Relaxed machining How to successfully finish milling bimetal without any headaches

Worldia has developed a method in which PCD (polycrystalline diamond) and PCBN (polycrystalline cubic boron nitride) indexable inserts are combined in a standard milling cutter. In practice, this has reduced machining time by 25 to 30%.

The vastly different and nearly opposite machining properties of aluminum and cast iron often pose challenges for technologists, especially when tight roughness and flatness tolerances must be achieved in mass production, such as in the finish milling of engine block deck surfaces (Image 1). However, the new solution from Worldia may alleviate these issues, as will be demonstrated here.

Tool life can be significantly increased

Aluminum engine blocks with gray cast iron cylinder liners are today's standard to reduce weight and improve heat dissipation. However, as mentioned, aluminum and cast iron are two materials with very different physical and mechanical properties, making their machinability quite distinct. At the same time, the top surface of the cylinder block, as a sealing surface, imposes stringent requirements on surface roughness (Ra 1.25 to 2), waviness (Wt max 10), and overall flatness (50 μm). These specifications can be particularly challenging to meet on bimetallic surfaces where both materials must be machined to high precision. The process needs to accommodate the disparate cutting characteristics of each material to achieve a uniformly finished surface that meets all these critical specifications.

When milling the deck surface, the inserts continuously engage and disengage with the workpiece as the cutter rotates, resulting in varying chip thickness. In this highly interrupted cut, cutting forces change constantly. The engine block, with its thin wall thicknesses, is prone to vibrations that can cause marks on the sealing surface. The cast iron liners are susceptible to burring or chipping. The cast iron chips tend to stick to the cutting edges, leading to scratches on the aluminum surface. By carefully selecting the cutting material and designing the cutting edges, in conjunction with an effective supply of cooling lubricant, these undesirable effects can be minimized. Consequently, tool life can be improved by up to 40% compared to previous tools, enhancing both the quality of the machining and the operational efficiency.

Advantages and disadvantages of PCD and PCBN tools

Polycrystalline Diamond (PCD) has a structure composed of fine-grained diamond particles with various orientations. Despite containing a binder, its hardness reaches up to 8,000 HV, and its wear resistance is still eight to twelve times higher than that of carbide. With its sharp cutting edges, PCD is particularly suitable for non-ferrous metals and non-metallic materials, although it reacts with iron-based materials in air at high temperatures. Polycrystalline Boron Nitride (PCBN), on the other hand, shares similar structural properties and performance as PCD. However, it offers greater resistance to chemical reactions with iron-based materials. Coupled with its high hardness, PCBN is more suitable for machining hardened steel and cast iron. To optimally utilize the properties of both cutting materials, Worldia employs a combination of PCD and PCBN inserts for bimetal machining, as shown in Figure 2. This blend ensures enhanced performance across a diverse range of materials, balancing the benefits of both PCD’s superior hardness and wear resistance with PCBN's resistance to chemical interactions and high-temperature stability, thereby addressing the complex demands of bimetal machining.