Thursday, April 21, 2011

Open Mind Discusses Five-Axis Simultaneous Milling

For years, five-axis milling with the simultaneous movement of all machine axes was reserved for special applications such as turbines and condensers or aerospace tasks, according to Open Mind. It is only in recent years that five-axis technology has also caught on for standard milling tasks. Even today, five-axis machines are frequently used only for milling with fixed tool inclinations. Technologies used in three-axis milling, high-speed cutting and EDM processes are relatively mature.

In comparison, the five-axis simultaneous techniques used in general CAD/CAM solutions have historically been too limited, too expensive and/or too unreliable owing to the lack of efficient collision checking and avoidance. The quality of five-axis multiple-surface machining, for example, depended to a large extent on the quality of the surfaces. In addition, the tool's angle relative to the selected reference (point, axis, curve and surface) for the entire machining area could not be changed, which in many cases made it difficult to avoid collisions.

Rest machining necessitated breaking the job down into several machining steps, significantly increasing programming time. Ten years ago, Open Mind introduced a method for the easy programming of five-axis simultaneous machining for tool and mould manufacturing. In addition to real-world strategies for tool orientation, fully automated collision avoidance is at the core of this five-axis technology. For complex milling areas, it is typically difficult and sometimes even impossible to find a constant definition for tool orientation. Five-axis simultaneous movement with the fully automated calculation of tool angles solved this problem.

The increasing demand for five-axis machining has led almost all CAM providers to press their own developments forward or purchase the technology so that they can offer five-axis simultaneous machining. However, there are major differences from system to system in terms of their functionality, ease of use and price, among other criteria. Whoever wants to purchase such a system should not just look for the cheapest proposal; it is much more important to learn about the cost effectiveness of such a system, according to Open Mind. The company added that what is important is the difference it makes to the bottom line at the end of the year.

Saving just five minutes' machining time per part over a year could result in total savings that make the system's purchase price seem like a secondary factor. In order to make the creation of a program as easy as possible for the user, Hypermill offers a uniform user interface concept, from turning to 2D and 3D machining, through to five-axis simultaneous machining. This is important for several reasons: there are very few workpieces that require only one type of machining (2D/3D/five-axis machining); the wide range of available strategies grants the user flexibility; the company can program many different parts with one piece of software; and the user only needs to keep in mind the control of the rotary and tilting axes for five-axis simultaneous machining.

This means that, with the help of task-oriented operating screens, the user is provided with feasible and secure options only, rather than having to choose from all theoretically conceivable possibilities. This reduces complexity by offering a minimum number of parameters for choosing from tried-and-true procedures from the individual areas of application. In order to make generating an NC program as easy as possible, Open Mind has developed Hypermill, a CAM system that exploits the performance range of each machine tool and 'considers' its kinematic particularities. Compared with three-axis milling, the movements of the tool reference point and the movements of the machine's linear axes (pivot point path) are different during five-axis simultaneous machining as movements of the rotary and tilting axes result in 'compensation movements' in the linear axes.

These compensation movements depend on factors such as the geometry to be machined, the setup position and the machine's kinematics. During five-axis machining, the focus is on cost-effective milling - not on the execution of 'spectacular movements'. This can be achieved if a machining job requires as little movement as possible. For example, one test showed that a changed setup position reduced machining time by a third. Reducing movements of the machine axes to the necessary degree also reduces machine loads and thereby compensatory shifting owing to heat generation and wear.

This prolongs machine life and increases machining precision. According to the company, just buying CAM software is not enough. Successful five-axis simultaneous machining requires experience and a delicate touch, added Open Mind. This applies both to the selection of the right machine and to the right choice of tool orientation and collision avoidance strategies. The time invested into learning and understanding successful five-axis simultaneous machining later translates into cash. A software partner's expertise can represent value compared with attempts to go it alone as the software partner can help to save money, quickly making up for the additional costs of software acquisition and training.

Hypermill software enables the user to control both rotation axes independent of one another. It is therefore possible to use only one of the two rotation axes to achieve continuous, collision-free machining. This is particularly beneficial because both rotation axes are typically different in terms of technical capabilities and precision. The causes for this include the masses to be moved and the installed drive power. Hypermill, however, is claimed to offer machining with automatically indexed axes. The tool angle is calculated in such a way that the tool orientation within a milling area on the surface is not changed.

If necessary, the milling area can be automatically subdivided further, or local simultaneous movements can be generated. In a metal injection mould, for example, more than 350 flat and in excess of 370 steep rest material areas with different angles were machined in one operation. All the user had to do was define the preferred lateral tilt, significantly reducing programming times. The forging die for a bevel gear was machined in a spiral that circled the part 273 times. By tilting the milling tool to the centre of the cavity, it was possible with comparable five-axis machining to halve the length of the narrow tool tip, thereby increasing the rigidity by a factor of eight. This allows significantly higher process parameters with less risk of tool breakage and clatter.

However, compared with 3D machining, the C axis required 273 revolutions with 22 braking and acceleration processes, which represents a major strain and results in the danger of erroneous synchronisation of the axes on less dynamic machines. Automatic indexing enables another machining strategy that moves only once around the C axis in small steps in a single 360deg rotation - a machining strategy that, compared with conventional five-axis machining, can always maintain the programmed feed rate even when using a slow worm drive. As unnecessary movements can thereby be avoided, this process is also easy on the machine - resulting in actual cost savings.

Five-axis machining strategies for tool and mould manufacturing enable the continuous machining of larger areas with short tool lengths on vertical walls or steep walls, thereby achieving better process parameters and improved surface quality. With top milling and swarf cutting, higher stock removal rates are attainable thanks to the greater step-over distances between paths. Chamfers, shaped grooves and engravings can also be manufactured more efficiently. Other types of workpieces, such as tread patterns, tubes, blades, impellers and blisks, place widely differing demands on CAM systems. Details that are impossible or hard to implement using general approaches often decide whether programming and machining times are effective.

The user is often confronted with materials that require a special approach to machining. As a result, Open Mind offers specially designed milling strategies for different types of parts. In this way, complicated machining jobs can be programmed efficiently and in a user-friendly way with a limited number of parameters, according to the company. An example for this is the tyre package, which cuts machining times for such shapes in half and almost completely avoids rework tasks. Although machining becomes more detailed, the programming times do not increase. For turbines and compressors, machining times are reached that are equal to special systems in every way - only that the machining of the housings can be done with the same CAM system, as well as the turning operations of the shafts and the stock.

The range of machining strategies in the CAM software and the integration of special packages into the user interface provide the foundation for this. Hypermill includes an analysis feature and two effective simulation options to support programming. The analysis function can check distances, angles and radii, as well as test accessibility with the planned tools. The simulation options offer exclusive tool simulation or can be used to perform a check including the entire machine plus material removal. The focus of the simulation is moving more and more away from collision control and towards providing support for selecting the machine, identifying the optimal setup and defining the most stable tool possible. To accommodate the customers who want to perform an additional simulation after the post-processing stage, Hypermill offers an interface to Vericut and NCSimul.

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