Feed Drives 4xcu

DRIVES:

Drives are employed for systems that require motion control . e.g. transportation system, fans,robots, pumps, machine tools, etc. In a CNC machine tool there are three major groups of elements:  Control and Electronics  Electric drives (Electromechanical drives)  Mechanical elements (table, slide,tool holder etc) Also, Hydraulic and Pneumatic systems are integrated with the CNC machine tool The primary function is to cause motion of the controlled machine tool member to conform as closely as possible to the motion commands issued by the CNC system

InCNC machines, the metal is removed as a result of the movement of the workpiece and the cutting tool as shown in following figure:

CNC machine tool …. To

maintain a constant metal removal rate, the spindle and tool movement to be coordinated such that the spindle has a constant power and the - slide has a constant torque. In order to ensure a high degree of consistency in production, CNC machines are built up with variable speed drives with higher control accuracies. The electric motor for spindle drive and feed drive are shown in the following figures.

Electric Drives on a CNC lathe

Various electric motors used in machine tools are given below: Spindle

drive motors 1. DC shunt Motor (Separately excited) 2. Three phase A/c induction motor Feed

drive motors 1. DC Servomotor 2. AC servo motor 3. Stepper motor Note: Spindle drives require constant power Feed drives require constant torque.

Spindle drives Spindle is an important part of the machine tool. It gets power from its drive unit and deliver to the work in case of lathe and to the tool in case of drilling and milling operations. Spindle is used to 1. Center the workpiece (or) tool 2. Hold the workpiece (or) tool To increase the stiffness and minimize the torsional strain on the spindles, they are designed as stiff as possible. In case of lathe, when a chuck (work holding device) is mounted on the spindle, the accuracy of rotation is important as it affects the roundness of the workpiece. The rotational accuracy of the spindle is mostly dependent on the design of the bearings used and preloading

Spindle is used to…. The

accuracy and the quality, of the workpiece depends directly on the geometric accuracy, running accuracy and stiffness of the spindle assembly. The various types of spindle bearings used in the design of a spindle for CNC machine tools are: 1. Hydrodynamic bearings 2. Hydrostatic bearings 3. Antifriction bearings The work (or) tools carrying spindles are rotating at the order of 30-6000 rpm. The main function of the drive is to cause motion to the spindle (or) slide to execute motion commands issued by the CNC system.

The following are the requirements CNC spindle drives: 1. 2. 3. 4. 5. 6. 7.

Excellent running smoothness High rotational accuracy Wide constant power band. Fast dynamic response . High speed up to 9000-20000 pm High load capacity Compactness

Normally,

the dc spindle drives are used in machine tools. With the availability of microprocessor-based AC frequency inverter, the ac drives are preferred to DC drives Be used for C- axis applications for speed control in the range of 1:10, 00, 000 with positioning.

Feed drives A feed drive comprises of a feed servomotor and an electronic controller. •Feed motor has special characteristics like constant torque and positioning. •In continuous path (contouring) operations, several feed drives have to Work simultaneously since the desired profile path has to be followed continuously. The following are feed drive used in CNC machine tools. 1. DC servomotors 2. Brush DC servomotors 3. AC servomotors 4. Stepper motors 5. Linear motors •

Requirements of CNC feed drive: 1. The CNC feed drive should have constant torque for overcoming frictional and working forces. 2. It should be able to provide variable speed with a speed range of 0.1 rpm to 2000 rpm without waviness. 3. It should be able to position the tools with the smallest increment of 1-2 .tm. 4. Low electrical and mechanical time constants. 5 High torques to weight ratio. 6. High torque for quick responses. 7. It should be totally enclosed non-ventilated design.

DC servomotors DC

servomotors are the high performance motor and are used in CNC machine tools where the start and stop must be made quickly and accurately. The light weight and low - inertia armatures of DC servomotors respond quickly to the excitation voltage changes. Brushless DC servomotors In this type, the construction of iron cored motor is turned inside out: The rotor becomes a permanent magnet and the stator becomes a wound iron core.

Stepper motor A

stepper motor steps (rotates slowly) in- fixed angular increments. Step size (or) step angle (or) angular increment is determined by the construction of the motor and the type of drive system used to control it. Normally the step angle is 1.80. i.e., 200 steps per revolution (step angle= 1.8°) Nowadays, micro step motors are capable of giving angular increment of 0.0144° step resolution. Advantages of stepper motor 1. Low cost 2. Ruggedness 3. Simplicity in construction 4. High reliability 5. No maintenance.

Linear motors Most

of the CNC machine tools use-the linear motors. The linear motor consists of a series of magnets attached to the machine base and a set of electrical coils plotted in a steel laminate core attached to the moving slide. Machines built up with linear motors can produce parts with higher accuracy and close tolerances at higher speeds and feeds. Linear motors reduce the non machining time with high acceleration and deceleration rates.