25-08-2017, 09:32 PM
Chip-Type Machining Processes
Cutting2.ppt (Size: 490 KB / Downloads: 88)
Basic Mechanics of Metal Cutting
Metal ahead of the cutting tool is compressed. This results in the deformation or elongation of the crystal structure—resulting in a shearing of the metal. As the process continues, the metal above the cutting edge is forced along the “chip-tool” interference zone and is moved away form the work.
Chip Formations
During this process (3) basic types of chips are formed:
Discontinuous
Continuous
Continuous with a built-up edge (BUE)
Discontinuous
Typically associated with brittle metals like –Cast Iron
As tool contacts work, some compression takes place
As the chip starts up the chip-tool interference zone, increased stress occurs until the metal reaches a saturation point and fractures off the workpiece.
Continuous with a built-up edge(BUE)
Same process as continuous, but as the metal begins to flow up the chip-tool zone, small particles of the metal begin to adhere or weld themselves to the edge of the cutting tool. As the particles continue to weld to the tool it effects the cutting action of the tool.
This type of chip is common in softer non-ferrous metals and low carbon steels.
Problems
Welded edges break off and can become embedded in workpiece
Decreases tool life
Can result in poor surface finishes
Coolants/Cutting fluids
Cutting fluids are used extensively in metal removal processes.
Act as a coolant, lubricant, and assist in removal of chips.
Primary mission of cutting fluids is to extend tool life by keeping keep temperatures down.
Most effective coolant is water….BUT is hardly ever used by itself. Typically mixed with a water soluble oil to add corrosion resistance and add lubrication capabilities.
Tool Materials
They include:
Tool steels - low end of scale. Used to make some drills, taps, reamers, etc. Low cost equals low tool life.
High speed steel(HSS) - can withstand cutting temperatures up to 1100F. Have improved hardness and wear resistance, used to manufacture drills, reamers, single point tool bits, milling cutters, etc. HSS cutting tools can be purchased with additional coatings such as TiN which add additional protection against wear.
Tool Materials
Cobalt - one step above HSS, cutting speeds are generally 25% higher.
Carbides - Most widely used cutting tool today. Cutting speeds are three to five times faster than HSS. Basic composition is tungsten carbide with a cobalt binder. Today a wide variety of chemical compositions are available to meet different applications. In addition to tool composition, coatings are added to tool materials to incerase resistance to wear.
Summary Positive vs. Negative Rake Angles
Positive rake angles
Reduced cutting forces
Smaller deflection of work, tool holder, and machine
Considered by some to be the most efficient way to cut metal
Creates large shear angle, reduced friction and heat
Allows chip to move freely up the chip-tool zone
Generally used for continuous cuts on ductile materials which are not to hard or brittle