cutting tool geometry for determiniung surface finish

A single point tool geometry to to study to analyse the dependance of tool gemetry on surface roughness

1. Department of Collegiate and Technical Education TOOL GEOMETRY (Week 3) Session-III MACHINE TOOL TECHNOLOGY ( IIIrd Semester) Mechanical Engineering Mechanical Engineering – 20ME32P

2. Tool Geometry Tool Materials – High carbon steel, HSS, Cemented carbides, Medium alloy, Abrasives, Diamonds, Stellite ceramics Tool Designation (Tool Signature) Tool Life - Definition, Taylors Tool life equation Tool Wear – Mechanical wear, Thermo Chemical wear, Chemical wear, Galvanic wear Cutting Speed, Feed, Depth of Cut & Machining Time Contents

3. TOOL GEOMETRY The typical right hand single point cutting tool terminology is as shown in fig

4. Shank: The shank is that portion of the tool bit which is not ground to form cutting edges and is rectangular in cross section. Face: The face of cutting tool is that surface which faces the w/p. Heel: The heel is the lowest portion of the side cutting and end cutting edge. Nose: The nose is the conjunction of the side and end cutting edges. Base: The base is the underside of the shank. Contd.,

5. Rake: The rake is the slope of the top away from the cutting edge. Each tool has a side and back rake. Back rake indicates that the plane which forms the face or top of a tool has been ground back at an angle sloping from the nose. Side rake indicates that the place that forms the face or top of a tool has been ground back at an angle sloping from the side cutting edge. Contd.,

6. Side clearance or side relief angle: The side clearance or side relief indicates that the plane that forms the flank or side of a tool has been ground back at an angle sloping down from the side cutting edge. End cutting edge angle: The end cutting edge angle indicates that the plane which forms the end of a tool has been ground back at an angle sloping from the nose to the side of the shank. Chips are removed by its cutting edge. Contd.,

7. Lip or cutting angle: The lip or cutting angle is the included angle when the tool has been ground wedge shaped. Rake angle: It is the angle made by the face of tool and the plane parallel to the base of cutting tool. If the rake angle is measured in the direction of tool shank, it is called back rake angle and if measured in a direction at right angle to it, then it is called side rake angle. Side cutting edge angle: It also known as lead angle. It is the angle between the side cutting edge and the side of the tool shank. Contd.,

8. 1. High Carbon Steel: It is one of the earliest cutting materials used in machining. It is however now virtually superseded by other materials used in engineering because it starts to temper at about 220 C. This softening process continues as the temperature rises. As a result cutting using this material for tools is limited to speeds up to 0.15 m/s for machining mild steel with lots of coolant. Cutting Tool Materials

9. High carbon steels are oil- or water-hardened plain carbon steels with 0.9 to 1.4 percent carbon content. They are used for hand tools such as files and chisels, and only to a limited extent for drilling & turning tools. These tools, however, tend to soften at machining speeds above 50 feet per minute (fpm) in mild steels. Contd.,

10. 2. High Speed Steel (HSS): This range of metal contains about 7% C, 4% Cr plus additions of tungsten, vanadium, molybdenum and cobalt. These metals maintain their hardness at temperature up to about 600, but soften rapidly at higher temperatures. These materials are suitable for cutting mild steel at speeds up maximum rates of 0.8 m/s to 1.8 m/s HSS may be used at higher cutting speeds It is sometimes used for lathe tools when special tool shapes are needed, especially for boring tools. However, HSS is extensively used for milling cutters. These cutters usually have a longer working life

11. 3. Cast Alloys: - These cutting tools are made of various nonferrous metals in a cobalt base. They can withstand cutting temperatures of up to 7600C and are capable of cutting speeds about 60% higher than HSS. 4. Stellite : - This is a cast alloy of Co (40 to 50%), Cr (27 to 32%), W (14 to 19%) and C (2%). Stellite is quite tough and more heat and wear resistive than the basic HSS (18–4–1) but such stellite as cutting tool material became obsolete for its poor grindability and especially after the arrival of cemented carbides.

12. 5. Cemented Carbides (Cermets or Sintered Carbide): - This material usually consists of tungsten carbide or a mixture of tungsten carbide, titanium, or tantalum carbide in powder form, sintered in a matrix of cobalt or nickel. The term Carbide is commonly used to re-present to cemented carbides, the cutting tools composed of tungsten carbide, titanium carbide, or tantalum carbide & cobalt in various combinations. A typical composition of cemented carbide is 85 to 95 percent of tungsten & the remainder a cobalt binder for the tungsten carbide powder

13. Contd., Cemented carbides are extremely hard tool materials (above RA 90), have a high compressive strength & resist wear & rupture. Coated carbide inserts are often used to cut hard or difficult-to-machine work pieces. These are the most widely used in the machining industry, particularly useful for cutting tough alloy steels, which quickly break down HSS. As this material is expensive and has low rupture strength it is normally made in the form of tips which are brazed or clamped on a steel shank

14. 6. Coated Carbides The cutting system is based on providing a thin layer of high wear-resistant titanium carbide fused to a conventional tough grade carbide insert, thus achieving a tool combining the wear resistance of one material with the wear resistance of another. These systems provide a longer wear resistance and a higher cutting speed compared to conventional carbides

