Metal cutting

Introduction Metal cutting or “Machining” is a process which removing unwanted materials from the work piece by the form of chips. The cutting action is done by having suitable relative motion between a hard edged cutting tool and the work piece. TurningDrillingMilling

In the figure the tool is moving from right to left taking a depth of cut and producing a chip. - Rake angle (  ): Angle between the plane perpendicular to work- part and the tool face.

Three dimensions of a machining process 1.Cutting speed v- Primary motion-m/s (The speed which the work material passes the major cutting edge) 2.Feed f- Secondary motion-mm/rev (The velocity at which the cutter is fed ) 3.Depth of cut d- (The amount of penetration to the work surface) For certain operations, material removal rate can be found as: MRR = vfd where v = cutting speed(mm/s) ; f = feed(mm/rev); d = depth(mm) of cut

Chip formation in metal cutting The chip is formed by a process of shearing. The shearing is take place along in the shear plane. This plane is at an angle Ø to the finished surface called the shear angle. Below the shear plan the work piece is unreformed and parallel shear plates moving up the face of the tool when cutting progress. Geometrical illustration of chip formation Chip formation Shearing: Relative motion in between two different planes in a material

Three basic type of chip in machining The various types of chips found in metal cutting are classified under following heads 1. Continuous chip. 2. Continuous chip with built-up edge (BUE chip) 3. Discontinuous chip (1) (2)(3)

Such chips are in the form of long ribbons of uniform thickness as shown in figure. 1. Continuous chip Usually form with ductile materials at high cutting speeds and/or high rake angles. Generally produce good surface finish. Usually following conditions are result for their formation. 1. Ductile work piece material 2. Fine feed 3. Sharp cutting edge 4. Effective use of cutting fluid 5. Large rake angle Ductile: Capable of having its shape changed permanently by means of applied mechanical force.

Such chips are in the form of long but not smooth as shown figure 2. Continuous chip with built-up edge (BUE chip) Under above condition the friction between the chip and the tool is so great that the chip material welds itself to the face. The presence of the welded material further increases the friction and this leads to the building up of layer upon layer of chip material. Usually following conditions are result for their formation. 1. Ductile work piece material 2. large depth of cut 3. Low rake angle 4. Lower cutting speed 5. Ineffective use of cutting fluid 6. High friction at tool face

Such chips are in the form short as shown figure 3. Discontinuous chip During the chip formation material undergoes severe strain and if the work material is brittle, then fracture will occur in the primary deformation zone. Under this condition the chip is broken in to pieces. Usually following conditions are result for their formation. 1. Brittle work piece material 2. Lower cutting speed 3. Insufficient rake angle 4. Vibration on the machine tool

Classification of metal cutting The metal cutting process has been classified in to two groups Orthogonal cutting (two dimensional cutting) Oblique cutting (three dimensional cutting) Orthogonal cuttingOblique cutting

Orthogonal cutting (two dimensional cutting) It is the cutting operation in which the cutting edge is straight, parallel to the original plane surface of the work piece, and perpendicular to the direction of cutting, ` The chip flows over the tool face. The chip coils are formed Only two components of the cutting forces are acting on the tool Act on a smaller area and, therefore heat development per unit area is too high and tool life is less

Oblique cutting (three dimensional cutting) In this type of cutting, the cutting edge is placed at an angle to the direction of tool travel in order to allow a free flow of chips as the tool is fed into the work piece. It acts on large area and therefore tool life is more The chip flows on the tool face making an angle. The chip flows sideway in long curl. Three components of the forces act at cutting edge

Different type of chips produced in two different machining operations

1.Toughness: Impact forces on the tool in machining operations, vibration, during machining do not chip or fracture the tool. 2.Hot hardness: Hardness, strength and wear resistance of the tool are maintained at elevated temperatures. Essential properties of cutting tool 3. Wear resistance: Acceptable tool life is obtained before the tool is replaced. 4. Chemical stability: No reactions between tool material and work part are allowed to prevent tool wear.

Cutting tool materials 1. High speed steels 2. Cast cobalt alloys 3. Carbides 4. Alumina based ceramics 5. Coated tools 6. Cubic boron nitride 7. Silicon nitride (SiN) Ceramics 8. Diamond

Hardness of Cutting Tool Materials as a Function of Temperature The hardness of various cutting- tool materials as a function of temperature (hot hardness). The wide range in each group of materials is due to the variety of tool compositions and treatments available for that group.

General Characteristics of Cutting-Tool Materials

The major independent variables in the cutting process are as follows Tool material, coatings, and tool condition Tool shape, sharpness, tool surface finish Work piece material, condition, temperature Cutting parameters, speed, feed, depth of cut Application of cutting fluids Characteristic of the machine tool, stiffness, damping Work holdings and fixtures The major dependant variables in the cutting process are as follows Surface finish produced on the work piece after machining Type of chip produced Temperature development in the work piece,the chip, tool Wear and failure of the tool Force and energy dissipated in the cutting process Variables in metal cutting

Determining shear plane angle D

Velocity relationship in metal cutting There are three types of velocities involved in metal cutting (1)Cutting velocity - Velocity of the tool relative to the work (2)Chip velocity - The velocity of the tool relative to the tool (3)Shearing velocity - The velocity of the chip relative to the work piece

Velocity relationship in metal cutting cont.. According to the principles of relative motion According to the sine rule

Forces acting on metal chip in metal cutting

Force acting on tool in metal cutting Let forces in tool holder (applied load to the tool) is and Thrust force (forces acting perpendicular to the work surface) Cutting force (force acting along the cutting velocity) Tool and can be measured using dynamometer

Merchant circle diagram for represent cutting forces All these forces can be represented with the help of a circle known as the merchant force circle’

Power consumption in metal cutting The power consumption in metal cutting is given by the following expression The energy consumed per unit volume of metal removed is given by the following expression Un-deformed chip thickness With of the chip to be removed

Home work Read the Chapter 20 Fundamental of cutting, chapter 21 Tool materials and cutting fluids Manufacturing Engineering and Technology By Serope Kalpakjian Steven R. Schmid