Ultra-small holes with diameters down to 2.5mm in hardened materials can be finish-bored and faced using Sumiboron cubic boron nitride (CBN) tools; this follows the development by Sumitomo Electric Hardmetal Ltd, Princes Risborough (www.sumitomo-hardmetal.co.uk), of its BSME and SEXC series of boring-bar tooling.

When turning hardened, tough and cast materials, CBN allows engineers to use cutting data that are beyond the scope of carbide; it also offers predictable in-cut life.

The BSME tool has a brazed CBN cutting edge on a carbide tool shank, which creates added stiffness for bore sizes from 2.5mm in diameter x 5.3mm deep to 5mm x 20.3mm deep.

Meanwhile, the SEXC version features a two-corner indexable CBN insert that can machine bores from 4 to 6mm in diameter x 8-18mm deep. Both tool types have through-tool coolant, and each tool shank in the range can be held in a common 6mm-diameter sleeve by two screws.

A location dead-stop pin ensures repeatability when setting depths, which can be maintained within 0.02mm.