A cursory look at the machine, often do not see the details of most of the drive technology installed. The illustration of a machining center shows in principle the various possibilities for selecting the main drive, the feed drive and the auxiliary drive for the movement to be achieved. Here, Rexroth offers a complete solution for the drive and control system for the machine.
Main drive
For the main drive, the most important are synchronous and asynchronous motors with closed-loop control. They are used in combination machines and enclosed motors in lathes, milling machines, grinding machines, and machining centers.
Conventional spindle drives driven by closed motors are mostly air cooled and their use is very common. Compared to the motor spindle, it is considered an economical solution considering the two system sub-costs. The intermediate shifting of the gearbox allows the speed and torque to be matched to the machining needs on the one hand and the undesired radial forces, noise and increased wear on the other hand.
The spindle drive concept using a combined motor with spindle integration is fully mature at the technical level. Because the gearbox and clutch can be removed, there is no radial force and a completely central swivel motion in this drive. It is marked by long-term smooth motion and minimum wear, making it ideal for high-power cutting operations.
The generation of high torque is still complicated, either by integrating a (planetary) transmission on the main shaft or by selecting a high-powered motor. For preventive maintenance and repair, integrated on-axis monitoring sensors as standard components have been developed for measurement data acquisition, while cooling with oil, air or glycol is essential!
Feed drive
The feed drive can be implemented using a mechatronic or hydraulic system. Choose according to the advantages and disadvantages of different specific drive technologies.
Most of the world's mechatronic feed drives use servo motors with ball screw drive systems to convert rotary motion into linear motion. Unlike the main drive, here, the synchronous closed motor is preferred from the standpoint of high requirements for positioning accuracy, synchronization and dynamic performance.
Due to its high static stiffness, this drive system can be used in many fields and has become a traditional solution. However, it is very wear-resistant. Depending on the installation and the amount of torque to be generated, the servo motor is directly or through a tooth type. The belt is connected to the spindle.
Although the principle of motor linear motors was invented in the nineteenth century, this technology was not applied to machine tools until the 1990s. At the time, Rexroth equipped the first series of machines with linear motors. The wear-free, high-strength and high-dynamic combination of this drive is generally welcomed. As a result, it guarantees long-term trouble-free operation with high precision compared to comparable ball screw drive systems with indirect displacement measuring systems.
The limitation of use is on the one hand the load carrying capacity of the drive: therefore, it is not possible to abandon the use of a ball screw drive system or a hydraulic drive solution for applications with large cutting forces. On the other hand, with regard to other machine parts involved, for example, the maximum allowable moving speed of the cutting shroud and the damping performance of the guide rail also limit the use limit of the drive. The advantages of linear motor drive technology compared with the associated investment costs have also hindered the worldwide breakthrough of this drive technology.
This type of drive is used when considering the advantages of hydraulic feed drive. It is mainly used in small installation spaces, where high dynamic characteristics or large feed driving forces are required. It goes without saying that for hydraulic feed drive systems, precise positioning at the micron level must be possible.
The specific practical application has always required that the hydraulic linear drive must be able to work without gaps for a long time and have a longer service life than the ball screw drive system. For electric feed drives, the corresponding power (torque and speed) must be installed, while the hydraulic drive shaft is energized as needed in the pressurized liquid accumulator, so the installation power can be reduced by 80%.
Auxiliary drive
The auxiliary drive scheme is very diverse, and the obvious trend can be seen from the auxiliary drive function on the machine tool. The proven performance scheme is widely adopted, and the choice can be made according to whether it can best meet the current drive requirements. Not surprisingly, different types of drive technology are used in a mechanical manufacturing unit to implement a closed-loop functional flow, for example, using an electromechanical drive on an upright or tilting machine slide, and then with a hydraulic or Pneumatic weight compensation for use. Further, the weight compensation device can be considered as a passive auxiliary drive whose task is to compensate for the gravity of the moving parts. There are many types of hydraulic solutions with pressure oil accumulators that are widely used. For smaller compensated gravity, a pneumatic gas spring can be used to achieve the compensation function. The advantages of this solution are good dynamic matching characteristics and low energy consumption.
The pneumatic drive system is ideal for installation in handling equipment due to its small weight, simple control structure and fast moving characteristics. It is suitable for the loading and loading of small weights and, therefore, can be integrated into the workpiece transfer of the machining process.
In machine tools, the clamping of tools and workpieces is of great importance because the accuracy and repeatability of the clamping process have a decisive influence on the quality of the machining results.
The hydraulic clamping system is also a type of auxiliary drive and is mainly used on unmanned workpiece loading and unloading machines due to its excellent automation performance.
The large clamping force density of the clamping members allows the construction of the clamp to be minimized.
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