24-12-2012, 05:52 PM
THEORY OF METAL MACHINING
THEORY OF METAL.ppt (Size: 1,008 KB / Downloads: 38)
Material Removal Processes
A family of shaping operations, the common feature of which is removal of material from a starting workpart so the remaining part has the desired geometry
Machining – material removal by a sharp cutting tool, e.g., turning, milling, drilling
Abrasive processes – material removal by hard, abrasive particles, e.g., grinding
Nontraditional processes - various energy forms other than sharp cutting tool to remove material
Machining
Cutting action involves shear deformation of work material to form a chip
As chip is removed, new surface is exposed
Why Machining is Important
Variety of work materials can be machined
Most frequently used to cut metals
Variety of part shapes and special geometric features possible, such as:
Screw threads
Accurate round holes
Very straight edges and surfaces
Good dimensional accuracy and surface finish
Disadvantages with Machining
Wasteful of material
Chips generated in machining are wasted material, at least in the unit operation
Time consuming
A machining operation generally takes more time to shape a given part than alternative shaping processes, such as casting, powder metallurgy, or forming
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 workpart
Machining provides the final shape, dimensions, finish, and special geometric details that other processes cannot create
Machining Operations
Most important machining operations:
Turning
Drilling
Milling
Other machining operations:
Shaping and planing
Broaching
Sawing
Milling
Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surface
Two forms: peripheral milling and face milling
Cutting Tool Classification
Single-Point Tools
One dominant cutting edge
Point is usually rounded to form a nose radius
Turning uses single point tools
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
Cutting Conditions in Machining
Three dimensions of a machining process:
Cutting speed v – primary motion
Feed f – secondary motion
Depth of cut d – penetration of tool below original work surface
For certain operations, material removal rate can be computed as
RMR = v f d
where v = cutting speed; f = feed; d = depth of cut
Roughing vs. Finishing
In production, several roughing cuts are usually taken on the part, followed by one or two finishing cuts
Roughing - removes large amounts of material from starting workpart
Creates shape close to desired geometry, but leaves some material for finish cutting
High feeds and depths, low speeds
Finishing - completes part geometry
Final dimensions, tolerances, and finish
Low feeds and depths, high cutting speeds
Cutting Temperatures are Important
High cutting temperatures
Reduce tool life
Produce hot chips that pose safety hazards to the machine operator
Can cause inaccuracies in part dimensions due to thermal expansion of work material