The sophistication of multitasking machining systems has made it expensive and the selection of a system for the right application an important decision becomes all the more complex.
Across all industrial markets and sectors the production challenges facing manufacturers revolve around cutting lead times, increasing throughput and getting products to market quicker. Other issues facing mass production shops include shorter runs, higher product mix, tighter tolerances, more complex geometries in tougher materials, and complete machining in a single handling.
The introduction of new technologies and innovations has made a new generation of machine tools known as Multitasking Machining (MTM) systems more responsive to these challenges. MTM systems are Computer Numerical Control (CNC) systems capable of performing a variety of operations with multiple tools and/or spindles in single setup.
The tangible benefits of MTMs have drawn the attention of a large sector of the manufacturing industry including automobile, mould and die manufacturers, and aerospace companies as a means to gain substantial productivity, quality improvement and profit.
Framework Of The Selection
There is a framework of factors involved in the decision-making process for a selection of a MTM system. Determination of production goals and available budget represent the starting point for MTM-selection process.
Next, a decision is made to consider a basic two-five-axis machine or a more complex one with a higher number of axes, spindles and/or tool systems. At this stage a detailed analysis of needs and constraints determine the number of axes, spindle and type of tool systems that match the production goals. In final stage, general machine characteristics are determined to compliment the MTM-selection process. These factors are discussed in the following sections along with application recommendations.
Before adopting a multitasking system a decision maker must have a clear understanding of the production goals for using such complex technology and major capital investment:
(1) Production flexibility – A key feature in MTM systems is flexibility, which takes various forms from machines that can do more because of multi-axis, multi-tool, and multi-spindle capabilities to those that can adapt to changes in product and market demand.
(2) Cycle time reduction – Most MTM systems are capable of processing multiple parts simultaneously using multiple spindles and/or axes of motion. Because of these attributes, once material is loaded on the machine, a completely finished workpiece is output, allowing substantial reductions of in-process time.
(3) Lean manufacturing – Multitasking can contribute to a leaner operation by reducing non-value added times such as setup time as well as process simplification and streamlining.
(4) Quality control – MTM systems have the potential to improve part accuracy by not having to handle parts between operations, which eliminates the risk of stacked tolerances, reduces scrap rates and eliminates re-fixturing.
The determination of general attributes of MTM systems includes the machine’s body, way system and spindle drive system, among others. is not much different from conventional machine tools. A brief recommendation is provided for each feature:
- Machine’s body – For the most part, machine tools utilise castings in their body because they exhibit good overall strength and vibration damping characteristics at a low cost. It is necessary for castings to have uniformly thick walls as variation in wall thickness can cause cooling and distortion problems.
- Way system – The way system of a machine tool is one of the most important factors affecting characteristics such as part surface finish and overall accuracy. The way system has two main functions. The first function is to support the spindle and table, and the second is to guide the movement of the machine components. There are two primary types of way systems: box ways and linear guides. Of the two primary types linear guides provide faster positioning. This speed comes at a price as linear guides offer less vibration damping, less ability to withstand side thrust, and less ability to resist damage from crashes.
- Controls – The control of any machine is the brain, which translates all the written code into precise movements and actions. The control interprets programmed code, sends commands to the machine to move its various components, monitors machine response, processes part programs and offers the ability to edit and fine tune existing programs.
- Spindle drive system – The spindle is considered the heart of a MTM system. For consistent parts quality the spindle must maintain proper stiffness, rolling torque, spindle run-out, low-heat generation, and thermal stability. Machining different materials will affect these areas differently so there are many types of spindles, each for specific applications.
- Ergonomics – Ergonomics may seem like a low priority and is easy to ignore but the operator must have ready access to various areas of the machine to work effectively. For instance, the machine table should be within the reach of the control and located at an accessible height.
- High-speed machining – The decision for adopting a high-speed MTM system is influenced by two major factors. First, the reduction of cycle time when cutting time accounts for a significant portion of overall machining time. Second, the faster dissipation of cutting heat through faster chip removal, which results in less thermal expansion and high-dimensional tolerances.
- Materials of workpiece – One of the basic requirements evaluated when determining performance needs is the type of materials to be machined. This will determine the levels of critical features such as spindle RPM, torque, and high-speed horsepower.
Most MTM systems contain five or more axes of motion, capable of utilising any combination of x, y, z, a, b, and/or c-axes. This group of machines is commonly equipped with two or more tool systems and spindles and can operate in synchronous or asynchronous machining modes.
With the non-stop development of new machining methods and options, users must determine what type and configuration of MTM will best match their needs. While most MTMs can be identified as milling or turning type machines a hybrid MTM may comprised of a wide range of functions such as turning, milling, contouring with the c-axis, off-centre machining with the y-axis, milling of angled surfaces with the b-axis, heat treatment by laser, and grinding.
MTMs can be also subdivided into machines with vertical and horizontal spindle orientations. This structural feature which is inherited from conventional milling machines affects other machine features such as rigidity, chip excavation, machine size and fixturing capability.
The number of axes of motion influences the performance of a MTM system in a number of ways. For instance, a five-axis machine can often equal the output of two three-axis machines and generate additional savings in inventory, floor space, and energy consumption.
- Part geometry – One of the major factors that influence the number of axes is the complexity of part geometry. Typically a multisided part or multisurface part requiring multiple setups is a good candidate for MTM.
- Accuracy – For some applications the selection of a MTM system depends on how much movement or re-fixturing of the part can be eliminated rather than just looking at complex geometry. This is because every time a part is moved, there is a potential for error.
- High-speed advantage – Conventional three-axis high-speed machining has limitations when milling deeper cores or cavities. The length of the cutters that have to be used with a three-axis system can result in chatter and poor surface quality. Five-axis machining enables shorter cutters to be used since the head can be lowered toward the job and the cutter-oriented toward the surface. As a result, higher cutting speeds can be achieved with no loss in accuracy.
- Many-axis advantage – When the number of axes in a MTM system goes beyond a common five-axis machine the ability to streamline the process by combining several operations can be enhanced significantly. Turning, milling, contouring with the c-axis, and off-centre machining with the y-axis, are among many capabilities offered by such advanced hybrid MTM systems. One of the recent developments in design of multitasking systems is the addition of y-axis spindle and its associated rotary b-axis.
MTM systems provide the capability for manufacturing a wide range of products at a cost, which will be insignificant over the life of the system. While these systems are capable of meeting several production goals such as cycle time reduction, minimising non-value added times and concurrent processing of multiple parts, they possess inherent programming and reliability challenges due to their complex configuration and simultaneous machining functions.
Closely related issues are the tooling and fixturing aspects of MTM systems, which were not included in this study. Although, it typically represents a very small percentage of total system cost, tooling can have a disproportionate impact on throughput and total parts cost. In recent years, machine tool builders and cutting tool suppliers have paid more attention to new tool designs that can take better advantage of the MTM unique capabilities.
Article adapted from Manocher Djassemi, Industrial Technology Area of Orfalea College of Business, California Polytechnic State University, San Luis Obispo, California, USA.
APMEN Metal Cutting