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Cutting Tool Application s Cutting Tool Applications By George S chneider, J r. CMfgE 2 Tooling & Production/Chapter 3 www.toolingandproduction.com 3.1 Introduction The condition and physical properties of the work material have a direct influence on the machinability of a work material. The various conditions and characteristics described as ‘condition of work material’, individually and in combinations, directly influence and determine the machinability. Operating conditions, tool material and geometry, and work- piece requirements exercise indirect effects on machinability and can often be used to overcome difficult conditions presented by the work material. On the other hand, they can create situations that increase machining difficulty if they are ignored. A thorough understanding of all of the factors affecting machinability and machining will help in selecting material and workpiece designs to achieve the optimum machining combina- tions critical to maximum productivity. 3.2 Condition of Work Material The following eight factors determine the condition of the work material: microstructure, grain size, heat treatment, chemical composition, fabrication, hardness, yield strength, and tensile strength. Microstructure: The microstructure of a metal refers to its crystal or grain structure as shown through examination of etched and polished surfaces under a microscope. Metals whose microstructures are similar have like machining properties. But there can be variations in the microstructure of the same workpiece, that will affect machinability. Grain Size: Grain size and structure of a metal serve as general indicators of its machinability. A metal with small undistorted grains tends to cut easily and finish easi- ly. Such a metal is ductile, but it is also ‘gummy’. Metals of an intermediate grain size represent a compromise that permits both cutting and finishing machinability. Hardness of a metal must be correlated with grain size and it is generally used as an indicator of machinability. Heat Treatment: To provide desired properties in metals, they are sometimes put through a series of heating and cooling operations when in the solid state. A material may be treated to reduce brittleness, remove stress, to obtain ductility or toughness, to increase strength, to obtain a definite microstructure, to change hardness, or to make other changes that affect machinability. Chemical Composition: Chemical composition of a metal is a major factor in deter- mining its machinability. The effects of composition though, are not always clear, because the elements that make up an alloy metal, work both singly and collectively. Certain generalizations about chemical composition of steels in relation to machinability can be made, but non-ferrous alloys are too numerous and varied to permit such general- izations. Fabrication: Whether a metal has been hot rolled, cold rolled, cold drawn, cast, or forged will affect its grain size, ductility, strength, hardness, structure - and therefore - its machinability. The term ‘wrought’ refers to the hammering or forming of materials into premanfac- Chapter 3 Machinability of Metals Upcoming Chapters Metal Removal Cutting-Tool Materials Metal Removal Methods Machinability of Metals Single Point Machining Turning Tools and Operations Turning Methods and Machines Grooving and Threading Shaping and Planing Hole Making Processes Drills and Drilling Operations Drilling Methods and Machines Boring Operations and Machines Reaming and Tapping Multi Point Machining Milling Cutters and Operations Milling Methods and Machines Broaches and Broaching Saws and Sawing Abrasive Processes Grinding Wheels and Operations Grinding Methods and Machines Lapping and Honing George Schneider, Jr. CMfgE Professor Emeritus Engineering Technology Lawrence Technological University Former Chairman Detroit Chapter ONE Society of Manufacturing Engineers Former President International Excutive Board Society of Carbide & Tool Engineers Lawrence Tech. Univ.: http://www.ltu.edu Prentice Hall: http://www.prenhall.com tured shapes which are readily altered into components or products using tra- ditional manufacturing techniques. Wrought metals are defined as that group of materials which are mechani- cally shaped into bars, billets, rolls, sheets, plates or tubing. Casting involves pouring molten metal into a mold to arrive at a near component shape which requires mini- mal, or in some cases no machining. Molds for these operations are made from sand, plaster, metals and a variety of other materials. Hardness: The textbook definition of hardness is the tendency for a mater- ial to resist deformation. Hardness is often measured using either the Brinell or Rockwell scale. The method used to measure hardness involves embedding a specific size and shaped indentor into the surface of the test material, using a predetermined load or weight. The