Successful high-performance milling is premised on the task requirements, reliable spindle technology, flexible machine tool manufacturing and reliable cooperation of all components. Is the spindle as the core component, the machine itself, or its peripherals offering additional capabilities in HSC milling? Automation and electronic monitoring can provide answers.
High-performance milling features two features: machining with high cutting speeds (HSC milling, also known as HSM) and high-efficiency machining (HPM) characterized by high torque and high cutting power. In the meantime, there are also some cutting tasks that require high cutting efficiency and surface finish.
On the one hand, the extremely high rotational speed is required for the aviation industry. It is mainly used for processing aluminum assembly; on the other hand, high rotational speed is also used in the mold processing. As products such as cameras or mobile phones are becoming more and more compact, the processing of molds has become more and more sophisticated. For this reason, it is necessary to use a milling cutter having a diameter of 0.5 mm or less for machining, and also for processing narrower fine dies. in this way.
The higher the cutting speed, the lower the cutting pressure - this is extremely important for electrode milling in EDM. When machining thin-walled graphite electrodes, diamond-coated diamond cutters with a diameter of 0.2 mm can be used for machining. The blade length is 2 to 4 mm (angle ratio 10 to 20). As a result, machining spindles have always occupied a key position in the development of machine tools. Due to the trend of miniaturization of technology, it is only meaningful if the spindle speed approaches 60 000 r/min.
Step-Tec has developed, manufactured and sold electric spindles since 1995. The company joins Swiss GF AgieCharmilles (Mikron machining center manufacturer) with approximately 50% of the shares, and joins another famous machine tool manufacturer with a 50% share. A total of about 60 different types of spindles were manufactured. Due to the different requirements of mold manufacturing, machine manufacturing, and aerospace industries, the characteristics of each motor are different. Speed ​​range up to 54 000r/min, power up to 70kW. The goal of Step-Tec's production is to combine acceptable spindle dimensions, high precision, controllable temperature levels, and long service life. In this regard, manufacturers need to rely on meaningful planning, reliable calculation and design as well as the most advanced production methods and good assembly techniques. Finally, each component must be inspected before leaving the factory (Figure 1).
Figure 1 Spindle is a key part of machine tool development. Increased speed is a challenge for spindle manufacturers such as Step-Tec in Switzerland
Competent employees play a key role, just as the watch industry or tool grinding operations, these employees have the necessary manual skills. About 25% of Step-Tec company processes itself. All parts in contact with the spindle bearing belong to this category, especially the main shaft and the front and rear bearing flanges. High-quality composite bearings are used, the rolling elements (balls and cylindrical rollers) are made of ceramic, and the sealing rings are made of high-quality steel.
Composite bearings are a groundbreaking technological innovation that companies have assigned to it from the very beginning. The current electrospindle can achieve the highest surface accuracy, but if the ball bearing with a life of more than 20 000h is used, this surface quality cannot be achieved. From the angle of friction characteristics (small centrifugal force), thermal expansion and lubrication (usually oil-air lubrication), composite bearings have great advantages.
One successful way to maintain a long service life is to see how to isolate the bearings. It is only 1μm in size and roundness. In the past 12 years, electric spindles have experienced a large power leap, not just for HSC devices with a capacity of 54 000 r/min. The HPM jumped from a maximum speed of 12 000 r/min (160 Nm) to 20 000 r/min (160 Nm) and increased from 20 kW to 66 kW. Both of these influence the manufacture of equipment corresponding to this speed and power. The HPM spindle is suitable for difficult rough machining as well as for finishing.
Bearing is the key to efficient spindle
One of the ways for the spindle manufacturer to succeed is through continuous and transparent information transfer among employees, so that the staff can achieve close cooperation, and at the same time adopt real-time observation pictures to improve the processing technology (KVP).
There is a diagnostic module similar to the drive recorder on the electric spindle. The data shown can help to ensure the stability of the process. Individual information, such as vibration and bearing position, can be transmitted to the Mikron machining center operator via the Smart Machine module controlled by the milling process. The operator can identify whether the spindle is working properly and avoid overload damage in the job.
Figure 2 Mikron's HSM 400U 5-axis HSC machining center is an efficient spindle application. This kind of processing equipment for finer mold manufacturing supports the combination of milling/submerged etching processes
For the machining centers produced by the Swiss GF AgieCharmilles Group, spindles are the first choice, and the range of products processed by the company covers 42 types of equipment. The Mikron HSM 400U machining center is a top model (Fig. 2), which can be equipped with a spindle with a maximum speed of 54 000 r/min. In this way, the device can be used not only for the HSC milling of steel and aluminium parts, but also for the milling or grinding of ceramic materials, as well as the milling of graphite. For applications in the medical industry, the device has another name, Prod Med or Prod Med Dental. Different from the basic configuration, the equipment used in the medical industry uses oil cooling. The working range of the Step-Tec spindle is 20 000 to 30 000 r/min when making implants, plates, or plates (Figure 3). The diameter of the cutter is 1~16mm. The processing of quenched and tempered injection molds also saves time.
Figure 3 Example of application in the medical industry: titanium Ti-6Al-4V implants (top)
And bone plates (middle) and cross plates (lower). The latter is a high alloyed austenitic steel
When purchasing a machine, it must be clear that the automation trend with tray systems is getting stronger. The user of the device is not producing millions of pieces, but small and medium quantities, and sometimes even single pieces. Germany's production base can make full use of the potential of automation, that is, the work system to achieve the operation of no one or very few people. This potential has been recognized and used, especially when rapid delivery orders are received and repetitive production of small batches of parts is performed. The actual situation shows that about 70% of the purchased Mikron machining centers install the pallet changer for the first time. The Smart Machine module optimizes the milling process from the aspect of man-machine dialogue, improves the reliability of unmanned operation, reduces the production cost, and plays a role in achieving a good economical operation. Here are two examples to illustrate: APS module (Advanced Process System) uses the signal of the spindle to react. Through the OSS module (operator support system), the speed, accuracy and surface quality of the machining process can be prioritized to ensure that the design and work planning requirements are met, even for individual workpieces.
Will synchronous HSC 5-axis machining applications be enhanced? This mainly depends on the degree of research and development of CAD/CAM systems in terms of operability. In the machine tool and mold manufacturing industry, it is often the case that a single product is processed. In this case, the time-consuming disadvantages of programming are very significant with respect to the processing time.
Figure 4 is also increasingly applied to automation components in small batches and single-piece processing. Through the use of the workpiece tray,
Can develop the potential for unattended or virtually unattended operations
Combination of milling and erosion
In the future, the cost-saving combination of high-speed milling and submerged electric erosion will also lead to many application opportunities. On the one hand, these application possibilities can improve the company's competitiveness in tool and mold manufacturing, and on the other hand, it can also be The processing of precision machined parts injects new vitality.
Another potential for development will be demonstrated in the mid-term through linearly driven 5-axis machine tools with all axes. If the equipment manufacturer maintains contact with the electric spindle manufacturers, it will benefit the user in HSC and HPM processing operations. This potential also contributes to the realization of the prerequisites for the coordination and cooperation of various components on complex HSC 5-axis machining systems.