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INTRODUCTION:
Numerical control (NC) refers to the automation of machine tools that are operated b abstractly programmed commands encoded on a storage medium, as opposed to manually controlled via hand wheels or levers, or mechanically automated via cams alone. Flexible automation is implemented implemented in machine tools in the form of digi tal control. The programs are in binary, in numerical form; strictly speaking alphanumeric. These instructions when read by the system, regulate the various slides of the machine tool to enable the tool, to shape the objects to required profiles by positioning and continuous control. Such systems are known as numerical control (NC) systems. As would be obvious, special skill would be required for writing programs and maintenance of the machines. These were the mai factors which delayed the quick acceptance of these systems but with the presen advancement in technology, these factors are no longer then important. Computer-assisted programming using high level languages, manual data input system, computer numerica control systems and self diagnostics have made the systems so useful and easy to learn and maintain that there has been phenomenal rise rise in manufacture of NC machines. Th revolution revolutio n in electronic industry industry has not only brought brought down the cost for t he control system appreciably but also vary largely miniaturised the controls. The use of microprocessors ha brought a very significant increases in the reliability in present systems decides reducing costs tremendously and also increasing the versatility of the control systems. The lates systems are in interactive type in which the operator can carry on a dialogue with th system with the assistance of a keyboard and a VDU screen enabling him to enter and edit programs to deal with a variety of production, modifications, in product design in case o prototypes, etc. while the manufacturing is in process. In fact, this makes the automation really flexible.
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NOMENCLATURE OF NC MACHINE AXIS:
This is the most important element in any NC manufacturing system.Before going into the details of machine tools and machine control units , its worth understanding the designation designatio n of the axis of these machines. -Nomenclature of the NC machine axis:
The trend in this respect is to follow the International Standard ISO/R841.The ISO/R841.The information in the following paragraphs accordingly pertains to understanding the ISO standard. -Coordinate system:
The guiding coordinate system for designating the axis is the conventional mathematica right-hand coordinate system. Some possible dispositions dispositions of these coordinates are as shown in the figure.
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-Designating the motions:
First of all, Z-motion shall be designated. This shall be followed by X and Y motions respectively.
Positive directions of motion
Finding positive directions for rotary motions
Z axis and motion :
a) Location: Z-a Z-axis xis motion is either along the spindle axis or or parallel tp the spindle spindle axis Incase of machine groups III and IV it is recognised as the one perpendicular to the work holding holding surface which may or m ay not be passing through the c ontrol ontrolled led point. b) Direction: The principle for the machines of groups I and II where drilling type motion can be performed is that for moving moving a drill into the work piece the cuttin tool should move in (-) z direction. For other machines the positive (+) z motion increases the clearance between the work surface and the pen holder. Th Read Free Foron 30this Days Sign up to vote title designation designatio n of Z -axis is demonstrated in the figure below. Useful Not useful c) Where there are several spindles and slide ways: In such Cancelcases, anytime. one of the spindles Special offer for students: Only $4.99/month. preferably perpendicular perpendicular to the work holding surf ace may m ay be chosen as the princip spindle. The primary Z motion is the near to the primary spindle. the tool motion o
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Turret lathe (Group II)
X- motion:
The X motion is principle motion in the positioning plane of the cutting tool or the work piece.
