Regardless of the part, material or process, all manufacturers aim to create a certain number workpieces of a desired quality, in a specified time and at an appropriate cost. By Patrick de Vos, Corporate Technical Education Manager, Seco Tools

Many manufacturers achieve this by following a narrow-perspective model that begins with tool selection and application and solving problems on a reactive basis. Reversing that approach, however, can reduce costs and increase efficiency. Instead of waiting for problems to arise and then making adjustments, manufacturers should focus first on proactive preplanning aimed at eliminating rejected parts and unplanned downtime.

Once a stable, reliable process has been established, applying the concepts of production economics can help manufacturers find a balance between production rate and manufacturing costs. Building on the foundation of secure, economically strong operations, manufacturers can select tools and cutting conditions that will fully optimise the machining process.

Production economics

Before taking steps to optimise metal cutting, it is essential that processes are secure and reliable, minus defective parts or unplanned downtime. Process security requires a stable production environment. Manufacturers must analyse machine tool maintenance, CAM programming, tool holding systems and coolant application. Work-handling automation such as pallet or robotic part load/unload systems could also be part of the evaluation.

Production economics focuses on assuring maximum security in, and predictability of, the manufacturing process, while maximising productivity and minimising production costs. When the metal cutting process and environment are secure and predictable, production economics becomes a two-dimensional pursuit: finding a balance between production output and manufacturing costs that is appropriate for a manufacturer’s situation. For example, in mass production of simple parts, maximising output at minimal cost may be the primary consideration. In high-mix, low-volume manufacturing of valuable complex parts, the emphasis must be on total reliability and accuracy.

Minimising downtime

Maximum utilisation of manufacturing resources requires minimisation of downtime. Some downtime is necessary and planned, including time spent programming and maintaining the machine tool, installing fixturing, loading and unloading workpieces, and changing tooling. Manufacturers account for planned downtime in their production schedules. However, production of unacceptable parts results in unplanned downtime. When a rejected workpiece must be remachined, the time spent originally machining it is wasted.

Traditionally, shops take a reactive approach to reducing downtime. When a problem halts production, the search for a solution begins. Rather than waiting to react to a negative situation, a better approach is proactive planning that recognises key targets and steers the process toward them from the beginning. Most shops spend 20% in preparation efforts followed by 80% implementation and testing. The ideal would be to invest 80% in preparation and the rest in implementation and adjustments if necessary.

In preparing for machining, a shop should analyse its targets and develop reliable processes to achieve them. The primary target is not always increased production. Although some examples of high-volume mass production remain, such as automotive part production, manufacturing in general is moving more towards high-mix, low-volume scenarios.

In mass production, losing 50 or 100 parts developing a machining process that will turn out hundreds of thousands of parts over a long time represents a tiny percentage of overall volume and can easily be absorbed. However, in a high-mix, low-volume situation, the process must be as fully developed as possible before part-making begins. High-mix, low-volume scenarios can involve small batches, single-digit lot sizes or even custom one-part runs. In these cases, rejection of a few parts represents the difference between a profit and a loss.

Micro versus macro

The traditional approach to maximising metal cutting output involves a narrow-perspective micro model based on optimisation of individual tools in individual operations. Macro models, however, consider processes from a broader perspective, concentrating on the total floor-to-floor time required to produce a workpiece.

The relationship between the micro and macro models can be compared to an artist’s perspective when creating a painting. The micro model concentrates on individual details, or individual brush strokes. The macro model steps back and views the overall process, as in viewing a painting in its entirety. Attention to detail is necessary, but not at the price of ignoring overall purpose.

Fixation on detail can distract attention from the final outcome. For instance, it is a disadvantage to reduce cutting time by ten seconds when it is achieved with an extra tool that adds ten minutes in set-up and indexing. Similarly, working to achieve product quality beyond customer requirements will increase costs and production time. Almost seriously, one could ask: “How long would it take, and how much would it cost, to produce the worst workpiece possible that is still functionally acceptable?”

Operating costs

Models for machining costs can also represent micro- and macro-perspectives. Micro models consider processes from a narrow viewpoint, linking cutting conditions directly to costs. Macro models work from a broader perspective, emphasising the overall time to produce a workpiece.

