Knob cutters from kaneshin. A very useful tool in bonsai and used often.
A knob cutter is a bonsai tool used to create a concave cut in a branch or trunk to help the tree heal smoothly:
Create a concave cut
Knob cutters are used to create a cut that's just past flush, which allows the tree to heal flat. This is done by "biting" a chunk off the branch, which creates a divot in the trunk.
Remove excess material
Knob cutters can be used to remove undesirable material from the trunk or root area, such as excess callous.
Smooth flat cuts
Knob cutters can be used to smooth out flat cuts left on the tree after trunk chopping or removing a large branch.
Finish delicate work
Smaller knob cutters can be used for finishing delicate work or for show hin trees.
Knob cutters are also known as concave cutters. They have spherically shaped cutting blades that resemble two halves of a sphere coming together. This shape allows the tool to aggressively bite into wood. While knob cutters can be precise, they can also easily remove more material than intended, so caution is advised.