a) Location: It is perpendicular to the axis and should be horizontal and parallel to the work holding surface wherever possible. b) Direction: For group I(a-i) machines when looking from principle tool spindle to the column the positive X- axis is to the right as shown in fig. for group I(a-ii), when looking from principle spindle to the left hand gantry support the positive (+) X to the right. From group I(b) when looking from the principle tool spindle towards the workpiece, the positive X is to the right. For group II, it is radial and parallel to th Read Free Foraxis 30this Days Signfrom up to the vote on cross slide. X is positive when the tool recedes oftitle rotation of th workpiece. The figure below shows two possibilities possibilities of slides slide s and turret placements Not useful Useful Cancel anytime. thereby affecting the positive positive direction of X and U axis. For group III and IV, the X axi Special offer for students: Only $4.99/month. is parallel to and positive in the principle direction of movement or cutting of the
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Y -motion:
Its designation is derived from the already recognized Z and X axis. it is perpendicular to both X and Z axis and positive Y is in the direction which completes with the positive X and the positive Z motion and a right hand Cartesian coordinate system. The first two columns under Z and X show the designation designation of Z and X axis as per principle principle mentioned earlier. Th column under coordinate system shows the relevant right hand coordinate system. From the third column the Y axis designation designation is derived and is men tioned in the column column unde r Y. Rotary Motions:
A, B, C define the primary rotary motions
a) Location: These motions are located about the axis parallel to X, Y, Z axi respectively. If in addition to the above mentioned primary rotary motion, there exists secondary rotator motion, whether parallel or not to A,B,C should be designated as D and E b) Direction Direction:: Positive A,B and C are in directions which advance right h and screws i the positive X,Y,Z directions respectively. In figure the fingers of right hand poin towards the positive direction direction of the rotary motion. All the above mentioned motions viz viz X,Y,Z; U,V,W; P,Q,R; P,Q,R; A,B,C and D,E are with reference to a poin movement of which is being controlled. This point is mostly thee tip of the cutting tool. Many times the tool may not be moving in some directions. Ex the quill of spindle of a vertical milling machine is moving in Z direction but not in x and Y direction. In such cases the work surface is generally moved in a direction opposite to the one intended for thee tool, eg the table of milling machine holding the Such workpiece may be moved in negative X and negative Y axis direction. Read Free Foron 30this Days Sign up to vote title movements of machine elements say X and Y are denoted as + X and +Y Useful Not useful respectively. The prime letters can thus be used for allCancel the aanytime. bove mentioned motion Special offer for students: Only $4.99/month. to the indicate the corresponding revers ed directions for moving work surfac surfaces. es. Th various illustrations of the machines indicate the motions using prime as well a
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TYPES OF NC MACHINE TOOLS:
One important distinguishing feature of NC machine tool is the nature and extent to which the movement of the tool point is automated. It is essentially the way the motions along the various axis are controlled. The distingu distinguishing ishing features are as below. 1. Capability with reference to slide control :
It has been seen that the right hand Cartesian system provides a familiar method fo designating the slide movements. It also provides a simple method for the definition of the tool point in 3D space. NC coordinate System :
The NC coordinate system considers only a finite number fo subdivisions or unit. the contro of motion along any axis is built up of these small units of motion. This unit may be as smal as 0.0001 mm.NC machine is as accurate as this small prefixed interval. An accuracy o +.025mm is quite common. Further motion of each slide has a fixed range. Every NC machine thus has a constraint on its working working space. 2. Positioning system of NC (P type):
The figure shows a job in which three holes need to be drilled at point p1,p2 and p3.If the drill point can be automatically guided along X axi s in X-Y X -Y plane, then the holes p1 and p2 can be drilled by first bringing automatically the tool point from p0 to p1,and then moving the drill point point manually through the workpiece at the desired feed. Similarly the hole p2 ca be drilled. For drilling p3 the automatic control of Y slide is also required. With such a X-Y Read Free Foron 30this Days Sign up to vote title position control, any hole on any X-Y plane within the work envelop can be drilled. The NC Useful or Not useful machine with this capability is known as point to point positioning Cancel anytime.NC system which is Special offer for students: Only known as$4.99/month. 2P for two axis system.
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3. Line cutting NC/Straight cutting NC (L-type):
In the above mentioned example, if the hole p1 is to be drilled automatically, then th movement of slide along Z axis has to be controll controlled ed so that the drill po int moves along th line Z1-Z2.Not only that the speed of movement is also desired to be controlled. A positio control from Z1 to Z2 w ould not do because in point point to point system, the path and speed o tool point from 1 point to another cannot be controlled .It should be be remembered rememb ered that it i unusual for for a p -type machine to take the shortest straight line path from 1 point to another The two typical paths from P1 to P3 for such system are shown in the figure. Line cutting control on the other hand permits cutting moves at desired speed along lines parallel to the machine slides (or for a circular cut about the axis of a rotary table) .
