Customer satisfaction depends on reliable machining processes

To a great extent, customer satisfaction is based on minimising the time between the placement of the customer’s order and delivery of the finished product. In the past, manufacturers minimised lead times by machining thousands of identical parts and creating large inventories from which they could ship products immediately. This low-mix, high volume production (LMHV) scenario enabled manufacturers to meet customer needs in a timely way throughout gradual development of the machining process and unanticipated production errors and interruptions.

Today’s market requirements, however, are radically different. Customers increasingly order small batches of products tailored to specific needs. As a result, manufacturers rarely make long production runs. Groups o

These high-mix, low-volume (HMLV) scenarios leave no room for ongoing process development or unanticipated interruptions. Manufacturers are under pressure to develop machining processes that are totally reliable beginning with the first part. Immediate speed, consistency and predictability are paramount.

Nevertheless, many manufacturers continue to focus on what they call “efficiency,” developing manufacturing processes aimed nearly exclusively at maximum output and minimal cost. They unintentionally ignore “the elephant in the room” – the crucial priority of satisfying their customers, especially customer demands for timely delivery.

QRM

Conceived in the early days of the HMLV era, a concept called Quick Response Manufacturing (QRM) underscores the critical role of time in the manufacturing process. QRM strategies, along with zero-waste and process optimisation efforts, provide a roadmap that can put manufacturers on a path to minimise lead time and thereby maximise customer satisfaction.

Rajan Suri, a professor of industrial engineering at the University of Wisconsin-Madison in the 1990s, recognised looming changes in manufacturing markets, particularly the trend towards HMLV production. In 1993, he founded the Center for Quick Response Manufacturing. The center’s purpose is to create partnerships between the university and manufacturing companies to develop and implement ways to reduce lead times. QRM strategies are often applied in addition to lean, Six Sigma and similar process improvement initiatives.

The traditional approach

Production managers in traditional machining environments seek maximum machine utilisation above all. If a machine is standing still, it is not efficient and is costing money, not earning it. The goal is to produce large batches for inventory. Parts in stock buffer fluctuating customer demand.

In HMLV manufacturing, however, a job is put into production not for stock, but to fulfil a customer order for a limited number of specific components. There is no buffering inventory.

Further complicating the situation are factors such as so-called “hot jobs” that arrive unexpectedly in response to emergency circumstances or special requests by important customers. If all of a facility’s machines are running, other jobs will be delayed to deal with the hot jobs. Then the delayed jobs themselves become hot jobs, lead times increase, and chaos begins to creep into the production process.

Another issue is the tendency of manufacturing staff to concentrate on finding ways to meet internal goals, such as achieving 100 percent on-time delivery. Planning often is carried out with those internal goals in mind. For example, shop personnel may know that completing a certain job takes one day, but will allocate two days to account for interruptions by hot jobs or other possible delays.

Planners add a time cushion to avoid incidents of “acoustic management” – being reprimanded by management. However, if similar behaviour is common throughout a shop, two weeks of lead time can grow to perhaps seven weeks. On-time delivery performance as measured internally may be 98 percent, production personnel are happy to meet internal goals, but the customer who needed the product in two weeks is not happy at all.

The traditional manufacturing environment has systemic limitations (see figure 2). In the figure at left is a highway with minimal traffic that symbolises underutilisation of resources and, as applied to manufacturing, high production cost per finished workpiece. The over-utilised highway at right, jammed with stopped vehicles, represents the chaos and extended lead times that result when errors occur or unexpected jobs vie for space on the production highway. The middle image illustrates a balanced and cost-efficient approach to output and utilisation of resources.

Roadmap for HMLV production

In a HMLV production environment, first time part yield and consistent quality in production of non-identical workpieces is key. The objective is to provide customised products where the part in a one piece batch costs the same as a part in a million piece batch and immediate delivery is assured.

Producing good parts from the start depends on establishing a trouble-free and reliable machining process. It currently is fashionable to point to the newest production techniques and digitalisation technologies as solutions to machining problems. However, speed, consistency and flexibility always have been, and still are, based on a foundation of operational excellence as well an educated manufacturing staff with a positive mind set and motivation.

Before discussing digitisation and optimisation, it is necessary to look at the workshop operations overall, determine where waste of time and resources occurs, and develop methods to minimise it. After that, the emphasis shifts to process quality or reliability.

A zero-waste workshop

Reducing lead times requires elimination of waste in the manufacturing process. A zero-waste workshop does not over-produce parts, fully utilises workpiece material, and does away with extra movement for semi-finished parts. Wasteful and time-consuming activities in the machining process itself include production of burrs, bad surface finishes, long chips, vibration, and machining errors that create unacceptable parts. Bad parts must be reworked or rejected and remade, either of which adds waiting time to the production process.

Even producing part quality that exceeds customer requirements represents wasted time and money. Shops must realise that it is necessary to achieve only the lowest possible workpiece quality that meets customer specifications and functional requirements.

If a part tolerance is five microns, achieving three microns is wasteful. Higher quality tooling and more precise operating processes will be required to meet the tighter tolerance, but a customer will not pay for the unrequested higher quality. The job will be a money-losing proposition for the shop.

Respecting constraints

The first phase in establishing a balanced machining process is choosing tools with load capacity that meets or exceeds the mechanical, thermal, chemical, and tribological loads present in the metal cutting operation.

Phase two involves selecting cutting conditions that recognise the constraints put on a machining process by real-world factors. A cutting tool possesses broad capabilities, but specific realities constrain the range of effective application parameters.

For example, tool capabilities change according to the power of the machine tool in use. Machining characteristics of the workpiece material may limit cutting speed or feed rate, or complex or weak workpiece configurations may be prone to vibration. Although a vast number of cutting condition combinations will work in theory, reality-dictated constraints will narrow trouble-free choices to a certain selection of parameters.

Applying cutting conditions outside the constraints of the specific situation will have negative consequences, including higher costs and lower productivity. The majority of the problems experienced during machining result from a lack of respect for the constraints that physical realities place on the cutting process. When cutting conditions do not exceed real-world constraints, the operation is safe from a technical perspective.

However, not every technically safe combination of cutting conditions will produce the same economic result and changing cutting conditions will change the cost of the machining process. Aggressive but technically safe cutting conditions will speed output of finished workpieces. After a certain point, however, output will slow because the aggressive cutting parameters also will result in shorter tool life, and multiple tool changeovers will consume excessive time.

Accordingly, the third phase of achieving a balanced machining process involves determining the optimal combination of cutting conditions for a given situation. It is essential to establish a working domain where combinations provide the desired levels of productivity and economy. After the combinations are put into production, episodes of troubleshooting to solve specific problems are usually required, as well as ongoing process analysis and optimisation.

Versatile tooling

While high-performance, specialised tools can boost output speed, recognising process constraints may prompt the choice of tools developed for versatility. When tools are selected for maximum productivity and cost efficiency in machining a specific part, a change from one workpiece configuration to another may require emptying the machine turret completely and replacing all the tools. In HMLV situations where smaller runs of different parts change frequently, that changeover time can consume all of the productivity gains resulting from use of maximum-productivity tooling.

In cases where tool performance is stretched to the maximum, some operators will reduce cutting parameters in fear of tool failure and disruption. Versatile tooling, on the other hand, is applicable across a wider range of cutting conditions than productivity focused tooling, although at less-aggressive parameters. When versatile tooling is applied to process a variety of different workpieces, actual machining may be somewhat slower or more expensive, but the reductions in setup time, scrap, and lead time make up the difference and then some.