INTRODUCTION:

Numerical control (NC) refers to the automation of machine tools that are operated by 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 in machine tools in the form of digital 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 main factors which delayed the quick acceptance of these systems but with the present advancement in technology, these factors are no longer then important. Computer-assisted programming using high level languages, manual data input system, computer numerical control systems and self diagnostics have made the systems so useful and easy to learn and maintain that there has been phenomenal rise in manufacture of NC machines. The revolution in electronic industry has not only brought down the cost for the control systems appreciably but also vary largely miniaturised the controls. The use of microprocessors has 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 latest systems are in interactive type in which the operator can carry on a dialogue with the 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 of prototypes, etc. while the manufacturing is in process. In fact, this makes the automation really flexible.

CNC milling machine

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 , it’s worth understanding the designation 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 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 mathematical right-hand coordinate system. Some possible dispositions of these coordinates are as shown in the figure.

Right hand coordinate system

-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-axis motion is either along the spindle axis or parallel tp the spindle axis. Incase of machine groups III and IV it is recognised as the one perpendicular to the work holding surface which may or may not be passing through the controlled point.

b) Direction: The principle for the machines of groups I and II where drilling type motion can be performed is that for moving a drill into the work piece the cutting tool should move in (-) z direction. For other machines the positive (+) z motion increases the clearance between the work surface and the pen holder. The designation of Z-axis is demonstrated in the figure below.

c) Where there are several spindles and slide ways: In such cases, one of the spindles, preferably perpendicular to the work holding surface may be chosen as the principle spindle. The primary Z motion is the near to the primary spindle. the tool motion of thee other spindle quills or other slides, which are termed as secondary and tertiary motion, may be designated as U, V,W and P,Q,R respectively.

Vertical knee milling machine (Group I (a-i)) Gantry profiler (Group I (a-ii))

Horizontal boring machine (Group I (b)) 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 the cross slide. X is positive when the tool recedes from the axis of rotation of the workpiece. The figure below shows two possibilities of slides and turret placements, thereby affecting the positive direction of X and U axis. For group III and IV, the X axis is parallel to and positive in the principle direction of movement or cutting of the guided point.

Shaper (Group III) Drafting machine (Group IV)

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 of Z and X axis as per principle mentioned earlier. The column under coordinate system shows the relevant right hand coordinate system. From the third column the Y axis designation is derived and is mentioned in the column under 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 axis 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: Positive A,B and C are in directions which advance right hand screws in the positive X,Y,Z directions respectively. In figure the fingers of right hand point towards the positive direction of the rotary motion. All the above mentioned motions viz X,Y,Z; U,V,W; P,Q,R; A,B,C and D,E are with reference to a point, 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 workpiece may be moved in negative X and negative Y axis direction. Such movements of machine elements say –X and –Y are denoted as + X’ and +Y’ respectively. The prime letters can thus be used for all the above mentioned motions to the indicate the corresponding reversed directions for moving work surfaces. The various illustrations of the machines indicate the motions using prime as well as unprimed letters.

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 distinguishing 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 for 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 control of motion along any axis is built up of these small units of motion. This unit may be as small as 0.0001 mm.NC machine is as accurate as this small prefixed interval. An accuracy of +.025mm is quite common. Further motion of each slide has a fixed range. Every NC machine thus has a constraint on its 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 axis in 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 manually through the workpiece at the desired feed. Similarly the hole p2 can be drilled. For drilling p3 the automatic control of Y slide is also required. With such a X-Y position control, any hole on any X-Y plane within the work envelop can be drilled. The NC machine with this capability is known as point to point or positioning NC system which is known as 2P for two axis system.

Job requiring point to point job control

3. Line cutting NC/Straight cutting NC (L-type):

In the above mentioned example, if the hole p1 is to be drilled automatically, then the movement of slide along Z axis has to be controlled so that the drill point moves along the line Z1-Z2.Not only that the speed of movement is also desired to be controlled. A positional control from Z1 to Z2 would not do because in point to point system, the path and speed of tool point from 1 point to another cannot be controlled .It should be remembered that it is unusual 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:

Drilling jobs like the one mentioned above can be very effectively done with 2P,L type systems the which the position control along two axis say X and Y and line control in axis of cutting say Z axis is possible.

5. Contouring/Continuous path NC systems(C type):

The job shown in the figure requires profile milling around P1-P2-P3-P4.With a 2L system type of control it can be done though for profile milling around P3-P4, it should be set parallel to any 1 of the axis say X and Y.A 2C type of control can easily do this job in a single set up. It will use linear interpolation to automatically guide the tool along CD, DE, GH, JK, LM and OP. Once the various points on the part (i.e. C, D, E etc) have been defined. The countering controller generates a path for the tool point between the defined points by interpolating intermediate coordinates. A shortest path straight line is thus generated.