dis- tance the indentor penetrates the mater- ial surface will correspond to a specific Brinell or Rockwell hardness reading. The greater the indentor surface pene- tration, the lower the ultimate Brinell or Rockwell number, and thus the lower the corresponding hardness level. Therefore, high Brinell or Rockwell numbers or readings represent a mini- mal amount of indentor penetration into the workpiece and thus, by definition, are an indication of an extremely hard part. Figure 3.1 shows how hardness is measured. The Brinell hardness test involves embedding a steel ball of a specific diameter, using a kilogram load, in the surface of a test piece. The Brinell Hardness Number (BHN) is determined by dividing the kilogram load by the area (in square millimeters) of the circle created at the rim of the dimple or impression left in the workpiece sur- face. This standardized approach pro- vides a consistent method to make com- www.toolingandproduction.com Chapter 3/Tooling & Production 3 parative tests between a variety of workpiece materials or a single material which has undergone various hardening processes. The Rockwell test can be performed with various indentor sizes and loads. Several different scales exist for the Rockwell method or hardness testing. The three most popular are outlined below in terms of the actual application the test is designed to address: Rockwell Testing Scale Application A For tungsten carbide and other extremely hard materials & thin, hard sheets. B For medium hardness low and medium carbon steels in the annealed condition. C For materials > than Rockwell ‘B’ 100. In terms of general machining prac- tice, low material hardness enhances productivity, since cutting speed is often selected based on material hardness (the lower the hardness, the higher the speed). Tool life is adversely affected by an increase in workpiece hardness, since the cutting loads and tempera- tures rise for a specific cutting speed with part hardness, thereby reducing tool life. In drilling and turning, the added cutting temperature is detrimen- tal to tool life, since it produces excess heat causing accelerated edge wear. In milling, increased material hardness produces higher impact loads as inserts enter the cut, which often leads to a pre- mature breakdown of the cutting edge. Yield Strength: Tensile test work is used as a means of comparison of metal material conditions. These tests can establish the yield strength, tensile strength and many other conditions of a material based on its heat treatment. In addition, these tests are used to compare different workpiece materials. The ten- sile test involves taking a cylindrical rod or shaft and pulling it from opposite ends with a progressively larger force in a hydraulic machine. Prior to the start of the test, two marks either two or eight inches apart are made on the rod or shaft. As the rod is systematically sub- jected to increased loads, the marks begin to move farther apart. A material is in the so-called ‘elastic zone’ when the load can be removed from the rod and the marks return to their initial dis- tance apart of either two or eight inches. If the test is allowed to progress, a point is reached where, when the load is removed the marks will not return to their initial distance apart. At this point, permanent set or deformation of the test specimen has taken place. Figure 3.2 shows how yield strength is measured. Yield strength is measured just prior to the point before permanent deforma- tion takes place. Yield strength is stated in pounds per square inch (PSI) and is determined by dividing the load just prior to permanent deformation by the cross sectional area of the test speci- men. This material property has been referred to as a condition, since it can be altered during heat treatment. Increased part hardness produces an increase in yield strength and therefore, as a part becomes harder, it takes a larger force to produce permanent deformation of the part. Yield strength should not be con- fused with fracture strength, cracking or the actual breaking of the material into pieces, since these properties are quite different and unrelated to the current subject. By definition, a material with high yield strength (force required per unit of area to create permanent deformation) requires a high level of force to initiate chip formation in a machining opera- tion. This implies that as a material’s yield strength increases, stronger insert shapes as well as less positive cutting geometries are necessary to combat the additional load encountered in the cut- ting zone. Material hardness and yield strength increase simultaneously during heat treatment. Therefore, materials with relatively high yield strengths will be more difficult to machine and will reduce tool life when compared to mate- Chap. 3: Machinability of Metals Load 500 kg Large indentation Small indentation Soft part Load500 kg Hard partt Figure 3.1 