Unit-3 Constructional features of CNC machines.pptx

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1. Unit - 3 Constructional features of CNC Machines 1

2. CNC Machines CNC machines are meant for achieving precision in a more flexible manner. So the design and construction of CNC machines are vastly different from those of conventional machineries. They incorporate lot of electronic devices for control and adopt computing technologies, augmenting the hardware resources . 2

3. CNC Machines - requirements The structure should exhibit rapid response against substantial cutting forces. The accuracy should be maintained in the order of a hundredth of a millimetre. The structural members are treated properly to maintain their rigidity and hardness. As accuracy of machining is of prime concern , care is taken to see that wear is kept to a minimum . For this purpose, proper anti-friction elements are provided which reduce the friction in members having relative movement in contact. 3

4. CNC Machines 4

5. Elements (Components) of a CNC system Machine structure and frame Slideways Drive system Spindle supports Feedback system Control system. Tool holding arrangements. 5

6. 1. Machine structure Accuracy depends on the stability of structure .   Causes of instability   a ) Elastic deflection due to static or dynamic loads Static Load: The weight of the slides and the job leads to static loading and causes deformation. The structure should have adequate stiffness and proper frame setup.   Dynamic Load: The rapid acceleration/deceleration of moving parts and the cutting force generate dynamic loads. Interrupted cuts or unbalanced rotating parts also generate dynamic loads. The effect of these can be reduced by providing proper damping and reducing the mass of the structure. . 6

7. Thermal Load 7 Heat Expansion due to heat Cause: Temperature Gradient Schemes to reduce the effect of thermal loads External mounting of drives Proper lubrication for friction prone areas Adequate coolant supply and swarf removal Re-arrangement of parts for reduction in thermal gradient Providing large areas of heat removing surfaces Use of low friction bearings

8. 2. Guideways Guideways are the machine elements having slots through which a carriage is moved in different axes for the purpose of machining. The guideways should not only control the movement of the table but also should absorb all the static and dynamic loads . They should enable rapid response to the command signals and offer steady frictional resistance. Straightness, parallelism, flatness are other parameters which determine the accuracy. Wear of the guideways also affects the accuracy. . 8

9. Factors affecting design of Guideways Rigidity Damping capacity Geometric and kinematic accuracy Friction characteristics Velocity of slide Wear resistance Provision for adjustment of play Protection against swarf and damage . 9

10. Types of Guideways Friction guideways Vee guideway Flat and Dovetail guideway Cylindrical Guideway Antifriction guideways Hydrostatic Guideways Aerostatic Guideways 10

11. Friction guideways – Stick slip friction 11 Velocity of slide Coeff. of friction Siding friction Rolling friction Stick slip friction Stick slip region Coefficient of friction is not constant , but varies as shown in the figure Upto a certain lower range of velocity, the co-efficient of friction is very high. Accordingly, when fine adjustment is needed with lower speeds , the oil lubricated sliding surfaces stick to the guides and when movement takes place at low velocities, jerky action is initiated. This phenomenon is known as stick-slip phenomenon

12. Vee Guideway 12 In the inverted Vee guideways , the slant surfaces of the carriage come onto the exposed surfaces of the guideway, thus maintaining alignment with the spindle axis. The carriage should be designed to avoid any lift-off the carriage during movement. A combination of vee and flat guideways can also be useful in some lathes.

13. Flat and Dovetail guideway 13 the flat type of guideways have greater surface area, they have better load bearing capacity compared to other guideways. Therefore, they are commonly used in CNC machine tools. As the wear rate is more compared to Vee type guideways, tapered strips called jibs are inserted at the interface of the carriage and the guideways. The guideway surfaces are heat treated for hardness and ground to reduce friction. Jib strip Guideway Guideway Carriage Carriage Jib strip

14. Cylindrical guideway 14 guideway Carriage Cylindrical guideway Here, the guideway is cylindrical in shape and is surrounded by the carriage as shown in the figure. Due to the limited load bearing capacity , these are useful for short traverses under light loads. Guideways of hardened steel are machined separately and fastened to the machine bed. They can be replaced, once worn out .