Typical component requiring profiling work

Typical component requiring profiling work Typical component 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 contour milling to be carried out, usually for any pair out of the three axis, with alternative controlled cutting feed rate in depth in the 3rd 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?

FEATURES OF NC MACHINE TOOLS:

The development of NC machine tools dates from 1945.The general objective for NC technology continues to be the reduction of cost of production time. This in turn reduces the time used in setups, handling, removing, changing, lead time.

Machining Centres:

A machining centre at the movement appears to be the most capable and the versatile NC cutting tool which can perform milling, drilling, boring, reaming and taping operations. The NC machines used in the industries are patterned after their conventional counterparts though the inner construction is often quiet different

General design features

NC machines centres, in general, are designed for long hours of continuous production. Thus accurate performance and productivity are expected. These centres therefore tend to be massive construction to provide enough stiffness so that minimum deviations results due to large cutting dynamics forces and environmental changes as well as due to thermal effects due to chips formation.

To minimize wear, sliding components have given away to the rolling components. Several types of linear motion bearings provide friction free movement of the slides. Screw thread drives of the conventional machines are replaced by recirculating ball screws. These overcome backlash problems (by preloading) as well as provide efficiencies of the order of 90%. As regards configuration centres are horizontal bores, milling machine or drilling machine. Now a day’s machines claimed the full potential of machining centres is that which automatically changes the cutting tool.

Recirculating ball leadscrew nut

Automatic tool changers:

To perform the variety of operations, mentioned above, machining centre require several drills, reamers, cutters ,counter-borers, taps and boring bars ,etc. For automatic tool changing, these cutting tool need to be stored at the machining centres so that the desired tool can be selected from these as per requirements. These tools are usually stored on drum, chain and “egg box” type of magazines. The chains and the drum can be moved to the selected positions. The 16/24 tools magazines are quiet common but versions upto 150 tools capacity. The chain may follow “M” type of path instead of rectangular path.

The position of each tool is identified by the rectangular coordinates of its location. In this the movement of spindle and slides bring the tools to the desired positions.

Vertical machining centre with a chain type tool magazine

Tool transfer in ATC:

To select the correct tool automatically from the magazine and replace it with the one already existing in the machine spindle a tool transfer arm is generally employed. This arm shuttles to and fro between the spindle and the magazine. The replaced tool carried out by arm can be returned back to the same fixed place. Else the tool can be returned by locating newly selected tool. The second method has the advantage that no time is lost in indexing the magazine from the position of the required tool to the vacant station needed replaced the used tool. The tool changing time as could be as small as 4 sec.

Additional features of machining centres:

-Automatic 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 this 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 next 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.

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 is now being machined while the component already machined on table 1 can be dismounted making 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.

Turning Centres:

In addition to usual shaft or bar and a chuck work a turning centre is able to perform operations like 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 indexing of spindle is possible. The tailstock usually has a hydraulically powered ram and a revolving quill supported in preloaded anti friction bearing.

Bed construction: A Slant bed is quite common though versions with horizontal and vertical beds are available.

Drive: With current variable speed DC drives, the speeds are infinitely selectable and also the desired constant surface speed can be provided for turning operations.

Bar feeds: For turning applications from the bar, high speed bar feeds are available. The bar material can be contained in oil within the guide tubes so as to eliminate metallic friction by hydrodynamic lubrication.

Parting –off slide: The turning centres can be equipped with separate recessing slide for parting-off and recessing operation. This could result in saving of production time because recessing or parting-off operations are very time intensive machining.

MACHINE CONTROL UNIT FOR N.C MACHINE:

The controller unit is most vital parts part of the NC and CNC machines. The controller 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 machining 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 with each block resulting in certain sequence of 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 proper 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.

Block diagram representation of a typical buffered MCU

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.

2) Vary the spindle speed.

3) Change the direction of rotation of the spindle.

4) Change the desired tool and work piece.

5) Lock and unlock work pieces/fixtures.

6) Guide the cutting tool tip along the desired path.

7) Control the rate of movement of the tip.

8) Start and stop the coolant supply.

ACTUATION SYSTEM:

Corresponding to each of the automatic control function of an NC machine tool, an 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 of actuating mechanisms for the slides i.e. for positional control are listed below:

1) Stepper motor/ servomotor,

2) Ball screw and nut with 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 incremental steps. For each impulse of 10-20 V of digital input signal, its rotor moves precisely by one step. Usually a step angle of 1.8 degree is common. Thus one step of stepper motor rotate the screw by linear movement of 0.002 mm with lead of 4 mm if screw is coupled

The open loop application is generally restricted to smaller machines because of limited power output availability with the stepping motor. Further the number of pulse per second restricts the speed of drive. The maximum speed for stepping motors is 8000 pulses per second. So for high precision applications like Jig Boring where large accuracy is of 0.001 mm is maintained, an open loop servo system would not support the purpose.

