machine Ref. Plane, πX. γy = back rake: angle of inclination of the rake surface from the reference plane and measured on Machine Transverse plane, πY. • Clearance angles: αx = side relief angle: angle of inclination of the principal flank from the machined surface and measured on πX plane. αy = End relief angle: same as αx but measured on πY plane. • Cutting angles: φe = side cutting edge angle: angle between the principal cutting edge (its projection on πR) and πY and measured on πR φe = end cutting edge angle: angle between the end cutting edge (its projection on πR) from πX and measured on πR • Nose radius, r (in inch) r = nose radius : curvature of the tool tip. It provides strengthening of the tool nose and better surface finish. Tool signature : γy - γx- αy- αx- φe- φe-r Tool signature : 8 -14 - 6 – 6 - 6 – 15 -1/8 This system does not indicate the behaviour of tool in actual practice. Hence actual cutting condition include the side cutting edge OR principle cutting edge. πR = Refernce plane perpendicular to the cutting velocity vector, CV πC = cutting plane; plane perpendicular to πR and taken along the principal cutting edge πO = Orthogonal plane; plane perpendicular to both πR and πC and the axes; Xo = along the line of intersection of πR and πO Yo = along the line of intersection of πR and πC Zo = along the velocity vector, i.e., normal to both Xo and Yo axes. Planes and axes of reference πR = Refernce plane perpendicular to the cutting velocity vector, CV πC = cutting plane; plane perpendicular to πR and taken along the principal cutting edge πO = Orthogonal plane; plane perpendicular to both πR and πC and the axes; Xo = along the line of intersection of πR and πO Yo = along the line of intersection of πR and πC Zo = along the velocity vector, i.e., normal to both Xo and Yo axes. • Rake angles γo = orthogonal rake: angle of inclination of the rake surface from Reference plane, πR and measured on the orthogonal plane, πo λ = inclination angle; angle between πC from the direction of assumed longitudinal feed [πX] and measured on πC • Clearance angles αo = orthogonal clearance of the principal flank: angle of inclination of the principal flank from πC and measured on πo αo’ = auxiliary orthogonal clearance: angle of inclination of the auxiliary flank from auxiliary cutting plane, πC’ and measured on auxiliary orthogonal plane, πo’ as indicated in Fig. 3.8. • Cutting angles φ = principal cutting edge angle: angle between πC and the direction of assumed longitudinal feed or πX and measured on πR φ1 = auxiliary cutting angle: angle between πC’ and πX and measured on πR • Nose radius, r (mm) r = radius of curvature of tool tip Tool signature λ, γo, αo, αo’, φ1, φ, r (mm) •By far the most common cutting tools for holemaking •Usually made of high speed steel Toolbits generally made of seven materials ◦ ◦ ◦ ◦ ◦ ◦ ◦ High-speed steel Cast alloys (such as stellite) Cemented carbides Ceramics Cermets Cubic Boron Nitride Polycrystalline Diamond Hardness ◦ Cutting tool material must be 1 1/2 times harder than the material it is being used to machine. Capable of maintaining a red hardness during machining operation ◦ Red hardness: ability of cutting tool to maintain sharp cutting edge ◦ Also referred to as hot hardness or hot strength Wear Resistance ◦ Able to maintain sharpened edge throughout the cutting operation ◦ Same as abrasive resistance Shock Resistance ◦ Able to take the cutting loads and forces Shape and Configuration ◦ Must be available for use in different sizes and shapes. Highly alloyed tool steel capable of maintaining hardness at elevated temperatures better than high carbon and low alloy steels May contain combinations of tungsten, chromium, vanadium, molybdenum, cobalt Can take heavy cuts, withstand shock and maintain sharp cutting edge under red heat Especially suited to applications involving complicated tool geometries, such as drills, taps, milling cutters, and broaches Generally two types (general purpose) ◦ ◦ Molybdenum-base (Group M) Tungsten-base (Group T) Cobalt added if more red hardness desired Typical composition: Grade T1: 18% W, 4% Cr, 1% V, and 0.9% C Usually contain 25% to 35% chromium, 4% to 25% tungsten and 1% to 3% carbon ◦ Remainder cobalt Qualities ◦ High hardness ◦ High resistance to wear ◦ Excellent red-hardness Operate 2 ½ times speed of high-speed steel Weaker and more brittle than high-speed steel Class of hard tool material based on tungsten carbide (WC) using powder metallurgy techniques with cobalt (Co) as the binder • Two basic types: 1. Non-steel cutting grades - only WC-Co 2. Steel cutting grades - TiC and TaC added to WC-Co High compressive strength but low-to-moderate tensile strength •High hardness (90 to 95 HRA) •Good hot hardness •Good wear resistance •High thermal conductivity •Toughness lower than high speed steel Used for nonferrous metals and gray cast iron •Properties determined by grain size and cobalt content ◦ As grain size increases, hardness and hot hardness decrease, but toughness increases ◦ As cobalt content increases, toughness improves at the expense of hardness and wear resistance Used for low carbon, stainless, and other alloy steels For these grades, TiC and/or TaC are substituted for some of the WC Titanium carbide ◦ Addition provides resistance to tool cratering ◦ Content increased Toughness of tool decreased Abrasive wear resistance at cutting edge lowered Tantalum carbide ◦ Addition provides resistance to tool cratering Without affecting abrasive wear resistance ◦ Addition increases tool's resistance to deformation Combinations of TiC, TiN, and titanium carbonitride (TiCN), with nickel and/or molybdenum as binders. •Some chemistries are more complex •Applications: high speed finishing and semi finishing of steels, stainless steels, and cast irons Higher speeds and lower feeds than steel-cutting carbide grades Better finish achieved, often eliminating need for grinding Cemented carbide insert coated with one or more thin layers of wear resistant materials, such as TiC, TiN, and/or Al2O3 •Coating applied by chemical vapor deposition or physical vapor deposition •Coating thickness (0.0001 to 0.0005 in) Applications: ◦ cast irons and steels in turning and milling operations ◦ Best applied at high speeds where dynamic force and thermal shock are minimal Primarily fine-grained Al 2O 3, pressed and sintered at high pressures and temperatures into insert form with no binder •Applications: high speed turning of cast iron and steel •Not recommended for heavy interrupted cuts (e.g. rough milling) due to low toughness •Al 2O 3 also widely used as an abrasive in grinding Sintered polycrystalline diamond (SPD) - fabricated by sintering very fine-grained diamond crystals under high temperatures and pressures into desired shape with little or no binder •Usually applied as coating (0.5 mm thick) on WC-Co insert •Applications: high speed machining of nonferrous metals and abrasive nonmetals such as fiberglass, graphite, and wood Not for steel cutting cubic boron nitride (cBN) is hardest material •Fabrication into cutting tool inserts same as SPD: coatings on WC-Co inserts •Applications: machining steel and nickel-based alloys •SPD and cBN tools are expensive What is orthogonal cutting and oblique cutting ? List the assumptions in calculating cutting force? Orthogonal cutting In orthogonal cutting the tool approaches the work piece with its cutting edge parallel to the uncut surface and at right angle to the direction of cutting. Thus tool approach angle and cutting edge inclination are zero. Here only two component forces are acting cutting force Fc and thrust force Ft. So the metal cutting may be considered as 2 Dimensional cutting. wor k feed Oblique cutting The cutting edge is inclined at an angle i (known as inclination angle ) with the normal to the direction of tool travel . The cutting edge may are may not clear the width of the work piece. The chip flow on tool face making an angle with the normal on the cutting edge. The chip flows side ways in a long curl. Three components of the forces (mutually perpendicular act at the cutting edge). Oblique cutting occur when the major edge of the cutting tool is presented to the work piece at an angle which is not perpendicular to the direction of feed. Assumptions in calculating the cutting forces: - Establish a relation between three velocities in orthogonal cutting ? Vc=Chip velocity Vs =Shear velocity V= cutting velocity Φ=Shear angle α =Rake angle The cutting velocity V: it is the velocity of the tool relative to the work and directed parallel to Fh. The chip velocity Vc: it is the velocity of the chip relative to the tool and directed along the tool face. The shear velocity Vs: it is the velocity of the chip relative to the work piece and directed along the shear plane. With the help of sine rule we can write V = Vs = Vc Sin (90+α-Φ) Sin (90-α) Sin Φ Chip velocity Vc = V sin f Cos (Φ-α) Shear velocity Vs = V Cos a Cos (Φ-α) Derive an expression for the shear angle in orthogonal cutting In terms of rake angle and chip thickness ratio? The out ward flow of the metal causes the chip to be thicker after separation from the parent metal. Metal prior to being cut Is much longer than the chip which is removed. The chip thickness ratio or cutting ratio is the ratio of uncut chip thickness to the cut chip thickness. When metal is cut there is no change in volume of the metal cut therefore t1*b1*L1 = t2*b2*L2 where t1 & t2 are chip thickness before and after cutting L1 &L2 are length of chip before and after cut. b1 & b2 are width of cut before and after cut it is observed that b1 = b2 There fore t1 * L1 = t2 * L2 t1 = L2 = r t2 L1 From the triangle ABC BC = Sin f But BC =t1 AB AB = t1 ------ (1) Sin f From the triangle ABD BD = Sin 90-(f-a) AB But Sin 90-(f-a) =cos (f-a) And BD= t2 AB= t2 ----- (2) Cos (f-a) From (1) and(2) t1 = Sin f = r t2 Cos (f-a) r= Sin f Cos f . Cos a + Sin f Sin a r Cos a = 1 – r Sin a Tan f Tan f = r Cos a 1- r Sin a There fore shear angle can be shown from the above equation