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TOOL COATINGS

Tool coatings are applied by the use of chemical vapor deposition (CVD) or physical vapor deposition (PVD). Each coating process has distinct advantages in the proper application.Tool coatings are useful because of their high hardness and low coefficient of friction. Coatings improve tool performance whether the substrate is HSS or carbide.CVD coatings are produced in a high-temperature ( ) process. CVD is a gaseous process that coats every surface exposed to the gases during the process. CVD coatings are chemically and metallurgically bonded to
a base material such as carbide or high-speed steel.
Because of the high coating temperature, these tools need to be heattreated again to restore their metallurgical properties.
During this second heat treatment, the tool may change size. CVD-coated tools generally are not made with tight dimensional tolerances. CVD coatings normally are between .0002 and .0003 in. thick. CVD coatings are used where severe compressive stresses are encountered, such as in heavy roughing cuts. 1850°F PVD coatings are applied in a vacuum chamber at less than At this low temperature, there are usually no dimensional changes or loss of hardness during the coating process. PVD-coated tools usually retain sharper cutting edges, preferred for light cuts. PVD coatings can be directed to a specific surface, because the process uses a directed ion beam that affects only line-of-sight areas. Coating thickness is usually from .0001 to .00015 in. 750°F. TiN coating Titanium nitride gives high lubricity to the tool surface, which facilitates chip flow and resists built-up edge during machining. It is used on ferrous materials and cast iron and has excellent abrasion resistance, edge strength, and resistance to cratering.TiC coating Titanium carbide adds abrasion resistance.TiCN coating Titanium carbonitride has the same lubricity properties as TiN but is 30 percent harder. This increased hardness gives maximum tool life. Al2O coating Aluminum (oxide) and carbon dioxide coatings on tools are used to machine cast irons (nodular and malleable).
This gives resistance to abrasive wear and cratering and adds shock resistance.3 CrN coating Chromium nitride is used to machine
many nonferrous materials. This coating also has a high oxidation resistance.CrC coating Chromium carbide coatings can be used
effectively when machining titanium or aluminum.
Multilayered coatings These combine the best properties of single coatings. Multiple coatings are made up of layers of and repetitions of these.
TiN>TiCN>Al2O3 Efficient CNC machining requires attention to workholding, toolholding, and the tooling itself. Always use the most rigid setup feasible. Machining centers have accuracies Figure M-82 The chip splitter reduces cutting forces, gives smoother cutting action, and promotes longer tool life (IngersollCutting Tools).and repeatabilities of .0002 in. and better. To maintain this accuracy, the toolholder must fit the spindle virtually perfectly.
It is just as important that the tool-holding feature be concentric to the toolholder taper. Cutting forces can be reduced by using positive-rake tooling and the correct chipbreaker geometry.
Chip-breaker design can also reduce cutting forces and vibration (Figure M-82).
With more rigidity and lower cutting forces, higher speeds and feeds can be used.

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