Chapter 2 CNC HARDWARE BASICS STRUCTURE OF CNC

Chapter 2 : CNC HARDWARE BASICS Ø STRUCTURE OF CNC Ø SPINDLE DESIGN Ø DRIVES Ø ACTUATION SYSTEM Ø FEEDBACK DEVICES Ø AXES

STRUCTURE OF CNC i. High static stiffness - the large static stiffness would allow for very small deflection of the structural element under the operative load of the machine tools ii. Stiffness to weight ratio - to achieve better dynamic response iii. Cast iron based - to provide high stiffness and low weight - cast iron structure provide the necessary material clamping to reduce the vibrations which is essential for large material removal rates and speed machining iv. Used concrete as bed material - low cost and better clamping capacity

STRUCTURE OF CNC Figure : Concrete bed of CNC turning centre

SPINDLE DESIGN Spindles are rotating drive shafts that serve as axes for cutting tools or hold cutting instruments in machine tools. Very important element. Subjected to higher cutting forces. Proper cooling to remove the heat generated. Oil cooling with heat exchanger. Spindle motor and gearbox can be isolated to avoid heat transmission to machine tool structure. Figure : A typical spindle design

Spindle

DRIVES There are two types of drive in CNC machines 1. Spindle drive 2. Feed drive

SPINDLE DRIVES Provide the main spindle power for cutting Two types of motors used are: i. DC motors : are used to control the speed by varying voltage infinitely. ii. AC motors : The use of AC motors is being preferred with the developments in the microprocessor controlled converter. - The AC drive can also be used for positioning the spindle axis (C axis) such as in Turn Mill Centers

FEED DRIVES To drive the axis as per the program. The feed drives that are used in CNC machine tools are the following: 1. 2. 3. 4. 5. DC servomotors Brushless DC motors AC Servomotors Stepper motors Linear motors

DC servomotors The force that rotates the motor armature is the result of interaction between two magnetic fields (the stator field and armature field). DC servomotors are high performance motors and are useful as prime movers in numerically controlled machine tools where starts and stops must be quickly and accurately. The lightweight and low inertia armatures of DC servomotors respond quickly to the excitation voltage changes Figure : Structure of DC Motor

Brush less DC servomotor In brush less motor, the construction of that iron cored motor is turned inside out, so that the rotor becomes a permanent magnet and the stator becomes a wound iron core The major difference is that the brush less motor maintains position by electrical commutation rather than by mechanical commutation

Stepper motors 1. A stepper motor rotates in fixed angular increments. 2. Stepper motors are usually used in open loop control system 3. The benefits of stepper motors are: i. Low cost ii. Ruggedness iii. Simplicity in construction iv. High reliability v. No maintenance

Linear motors Use in high performance CNC machine tools Machine built with linear motors and all digital drive system can produce part with higher accuracy and tighter tolerance at higher speeds and feeds Reduce the non-machining time with high acceleration and deceleration rate No mechanical parts in contact No wear or periodic maintenance required

ACTUATION SYSTEMS Lead screw As a power screw or translation screw, is a screw designed to translate turning motion into linear motion. Greater power requirement. High speed of movement where friction will be very high. i. Acme with metal nut • Normally used in conventional machine tools • In view of the metal to metal and sliding contact between the nut and the screw, the friction is very high • The clearance provided between nut and the screw to reduce friction, there is the problem of backlash. • If any attempt is made to reduce the backlash, the friction increases.

Figure: Lead screw with Acme nut

ii. Recirculating ball screw The nut is replaced by a series of balls which circulate in the channel in the form of threads This results in a highly efficient rolling motion of balls in the space between the screw shaft and nut The ball at the end of the thread portion in the nut will be repositioned back into the beginning of the thread form by a deflector. Another type of nut used is where the balls at the end of the thread will be picked up by a return tube which recirculates the balls to the beginning of the load zone by providing continuous rolling motion. The ball screws can be preloaded to eliminate the axial displacement which is consequently also reduces the backlash.

Advantages of recirculating ball screws No. 1 Advantages Longer life. 2 Relatively small wear. So will maintain the accuracy through the entire life of the screw 3 The frictional resistance offered is small, hence can be used for carrying heavier loads at faster speeds 4 Small power requirement

Figure : Recirculating ball screw with deflector Figure : Recirculating ball screw for sideways

FEEDBACK DEVICES Feedback system The feedback system is also referred to as the measuring system. n. It uses position and speed transducers to continuously monitor the position at which the cutting tool is located at any particular time. It provides the control with information about the status of the motion control system n n. The control can compare the desired condition to the actual condition and make correction n. The most obvious information to be fed back to the control on a CNC machine tool is the position of the table and the velocity motor.

FEEDBACK DEVICES Rotary Feedback Elements Linear Feedback Elements

AXES To produce any machining profile, it is necessary to follow a proper co-ordinate system All the machine tools make use of the Cartesian coordinate system for the sake of simplicity The guiding co-ordinate system followed for designating the axes is the familiar right hand coordinate system NC system can be classified on the number of directions of motion they are capable to control simultaneously on a machine tool.

NC system can be classified on the number of directions of motion they are capable to control simultaneously on a machine tool. Identification of controlled axes for a: (a) lathe (b) vertical spindle milling machine (c) and horizontal spindle milling machine

RIGHT HAND RULE Vertical Machine Horizontal Machine

AXES Z-axis and motion 1. Location The z axis motion is either along the spindle axis or parallel to the spindle axis 2. Direction the tool moving away from the workholding surface towards the cutting tool is designated as the positive Z direction e. g: Drill 3. When there are several spindles and slide ways In such cases, one of the spindles, preferably the motion is then near to the primary spindle.

AXES X-axis and motion 1. Location It is perpendicular to the Z axis and should be horizontal and parallel to the work-holding surface wherever possible 2. Direction For turning machines, it is radial and parallel to the cross slide. X is positive when the tool recedes from the axis of rotation of the workpiece. For other machine tools, the X axis is parallel to and positive along the principle direction of movement of the cutting or the guided point Y axis and motion 1. It is perpendicular to both X and Z axes and the direction is identified by the right hand Cartesian co-ordinate system

AXES Rotary Motions 1. Location These motions are located about the axis parallel to X, Y and Z respectively 2. Direction positive A, B and C in the directions which advance right hand screws in the positive X, Y and Z directions respectively. Z C B Y A X

AUTOMOTIVE INDUSTRY Engine Block

AUTOMOTIVE INDUSTRY (Different Products)

AEROSPACE INDUSTRY (Aircraft Turbine Machined by 5 -Axis CNC Milling Machine)

ELECTRONIC INDUSTRY

RAPID PROTOTYPING PRODUCTS