15. 7. Ceramics Ceramic or “cemented oxide” tools are made primarily from aluminum oxide. Ceramics are made by powder metallurgy from aluminium oxide with additions of titanium oxide and magnesium oxide to improve cutting properties. These have a very high hot resistance and wear resistance and can cut at very high speed. However they are brittle, little resistance to shock. Their use is therefore limited to tips used for continuous high speed cutting on vibration-free machines

16. 8. Diamond Tools Diamonds have limited application due to the high cost and the small size of the stones. They are used on very hard materials to produce a fine finish and on soft materials especially those inclined to clog other cutting materials. They are generally used at very high cutting speed with low feed and light cuts and vibration free. The tools last for 10 times longer than carbide based tools. Industrial diamonds are sometimes used to machine extremely hard work pieces.

17. Contd., Only relatively small removal rates are possible with diamond tools, but high speeds are used and good finishes are obtained. Diamond tools are particularly effective for cutting abrasive materials that quickly wear out other tool materials. Nonferrous metals, plastics, and some nonmetallic materials are often cut with diamond tools.

18. Tool Designation (Tool Signature) α b – α s – ϕ e – ϕ s – C e – C s – r Where, α b = Back rake angle, α s = Side rake angle, ϕ e = End relief angle, ϕ s = Side relief angle, C e = End cutting edge angle, C s = Side cutting edge angle, r = Tool radius

19. Contd.,

20. Tool Life Tool life generally indicates the amount of satisfactory performance or service rendered by a fresh tool or a cutting point till it is declared failed. Tool life is defined in two ways (a) In R & D: Actual machining time (period) by which a fresh cutting tool (or point) satisfactorily works after which it needs replacement or reconditioning. (b) In industries or shop floor: The length of time of satisfactory service or amount of acceptable output provided by a fresh tool prior to it is required to replace or recondition

21. Taylor’s Tool Life Equation Wear and hence tool life of any tool for any work material is governed mainly by the level of the machining parameters i.e., cutting velocity (VC), feed (f) and depth of cut (t). Cutting velocity affects maximum and depth of cut minimum. The usual pattern of growth of cutting tool wear (mainly Flank wear VB), principle of assessing tool life and its dependence on cutting velocity are schematically shown in Fig

22. The tool life obviously decreases with the increase in cutting velocity keeping other conditions unaltered as indicated in above Fig. If the tool lives, T1, T2, T3, T4 etc are plotted against the corresponding cutting velocities, V1, V2, V3, V4 etc as shown in above Fig. Contd.,

23. A smooth curve like a rectangular hyperbola is found to appear. When F. W. Taylor plotted the same figure taking both V and T in log-scale, a more distinct linear relationship appeared as schematically shown in previous Fig With the slope, n and intercept, c, Taylor derived the simple equation as, V C T n = C Where, n = Taylor’s tool life exponent. The values of both ‘n’ and ‘C’ depend mainly upon the tool-work materials and the cutting environment (cutting fluid application) Contd.,

24. The common mechanisms of cutting tool wear are: (a) Mechanical wear Thermally insensitive type; like abrasion, chipping and de-lamination. Thermally sensitive type; like adhesion, fracturing, flaking etc. (b) Thermo chemical wear Macro-diffusion by mass dissolution. Micro-diffusion by atomic migration. Tool Wear

25. (c) Chemical wear: Chemical wear, leading to damages like grooving wear may occur if the tool material is not enough chemically stable against the work material and/or the atmospheric gases. (d) Galvanic wear: Galvanic wear, based on electrochemical dissolution, seldom occurs when the work and tool materials are electrically conductive, cutting zone temperature is high and the cutting fluid acts as an electrolyte. Contd.,

26. Contd.,

27. CUTTING SPEED: Cutting speed for lathe work may be defined as the rate in meters per minute at which the surface of the job moves past the cutting tool. Cutting speed is given by the equation V = m/min Where, D – Mean diameter of the work piece (mm). N – Rotational speed of the work piece (rpm). Cutting speed, Feed, Depth of Cut, Machining Time

28. FEED: In a lathe, the feed of a cutting tool is the distance the tool advances for each revolution of the work piece, it is expressed in ‘mm/rev’. DEPTH OF CUT: It is the perpendicular distance measured from the machined surface to the uncut surface of the work piece. Depth of cut = (D-d)/2 Where D = Dia of the work piece before machining. d = Dia of the machined work piece. Contd.,

29. MACHINING TIME: The machining time in lathe work can be calculated for a portion operation, if the speed of the work piece and feed length of the work piece is known. Let f = Feed of the work piece in mm/rev L= length of the work piece in mm. N = Speed of the work piece in rpm. T = Machining time in minutes. Machining time is given by: T = L/(f x N) minutes. Contd.,

30. REFERENCES Elements of Workshop Technology (Vols. 1 and II) by Hajra Chaudhary . Production Technology By R.K. Jain Foundry Technology By O.P.Khanna Engineering Drawing Vol-2 By K.R.Gopala Krishna Engineering Drawing By N.D.Bhat