FOR IMMEDIATE RELEASE

Transcription

1 FOR IMMEDIATE RELEASE Contact: Seco Tools AB Björnbacksvägen Fagersta Sweden Bettina LIEBL Phone: bettina.liebl@secotools.com The challenges of cast iron turning Fagersta, September 2012 Turning in cast irons involves multiple properties and applications, so alternative machining strategies may be necessary to meet the demands for quality, cost and productivity. TK_1.jpg Cast irons are used in many products, from large pumps in relatively short production runs to massproduced items such as brake disks. The variety of components and tool applications combined with a spectrum of material properties for different types of cast iron (see factbox) creates challenges for achieving productivity and quality requirements. With better understanding of the factors involved, an optimal approach can be designed in terms of operations, cutting strategies, tools and cutting data. I think one response to challenges met is that manufacturers and users of cutting tools work more closely together and combine their know-how, says Mikael Lindholm, product manager ISO turning at cutting specialist Seco Tools. Together they can use the whole map to find the best solution, instead of just a part of it, which will create more opportunities for improvement. Turning in grey cast iron is one example. Grey cast iron contains carbides in the form of lamellar graphite particles, which give the cast iron excellent vibration damping properties; this makes it a common choice for engine components. However, its machinability is affected by variations in the 1/5

2 surface and by other near-surface conditions, such as mould residues or free ferrite (iron in its purest form) that disturb the manufacturing process in different ways. The former creates harder and randomly located zones, while the latter results in softer areas on the workpiece. Both cause predictability deviations that influence the grey cast iron s machinability, so carefully planned logistics from casting through storage to machining is crucial for ensuring consistent workpiece batches that are large enough for the application. One must have the properties of the workpiece under the best possible control at any given time, says Lindholm. Any variations will negatively affect the total productivity, either directly or indirectly. When components are machined in a just-in-time supply chain, however, there may be times when a batch of as-cast workpieces is out of specification in terms of near-surface conditions but must be machined anyway even at the higher cost of lower tool life and productivity. Component manufacturers must decide how to handle this situation, often by making a delicate choice between different types and grades of inserts, for example choosing between cemented carbide and PcBN (Polycrystalline Cubic Boron Nitride). One can always rely on carbide inserts to do an excellent job, but PcBN will give maximum productivity, says Lindholm (see fig.1). Another strategic consideration is the number of operations, which has a direct impact on lead time. If the properties of a workpiece are not fully known, one may choose to include an extra finishing cut. In this case a forgiving wear- and heat-resistant coated carbide insert could support a machining strategy that minimises the number of operations. Other cast irons offer other challenges. The carbides in ductile (nodular) cast iron, for example, take the form of spheroidal graphite particles. This affects the cast iron s properties, which include a more pronounced abrasive wear mode than for grey cast iron. Wear is of course an issue to consider for turning in all cast irons, not least when it comes to the choice of machining strategy. It s interesting to take a closer look at tool life from an economic perspective, says Lindholm. While turning at high feed and cutting speed will limit the tool life, it could still be the most profitable strategy per component. There are many cases in which the optimal time in cut for a tool should be less than ten minutes, instead of 30 or more. All manufacturers of cutting tools have their specific recommendations and can help manufacturers to optimise their particular applications. This involves a closer look at the machining window for a particular insert, showing allowed cutting data in relation to wear resistance and toughness. One could go for a large number of types and grades to optimise every application for maximum productivity, says Lindholm. Or one could choose a limited selection of all-round character that is easier to manage, at the price of lower productivity. This is yet another strategic choice to make. Pull-quote: I think one response to challenges met is that manufacturers and users of cutting tools work more closely together and combine their know-how. Mikael Lindholm, product manager ISO turning at cutting tool specialist Seco Tools. 2/5

3 Factbox 1: Cast irons Grey cast iron: Lamellar graphite shape provides excellent vibration damping properties. This makes grey cast iron a common choice in engine components. Estimated Machinability Index = 100 Vermicular cast iron: Vermicular refers to the worm-like appearance of graphite in this material. Suitable for components subjected to both mechanical and thermal stress, such as cylinder heads and brake parts. Estimated Machinability Index = 85 Silicon alloyed ferritic ductile cast iron: With improved machinability and mechanical properties, this cast iron is expected to grow in usage, for example in wheel hubs and axles. Estimated Machinability Index = 80 Nodular (ductile) cast iron: Graphite particles have a nodular shape. It is often used for hightemperature components such as exhaust pipes and turbocharger housings. Estimated Machinability Index = 70 Austempered ductile iron: With a combination of high strength, high fatigue strength, good wear resistance and high values of elongation to fracture, this is a very competitive material in relation to many cast, and even forged, steels. Estimated Machinability Index = 45 Microstructure_TK_grades.jpg Grey_cast_iron_microstructure_TK1001.jpg Nodular_cast_iron_microstructure_TK2001.jpg Compacted_graphite_iron_microstructure.jpg 3/5

4 Austempered_ductile_iron_microstructure.jpg Factbox 2: TK1001 and TK2001 grades for turning in cast irons Type: Duratomic cemented carbide grades. Use: Turning of a wide range of applications in cast iron. Range: Available for a number of insert types and chipbreakers. Main feature: Long and reliable tool life. Result: Increased productivity and predictability. Applications: Cylinders, brake drums, brake discs, pump housings, cam shafts, wheel hubs, etc. TKx001_inserts.jpg Fig. 1: Principal division of cutting materials from an application perspective. 4/5

5 Fig. 2: Case 1 (Brake drum) Fig. 3: Case 2 (Housing) About Seco Tools: Seco Tools is a leading manufacturer of high performance metal cutting tools. Seco s product range includes a complete programme of tools and inserts for turning, milling, drilling, reaming and boring as well as complementary tool holding systems. With more than 25,000 standard products, Seco is a complete solutions provider for the metal cutting industry and equips machine tools from the spindle down to the cutting edge. The company is headquartered in Fagersta, Sweden and represented in more than 50 countries worldwide with 40 subsidiaries, distributors and channel partners. More information can be found at: # # # 5/5