Manufacturers measure production rate in various ways, from workpieces completed over time, to the total time required to finish an operation. Many factors affect production rate, including workpiece geometry requirements and material characteristics, product flow throughout a facility, personnel input, maintenance, peripheral equipment and environmental, recycling, and safety issues.

Some cost elements are fixed. Workpiece complexity and material generally dictate the type and number of machining operations required. Acquiring, maintaining and running machine tools are basically fixed costs. Labour costs are somewhat more flexible, but effectively fixed for the short term at least. These costs must be offset with revenue from the sale of machined components. Raising production rate – the speed at which workpieces are converted into products – can offset fixed costs.

Individual optimisation

After the overall productivity and cost efficiency of a process are balanced and optimised on a macro basis, manufacturers can achieve further improvements by optimising individual operations. Cutting conditions play a key role. Depth of cut, feed rate and cutting speeds can contribute to reductions in machining time, but the impact of each on reliability varies widely. Depth of cut essentially has no effect on tool life. Feed rate affects tool life slightly. However, the impact of cutting speed on tool life, and on process reliability, is significant.

Many shop managers believe that simply increasing cutting speeds will produce more parts and thereby reduce costs. Usually that is true, but trade-offs are involved. In general, the faster an operation runs, the less stable it becomes. High speeds generate more heat that affect both tool and workpiece. Tool wear occurs faster and is less predictable, and tool wear or vibration can cause part dimensions to vary and surface finish to decline.

A tool may break and mar the workpiece. In addition, a process operating at the outer boundaries of reliability is typically unable to run untended or semi-tended, eliminating potential labour savings. Extremely high cutting speeds and aggressive machining parameters can increase machine maintenance costs and even downtime from machine failures.

At the beginning of the 20th century, US mechanical engineer FW Taylor developed a model for determining tool life. For a given combination of depth of cut and feed there is a certain window for cutting speeds where tool deterioration is safe, predictable and controllable. Taylor’s model makes it possible to quantify the relationship between cutting speed, tool wear and tool life, balancing cost efficiency and productivity, and providing a clear picture of optimum cutting speed.

In general, manufacturers should select the largest depths of cut and highest feed rates possible for each operation, subject to the stability of the tool clamping, workpiece fixturing and machine tool, as well as the machine tool’s power. Operational safety, in regard to chip formation and evacuation, vibrations and workpiece deformation, also must be considered. A balanced approach involves reduced cutting speeds matched with proportional increases in feed rate and depth of cut. Utilising the largest depth of cut possible reduces the number of cutting passes required and thereby reduces machining time. Feed rate should be maximised as well, though workpiece quality and surface finish requirements can be affected by excessive feed rates. In most cases, increases in feed rate and depth of cut while maintaining or lowering cutting speeds will produce metal removal rates equal to that achieved by higher cutting speeds alone.

Production costs are the sum of tool costs and machine costs. With increasing cutting speeds, machining times become shorter and machine costs decrease. However, from a certain point overall costs rise because shorter tool life increases the cost of tooling and tool change times enough to surpass the savings in machine cost. When a stable, reliable combination of feed rate and depth of cut has been reached, cutting speeds can be used for final calibration. The target is a higher cutting speed that reduces machine time costs but does not excessively raise cutting tool costs via tool wear.

Non-cutting issues

Environmental and safety issues are increasingly important in production economics. Manufacturers are under pressure to conserve energy. Use and disposal of coolants and oils are increasingly regulated and expensive. A balanced approach to cutting conditions can help manufacturers deal with these and similar concerns. Lower cutting speeds combined with increased feed rate and smaller depths of cut require less energy. Balanced conditions also increase tool life, reducing tool consumption and disposal issues. Lower energy consumption results in reduced heat, offering opportunities for minimal- or zero-coolant machining.

Adopting production economics concepts requires an overall analysis of the machining environment and ways of thinking that are counter to many established practices. Carrying out the recommended strategies can improve cost savings and workpiece quality and enable more environmentally friendly production, while maintaining productivity and profitability in a stable, reliable overall process.