15. 15 One of the main drawbacks of sliding type guideways is the metal-to-metal contact area , which results in increased friction and consequent heat generation. By incorporating rolling element in between the guideway and the carriage, the effective area of contact is vastly reduced. This helps in avoiding the stick-slip phenomenon to a great extent. Rapid slide response can be also achieved with increase in speed.   Vee and Flat roller Linear ball bushing Roller Ball Carriage Cage Shaft Antifriction linear motion guideways

16. Hydrostatic guideways 16 In hydrostatic guideways, the guideway is separated from the slide by a continuous supply of thin fluid film at a pressure of 30000 kN /m 2 . Friction and stick-slip are fully eliminated. The guideway remains clean as the swarf will not enter the interface due to high pressure of the fluid in between.   The slide can take heavy loads like in milling operation. Machining capability is improved as higher stiffness and good damping are achieved. To maintain positive contact on the side surfaces of the slideway, a thermoplastic insert with a very low co-efficient of friction called poly tetra fluoro ethylene (PTFE) is used. Hydrostatic guideways Carriage Hydraulic supply lines Oil seal

17. Aerostatic guideways 17 In aerostatic guideways, slide and the guideway are separated by a film of compressed air. Due to its lower stiffness, the aerostatic guideways are used for positioning applications like Co-ordinate Measuring Machine ( CMM ). Aerostatic guideways Compressed air Carriage Compressed Air lines Oil seal

18. Feed drives The machining in CNC machines, as in conventional machining involves relative motion between the work piece and cutting tool. Feed drives are used to provide the movements to the slide. The movements imparted by the feed drives should facilitate controlled movements with accurate positioning 18

19. Requirements of Feed drives Constant torque : The drive must deliver constant torque during machining. Positioning accuracy: The drive must be able to position in very small increments, of the order of 2-5 minutes of angular rotation. The linear positioning accuracy will of of the order of 2-5 mm. Range of speed: The speed of the drive should be indefinitely variable upto a very high RPM (about 3000 RPM) and the variation allowed in speed is as low as 0.1-0.2 RPM. Torque to weight ratio: The toque to weight ratio must be high which means that the machine will be smaller for a given torque. This makes the machine compact Quick response: The drive must respond very quickly to input signals. TENV design: The design is to be of totally enclosed non ventilated type. Low inertia: The armature and rotor inertia must be low. Integral mounting : The feedback devices are to be integrally mounted with the machine. 19

20. Types of Feed drives DC Servomotor AC Servomotor Variable reluctance stepper motor Permanent Magnet Stepper motor. Feed drives will consist of Servomotor Mechanical transmission system 20

21. Servomotors Both DC and AC servomotors are used as Feed drives. DC servomotors are usually constructed with a permanent magnet field and a wound armature, giving them characteristics similar to DC shunt motors. Servomotors can produce very precise motions with controlled accelerations and decelerations. The figure shows the working principle of a servomotor. Here a tachogenerator is coupled the motor. It will give a feedback voltage based on the rotation of the motor which is used to further control the speed of the motor 21

22. Mechanical Transmission systems Consists of all the elements starting from the motor to the slide. They can be broadly classified as follows 1. Elements to convert rotary motion to linear motion Screw and nut Recirculating ball screws Rack and pinion 2. Torque Transmission systems Gears Timing Belts 22

23. 1. Screw and nut The acme threaded drives used in conventional machines are very inefficient due to the high frictional resistance between the flank of the screw and the nut. Backlash also will be predominant. There are two types of screw and nut used in CNC systems , namely re-circulating ball screws and roller screws. 23

24. 2. Recirculating ball screws Here, the sliding friction is replaced by rolling friction due to the presence of balls in the grooves provided in the assemblage. The screw and the nut are connected by an endless chain of recirculating steel balls. These have got very high efficiency to the order of 90%. 24 Reciprocating balls Ball return take ground screw thread Nut

25. 3. Rack and Pinion Rack and pinions are used to convert linear motion to rotary motion and vice versa. The rack and pinion drives are suitable for slides of large machines. 25   Pinion Rack Rack and Pinion