Hardness is measured by depth of indentations made. Chap. 3: Machinability of Metals 4 Tooling & Production/Chapter 3 www.toolingandproduction.com rials with more moderate strengths. Tensile Strength: The tensile strength of a material increases along with yield strength as it is heat treated to greater hardness levels. This material condition is also established using a ten- sile test. Tensile strength (or ultimate strength) is defined as the maximum load that results during the tensile test, divided by the cross-sectional area of the test specimen. Therefore, tensile strength, like yield strength, is expressed in PSI. This value is referred to as a material condition rather than a property, since its level just like yield strength and hardness, can be altered by heat treatment. Therefore, based on the material selected, distinct tensile and yield strength levels exist for each hard- ness reading. Just as increased yield strength implied higher cutting forces during machining operations, the same could be said for increased tensile strength. Again, as the workpiece tensile strength is elevated, stronger cutting edge geometries are required for productive machining and acceptable tool life. 3.3 Physical Properties of Work Materials Physical properties will include those characteristics included in the individ- ual material groups, such as the modu- lus of elasticity, thermal conductivity, thermal expansion and work hardening. Modulus of Elasticity: The modulus of elasticity can be determined during a tensile test in the same manner as the previously mentioned conditions. However, unlike hardness, yield or ten- sile strength, the modulus of elasticity is a fixed material property and , therefore, is unaffected by heat treatment. This particular property is an indicator of the rate at which a material will deflect when subjected to an external force. This property is stated in PSI and typi- cal values are several million PSI for metals. A 2” x 4” x 8 ft. wood beam supported on either end, with a 200 pound weight hanging in the middle, will sag 17 times more than a beam of the same dimensions made out of steel and subjected to the same load. The dif- ference is not because steel is harder or stronger, but because steel has a modu- lus of elasticity which is 17 times greater than wood. General manufacturing practice dic- tates that productive machining of a workpiece material with a relatively moderate modulus of elasticity normal- ly requires positive or highly positive raked cutting geometries. Positive cut- ting geometries produce lower cutting forces and, therefore chip formation is enhanced on elastic material using these types of tools. Sharp positive cut- ting edges tend to bite and promote shearing of a material, while blunt neg- ative geometries have a tendency to cre- ate large cutting forces which impede chip formation by severely pushing or deflecting the part as the tool enters the cut. Thermal Conductivity: Materials are frequently labeled as being either heat conductors or insulators. Conductors tend to transfer heat from a hot or cold object at a high rate, while insulators impede the flow of heat. Thermal conductivity is a measure of how efficiently a material transfers heat. Therefore, a material which has a rela- tively high thermal conductivity would be considered a conductor, while one with a relatively low level would be regarded as an insulator. Metals which exhibit low thermal conductivities will not dissipate heat freely and therefore, during the machin- ing of these materials, the cutting tool and workpiece become extremely hot. This excess heat accelerates wear at the cutting edge and reduces tool life. The proper application of sufficient amounts of coolant directly in the cutting zone (between the cutting edge and work- piece) is essential to improving tool life in metals with low thermal conductivi- ties. Thermal Expansion: Many materi- als, especially metals, tend to increase in dimensional size as their temperature rises. This physical property is referred to as thermal expansion. The rate at which metals expand varies, depending on the type or alloy of material under consideration. The rate at which metal expands can be determined using the material’s expansion coefficient. The greater the value of this coefficient, the more a material will expand when sub- jected to a temperature rise or contract when subjected to a temperature reduc- tion. For example, a 100 inch bar of steel which encounters a 100 degree Fahrenheit rise in temperature would measure 100.065 inches. A bar of alu- minum exposed to the same set of test conditions would measure 100.125 inches. In this case, the change in the aluminum bar length was nearly twice that of the steel bar. This is a clear indi- cation of the significant difference in thermal expansion coefficients between these materials. In terms of general machining prac- tice, those materials with large thermal expansion