Chapter 4

Elements of CNC MachinesbyG.M.Gadad

CNC differs greatly from conventional machine toolsThey employ various mechatronic elementsThe quality of cnc machine tools greately depends upon machine elements and subsystems

Aspects of CNC machines to be considered in their designing

Machine Structure

Guideways

Feed drives

Spindle and spindle bearings

Measuring systems

Controls, software and operator interface

Gauging

Tool monitoring

Machine StructureMain structure is load carrying and supporting member of machine toolAll motors, drive mechanisms and other functional assemblies are aligned to each other and fixed rigidly to structure.All components must remain in correct relative positions to maintain geometric accuracy.

Basic design considerations involved are:Static Load - include weights of slides, work pieces - forces due to cutting to keep the deformation within limits, the structure should have - adequate stiffness - proper structural configuration to meet these demands there are two basic configurations

Commonly used configuration of machine tools

Dynamic Load - These are constantly changing forces - These cause the whole machine to vibrate - origin of such vibrations - unbalanced rotating parts - improper meshing of gears - bearing irregularities - interrupted cuts while machining - The effect of these vibrations on m/c performance can be reduced by: - reducing the mass of the structure - increasing the stiffness of the structure - improving the damping properties

Thermal LoadHeat sources which setup the thermal gradient include: a. Electric motor b. Friction in mechanical drives and gears c. Friction in bearing and guideways d. Temperature of surrounding objectsWhat these heat sources cause? i. localized deformation ii. Considerable inaccuracy in m/c performance

Steps to reduce thermal deformation 1. External mounting of drives and gear boxes 2. lubricating bearings and guideways 3. Efficient coolant and swarf removal for dissipation of heat generated 4. Thermo-symmetric designing of the structure

Guideways - control the direction of the carriage/ table on which the tool / workpiece held - to absorb all the static and dynamic forcesThe shape of the workpiece depends on the accuracy of movement and kinematic accuracy of guidewaysThe geometric relationship of slide and the guideway to the machine base determines the geometric accuracy of the machine.

Kinematic accuracy depends the straightness, flatness, and parallelism errors in guideway.These errors further result in variety of tracking errors like pitch, yaw and roll that are difficult to measure and correct.Over a period of usage any kind of wear in the guideway reduces the accuracy of guide motionThis results in movement and positioning errors.When machining feed rate can vary from 20 mm/rev to 50 mm/rev

Points to be considered while designing guideways - rigidity - damping capability - geometric and kinematic accuracy - velocity of slide - friction characteristics - wear resistance - provision for adjustment of play - position in relation to work area - protection against swarf and damageThe positioning of drive mechanism relative to guiding surface of the slide way is also important.Drive mechanism should be placed in such a way that the reaction and hence frictional forces are uniform in the guiding system.This ensures uniform wear on guideways.

Types of guideways i) Friction guideways ii) Antifriction linear motion (LM) guidewayFriction guideways:Widely used in conventional machine toolsHave good damping propertiesLow manufacturing costOperate under conditions of sliding friction and do not have constant coefficient of friction.Coefficient of friction varies with sliding velocity

Coefficient of friction is high when movement commences and as speed increases, it rapidly falls and beyond certain velocity it remains almost constantTherefore to start the movement high force is required.This results in elastic deformation of drive mechanism ( such as screw)The energy stored in screw together with applied force causes carriage to slip and move at a faster rate.As the speed increases the friction decreases and a greater amount of movement than intended for the slide takes place.There is a possibility of repetition of this cycle of events and results in positioning errors.Results in jerky motion.This phenomenon is known as stick-slip phenomenon

To reduce the possibility of stick-slip there should be minimum but constant frictionThis is achieved by using strips of poly tetra fluoro ethylene (PTFE) or turcite lining at the guideway interfaceTurcite is a special type of plastic with particles of graphite embedded on its surfaceThese materials have low coefficient of friction. (of the order of 0.1)

The strips can be replaced when worn and the accuracy can be restored.Coating can be carried out on flat, dovetail and vee guideways.

Coated guideways

Antifriction Guideways:

These guideways use rolling elements in the form of balls or rollers in between the moving and stationary parts of the machineAdvantages of Antifriction guideways - Easy controlling of slick-slip motion of slides - Low kinetic frictional resistance - High durability - High accuracy of movement - High load carrying capacity - High traverse speed - Ease of assembly - Operation without external hydrodynamic lubrication

Disadvantages Low damping capacity Quite expensive.Combined rolling & sliding guidewaysUse of balls and rollers requires a highly finished and hardened guiding surface.As the contact is on a curved line or point, the waviness of the surface impairs the accuracy of the motion. To combine the advantages of both the sliding (as regards to the accuracy of guided motion) and roller guides (as regards to minimization of motion) combined rolling and sliding guideways are designed for various machine tools.

