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Most modern carbide inserts and solid carbide tools are coated with a thin film (between 3 and 20 microns, or 0.0001 to 0.0007 inch). The coating is typically composed of titanium nitride, aluminum oxide and titanium carbon nitride layers. This coating increases the hardness and creates a heat barrier between the cut and the substrate.

Source: Choosing Your Carbide Grade: A Guide

Counterboring is a machining operation that will enlarge an existing hole’s diameter to a specified depth. Traditionally, counterboring is done with either a special counterboring tool that has a pilot, or it is done with an end mill. In either case, the diameter of the tool (end mill or counterbore) must match the diameter that is counterbored.

Traditional counterboring is great for smaller holes, but as holes get larger, the cutting tools get more expensive.

Source: Custom Macro To Helically Mill A Counterbore

A countersink is a functional surface upon which a fastener head bears. Because fastener performance is so important, countersink tolerances are critical.

Source: Gaging Countersunk And Chamfered Holes

Hole chamfers are usually specified simply to make it easier to insert a screw, pin, bushing, or other assembly component. The component doesn't bear on the chamfer, so diameter and angle tolerances are usually not critical to the part's performance.

Source: Gaging Countersunk And Chamfered Holes