There is an ever-growing need to improve productivity, reduce production lead times and respond to increased demands for savings when machining fully hardened steels, hard sintered metals and heat-resisting super-alloys.

This need is being met with the latest advances in cubic boron nitride (CBN) tooling, which include the introduction of special compounds, high-efficiency coatings and new geometries designed to withstand heat, improve surface finish and accommodate greater mechanical shocks — such as those encountered in interrupted cutting.

Sumitomo Electric Hardmetal (www.sumitomotool.com) is introducing a ‘hybrid’ series of CBN hard-turning inserts, new chip-breaker designs and a new range of micro-boring bars for holes as small as 2.5mm in diameter.

The new product launches add to the company’s SumiBoron range of CBN tooling, which now covers milling cutters and end-milling cutters, as well as tools for turning, grooving, threading and boring.

Both the latest SumiBoron hybrids feature multi-layered heat- and wear-resistant micro-structured coatings on sintered CBN powder blanks with a substrate that incorporates a carbo-nitride binder.

To further improve the performance and application range of the new-grade inserts, Sumitomo has also developed a totally new series of Break Master chip-breakers.

Hot hardness

CBN has excellent hot hardness characteristics, allowing it to be used at very high cutting speeds. The SumiBoron material offers high levels of toughness, as well as resistance to thermal shock and chemicals; and with Sumitomo’s second generation of CBN, the material is used to machine not only ferrous materials, but also grey cast irons, challenging heat-resisting super-alloys and hard powdered metals containing tungsten and titanium carbide (for which diamond would be unsuitable).

On cast iron, the latest CBN gives a longer cutting life and a better finishing capability than carbide, cermets or ceramics.

As a result, users can now access the higher-productivity advantage of single-operation strategies on materials over 45HRC, and thus eliminate grinding, EDM processes, hand polishing and other time-consuming skill-dependent and difficult-to-control finishing processes.

Indeed, Sumitomo says that above 55HRC, CBN is the only cutting tool that can effectively replace the traditional grinding process. The resulting single-cycle finish machining reduces lead times, helps control important geometric relationships, lessens the need for tool change interruptions (due to the longer in-cut life), and reduces work handling — as well as the cost of grinding fluid and sludge disposal.

The latest hybrid PVD-coated SumiBoron BNC2010 high-precision turning grade incorporates seven layers of coating. Three of these are highly secret developments, and each is sandwiched between layers of TiCN in order to ensure the integrity of the cutting edge and resistance to flank wear.

In order to help create a superior surface finish, the insert is topped with a layer of TiN.

The new BNC2020 insert is designed for more-general applications of continuous cutting, as well as impact-type cuts where a higher resistance to chipping around the edge of the insert is needed.

The insert incorporates an ‘adhesive layer’ that bonds its tough CBN substrate to the outer coating layer of TiAIN.

The addition of the newly developed Break Master N-FV, N-LV and N-SV chip-breakers enables chip control to be improved, with N-FV suitable for finishing with depths of cut up to 0.2mm.

N-LV delivers better performance when depths of cut are up to 0.3 mm, while N-SV is targeted at the removal of carbonised layers on hardened as well as non-hardened parts.

SumiBoron One Use WG and WH wiper inserts are primarily for continuous cutting cycles on hardened steel, where finishes comparable to grinding are required. WG is used on low-feed-rate turning applications, while WH is for higher-speed continuous-cutting conditions.

Micro-boring

The use of CBN on micro-boring tasks offers new opportunities to achieve consistency of finish and increased tool life when machining holes as small as 2.5mm in diameter in hardened materials.

Moreover, the same tool can be used to finish bore and face. The BSME tool has a brazed CBN cutting edge on a carbide tool shank (for added stiffness) for bore sizes between 2.5mm in diameter x up to 5.3mm deep, and up to 5mm in diameter x 20.3mm deep.

Meanwhile, the SEXC version features a two-corner indexable CBN insert that can machine bores from 4mm in diameter x 8mm deep to 6mm in diameter x 18mm deep. For this insert, there are two grades of CBN available — BN2000 and BN7000 — each with a 0.2mm nose radii. BN2000 has a honed preparation to the insert’s sharp edge, while BN7000 features
a normal sharp edge.

Both tool types have through-tool coolant, and each tool shank across the range can be held in a common 6mm-diameter sleeve with two clamp screws.

A location dead-stop pin ensures repeatability when setting depths (these can be maintained to within 0.02mm).

Recent trials on a CNC sliding-head turn-mill centre involved precision finish-turning of a cone in hardened (60HRC) material using neat-oil coolant.

The brazed SumiBoron BSME tool produced 3,600 parts; a competitor tool had achieved a tool life that was inconsistent, varying between 400 and 1,500 pieces. The cutting speed was 48m/min with a feed rate of 0.2mm/rev and depths of cut between 0.02 and 0.05mm.

When finish-boring the 7.1mm-diameter holes with emulsion coolant in hardened automotive components, the use of Sumitomo’s SEXC indexable insert grade BN2000 increased the number of parts produced per corner by over 400% to 2,700. The cutting speed was 156m/min (7,000rev/min), the feed rate 0.03mm/rev and the depth of cut 0.10mm.