Instructor: Shantanu Bhattacharya TA 202-A (Lecture 8) Instructor: Shantanu Bhattacharya

Introduction (Mechanics of machining operation) Idealized model; Orthogonal; 2-D cutting with a well-defined shear plane; also called Merchant model In idealized model, a cutting tool moves to the left along the workpiece at a constant velocity, V, and a depth of cut, to, Chip thickness, tc 2

Orthogonal and Oblique Cutting When the cutting edge is perpendicular to the direction of velocity of the tool, the cutting is orthogonal. When the cutting edge is at an angle not equal to 90 deg. to the direction of velocity of the tool, the cutting is oblique.

Mechanics of Cutting: Types of Chips Produced in Metal Cutting Types of metal chips commonly observed in practice (orthogonal metal cutting) There are 4 main types: Continuous chip (with narrow, straight, primary shear zone) Continuous chip with secondary shear zone at the tool-chip interface Built-up edge, BUE chip Serrated or segmented or non-homogenous chip Discontinuous chip 4

Mechanism of Chip Formation The uncut layer undergoes severe plastic deformation in the primary shear zone. Just after formation, the chip flows over the rake surface of the tool and strong adhesion between the high temperature chip and the rake face results in some sticking. This is secondary shear zone. At low speed, lower uncut chip thickness, large rake angle and suitable cutting fluid chips get produced as continuous and ribbon like. At higher speeds, uncut chip thickness and smaller rake angle, the temperature increases and the tendency of the plastically deformed material to adhere to rake face increases and a lump is formed at the cutting edge called built up edge (BUE). After the BUE builds up to a certain size it gets deadhered due to the increased force exerted on it by the surrounding flowing material. The broken surface adhete to the finished surface and makes it rough. If the material is brittle ruptures during high speed cutting occur intermittently resulting in discontinuous chips. 5

Mechanics of Cutting: Types of Chips Produced in Metal Cutting BUE deposited Cutting tool work piece Built-up Edge (BUE) Chips Consists of layers of material from the workpiece that are deposited on the tool tip As it grows larger, the BUE becomes unstable and eventually breaks apart BUE: partly removed by tool, partly deposited on workpiece BUE can be reduced by: Increase the cutting speeds Decrease the depth of cut Increase the rake angle Use a sharp tool Use an effective cutting fluid Use cutting tool with lower chemical affinity for workpiece material Built up edge formation BUE: turning BUE: milling Hardness distribution with BUE chip Note BUE chip much harder than chip 6

Mechanics of chip formation

Mechanics of chip formation

Mechanics of chip formation

Numerical Problem During an orthogonal machining operation with a cutting tool having a rake angle of 10 deg., the chip thickness is measured to be 0.4mm, the uncut thickness being 0.15mm. Determine the shear plane angle and also the magnitude of the shear strain.

Numerical Problem During an orthogonal machining operation with a cutting tool having a rake angle of 10 deg., the chip thickness is measured to be 0.4mm, the uncut thickness being 0.15mm. Determine the shear plane angle and also the magnitude of the shear strain. Sol. The cutting ratio is 0.15/0.4 = 0.38

Temperatures in Cutting Temperature rise (due to heat lost in cutting ⇒ raising temp. in cutting zone) Its major adverse effects: Lowers the strength, hardness, stiffness and wear resistance of the cutting tool (i.e. alters tool shape) Causes uneven dimensional changes (machined parts) Induce thermal damage and metallurgical changes in the machined surface (⇒ properties adversely affected) Sources of heat in machining: Work done in shearing (primary shear zone) Energy lost due to friction (tool-chip interface) Heat generated due to tool rubbing on machined surface (especially dull or worn tools) 12

Temperatures in Cutting Temperature Distribution Sources of heat generation are concentrated in primary shear zone, and At tool–chip interface ⇒ v. large temp. gradients in the cutting zone (right) 13

Temperatures in Cutting Temperature Distribution Note: Highest temp.: 1100ºC High temp. appear as dark- color on chips (by oxidation at high V ) Reason: as V ↑ ⇒ time for heat dissipation ↓ ⇒ temp. ↑ Temperatures developed in turning 52100 steel b) tool-chip interface temp. distribution a) flank temperature distribution 14

Temperatures in Cutting “Techniques for Measuring Temperature” Temperatures and their distribution can be determined using thermocouples (placed on tool or workpiece) Measuring infrared radiation (using a radiation pyrometer) from the cutting zone (only measures surface temperatures) 15

Tool Life: Wear and Failure Tool wear is gradual process; created due to: High localized stresses at the tip of the tool High temperatures (especially along rake face) Sliding of the chip along the rake face Sliding of the tool along the newly cut workpiece surface The rate of tool wear depends on tool and workpiece materials tool geometry process parameters cutting fluids 16

Tool Life: Wear and Failure: Flank Wear Flank wear occurs on the relief (flank) face of the tool It is due to rubbing of the tool along machined surface (⇒ adhesive/abrasive wear) high temperatures (adversely affecting tool-material properties) Taylor tool life equation : V = cutting speed [m/minute] T = time [minutes] taken to develop a certain flank wear . n = an exponent that generally depends on tool material (see above) C = constant; depends on cutting conditions note, magnitude of C = cutting speed at T = 1 min Also note: n, c : determined experimentally 17

Tool Life: Wear and Failure: Flank Wear To appreciate the importance of the exponent, n, Taylor tool life equation, rearranged: Thus, for constant C : smaller n ⇒ smaller tool life For turning, equation can be modified to where, d = depth of cut (same as t0) f : feed of the tool [mm/rev ] x, y: must be determined experimentally for each cutting condition 18

Tool Life: Wear and Failure: Flank Wear Tool-life Curves The exponent n can be determined from tool-life curves (see right) Smaller n value ⇒ as V increases ⇒ tool life decreases faster n can be negative at low cutting speeds Temperature also influences wear: as temperature increases, flank wear rapidly increases 19

Cutting Fluids Any liquid or gas applied directly to machining operation to improve cutting performance Two main problems addressed by cutting fluids: Heat generation at shear zone and friction zone Friction at the tool‑chip and tool‑work interfaces Other functions and benefits: Wash away chips (e.g., grinding and milling) Reduce temperature of work surface for easier handling Improve dimensional stability of work surface. Reducing the coefficient of friction at the chip tool interface due to the formation of a weaker compound at the interface. Protection of the finished surface from corrosion 20

Characteristics of An ideal cutting fluid Have a large specific heat and thermal conductivity Have a low viscosity and small molecular size Contain a suitable reactive constituent. Be nonpoisonous and non corrosive Be inexpensive and easily available 21

Cutting Fluid Functions Cutting fluids can be classified according to function: Coolants - designed to reduce effects of heat in machining Lubricants - designed to reduce tool‑chip and tool‑work friction Types of cutting fluid: Water based fluids (Coolant) Mineral oil based fluids (Lubricants) 22

Coolants Lubricants Water used as base in coolant‑type cutting fluids Most effective at high cutting speeds where heat generation and high temperatures are problems Most effective on tool materials that are most susceptible to temperature failures (e.g., HSS) Lubricants Usually oil‑based fluids Most effective at lower cutting speeds Also reduces temperature in the operation 23

Economic Advantages Using Cutting Fluids Reduction of tool costs Reduce tool wear, tools last longer Increased speed of production Reduce heat and friction so higher cutting speeds Reduction of labor costs Tools last longer and require less regrinding, less downtime, reducing cost per part Reduction of power costs Friction reduced so less power required by machining 24 24