What are material removal processes?
A family of shaping operations, the common feature of which is the removal of material from a starting work part so that the remaining part has the desired geometry.
Conventional machining:
material removal by a sharp cutting tool
e.g., turning, drilling, milling, sawing.
Abrasive processes:
material removal by hard, abrasive particles
e.g., grinding
Non-traditional processes:
use various energy forms other than a sharp cutting tool to remove material
e.g., laser beam cutting, water jet cutting, electrical discharge machining.
categories: mechanical energy processes, electrochemical machining, thermal energy processes, chemical machining.
Machining:
Cutting action involves shear deformation of the work material to form a chip; as the chip is removed, a new surface is exposed.
- Relative motion between the tool and work is achieved by:
– Primary motion – cutting speed.
– Secondary motion – feed (much slower).
Why machining is important:
Variety of work materials can be
machined.
– Most frequently used to cut metals.
Variety of part shapes and special geometrical features:
– Regular geometry: cylinders, round holes, flat planes.
– Irregular geometry: screw threads, T-slots.
– Very complex geometry – by computer numerical control (CNC)
Good dimensional accuracy and
surface finish.
Disadvantages of machining:
Wasteful of material:
– Chips generated in machining are wasted material, at least in the unit operation (although usually recycled).
Time consuming:
– A machining operation generally takes longer to shape a given part than alternative shaping processes.
Machining in manufacturing sequence:
Generally performed after other manufacturing processes, such as casting, forging and bar drawing.
– Other processes create the general shape of the starting work part.
– Machining provides the final geometry, dimension and finish.
Most important machining operations:
– Turning
– Drilling
– Milling
Turning:
- Single point cutting tool removes material from a rotating workpiece to form a cylindrical shape.
– Speed motion is provided by the rotating work part.
– Feed motion is achieved by the cutting tool.
- Traditionally performed on a machine tool called a lathe.
*The cutting tool moves slowly in a direction parallel to the axis of rotation of the workpiece.
Drilling:
- A rotating cylindrical tool (drill bit) with two (typically) cutting edges removes material to create a round hole.
– Speed motion and feed motion are provided by the tool.
– The workpiece is stationary.
- Customarily performed on a drill press.
*The drill is fed in a direction parallel to the axis of rotation into the workpiece.
Milling:
- A workpiece is slowly fed past a rotating cylindrical tool with multiple cutting edges to generate a plane or straight surface.
– Speed motion is provided by the rotating milling cutter.
– Feed motion is provided by the workpiece.
- Performed in a milling machine.
*The direction of feed motion is
perpendicular to the tool’s axis of rotation.
Types of milling:
Peripheral milling
Face milling
Peripheral milling:
The axis of the tool is parallel to the surface being machined. Cutting edges on the outside periphery of the cutter.
Face milling:
The axis of the tool is perpendicular to the
surface being machined. Cutting edges on both the end and outside periphery of the cutter.
Cutting tool:
- A cutting tool has one or more sharp cutting edges, and is made of a material harder than the work material.
– Cutting edges: separate a chip from the work material
– Rake face: direct the flow of the newly formed chip
– Flank: provide a clearance between the tool and the newly generated work surface.
a) Single-point tools (for turning etc.)
– The point is usually rounded to form a nose radius.
b) Multiple-cutting-edge tools (for drilling and milling etc.)
– Motion relative to the workpiece is achieved by rotating.
Cutting conditions:
- Three dimensions of a machining process:
– Cutting speed v – primary motion
– Feed f – secondary motion
– Depth of cut d – penetration of the cutting tool below the original work surface.
- In turning, material removal rate (RMR: mm3/s) can be calculated as: SEE EQUATION SHEET
Roughing cuts:
Remove large amounts of material from the
starting workpiece.
– Some material remains for finish cutting.
– High feeds and depths, low cutting speeds.
Finishing cuts:
Complete part geometry
– Final dimensions, tolerances and surface finish.
– Low feeds and depths, high cutting speeds.
Machine tools:
- A power-driven machine that performs a machining operation (turning, drilling, milling, grinding, etc.).
- Functions in machining:
– Hold work part
– Position the tool relative to the work
– Provide power at speed, feed and depth that have been set.
- The term also applies to machines that perform metal forming operations.
Machining centres:
A machining centre is a highly automated machine tool capable of performing multiple machining operations under computer numerical control (CNC) in one step with
minimal human attention.