Typical paths from P1 to P3 for 2P type of control system
4. Combination of P and L types:
Master your semester systems system s the which thewith positionScribd control along two axis say X and Y and line controlin axis o Read Free Foron 30this Days Sign up to vote title cutting say Z axis is possible. & The New York Times Useful Not useful
Drilling jobs like the one mentioned above can be very effectively done with 2P, L type
Special offer for students: Only $4.99/month. 5. Contouring/Continu Contour ing/Continuous ous path NC systems(C type):
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Typical compone nt requiring profiling work
6. Some combinations of C and L system:
The milling machines are usually fitted with 2C, L type of control. This enables contou milling to be carried out, usually for any pair out of the three axis, with alternative rd controlled controll ed cutting feed rate in depth in the 3 axis. The term 2½ axis has often been used to describe such a capability, but its use is recommended to be avoided as half an axis does not exists. And what happens on a 4 axis machine with 2C, 2L controlled would it be called a 2+2½=3 axis machine?
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RE
OF
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OO
The eve e chin e s es 1945 The en e jec ive echnolog continues to b e the re uction of cost of production time This in turn reduces the time use setups handling, removing, changing, lead time Machining Machin ing Centres Centres::
machining centre at the movement appears to be the most capable and the versatile NC cuttin tool which can perform milling, drilling, boring, reaming and taping operation s The NC machin used in the industries ar e patterned after their conventional counterparts though the construction is often quiet diff erent A
General Gen eral des design ign fea featur tures es
NC machines centres, in general, are designed for long hours of continuous produ ction. Thu accurate performance and productivity are e pected. These centres ther efore tend to be ma construction to provide enough stiffness so that minimum d eviations results due to large cuttin dynamics forces and environmental changes as well as due to th ermal eff ec ects du e to formation.
To minimize wear, sliding components ha ve given away to the rolling components. Several typ es linear motion b earings pro vide friction free movement of the slides. Screw thread drives of th conventional machines are replaced by recirculating ball screws. These overcome bac lash probl (by preloading) as well a s provide efficiencies of the order of 90%. As regards configuration centr are horizontal bores, milling machine or drilling machine. Now a day s machines claimed the potential of machining centres is that which automatically changes the cutting tool.
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Auto utomati maticc too tooll cha changer ngerss:
To perform the variety of operations, mentioned above, machining centre require several dril reamers, cutt ers ,counter-bor ers, taps and boring bars ,etc. For automatic tool changing, th cutting tool n eed to be stored at the machining centres so that the d esired tool can be selected from these as per requirements. These tool s are usually stored on drum, chain and egg box typ magazines. The chains and the drum can be moved to the sel ected positions. The 16 24 magazines are quiet common but versions upto 150 tools capacity. The chain may follow M t of path instead of rectangular path. The position of each tool is identified by the rectangular coordinates of its location. In this movement of spindle and slides bring the tools to the desired positions.
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nt s:
pallet changers: Savings in setting times can be achieved by minimising the time the machining centre stands idle during th individual setting of each unmachined component. With thi objective, the machining could be provided with more than one table. While machining is being performed on work part, the operator could be unloading the completed piece or setting of the nex one called pallets. Twin pallet features is common but any number of this could be provided as linear pallet shuttle system. Alternatively two, three and four version of rotary pallets, offers more flexible automated loading. -Automatic
In the figure, it can be observed that in position 1, while the work piece is being machined on table 1, table 2 is available for setting up an unmachined work piece .Position 2 show that the table 1 with the machined component moves to setting station 1. In position 3 the unmachined component i now being machined while the the component already machined on table 1 can be dismounted makin table 1 available for setting up a new unmachined component.
-Multi-axis machining: In addition to X, Y, Z machining, rotation of the table and tilting of table can also be provided to facilitate the presentation of work piece at the desired orientation.
-Multiple spindle heads: For jobs that require a number of pattern holes to be drilled repeatedly specially designed multispindle drilling and tapping heads can be incorporated as one of the tools to be handled by automatic too changer.
-In-Process gauging: A probe sensor carried in the tool magazine could be employed to check the exact position of components features such as bosses, bores; faces.This information when fed to the control could be used to correct the axis movement so that a machining detail is correctly located and finished. The probes sensor can also be used to compensate for too wear.
Master your semester with Scribd Tu nin C nt s: Read Free Foron 30this Days Sign up to vote title & The NewIn York Times Useful Not useful addition to usual shaft or bar and a chuck work a turning centre is able to perform operations lik Cancel anytime.
