Cutting the cost-per-part

With a survey by the US National Association of Manufacturers (NAM) showing 53% of manufacturers expect COVID-19 to impact their operations, increasing the pressure to be competitive. An alternative approach to steel turning operations helps optimise costs-per-part, and overall profitability, writes Rolf Olofsson.

Manufacturing economics determines a company’s profitability. With metal cutting, production economics should focus on ensuring these processes and environments are secure and predictable. There should be two ultimate goals: firstly, maintaining the highest production output; and secondly, the lowest production cost. These goals present challenges in steel turning operations, with bottlenecks, production slow-downs, or restrictions in the number of components produced per run. Now, there are the wider industry challenges posed by COVID-19. Manufacturers, particularly in mass production, are especially conscious of cost-per-part when managing steel turning operations. The basic principle of cost-per-part is calculated by the total fixed costs plus total variable costs, divided by the total units produced. The parameters of steel turning also depend very much on market demand, with a view to either reducing production costs or increasing output. Companies that manufacture automotive components, for instance, could face either high or low demand scenarios going forward. Low demand scenarios require tools that can produce more pieces per edge, while providing process security with fewer component rejections. High demand scenarios need tooling solutions that allow increased metal removal rates, reduce cycle times and increase machine utilisation with minimal production interruptions. Whatever scenario they face, manufacturers should strive to maximise machining output. According to Sandvik Coromant’s findings, this can reduce component costs by 15%. Achieving this while maximising process security may necessitate an alternative approach to tooling. Reduced non-cutting

Sandvik Coromant calculates that the cost of tools can account for 3% to 5% of the overall manufacturing cost. When considering the purchase of a tool that wears down over time, like a carbide insert for steel turning, it is natural to consider only the initial unit cost. Sandvik Coromant recommends customers look at things differently and reassess how to factor cost-of-tools into managing the whole production costing process, which includes overheads like machinery depreciation. If we examine a typical working day in a machining shop, let’s say that during two shifts totalling 14.4 hours, 60% of time is devoted to production, or cutting time, while 40% of time is devoted to other things, or non-cutting time. The goal , of course, should be to reduce the non-cutting time and maximise machining time. The best way to achieve this is to keep the production time shortened, while increasing the utilisation of the machine tool. Indeed, Sandvik Coromant has found that a 20% increase in machine utilisation can provide a 10% higher gross profit margin. Longer tool life

Manufacturers measure production rates in various ways, one being the number of workpieces completed in a certain time. However, several factors can stop manufacturers from reaching the desired number of workpieces per shift. The need for frequent insert changes, production interruptions, and not finding the right insert for each application or material, are all considered the biggest timekillers in modern production. How can manufacturers overcome these challenges while working with tough workpieces made from aluminium, unalloyed steels and other materials? In such cases, the insert grade should be selected primarily for its suitability to the workpiece. This is a challenge because so many variables impact on cutting tool insert performance, so sourcing a single grade to accommodate the wideranging demands of P15 to P25 areas can be a thankless task. P15 to P25 refer to the demands that different working conditions impose on machining parameters — cutting data, surface finish, depth of cut, machined or rough surfaces, continuous or interrupted cuts are all affected. There are many prerequisites for any grade making such claims. Fracture resistance is paramount, as is a cutting edge capable of delivering the hardness needed to resist plastic deformation induced by the extreme temperatures present in the cutting zone. Furthermore, the insert coating must be able to prevent flank wear, crater wear and edge build-up. Crucially, the coating must also adhere to the substrate. If the coating fails to stick, the substrate is exposed, and this can lead to rapid failure. To avoid these outcomes, it can be said that limiting continuous, controllable wear and eliminating discontinuous, often uncontrollable wear are the keys to success. Predictable tool wear is the goal. Not that complete predictability is easy to achieve, especially considering the current market trend for machining with limited, or no, supervision. In all instances, the optimum wear pattern for any insert is controlled flank wear, as it results in predictable life of cutting edges. The ideal grade is one that limits development of unwanted types of wear — and in some operations, prevents it from developing at all. To maximise the number of pieces produced, it is vital to select the right carbide insert — and this is also why Sandvik Coromant is launching a pair of new ISO P-turning carbide grades in its range, designated GC4415 and GC4425, which refer to P15 and P25 respectively. GC4425 delivers improved wear resistance, heat resistance and toughness, while grade GC4415 is designed to complement GC4425 when enhanced performance and more heat resistance is needed. Both grades are ideal for use with low-alloyed and unalloyed steel. They can machine a larger number of pieces within a mass and batch production set-up and contribute towards extended tool life, eliminating sudden breakages and reducing reworking and scrap. The GC4415 and GC4425 grades each contain the second generation Inveio technology, uni-directional crystal orientation in