26. Toque Transmission – 1. Gear When gears are used to transmit torque, the backlash in gear drives must be reduced by either in the manufacturing stage or in the assembly stage. The backlash can also be eliminated by pre-loading the gear combination. In this case, one part of the gears is rigidly fastened to the shaft and the other part is spring loaded against the mating teeth by means of circumferencial springs. Gears are used when power is to be transmitted between non-parallel shafts or speed reduction is required. 26

27. Toque Transmission – 1. Timing belt The teeth of an endless belt engage with a timing pulley, is shown in the figure. The advantages are : low cost, higher efficiency and lubrication free. It is a positive power transmission system. The belt is strengthened by a steel re-enforcement and the belt is set to an initial tension. 27

28. Spindle Bearings As the spindle carrying the tool rotates at high speeds, during the machining operations, it is subjected to torsional vibrations, bending and sometimes to thrust loading. Hence, the spindles should be supported by bearings such that the spindle attains more stiffness and the above stresses are reduced. The rotational accuracy of the spindle depends on the design of the bearing selected and the preloading. The accuracy of the component machined in turn depends on the rotational accuracy of the spindle. 28

29. Hydrodynamic bearing Here, the load on the bearing is supported by a continuous film of lubricant, usually oil which avoids direct contact between the journal and the bearing. Motion of the journal inside the bearing creates necessary pressure for the film to sustain the load. Hydrodynamic bearings are used for low load conditions and where frequent start/stop is not encountered. In the startup stage, when the journal starts rotating, the radial load applied to the bearing through the journal forces the journal off the center in the direction of the load. At the initial slow rotation, friction causes the journal to climb up the bearing amidst viscous shear stress. The converging wedge shaped area above the region of contact draws the oil, resulting in pumping action, which generates an oil film pressure. So, the journal will be lifted off from the bearing. The frictional force at this stage is greatly reduced, leading to steady state phase. The film thickness must be sufficient to eliminate metal-to-metal 29

30. Hydrostatic bearing In Hydrodynamic bearing, a steady rotational motion is required to maintain the oil film pressure in the clearance area . Under situations like reciprocating or oscillating devices or slow speed units and heavy load cases, it is difficult to maintain this pressure. Hydrostatic bearings are ideal under such situations, where an external pump delivers oil under pressure to the bearing The working of a hydrostatic bearing is shown in the figure. A positive displacement pump draws oil from a reservoir to be delivered under pressure to a manifold supplying oil to different bearing pads. The control element like a flow-control valve at each of the bearing pads permits balancing of the system. The film thickness should be large enough to avoid metal-to-contact but should be small enough to minimise the oil flow and power requirement of the pumping system. 30

31. Anti-friction bearings Antifriction bearings have got rolling elements ( balls or rollers ) in between the stationary part and rotating part. Because of the rolling contact, the area of contact is reduced and friction also is brought down. The other features of these are lesser space, reduced wear, higher reliability and lesser maintenance. They are ideal for high speed and high load applications Selection of the particular type of bearing depends in the speed limits, accuracy of the spindle and its fitness. 31 Outer race Inner race Ball Ball bearing Angular Ball bearing Roller bearing

32. Measuring system Measuring System is an integral part of CNC systems having closed loop control, where feedback information regarding the position/velocity of spindle/slide is continuously sent to the comparator. This information is helpful in controlling the slide displacement along each axis as well as the velocity of the spindle. 32 Measuring Device Reference value memory Drives C Control Signal Actual Value Comparator Measuring system layout

33. Functions of measuring system To identify the position of the slide on the slide way accurately To compare the actual position of the slide with the desired position To make corrections in further positioning of the spindle To measure the spindle speed To avoid inaccuracy due to deflection or misalignment 33

34. Types of measuring system Direct Measuring System Indirect Measuring System 34

35. 1. Direct measuring sysem In the direct measuring system, the measuring device is directly mounted on the moving machine element , thereby measuring the distance travelled by the slide directly. The backlash errors of the drive system do not affect the measurement. Linear scale is an example direct measuring systems. 35

