Lecture 4 - Cutting Tool Technology金属切削外教课程

1、CUTTING TOOL TECHNOLOGY1.Tool Life2.Tool Materials3.Tool Geometry4.Cutting FluidsThree Modes of Tool Failure1.Fracture failure Cutting force becomes excessive and/or dynamic, leading to brittle fracture2.Temperature failure Cutting temperature is too high for the tool material 3.Gradual wear Gradual

2、 wearing of the cutting toolPreferred Mode: Gradual WearFracture and temperature failures are premature failures Gradual wear is preferred because it leads to the longest possible use of the tool Gradual wear occurs at two locations on a tool: Crater wear occurs on top rake faceFlank wear occurs on

3、flank (side of tool)Tool WearWorn cutting tool, showing the principal locations and types of wear that occurCrater Wear and Flank WearCrater wear, (left), and flank wear (right) on a cemented carbide tool (photos by J. C. Keefe, Lehigh University)Tool Wear vs. TimeTool wear (flank wear) as a functio

4、n of cutting timeEffect of Cutting SpeedEffect of cutting speed on tool flank wear (FW) for three cutting speeds, using tool life criterion of 0.5 mm flank wearTool Life vs. Cutting SpeedNatural log log plot of cutting speed vs. tool lifeTaylor Tool Life EquationvTn = Cwhere v = cutting speed; T = t

5、ool life; and n and C are parameters that depend on feed, depth of cut, work material, tooling material, and the tool life criterion usedn is the slope of the plotC is the intercept on the speed axis at one minute tool lifeRelationship credited to Frederick W. TaylorTool Life Criteria in Production1

6、.Complete failure of cutting edge 2.Visual inspection of wear by the machine operator3.Fingernail test across cutting edge4.Changes in sound emitted from operation5.Chips become stringy and difficult to dispose 6.Degradation of surface finish7.Increased power8.Workpiece count9.Cumulative cutting tim

7、eTool MaterialsTool failure modes identify the important properties that a tool material should possess Toughness to avoid fracture failureHot hardness ability to retain hardness at high temperatures Wear resistance hardness is the most important property to resist abrasive wearHot HardnessTypical h

8、ot hardness relationships for selected tool materialsHigh speed steel is much better than plain C steelCemented carbides and ceramics are significantly harder at elevated temperatures.Typical Values of n and C Tool material n C (m/min) C (ft/min)High speed steel:Non-steel work0.125 120 350Steel work

9、0.125 70 200Cemented carbideNon-steel work 0.25 900 2700Steel work 0.25 500 1500CeramicSteel work 0.6 3000 10,000High Speed Steel (HSS) Highly alloyed tool steel capable of maintaining hardness at elevated temperatures better than high carbon and low alloy steels Especially suited to applications in

10、volving complicated tool shapes: drills, taps, milling cutters, and broachesTwo basic types of HSS (AISI)1.Tungsten type, designated T grades 2.Molybdenum type, designated M grades High Speed Steel CompositionTypical alloying ingredients:Tungsten and/or MolybdenumChromium and VanadiumCarbon, of cour

11、seCobalt in some gradesTypical composition (Grade T1):18% W, 4% Cr, 1% V, and 0.9% CCemented Carbides Class of hard tool material based on tungsten carbide (WC) using powder metallurgy techniques with cobalt (Co) as the binderTwo basic types:1.Non steel cutting grades - only WC Co2.Steel cutting gra

12、des - TiC and TaC added to WC Co Cemented Carbides General PropertiesHigh compressive strength but low to moderate tensile strengthHigh hardness (90 to 95 HRA)Good hot hardnessGood wear resistanceHigh thermal conductivityHigh elastic modulus 600 x 103 MPa (90 x 106 lb/in2)Toughness lower than high s

13、peed steelNon steel Cutting Carbide Grades Used for nonferrous metals and gray cast ironProperties determined by grain size and cobalt contentAs grain size increases, hardness and hot hardness decrease, but toughness increases As cobalt content increases, toughness improves at the expense of hardnes

14、s and wear resistance Steel Cutting Carbide GradesUsed for low carbon, stainless, and other alloy steelsTiC and/or TaC are substituted for some of the WC Composition increases crater wear resistance for steel cuttingBut adversely affects flank wear resistance for non steel cutting applicationsCermet

15、s Combinations of TiC, TiN, and titanium carbonitride (TiCN), with nickel and/or molybdenum as binders. Some chemistries are more complexApplications: high speed finishing and semifinishing of steels, stainless steels, and cast irons Higher speeds and lower feeds than steel cutting cemented carbide

16、grades Better finish achieved, often eliminating need for grinding Coated Carbides Cemented carbide insert coated with one or more layers of TiC, TiN, and/or Al2O3 or other materialsCoating thickness = 2.5 13 m (0.0001 - 0.0005 in)Coating applied by chemical vapor deposition or physical vapor deposi

17、tion Applications: cast irons and steels in turning and milling operationsBest used at high speeds where dynamic force and thermal shock are minimal Coated Carbide ToolPhotomicrograph of cross section of multiple coatings on cemented carbide tool (photo courtesy of Kennametal Inc.)Ceramics Primarily

18、 fine grained Al2O3, pressed and sintered at high pressures and temperatures into insert form with no binderApplications: high speed turning of cast iron and steel Not recommended for heavy interrupted cuts (e.g. rough milling) due to low toughnessAl2O3 also widely used as an abrasive in grinding Sy

19、nthetic Diamonds Sintered polycrystalline diamond (SPD) - fabricated by sintering very fine grained diamond crystals under high temperatures and pressures into desired shape with little or no binderUsually applied as coating (0.5 mm thick) on WC-Co insertApplications: high speed machining of nonferr

20、ous metals and abrasive nonmetals such as fiberglass reinforced polymer, graphite, and woodNot for steel cuttingCubic Boron NitrideNext to diamond, cubic boron nitride (cBN) is hardest material knownFabrication into cutting tool inserts same as SPD: coatings on WC Co inserts Applications: machining

21、steel and nickel based alloys SPD and cBN tools are expensiveTool GeometryTwo categories: Single point tools Used for turning, boring, shaping, and planingMultiple cutting edge tools Used for drilling, reaming, tapping, milling, broaching, and sawingSingle-Point Tool Geometry(a) Seven elements of si

22、ngle point tool geometry(b) Tool signature convention that defines the seven elementsHolding and Presenting a Single-Point Tool(a) Solid shank tool, typical of HSS; (b) brazed cemented carbide insert; and (c) mechanically clamped insert, for carbides, ceramics, and other very hard tool materialsComm

23、on Insert Shapes(a) Round, (b) square, (c) rhombus with 80 point angles, (d) hexagon with 80 point angles, (e) triangle, (f) rhombus with 55 point angles, (g) rhombus with 35 point anglesCollection of metal cutting inserts with various geometries and made of various materials (photo courtesy of Kenn

24、ametal Inc.)Twist DrillMost common cutting tools for hole makingUsually made of high speed steelShown below is standard twist drill geometryTwist Drill OperationRotation and feeding of drill bit result in relative motion between cutting edges and work material to form the chips Cutting speed varies

25、along cutting edges as a function of distance from axis of rotation Relative velocity at drill point is zero, so no cutting takes placeInstead, a large thrust force is required to drive the drill forward into the holeTwist Drill Operation - ProblemsChip removalFlutes must provide sufficient clearance to allow chips to move from bottom of hole during cuttingFriction makes matters worseRubbing between outside diameter of drill bit and newly formed hole Delivery of cutting fluid to drill point to reduce friction and heat is difficult because chips ar