To provide a tool assembly for boring with a wide adjustable range of processing diameters.

This tool assembly comprises a boring bar 31, and a tool cartridge 39 used as installed on a boring bar to support a cutting tip 49. A main body 41 of the tool cartridge 39 is pivotally installed on a recessed place 33 formed in the boring bar 31 through a pin 37, so an adjusting screw 51 is driven into a screw hole in the cartridge main body 41 on one side of a pivotal point to be abutted on a part of the boring bar 31. On the other side of the pivotal point, a strong compression spring 55 is provided between the boring bar 31 and the cartridge main body 41. The adjusting screw 51 is loosened or tightened to pivotally move the cartridge main body 41 in a wide adjustable range. This adjustment may be automatically performed by providing a draw bar 60.