There are several types of antifriction guideways that can be employed in machine tools.They areRecirculating ball bushingsLinear Bearings with ball and rollersRecirculating roller bearingsRecirculating cross roller bearings

Recirculating ball bushings

Figure illustrates the inner construction of recirculating ball bushVarious kinds of ball bush like open type, closed type are shown in figureThey are available with or without seals

Different methods of mounting the shafts and ball bushes

Linear bearings with balls and rollersThese are very effective in providing smooth and easy movementRequire an accurate form to be machined in the casting.The surface in contact with the rollers have to be hardened and should have a smooth structure.

Hardened steel rails with special guide forms may be fastened to the castings of the machine to reduce machining.Special blocks, a pair along each guide rail, with recirculating balls can move along the rails.The balls provide the rolling motion and the contact is line type, the coefficient of friction is reduced.There is no stick-slip phenomenon in this arrangement.

Linear bearing with balls

Various forms of linear guideways

Feed Drives:The function of a feed drive is to impart motion to the slide carrying the tool for removal of material from the job or workpiece. In the case of CNC machines the degree of precision and accuracy of motion of the slide are high and hence the feed drive should have efficiency and response. The feed drive generally adopted in conventional machine tools is rack and pinion. But the feed drives in the case of CNC machines are,Servo motorMechanical transmission system of very precise type.

Servo motor:Direct current (dc) servomotors and alternating current (ac) servomotors are the cornmonly employed feed drive motors in CNC machines.Earlier d.c servomotors and drives were employed in CNC machines, because they provided excellent speed regulation, high torque and high efficiency and therefore ideally suited for precise motion applications. AC servo motors quite cost effective, torque over the entire speed range, less maintenance due to brushless operation, have better response and dynamic stiffness and higher reliability compared to dc servomotors.

Mechanical Transmission system: The mechanical transmission system of a feed drive consists of all necessary elements required for transmission of force and motion from prime mover (motor) to the slide. They are,Elements to convert rotary motion to linear motion.Elements for transmission of torque.In the design of mechanical transmission system, the main criteria is to keep the transmission error to a minimum.

In achieving this, the essential requirements are,Natural frequency should be high.High stiffness.Sufficient damping.Very low friction.Backlash free motion.

Elements to convert rotary motion to linear motion:Many types of actuating mechanisms are used in CNC machines to convert rotary motion into linear motion. The accuracy of the job produced is influenced by the efficiency and responsiveness of the actuating mechanism employed.The two actuating mechanisms used in CNC machines are,Screw and Nut.Rack and pinion.

Screw and Nut:Screw and Nut system is one of the most common and oldest method adopted for conversion of rotary motion into linear motion. The simplest example being vice, screw jack, tail stock etc. This combination is effective for medium traverses, because for longer traverses the screw sags under its own weight. The traverse speed is low for longer screws due to reduction in critical speed.

Conventional thread forms like V, square, trapezoidal are not suitable for CNC machines, because of high friction and sliding action of the contacting surfaces of the threads on screw and nut, resulting in fast wear. The efficiency of these threads is 40 per cent.To achieve smooth and uniform motion of slides on guideways, recirculation ball and roller bearings are used to avoid surface to surface contact. Similarly, recirculation ball screws and roller screws are employed as screw and Nut system in CNC machines. These recirculating ball and roller screw and nut system provides low wear, smooth and uniform motion, reduced friction accuracy over a very long period, higher efficiency, better reliability, higher traverse speed, little temperature range and without stick-slip effect

Recirculating ball screwsRecirculating balls are extensively used in CNC machines because of the many advantages. Gothic arc and Circular arc are the two forms of threads employed on the screws and nut of the recirculating ball screws. Fig. below illustrates the two forms of threads used on screws of the recirculating ball screws.

Forms of threads used on screws

Recirculation of ball through external & internal path

The balls after rotating between nut and the screw comeback to the starting point of the thread in the nut hence called recirculating ball screwThe figure illustrates the two arrangements external path and internal path

Preloading of NutsThere should be not be any back lash and if any should be minimum amount between the screw and the nut so that the slide can travel on either direction without significant error in positioning. One method of achieving bear minimum backlash is by using two nuts, which are forced apart (tensional load) or alternatively squeezed (Compression load). This is called preloading. By doing so the balls in one nut contact one side of the threads in the nut and the balls in the other nut the opposite sides of the threads in the nut as shown in fig below.Efficiency as high as 90% can be achieved by this method.

