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What are the different types of expansive bit?

Shop for Expansive bits

Image to show both types of expansive bit: the Wright patent expansive bit and the Clark patent expansive bit There are two different types of expansive bit, each named after the man who filed the patent for their design. These two types are Wright patent expansive bits and Clark patent expansive bits.
Image to show that the way an expansive bit is classified depends on the way it is adjusted They are differentiated by the way the adjustable cutter is lengthened or shortened when attached to the bit.

Wright patent expansive bits

Close up of the head of a wright patent expansive bit showing the cog adjuster mechanism Wright patent expansive bits have adjustable cutters with teeth cut into the top.
Image showing how the diameter of a wright patent expansive bit is adjusted by rotating the adjuster screw The length of the cutter is altered by turning the adjuster screw clockwise or anti-clockwise. The screw itself turns a cog mechanism, which in turn pushes the cutter forwards or backwards using the teeth on its upper edge.
Image of the exterior of a hardware store, where Alfred M. Wright invented and patented a cog-adjusted expansive bit This type of bit is rarer than the Clark patent bit, and it was named after an American hardware company owner, Alfred M. Wright, when he invented it in 1939.

Clark patent expansive bits

Close up of the head of a clark patent expansive bit showing the clasp adjuster mechanism Clark patent expansive bits have adjustable cutters with no teeth along their upper edge.
Close up of the head of a clark patent expansive bit to show the adjustor mechanism They attach to the main body of the bit via a wedge and clamp system. To adjust the bit, a DIYer loosens the lock screw and slides the cutter to the correct length manually, before tightening the screw once again.
Image of a patent office, where William A. Clark filed his patent for an expansive bit with a clamp-based adjuster mechanism This is the oldest type of expansive bit currently in use, and was patented in 1858 by another American inventor, William A. Clark.