Friday, June 6, 2014

Resolving Opposing Concepts in Production Economics

By Patrick de Vos, Corporate Technical Education Manager, Seco Tools AB
To gain maximum productivity and economic benefits in your metal cutting operations, it’s essential to evaluate the processes in light of overall production economics. The goal is to ensure reliability of the machining process while maintaining high productivity and low production costs. Achieving those goals involves an examination of several opposing concepts.

Macro Versus Micro
The traditional approach to maximizing metal cutting results involves a narrow-perspective micro model based on 1:1 optimization of one tool in one operation. Macro models, however, consider manufacturing processes from a broader perspective. These models focus on the total floor-to-floor time required to produce a given workpiece. 

The macro perspective considers the interrelation of all the steps in a manufacturing process. A simplified example involves two machines employed in series to produce a component. If machine tool A is optimized to boost its output but the results from machine B can’t be improved, parts from the first machine will sit waiting for the second as semi-finished inventory, increasing costs. In this case, simply optimizing cutting costs (rather than output) on the first machine would lower machining cost overall while maintaining output. 

On the other hand, in a situation where machine B sits idle waiting to process parts from machine A, increasing the output of the first machine will increase total output. Much depends on whether the shop organizes production flow in a line, batch, or parallel sequence. 

Cost Versus Productivity
After evaluating the process from a macro viewpoint, shops can optimize operations on an individual basis with the goal of achieving high metal removal rates at the lowest possible cost. The process involves selecting tooling best suited to machine the part’s features and then employing the largest depth of cut and highest feed rate possible. Those parameters, of course, are subject to constraints regarding available machine power and torque, and the stability of workpiece fixturing and tool clamping. 

Shop personnel then can use cutting speed to fine-tune the machining process to achieve minimum cost, maximum productivity or a compromise result. Initially, machining time drops and productivity rises with increasing cutting speeds. After a certain point, however, costs again start to rise. Above a certain cutting speed, tool life becomes so short that the cutting edge requires frequent replacement. At that point, the decrease of the machining time cost has a smaller beneficial effect than the increasing cost of tools and changeover time. Somewhere between the extremes there is a cutting speed where the two costs balance to result in a minimum total cost.

Quality Versus Productivity
Standards for part quality continually become stricter, but the quest for quality sometimes is overdone. Pursuing unnecessary high quality wastes money. Manufacturers can drastically reduce costs and dramatically increase productivity just by fulfilling the given requirements for part precision. 

Productivity Versus Reliability
Similarly, focusing entirely on maximum productivity in terms of lower cycle times can negatively affect the reliability of a metal working process. A process that is run constantly at the limit may exceed that limit at a cost of scrapped workpieces and lost time. Understanding tool wear and failure modes is essential. Wear-related phenomena generally are gradual and predictable, while other failure modes, such as tool breakage, lack the predictability required to maintain a reliable cutting process.

Individual Factors Versus Overall Results
Typically, more than 50 to 70 individual factors can have an appreciable effect on product quality, production time, and cost. Typical factors that should be analyzed include tools/tooling systems, workpiece configuration and materials, equipment process capabilities and data, human factors, peripheral equipment and maintenance issues. Environmental factors such as energy consumption and disposal or recycling of worn tools and machining waste must also be examined, in addition to employee safety considerations. 

Universal Versus Specialized Tooling
An accepted way to increase productivity in an individual operation is to engineer specialized tooling for that specific process. Spending the time and money involved in such an effort may be worthwhile when the expense can be amortized over a long production run. However, balancing productivity, reliability and tool cost considerations in the small-batch situations that have become more frequent today requires versatile “universal” tooling that offers flexibility over a broad window of application. These tools reduce downtime by minimizing the time needed to switch in a new tool when the workpiece changes. They also eliminate the need to set up and test run a new tool. 

In summary, the choice of tooling and machining parameters for a single operation is dependent on how that operation fits into the total manufacturing picture. The choice should be biased toward what is desired in terms of productivity, cost efficiency and reliability, and what best fits a broad view of the production process.

About the Author 
Based in Sweden, Patrick is the corporate technical education manager for Seco Tools AB with global responsibilities for the technical education activities that help train Seco employees and customers worldwide. He led the creation of the Seco Technical Education Program (STEP) and since its launch more than 145,000 people worldwide have participated in the program. He has been with the Seco organization for more than 30 years, and during that time he has trained more than 50,000 people in over 55 countries. He is also the author of the book “Metal Cutting, theories in practice.”