Open loop machine slide control

Closed loop servo systems:

In this case, the control makes use of a position transducer referred to as a feedback device. The sensing element measures the actual position of the slide. The difference between the actual position and the required 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 resolution and speeds up to 20 m/min.

Closed loop machine slide control

TRANSDUCERS:

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 referred to as a feedback device. Any difference between the actual and required position is sensed by the feedback device and action is taken, within servo, to minimize error. Other information such as velocity, direction, acceleration is also desirable. Although classical device like LVDT, tachometer, accelerometer and potentiometer are essential for the measurement of these quantities are available, special transducers have been devised for use with NC machine tools.

Positioning feedback devices for NC machine can be classifies into two groups wiz rotatory transducer and linear transducers.

Rotatory transducers:

These are quite popular with NC machine manufacturers because of their compact size and flexibility with which these can be used. There are two primary types of transducers are resolvers and encoders.

Resolvers: It is an analogue device whose output is converted to digital form. Resolvers like 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 behave as rotating transformers and an inductive coupling exists between stator 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 widely used in NC machine tools as position and motion sensors. These can be classified as either incremental encoders or absolute encoders.

Incremental Encoders: These are pulse generating encoders. The optical incremental encoder consists of a glass disk with accurately etched lines at regular interval. Typically the diameter of the disks may vary from 60 mm to 150 mm with lines 10 to 2500.The disk rotates between a light source and one or more of photo diodes. The lines make and break this photoelectric beam and the generated pulse signal is amplified to give a square wave output.

Glass disk of an optical incremental encoder

Absolute Encoders: These are numerical encoders. The construction and operation of an optical absolute encoder appears to be similar to that of the incremental type but 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 glass 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 of conducting insulating surfaces. The conducting surfaces are connected to a collector ring. A series of brushes sre so arranged as to come in contact with these tracks so that rotation produces a train of signals relative to the particular code.

Linear transducers:

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, these do not need highly precise intermediate members like screw and nut and gearing. Further errors due to this interposing feed train are absent. The main types are:

Glass scales with line grating: The transparent index grating (sensing device) and the glass 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 regions is sensed by photodiodes. The resistance of these diodes changes with the intensity of falling light results in electrical pulses.

Ferranti system: The least count in the above mentioned grating system can be improved by utilizing the Moiré effect. When the index grating is set at a small angle, with reference to the scale grating, the interference effect will give rise to Moiré fringes. If the scale mounted on the machine tool slide is now moved the fringe pattern travels at right angle to the direction of slide displacement. This movement of fringes is detected by photocells placed across the width of the grating.

Scales with binary coding: It consists of many parallel strips with alternate bright and dark areas. The total width of strips is scanned by a fancing instrument which has a special sensor for each line of scale. The darker areas are connected by an electrical contact which remains open at the bright intervals. The lower most strip has the smallest increment of the scale say 0.01 mm.

TOOLS WITH CNC VARIANTS:

Spindle tooling for machining centers:

Spindle tooling provides an objective connection between the cutting tool and the spindle of the machine tool. The fact that the spindle is to be employed to perform a variety of cutting operations is only one of the factors influencing the need for adaptive spindle 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, since clearance between meeting surfaces are virtually eliminated when the 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 holder should be preferred. Minimum tool projection on drilling will reduce tool wander and improve hole positioning accuracy. A wide range of standard tool holders are available.NC tooling are offered by manufacturer to meet various machining requirement is 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. It 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.

5) Boring Bars: These are available in rough or finish boring versions. Finish boring version is fitted with micrometer adjustable inserts.

6) Screwed shank and mill holders: One may use the collect for grip and concentricity. Screw thread on the cutter shank increases the grip if the cutter tries to slip.

7) Taping head: The tape holders for NC taping must incorporate a system that allows for any minor difference between the advance rate of the tap (controlled by thread) and the advance rate of spindle.

8) Adjustable boring heads: These are suitable for bores upto 25.4 mm diameter.

Tooling for NC turning machines:

To take full advantage of the metal cutting capability of the machine, the tooling should be rigid and well supportive. Two basic types of tooling in use on NC turning machines are:

1) Qualified tooling: It has toleranced dimensions between the cutting edges and the tool location faces. The tool fits into locations on machine which are accurately positioned relative to known datum positions on the slides.

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 a qualified tool. The preset tool usually needs to be removed from the machine for adjustments required during batch production.

Typical spindle tooling for various machining requirements

Flexible tooling systems:

Tooling makers, over the last few years have come up with tooling systems that may cut the 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 tool dependent part, the adaptor. 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 tool heads can be used for internal and external machining which could reduce tooling inventory levels by extending the versatility of certain cutting tools.

Boring tool Fixture for a turning centrel

Fixture for machining centre shown with end mill adaptor