Material Removal

Discontinuous Chip

Brittle work materials Low cutting speeds Large feed and depth of cut High tool-chip friction

Continuous chip with Builtup Edge (BUE)

Ductile materials Low-to-medium cutting speeds Tool-chip friction causes portions of chip to adhere to rake face BUE forms, then breaks off, cyclically

Continuous Chip

Ductile work materials High cutting speeds Small feeds and depths Sharp cutting edge Low tool-chip friction

Serrated Chip

Semicontinuous - saw-tooth appearance Cyclical chip forms with alternating high shear strain then low shear strain Associated with difficult-tomachine metals at high cutting speeds

Material Removal

A family of shaping operations, the common feature of which is removal of material from a starting work part so the remaining part has the desired geometry

Sawing

A narrow slit is cut into the work by a tool consisting of a series of narrowly spaced teeth

Threading

A pointed tool is fed linearly across the outside surface of the rotating work part in a direction parallel to the axis of rotation at a large effective feed rate, thus creating threads in the cylinder

Shaping and Planing

A single-point cutting tool is a used to cut a flat surface. In shaping, the speed motion is accomplished by moving the cutting tool In planing, the speed motion is accomplished by moving the work part

Boring

A single-point tool is fed linearly, parallel to the axis of rotation, on the inside diameter of an existing hole in the part.

Centering

Also called center drilling, this operation drills a starting hole to accurately establish its location for subsequent drilling. The tool is called a center drill

Types of Chips in Machining

Discontinuous Chip, continuous chip, continuous chip with built up edge, and serrated chip

Variations of Turning

Facing. taper turning, contour turning, form turning

Form turning

In this operation, sometimes called forming, the tool has a shape that is imparted to the work by plunging the tool radially into the work.

Contour turning:

Instead of feeding the tool along a straight line parallel to the axis of rotation as in turning, the tool follows a contour that is other than straight, thus creating a contoured form in the turned part.

Taper Turning

Instead of feeding the tool parallel to the axis of rotation of the work, the tool is fed at an angle, thus creating a tapered cylinder or conical shape.

Milling Operation

Machining operation in which work is fed past a rotating tool with multiple cutting edges • Axis of tool rotation is perpendicular to feed • Cutting tool called a milling cutter Cutting edges called teeth • Machine tool called a milling machine • Interrupted cutting operation -the teeth of the milling cutter enter and exit the work during each revolution. This interrupted cutting action subjects the teeth to a cycle of impact force and thermal shock on every rotation. The tool material and cutter geometry must be designed to withstand these conditions. • Basic milling operation creates a planar surface Other geometries possible

Pros and Cons of Machining

Pros • Variety of work materials can be machined Most frequently used to cut metals • Variety of part shapes and special geometric features possible: Screw threads Accurate round holes Very straight edges and surfaces • Good dimensional accuracy and surface finish Cons • Wasteful of material Chips generated in machining are wasted material • Time consuming A machining operation generally takes longer to shape a given part than alternative shaping processes

Reaming

Reaming is used to slightly enlarge a hole, to provide a better tolerance on its diameter, and to improve its surface finish. The tool is called a reamer, and it usually has straight flutes.

Milling

Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surface

Turning

Single point cutting tool removes material from a rotating workpiece to form a cylindrical shape

Chamfering

The cutting edge of the tool is used to cut an angle on the corner of the cylinder, forming a "chamfer."

Facing

The tool is fed radially into the rotating work on one end to create a flat surface on the end.

Cutoff

The tool is fed radially into the rotating work at some location along its length to cut off the end of the part. This operation is sometimes referred to as parting.

Countersinking

This is similar to counterboring, except that the step in the hole is cone-shaped for flat head screws and bolts.

Tapping

This operation is performed by a tap and is used to provide internal screw threads on an existing hole.

Drilling

Used to create a round hole, usually by means of a rotating tool (drill bit) with two cutting edges

Turning

a machining process in which a single-point tool removes material from the surface of a rotating workpiece The tool is fed linearly in a direction parallel to the axis of rotation to generate a cylindrical geometry

Broaching

a tool, with successively increasing tooth size, is moved through the workpeice, creating the desired shape with a single pass

orthogonal cutting

a wedge-shaped cutting tool is used • The cutting edge is perpendicular to the direction of cut • The tool has only two elements of geometry Rake angle,: Determines the direction of the chip flow Clearance angle : Provides the clearance between the tool flank and work surface

Finishing

completes part geometry Final dimensions, tolerances, and finish Low feeds and depths, high cutting speeds

Abrasive processes

material removal by hard, abrasive particles, e.g., grinding

Machining

material removal by a sharp cutting tool, e.g., turning, milling, drilling

Counterboring

provides a stepped hole, in which a larger diameter follows a smaller diameter partially into the hole. A counterbored hole is used to seat a bolt head into a hole so the head does not protrude above the surface

Roughing

removes large amounts of material from starting work part Some material remains for finish cutting High feeds and depths, low speeds

Spot facing.

similar to milling. It is used to provide a flat machined surface on the work part in a localized area

Sawing

successive teeth, arranged in a narrow line, remove a small amount of material. Types of Blades • Hacksaws A rigid straight blade with limited teeth • Bandsaws A flexible long blade that is formed into a continuous band • Circular Saws A rigid disk with teeth on the circumference

Nontraditional processes

various energy forms other than sharp cutting tool to remove material

Peripheral milling

• Cutter axis parallel to surface being machined • Cutting edges on outside periphery of cutter Types (a) Slab milling, (b) slotting, (c) side milling, (e) straddle milling, and (e) form milling

Face milling

• Cutter axis perpendicular to surface being milled • Cutting edges on both the end and outside periphery of the cutter Types (a) Conventional face milling, (b) partial face milling, and (c) end milling (d) Profile milling, (e) pocket milling, and (f) surface contouring

Multiple Cutting Edge Tools

• More than one cutting edge • Motion relative to work achieved by rotating • Drilling and milling use rotating multiple cutting edge tools

Single-Point Tools

• One dominant cutting edge • Point is usually rounded to form a nose radius • Turning uses single point tools