Designing the rake face of an indexable insert requires a certain level of engineering skill: knowledge of metal cutting theory and the chip formation process, understanding specific features of machining different materials, knowing principles of powder metallurgy and limitations of manufacturing sintered products, experience and appropriate training in tool design. The rake face determines the cutting geometry of a tool - its total cuttability (cutting capability), and forming the rake face in an optimal way is a key element of the insert design.
Over the years, technological options for cutting tool manufacture have largely dictated the shape of the rake face. For example, in the earliest days of indexable tooling the inserts featured flat faces. Breaking a long chip when turning by tools carrying these inserts often required using additional cover parts that were mounted in the tools above the inserts. In certain tool designs, even an upper clamp, which secured the insert, acted like a chipbreaker. Another common solution for flat-face turning inserts was to produce a chip breaking dimple by grinding. The dimple promoted curling the chip in a spiral and then its breaking into smaller segments. Both these methods should be considered through the lens of time, but they were far from perfect.
The chip breaking cover part produced a natural obstacle for the chip flow. The chips caused intensive abrasion of the part and significantly reduced its tool life. The shape and dimensions of the dimple strongly depended on a grinding wheel that considerably reduced possible dimple forms. But the main problem was the necessity of long-term tests to develop a chipbreaker that would ensure stable performance when machining different types of material. To some extent, chipbreaker design was more like a path of trial and error.
Advances in powder metallurgy changed the situation dramatically, bringing new machines and computer-based control that substantially improved stability and reliability in a range of processes. The technology of sintered carbide products facilitated the shaping of insert rake faces in various forms and broke the dependence of a chip breaking surface on the dimple or the cover part. The rake face received a look of combined concave and convex portions, local protrusions etc. - this complex geometry was designed to provide the necessary chip formation and effective chip control. Rake faces of today’s indexable inserts feature the same surface texture.
Introducing computer aided design (CAD) systems into research and development (R&D) of cutting tools had a significant impact on shaping the rake face. CAD provides tool designers with a powerful tool for complicated 3-D modelling, engineering calculations and analyzing possible limitations of a designed insert and, of course, its rake face. The combination of state-of-the-art sintered product technology, advanced CAD systems and up-to-date CNC machines marked a quantum leap in the cutting tool industry. It not only allowed producing a wide variety of inserts with geometrically complex faces but substantially shortened the design process.
The totally new level of cutting tool design and technology reduced testing needs significantly. However, the time required for studying cutting capabilities of a new insert geometry with the use of machining trials remained considerable.
This holds for the design of all indexable inserts, not only turning. In the case of milling inserts, the rake face shape is considered mainly from the point of view of chip forming only - milling is a process of interrupted cutting and therefore chip breaking creates no difficulties. In milling inserts, the rake face is called a chipformer and not a chipbreaker as it is characterized for turning inserts. To be clear, the rake face of the turning insert is also intended mainly for chip formation though it should enable chip breaking. In the context of geometry, the rake face of every indexable insert is a combination of concave and convex areas.
3D MODELLING IMPACT
Scientific researches, numerous tests and the analysis of accumulated information in the field of metal cutting, combined with significant advances in computer technology, have provided the cutting tool industry with a new powerful design tool - three-dimensional modelling of chip formation. The first simplified models of chip formation were based on empirical and calculated data, and suffered from serious inaccuracies. Further development, based on the finite element method (FEM), raised cutting action modelling systems to a whole new level. Today, cutting tool designers utilize advanced software that enables simulation of chip formation processes with a sufficient approximation to reality. Even though the software still cannot replace machining tests, it is able to contribute greatly to the effective design of the indexable inserts and, most of all, their rake faces.
MATCHING GEOMETRY TO OPERATION
ISCAR, a leading company in the cutting tool industry, has implemented modelling practices that allow R&D engineers to determine which insert geometry is appropriate for which operation, even at the design stage.
CNMG TURNING INSERT
When designing the CNMG 120404-F3P turning insert, it was found that simulating cutting action was useful for shaping the insert’s top surface (Fig. 1). The fancy patterning was not devised to reflect the virtuosity of a R&D team, and in fact modelling proved to be an extremely valuable tool in realizing the team’s objective of ensuring the best cutting capabilities.
ICG HEADS FOR SUMOCHAM DRILLING TOOLS
Drilling hard-to-cut austenitic and duplex stainless steel (ISO M application group) presents difficulties, especially if the depth of a hole is substantial. To improve performance in these types of drilling operation, ISCAR developed ICG exchangeable carbide heads with chip splitting geometry. The range of the head diameter (D) is 14 - 25.9 mm (0.551 - 1.02 in). The heads are mounted in standard SUMOCHAM drill bodies and provide high-quality machining holes with depth up to 12×D. With head geometry featuring chip splitting notches on the cutting edge and a specially designed chipformer to ensure superior chip control, chip evacuation problems in deep drilling applications are simply solved.
Chip flow modelling has represented an important step in the chipformer shaping process, and has been integral in determining the success of the proposed design (Fig. 2).
NANMILL MILLING CUTTERS
In its recent “LOGIQ” campaign, which launched a range of new and enhanced cutting tool lines, ISCAR introduced a series of indexable mills in the small-diameter range (up to 20 mm or 0.75 in). Although this range is traditionally considered as more suitable for solid carbide tools, the new indexable mills represent an attractive and cost-beneficial alternative.
The NANMILL family of indexable milling cutters within the diameter range of 8-16 mm (0.315-0.625 in) integrates a new design concept: they feature a clamping screw located above the insert and a screw head that functions as a wedge. However, to prevent any contact between the screw head and the chips that are produced (a potential consequence of this design), the insert chipformer required additional adaption. Modelling the chip formation process was an important factor in successfully solving the problem (Fig. 3).
Chip forming simulation has already become a valuable tool in shaping the insert rake face efficiently. Further progress in modelling cutting action should bring tool designers closer to achieving optimal chip forming geometries and will significantly improve the quality of the designed tool.