Special offer for students: Only $4.99/month. that of centre drilling, counter milling, tapping making T slots and keyways etc. The C axis rotation is also provided at the spindle whenever working with rotating tools, so that necessary feeding and
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MACHINE CONTROL UNIT FOR N.C MACHINE:
The controller unit is most vital parts part of the NC and CNC m achines. The controller unit unit is made of the electronics components. It reads and interprets the program of instructions and converts them in the mechanical actions of the machine tool. Thus the controller unit forms an important link between the program and the machine tool. The control unit operates the machines as per the set of instructions given to it.
The typical control unit comprises of tape reader, a date buffer, signal output channels to the machine tools, feedback channel from the machine tool, and the sequence control to coordinate the overall machin ing operation.
Initially, the set of instructions from the punched tape are read by the tape reader, which is sort of the electromechanical devise. The data from the tape is stored into the data buffer in form of logical blocks of instructions instructions with each bl ock resulting in certain sequence o operations.
The controller sends the instructions to the machine tool via signal output channels that are connected to the servomotors and other controls of the machines. The feedback channels ensure that the instructions have been executed by the machine correctly. The sequence control part of the controller unit ensures that all the operations are executed in the prope sequence.
One important thing to note about the controller unit here is that all the modern NC machines are equipped with the microcomputer that acts as the controller unit. The program is fed into the computer directly and the computer controls the working the machine tool. Such machines are called as Computer Controller Machines (CNC) machines. Read Free Foron 30this Days Sign up to vote title
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Block diagram of a typical CNC machine
The Machine Control Unit (MCU) of a NC machine tool enables to control automatically one or more of the following functions: 1) Machine tool spindle start and stop. Master your semester with Scribd Vary the Times spindle speed. & The New2)York
Special offer for students: Only $4.99/month. 3) Change the direction of rotation of the spindle.
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ACTUATION SYSTEM:
Corresponding Corres ponding to each of the automatic control function of an NC machine tool, a
actuation system I provided. It acts on receiving a suitable command signal from MCU
Though electromechanical, hydraulic and pneumatic types of actuation systems are
available; the electro mechanical type is most common on NC machines. The elements o actuating mechanisms for the slides i.e. for positional control are listed below: 1) Stepper motor/ servomotor, 2) Ball screw and nut with support support bearings, 3) Feedback devices on closed loop systems,
4) Linear bearings / line motion systems wherever the sliding motion of guide ways is desired to be changed to rolling friction.
The group of above mentioned elements to control the position of a machine slide is also
known as servo. Servo control is basically approached in two ways either as an open loop servo system or as closed loop servo system. Open loop servo systems:
A stepping is generally employed as the driving component to provide the machine slide
motion. This motor responds in incremen incremental tal steps. For each impulse of 10 -20 V of digita digita
input signal, its rotor moves precisely by one step. Usually a step angle of 1.8 degree i Master your semester with Scribd Free Foron 30this Days common. Thus one step of stepper motor rotate the Read screw by linear movement of 0.002 Sign up to vote title with lead of 4 mm if screw is coupled & The NewmmYork Times Useful Not useful Cancel anytime.
Special offer for students: Only $4.99/month. The open loop application is generally restricted to smaller machines because of limited
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Closed loop servo systems:
In this case, the control makes use of a position transducer referred referred to as a feedback device. The sensing element measures the actual position position of the slide. The difference between the actual position position and the required position position is detected by the comparator circuit and the
necessary action is taken within the servo to minimize the error. Such a control can typically have capabilities of up to 0.0001 mm reso lution and speeds up to 20 m/min.
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Closed loop machine slide control
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Special offer for students: Only $4.99/month. As mentioned earlier, employment of closed loop servo systems for position control is quite common in NC machines. This control makes use of some position measuring device
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Resolvers: It is an analogue device whose output is converted to digital form. Resolvers lik synchros are simple, small, cylindrical AC motors in construction. The stator of simple synchros contains three windings 120 degree apart while that of resolver has two windings 90 degree apart. As rotor turns, synchros and resolvers resolvers behave as rotating transformers an an inductive inductive coupling exists between st ator and rotor. Typically by magnitude of the induced voltage is proportional to cosine of angle between rotor coil axis and stator coil axis.
Construction of a resolver
Rotatory Encoders : It is a numerical device that outputs digital data directly. These are Master your semester with Scribd widely used in NC machine tools as position and motion sensors. These ca n be classified a Read Free Foron 30this Days Sign up to vote title incremental encoders or absolu absolute te encoders. & The Neweither York Times Useful Not useful Cancel anytime.