Preloading of nuts

Roller ScrewsIn case of ball screw the intermediate element making physical contact with the threads of the nut and the screw are balls where as in the case of roller screw, it is the grooved roller elements which make physical contact with the threads on the nut and screw. The balls in the case of ball screw have highly polished spherical shape, where as in the case roller screw, the rollers have circumferential grooves which may have threads matching with the threads on the screw and nut

They are two types of roller screws generally in use. They are,Planetary andRecirculatingAdvantages of Roller screws:Roller screw provide backlash free movement unlike ball screw where preloading is necessary for avoiding or reducing backlashMore accurate position control is possible with a less complex electronic circuit because the pitch of the screw is smaller than the minimum pitch of the roller screw.

Differences between ball and roller screws:

Planetary Roller ScrewThis consists of roller with threads cut on it. At the end of the rollers are provided gear teeth which meshes with the annular provided in the nut, These screws are equally spaced circumferentially and held in their position with the help of spigot, provided at its end which is held in a ring.Because of the meshing of the gear teeth with the annular provided in the nut, the roller are kept rolling and there is no axial movement of the rollers relative to the nut, These types of roller screws are capable of transmitting high loads at high speeds. Fig below shows the cross sectional view of the assembled planetary roller screw and cross sectional view of the components used in it.

Planetary roller screw

Elements of planetary roller screw

Elements of recirculating roller screw

Recirculating Roller screwIn this case, the rollers have circular grooves in the form of thread shape of the screw or nut and do not have threads on it.Components of a recirculating roller screw, such as rollers are equally spaced around the screw and held in that position by a cage. There is axial relative motion between the nut and roller. A small axial gap is provided in the nut. At the end of one rotation of the screw, one of the roller gets into the gap provided in the nut, thereby disengaging itself both from screw and nut (fig next slide)

Recirculating roller screw

A cam provided on a ring inside the nut pushes the screw back into its starting point, there by recirculating the roller. When one of the roller is in the recess (i.e., not being engaged) the remaining rollers will provide the necessary driving power. This type of roller screws are slower in operation than planetary type but are capable of transmitting more loads with greater accuracy.

Spindle and spindle bearingThe speed attained by the spindle is of the order of 600 to 1800 m/minIt is subjected to radial and torsional deflectionsThey are also subjected to thrust forces depending upon the nature of machining operationDesired characteristics of spindles areStiffnessMinimum deflection under varying loadsRunning accuracyAxial load carrying capacityThermal stabilityHigh speed of operationLong service life

These characteristics can be achieved by proper bearing selectionThe different types of bearings used areHydrostatic bearingHydrodynamic bearingAntifriction bearing

Hydrostatic bearingSpindle is separated from the hydrostatic pockets by an oil film of high pressureOil is supplied to the bearing through a throttling system to the controlled pressure and volumeThe spindle is supported by a relatively thick film of oil.Leakage of oil is prevented by using oil seals.The load carrying capacity is independent of the rotational speed of the spindleThe bearing oil temp is maintained low using heat exchangers

Hydrostatic & Hydrodynamic bearings

These type of bearing have good damping propertiesHigh running accuracyHigh wear resistanceBut very expensiveUsed in grinding and boring machines

Hydrodynamic bearingsSpindle is supported on a thick film of oil called hydrostatic pocketsThe oil in the pockets is stationaryThe temperature is maintained using heat exchangersThere is constant flow of oil round the spindle maintaining thick film of oilPressure in the oil is created by the rotation of the spindleOil is forced into the wedge shaped cavities due to the centrifugal actionClearance should be maintained between spindle and casing of bearing for oil film (50 to 200 microns)Suitable for low load carrying capacityHave good accuracy and damping characteristics and are simple

Factors influencing the selection of spindle bearingsType of load (i.e axial, radial or combination)Intensity of loadSpeed of rotationThermal stabilitySpindle stiffnessAccuracy of spindle depends onRadial run out andAxial run outFor ideal conditions both should be zeroFor speed ranges of 10000 to 20000 rpm ceramic balls and rollers are used.They have low coefficient of friction, greater thermal stability, high wear resistance and high hardness

Tension preloadRequired amount of preload is exerted by providing spacer of specified width between nutsEach nut exerts pressure on its respective ball, thus forcing the balls away from each other

Compressive preloadPreload is achieved by insertion of spacer between two nutsPressure is applied to the nuts in opposite directionThickness of the spacer corresponds to the amount of preload

Preloading methods

Oversize ball preloadThis is accomplished by using balls which are just slightly larger than the space provided between the nut and the screw shaftBest suited for comparatively light preload ( to the extent of eliminating axial clearance)In this case balls have 4-point contact, thereby increasing the operational efficiencyIntegral preloadAppearance same as single nut oversize ball or non preloaded ball screwsHowever there is dimensional allowance in the nut internal rotating element which provides proper amount of preloadBest suited for light and moderate preloadsOther preload methodsCoil or belleville spring may be inserted between two nuts to provide a constant pressure preload

Integral and other preload methods