Machine tools have developed considerably with the introduction of High Speed Steel (HSS) in the twentieth century. Today, there are two types of cutting tools, rotary and indexable.
Indexable tools are predominantly made of inserts or tips, where the cutting edge is a replaceable shaped piece which is welded or clamped on to a tool body. These are generally used for turning, milling and hole-making operations.
Fabio Sala is product manager for round tools and Lukas Pavlis, product manager for indexables, at Dormer Pramet. Both discuss the key elements which make up the differences between the two types of cutting tools, including the material, performance and versatility.
Material – indexable
The harder the tool substrate the more wear resistant the cutting tool, resulting in longer tool life. The compromise here is that harder tool substrates are more brittle and more likely to fracture or break under adverse operating conditions.
Softer tool substrates mean the tool will be tougher, allowing successful machining even under adverse conditions. Although, tougher substrates are less likely to fracture or break, they operate at lower performance parameters and so tool life is not as long.
An ideal cutting tool material should resist flank and point wear, as well as deformation, be tough to resist breakage, does not chemically interact with the work piece material, resist oxidation and diffusion and have good resistance to sudden thermal changes.
Inserts are made from tungsten carbide, which is the most commonly used material for indexables. Other available material used for inserts includes cermet (CT), ceramics (CC), cubic boron nitride (CBN) or polycrystalline diamond (PCD). CT has good flank and crater wear resistance and is not prone to built-up-edge. Because of this, the cutting edge maintains its sharpness over a longer time.
Material – rotary
Round tools are traditionally made from High Speed Steel (HSS), High Speed Cobalt (HSCo or HSS-E) or Solid Carbide.
Carbon, Chromium, Tungsten, Molybdenum, Vanadium and Cobalt all make up the main alloy elements used within high speed steel cutting tools.
HSS is a medium alloy that has good machinability and performance. It offers toughness and is a cost-effective option for a wide variety of large scale drilling and threading applications in multiple materials. However, it is not very wear resistant and so not suited to demanding, difficult conditions.
Cobalt high speed steel offers a good combination of toughness and hardness, providing good machinability and wear resistance. This makes the material suitable for drills, taps, milling cutters and reamers.
Solid Carbide is the most widely used and wear resistant cutting tool material today and is suitable for both round tool and indexable applications.
Performance & versatility – indexable
From a cost perspective, indexable inserts and tool holders represent a significant investment, however, they have the potential to last for longer periods of time. Also, when a replacement is needed, this is more time and cost efficient because of the need to only rotate or change the insert.
To support the long-term investment of indexable inserts, they are also interchangeable and versatile. Machinists have the ability to keep the cutter (or tool holder) in place while easily switching the inserts with varying alternative grades more suited to machining different work-piece materials.
Performance & versatility – rotary
Although round tools generally involve a large initial investment compared to inserts, this investment can be spread over the long-term with the added possibility of regrinding. This is not an option with inserts.
The geometry, substrate and coating of a round tool determine performance. Application specific tools have the potential to produce greater accuracy than general purpose tools. Also, the overall design allows the machine operator to achieve much smaller diameters.
Both round tools and indexable tools have their own distinct advantages based on material, operation and user requirements. Each one continues to see on-going developments and improvements, including new geometries, coatings, substrates and manufacturing processes, such as edge preparations, surface finishing and other treatments.
As cutting tools, they present a collective offer to end-users by providing an overlap in diameters and applications, so that tooling engineers can assess and advise where and when it is appropriate to apply each.
Dormer Pramet has a range of support services, including printed publications, an E-shop and product selector tool, as well as face-to-face advice, to help make choosing the right cutting tool simple.