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What are expansive bits made of?

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Chrome carbon steel, which is used to make expansive bits Expansive bits are usually made of chrome carbon steel, a type of tool steel with a slight resistance to corrosion.
Image showing that expansive bits can bore through wood and plastic, but not ceramic, concrete, brick, metal or glass This type of material is not as hard as the high speed steel (HSS) used for some other drill bits, and so expansive drill bits are not suitable for boring through materials tougher than plastic.
Image showing steel being tempered in the same way as the cutting edges of an expansive bit The tip of an expansive bit and its adjustable cutters are tempered, a process that involves heating and cooling metal to increase its hardness. This helps the cutting edges to keep their sharp edge for longer.
Image showing steel with zinc plating, which is the material used to make the adjuster screws in expansive bits Most locking screws are made from zinc-plated steel. The zinc plating is added for cosmetic purposes, to prevent the screw from being too badly scuffed when it comes into contact with a screwdriver.
Image of a zinc-plated screw, the only part of an expansive bit with a protective coating Other than their zinc-plated screws, no coating is applied to expansive bits, as the bit itself is hard enough to stand up to the material through which it bores.