Special offer for students:yOnly $4.99/month. Incremental Encoders: These are pulse generating encoders. The optical incrementa
encoder consists of a glass disk with accurately etched lines at regular interval
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Encoders: These are numerical encoders. The construction and operation o an optical absolute encoder appears to be similar to that of the incremental type bu it is fundamentally different as far as the pattern of lines of the disk is concerned These encoders have a separate discreet patterns or codes for each line on the glas disk. Some of these codes are binary, decimal and gray codes. The light from a narrow radial silt passes through the transparent portions of the disk and fall on the photocells. The system of photocells scans all the tracks. The output signals from these cells represent the magnitude of rotation of the shaft as per the code on the disk. Contact Encoder: Instead of employing photoelectric principles of reading the information on the disk it is read through electrical contact in the case of this type of encoder. These contain a disk having a series of concentric coded tracks formed o conducting insulating surfaces. The conducting surfaces are connected to a collecto ring. A series of brushes sre so arranged as to come in contact with these tracks so that rotation produces produces a train t rain of signals relative to the particular code.
Absol ute
t
nsdu
s:
The principles involved in the operation of these transducers are quite similar to those discussed for rotary transducers. As these measure the position of the slide directly, thes do not need highly highly precise intermediate intermediate members like screw and nut and gearing. Furthe errors due to this interposing interposing feed train are absent. The main types are:
Glass scales with line grating: The transparent index grating (sensing device) and the glas scales in this case have similar line graduations. The relative movement between the two scales results in alternate transparent and opaque regions. Typical width of the lines and spaces is 20 microns each. The line passing through these is sensed by photodiodes Read Free For 30this Days Signregions up to vote on title The resistance of these diodes changes with the intensity of falling light results in electrica Not useful Useful Cancel anytime. pulses. Special offer for students: Only $4.99/month.
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TOOLS WITH CNC VARIANTS:
Spindl toolin fo
hinin
nt
s:
Spindle tooling provides an objective connection between the cutting tool and the spindle of the machine tool. The f act that the spindle is to be employed to perform a variety of c uttin operations is only only one of the factors influencing influencing the need for adapti ve spindle tooling. tooling. There is of course, a trend towards the greater use of ISO series of milling machine taper sockets A taper socket provides a rigid method of securing tool holder to the spindle nose, sinc clearance between m eeting surfaces are virtually eliminated when the holder holder is pulled back into the socket. Tool holders which provide minimum possible distance from the spindle bearing to the tool cutting edges and allow maximum projection of a tool from its supporting support ing holder should be preferred. Minimum tool projection on drilling drilling will reduce too wander and improve hole positioning positioning accura accuracy. cy. A wide range of standard tool holders are available.NC available.N C tooling are offered by manufacturer to meet various ma chini chining ng requirement i explained below: 1) Shell mill adaptor: An extensive range is available to suit different cutter diameter.
2 ) Collect chucks and extensions: For holding drills, this tooling should be preferred. I permits the adjustment of overhang of the drill.
3 ) End mill Adaptor: This is used where sufficient frictional grip is required which may sometimes be not available with collect chucks.
4 ) Morse taper Adaptor: Wherever the job necessitates the use of Morse taper shank drill these holders are suitable. Read Free Foron 30this Days Sign up to vote title
Master your semester with Scribd & The New5 )York Times Useful Not useful boring versions. Finish boring version is Boring Bars: These are available in rough or finish boring Special offer for students: fittedOnly with$4.99/month. micrometer adjustable inserts. inserts.
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Qualified tooling
2) Preset tooling: It has adjustable locating faces. It enables the dimensions between the tool cutting edges and locating faces to be preset to a much closer tolerance than that for qualified tool. The preset tool usually needs to be removed from the machine for adjustments required during batch production.
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xibl toolin syst
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s:
Tooling makers, over the last few years have come up with tooling systems that may cut the Tooling overall tooling requirements. In one such system for machining centres, the tool holder is split into two components: the machine dependent back end and the cutting too dependent part, the a daptor. This can result in fewer back ends being needed.
For CNC turning machines a typical flexible tooling system uses light weight tool heads fitting on to tool holders locked in the turret. The same sa me tool heads can be used for interna and external machining which could reduce tooling inventory levels by extending the versatility of certain cutting tools.
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