36. 2. Indirect measuring sysem In the indirect measuring system , measurement of the movement is done based on the rotation of feed screw/pinion or the drive motor. Even though the system is cheaper than the direct measuring systems, inaccuracies due to backlash and torsion may affect the accuracy. Encoders and resolvers are the examples of Indirect Measuring devices. Encoders are required when the spindle rotation has to be synchronised with the slide movement like in thread cutting. 36

37. Difference – Direct and Indirect measuring systems 37   Direct measuring system Indirect measuring system 1 Linear displacement is measured directly on the slide Linear displacement is measured indirectly through rotation of screw or motor 2 Accuracy is high Accuracy is low 3 Comparatively costlier Comparatively cheaper 4 Backlash errors do not affect measured value Backlash errors affect measured value 5 Mounting is difficult Mounting is easy 6 No error compensation available Error compensation available 7 E.g.: Linear scale E.g.: Rotary encoder

38. Gauging Gauging during machining operation is essential to maintain accuracy. Unless properly detected, inaccuracies may creep in due to thermal deformation, tool wear, etc. Hence, the job and tool are to be continuously monitored . The gauges trigger on contact . The gauges are normally switches, actuated by spring loaded stylus . Any abnormal deviation in the tool or the job results in deflection of the stylus and consequent actuation of the particular switch. These gauges may be mounted on the turret or the table. . 38

39. Tool monitoring system With the passage of machining time, the tool gets worn out . Sometimes, the tool breaks out due to variation in the hardness of the material or any cutting parameter. These factors result in inaccuracy of machining and chatter. The surface finish of the part is also affected. To avoid these problems , the condition of the tool is continuously monitored using a “Tool monitoring system”. . . 39

40. Types of tool monitoring system There are two types of tool monitoring systems Direct Tool monitoring system Indirect Tool monitoring system . . 40

41. Direct tool monitoring system In direct monitoring system , the condition of the tool is continuously monitored using a probe or stylus which is in direct contact with the tool. This is best done when cutting tool is not in contact with the workpiece . So this is an “ offline ” method. Direct method may also can also use various sensing and measuring instruments like microscope, machine camera etc. . 41

42. Indirect tool monitoring system Indirect monitoring is the most widely used , compared to direct monitoring system. Here, the cutting parameters which affect the tool are measured and used to monitor the condition of the tool. Some of the parameters used to monitor tool condition are Tool life Power of the drive Forces due to cutting Dimensions of the workpiece Noise emission during cutting . 42

43. Differences between direct & Indirect tool monitoring systems 43   Direct Tool monitoring system Indirect Tool monitoring system 1 Monitoring is done directly by using a touch probe. Monitoring is done indirectly by measuring changes in certain parameters 2 Here direct values are produced by the measuring system. Parameters like cutting force, noise, vibration etc. are the measured values. 3 Decisions are made manually based on the measurement of condition of tool Decision are made automatically by the control system 4 Usually does not produce alarm on reaching any abnormal condition Usually, alarms are produced on reaching any abnormal condition

44. Automatic Tool Changer Many operations like drilling, turning, milling, boring, reaming, tapping etc. may have to be performed on a work piece . These operations require the use of different tools . Manually changing the tools results in a lot of idle time. So Automatic Tool Changers are used in CNC machining centres. These tools are stored in a tool magazine and the required tool is grabbed and used according to the instructions in the program 44

45. Requirements of Automatic Tool Changer Requirements of ATC There should be location ( Tool magazine ) to store all the required Tool There should be a tool change position to which ATC will reach to pick up a tool There should be provision to replace the tool the tool back to a specific position in the tool magazine 45

46. Advantages of Automatic Tool Changer Increases productive time and reduces unproductive time Increases flexibility of the machine tool Makes easy to change heavy and large tool A large number of tools are readily available in one place (tool magazine) 46

47. Automatic Pallet Changer Setting up a job for machining takes considerable time an the machine remains idle during this time. To avoid this problem, more than one pallets are used to mount the workpiece. While the machining is being done on the first pallet , worker will be setting up the work on the second pallet. When the work is finished on the first pallet, second pallet will be automatically placed below the tool and the machining will go on. Then the worker can unload the work on the first pallet and fix the next work on that pallet. So setting up job on pallet is done without stopping the machine for set up. There are machining centres which come with 4, 6, 8 and even more pallets 47