the alumina coating layer. What makes Inveio unique can be seen by examining the material at a microscopic level: the material’s surface is characterised by a uni-directional crystal orientation. Every crystal in the alumina coating is lined-up in the same direction, which creates a strong barrier towards the cutting zone. The crystal orientation has been improved substantially within the secondgeneration Inveio coating. Inveio provides the insert with high wear resistance and longer tool life. Longer lasting tools are, of course, favourable for reducing costper-part. In addition, besides other selection parameters, engineers need to consider how the geometry of an insert affects chip control and machining performance. Better geometry

Geometry refers to the style of the insert itself, which is designed according to the types of machining: finishing, medium and roughing. Each has its own implications with regard to cutting speed — its own working area, based on acceptable chip breaking in relation to the feed rate and depth of cut. In turning, the three main cutting parameters of speed, feed, and depth of cut have a significant effect on tool life and therefore on cost-per-part, considering that a 20% increase in cutting data can decrease the cost of a component by 10%. A model by an American mechanical engineer, Frederick Winslow Taylor, developed at the beginning of the 20th century, established the relationship between cutting speed, tool wear and tool life. Taylor concluded that utilising the largest depth of cut possible reduces the number of cutting passes required and thereby reduces machining time. But he also advised that optimised steel turning depends on the stability of the clamp into which the tool is mounted, the fixturing of the workpiece and application of coolant for the machine tool, and the power of the machine tool. Taylor’s model shows us that optimised steel turning goes beyond grades and geometries. Instead, manufacturers should consider the whole tooling concept. Everything from the insert grade, clamp design and tool-holder can increase output, reduce costs and deliver higher levels of process security. This holistic and alternative approach was put to the test by a customer in the general engineering segment, using Sandvik Coromant’s GC4425 carbide insert to manufacture a track shaft. GC4425 is designed for improved wear resistance, heat resistance and toughness. In addition, it has the ability to run at higher cutting data. The insert was used on a 4140 pre-heat treated (PHT) steel, a chromium-molybdenum alloy steel with a hardness of 40 HRC (or Hardness Rockwell C). The grade is commonly used in everything from gears and pumps, to various applications in the automotive and construction industries. The 4140 PHT workpiece was subjected to multi-directional, external roughing. For the test, the performance of GC4425 was compared with a competitor’s ISO-insert used for the same process. Crucially, it was possible to increase the cutting speed and multiply the feed rate, from 183 metres/min and 0.33mm/rev with the competitor’s insert, to 244 metres/min and 0.51mm/rev with GC4425. In the end, Sandvik Coromant’s insert allowed a 100% productivity increase with a reduced cycle time of 50%. Overall, the customer achieved a 30% cost reduction. This result shows that, by considering the whole tooling concept, manufacturers can achieve more profitable production and a lower cost-per-part. This holistic approach to insert grades, geometry and overall manufacturing economics will be vital if manufacturers are to stay competitive amid the continuing impact of COVID-19.

Rolf Olofsson is a Product Manager at Sandvik Coromant. www.sandvik.coromant.com