coefficients will make hold- ing close finish tolerances extremely difficult, since a small rise in workpiece temperature will result in dimensional change. The machining of these types of materials requires adequate coolant supplies for thermal and dimensional stability. In addition, the use of positive cutting geometries on these materials will also reduce machining tempera- tures. Work Hardening: Many metals exhibit a physical characteristic which produces dramatic increases in hard- ness due to cold work. Cold work involves changing the shape of a metal object by bending, shaping, rolling or forming. As the metal is shaped, inter- nal stresses develop which act to harden the part. The rate and magnitude of this internal hardening varies widely from one material to another. Heat also plays an important role in the work hardening of a material. When materials which exhibit work hardening tendencies are subjected to increased temperature, it acts like a catalyst to produce higher hardness levels in the workpiece. The machining of workpiece materi- Test Specimen 2.000” Force = 0 lbsForce = 0 lbs Figure 3.2 Yield strength is measured by pulling a test specimen as shown. Chap. 3: Machinability of Metals www.toolingandproduction.com Chapter 3/Tooling & Production 5 als with work hardening properties should be undertaken with a generous amount of coolant. In addition, cutting speeds should correlate specifically to the material machined and should not be recklessly altered to meet a produc- tion rate. The excess heat created by unusually high cutting speeds could be extremely detrimental to the machining process by promoting work hardening of the workpiece. Low chip thicknesses should be avoided on these materials, since this type of inefficient machining practice creates heat due to friction, which produces the same type of effect mentioned earlier. Positive low force cutting geometries at moderate speeds and feeds are normally very effective on these materials. 3.4 Metal Machining The term ‘machinability’ is a relative measure of how easily a material can be machined when compared to 160 Brinell AISI B1112 free machining low carbon steel. The American Iron and Steel Institute (AISI) ran turning tests of this material at 180 surface feet and compared their results for B1112 against several other materials. If B1112 represents a 100% rating, then materials with a rating less than this level would be decidedly more difficult to machine, while those that exceed 100% would be easier to machine. The machinability rating of a metal takes the normal cutting speed, surface finish and tool life attained into consid- eration. These factors are weighted and combined to arrive at a final machin- ability rating. The following chart shows a variety of materials and their specific machinability ratings: 3.4.1 Cast Iron All metals which contain iron (Fe) are known as ferrous materials. The word ‘ferrous’ is by definition, ‘relating to or containing iron’. Ferrous materials include cast iron, pig iron, wrought iron, and low carbon and alloy steels. The extensive use of cast iron and steel workpiece materials, can be attributed to the fact that iron is one of the most frequently occurring elements in nature. When iron ore and carbon are metal- lurgically mixed, a wide variety of workpiece materials result with a fairly unique set of physical properties. Carbon contents are altered in cast irons and steels to provide changes in hard- ness, yield and tensile strengths. The physical properties of cast irons and steels can be modified by changing the amount of the iron-carbon mixtures in these materials as well as their manu- facturing process. Pig iron is created after iron ore is mixed with carbon in a series of fur- naces. This material can be changed further into cast iron, steel or wrought iron depending on the selected manu- facturing process. Cast iron is an iron carbon mixture which is generally used to pour sand castings, as opposed to making billets or bar stock. It has excellent flow proper- ties and therefore, when it is heated to extreme temperatures, is an ideal mate- rial for complex cast shapes and intri- cate molds. This material is often used for automotive engine blocks, cylinder heads, valve bodies, manifolds, heavy equipment oil pans and machine bases. Gray Cast Iron: Gray cast iron is an extremely versatile, very machinable relatively low strength cast iron used for pipe, automotive engine blocks, farm implements and fittings. This material receives its dark gray color from the excess carbon in the form of graphite flakes which give it its name. Gray cast iron workpieces have rela- tively low hardness and strength levels. However, double negative or negative (axial) positive (radial) rake angle geometries are used to machine these materials because of their tendency to produce short discontinuous chips. When this type of chip is produced dur- ing the machining of these workpi