Chapter 21

Chip breaker

A feature designed to prevent chips from forming into long pieces. Chip breakers are either indentations on the surface of the cutting insert or another wafer clamped above the insert in the tool holder.

Machine tool

A machine tool is a machine for shaping or machining metal or other rigid materials, usually by cutting, boring, grinding, shearing, or other forms of deformation. Machine tools employ some sort of tool that does the cutting or shaping. All machine tools have some means of constraining the workpiece and provide a guided movement of the parts of the machine

Shear plane

A shear plane is the plane in which shear stress occurs. A chip is produced ahead of the tool by plastic deformation, and shears the material continuously along the shear plane.

Built-up edge

Adhesion of workpiece material to the cutting edge of an insert due to high temperatures and pressure welding. Built-up edge is a common cause of tool failure when machining soft, gummy metal. Can be reduced by: Increase the cutting speed. Decrease the depth of cut. Increase the rake angle. Use a cutting tool that has lower chemical affinity for the workpiece material or use a sharp tool. Use an effective cutting fluid.

Serrated chip

Also called segmented or nonhomogenous chips. Semicontinuous chips with large zones of low shear strain and small zones of high shear strain. Sawtooth-like appearance Metals that have low thermal conductivity and have strength that decreases sharply with temperature exhibit this behavior, and is most notably observed with titanium and its alloys

Rake angle

An angle describing the tilt of the face from the cutter axis or radius. Positive rake angles reduce cutting forces and encourage chip removal.

Discontinuous chip

Consist of segments, attached either firmly or loosely to each other. Form under the following conditions: Brittle workpiece materials, b/c they donot have the capacity to undergo the high shear strains encountered in machining. Workpiece materials that contain hard inclusions and impurities, or have structures such as the graphite flakes in gray cast iron very low or very high cutting speed. Large depth of cut. Low rake angle. Lack of effective cutting fluid. Low stiffness of the toolholder or the machine tool, thus allowing vibration and chatter to occur.

Chip curl

Curvature that chips develop as they leave the workpiece surface. Affected by: The distribution of stresses in the primary and secondary shear zones. Work-hardening characteristics of the workpiece material. The geometry of the cutting material. Processing variables. Cutting fluids.

Machinability

Defined in terms of four factors: 1. Surface finish and surface integrity of the machined part 2. Tool life 3. Force and power requirements 4. The level of difficulty in chip control after it is generated.

Primary shear zone

Deformation of the material takes place along a narrow shear zone, called the primary shear zone.

Depth-of-cut line

line that describes how much material is removed from the surface.

Shear angle

phi. Angle between the shear plane and the normal plane.

Relief angle

present when there is a well-defined shear plane. angle between tool and depth of cut line The angle designed to eliminate interference and provide adequate space between the cutting tool and the workpiece.

Cutting force

Fc. It acts in the direction of the cutting speed and supplies the energy required for cutting.

What are the flank and rake faces of the tool?

Flank face-edge of the tool that is against the cut surface of the workpiece Rake face-edge of the tool that is against the chip surface of the workpiece.

Continuous chip

Formed with ductile materials machined at high cutting speeds and/or at high rake angles. Deformation of the material takes place along a narrow shear zone, primary shear zone, and continuous chips may develop a secondary shear zone b/c of high friction at the tool-chip interface.

Explain the difference between orthogonal and oblique cutting

In orthogonal cutting, the chip slides directly up the face of the tool and becomes like a spiral, whereas in oblique cutting, the chip becomes helical and leaves the workpiece surface at an angle i, called the inclination angle.

Machining

Machining is any of various processes in which a piece of raw material is cut into a desired final shape and size by a controlled material-removal process.

Secondary Shear zone

May be developed due to continuous chips because of high friction a the tool-chip interface. It becomes thicker as friction increases.

Chip

Piece of material being removed from surfaces of a workpiece.

How does the feed and speed of the tool affect the temperature rise in the tool?

Temperature increases with cutting speed and feed rate.

Clearance angle

The angle designed to eliminate interference and provide adequate space between the cutting tool and the workpiece.

Wear Land

The flattened section that forms on the flank of the tool behind the cutting edge due to abrasive wear with the workpiece.

Surface finish

The geometric features of a surface. influences not only the dimensional accuracy of machined parts, but also their properties and their performance in service.

Describe the mechanism by which temperature in a cutting tool rises during machining.

The main sources of heat in machining are: (a) the work done in shearing in the primary zone, (b) energy dissipated as friction at the tool-chip interface, and (c) heat generated as the tool rubs against the machined surface, especially for dull or worn tools.

Diffusion

The wearing away of material due to the exchange of atoms.

Crater wear

Tool wear characterized by a concave depression in the rake face of the cutting tool adjacent to the cutting edge. Crater wear is also called cratering.

Flank wear

Tool wear resulting in the gradual wearing away of the cutting edge. Flank wear is mostly caused by abrasion, is predictable, and is the most desired form of tool wear.

Turning

Turning is an engineering machining process in which a cutting tool, typically a non-rotary tool bit, describes a helical toolpath by moving more or less linearly while the workpiece rotates.

Tool life

adversely affected by tool wear (flank wear, crater wear, notching, plastic deformation, chipping, and gross fracture.

Allowable wear land

amount of wear on a tool that is allowed. different values for different machining operations and materials of tools

Oblique cutting

three dimensional cutting.

Cutting ratio

to(the depth of cut)/tc(chip thickness). chip thickness ratio

Orthogonal cutting

two dimensional and the forces involved are perpendicular to each other.

Chipping of tool

when a small fragment off the cutting edge of the tool breaks away.