In this article we will discuss about:- 1. Meaning of Numerical Control 2. Programmed Automation and Numerical Control 3. Advantages 4. Features 5. Machine Control Unit (MCU) 6. Design.
Meaning of Numerical Control:
The control of a machine tool by means of recorded information on punched tape or cards is known as numerical control, because information supplied to the control system consists of a series of numbers in binary (alpha-numeric form).
Thus numerical control is a method and a system of controlling a machine or process by instructions in the form of numbers. Machine control functions done by operator in conventional machines are translated into numeric instructions that can be understood by the machine control unit.
The information stored in the punched tapes or cards can be read by automatic means and converted into electrical signals, which operate the electrically controlled servo- system. The use of the cards and the electrically controlled servo system permits the slides of a machine tool to be driven simultaneously and at the appropriate speeds and direction so that complex shapes can be cut, often with a single operation, and without the need to reorient the work piece.
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Thus the numerical control system accurately positions the cutter with respect to the work piece, and in addition, it can operate a number of auxiliary facilities like vertical motion of the drill head etc. It also controls the switching functions relating to the management of the machine tool, like switching the coolant flow on and off, indexing a turret head etc.
Numerical control can be applied to milling machines, lathes, grinding, boring machines, flame cutters, drilling machines etc. However these machines should use hydrostatic lubricated slide-ways or rollers to reduce friction, and recirculating ball lead-screw and nut to avoid backlash problems.
The development of numerical control technology has brought about the concept of a “machining centre” on which a wide variety of machining tasks can be accomplished on the same machine tool.
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A machining centre is the most capable and versatile NC machine tool which can perform milling, drilling, boring, reaming and tapping operations. These are designed for long hours of continuous operation and thus are of massive construction to provide enough stiffness so that minimum deviations result from large cutting dynamic forces and environmental changes.
Programmed Automation and Numerical Control:
While the quality and delivery schedules can be ensured for large number of components by mass production techniques, difficulties were initially experienced, where the demand of the products was discontinuous and the batch sizes small. The use of conventional machine tools and manual skilled labour for batch production requirements resulted in poor consistency in quality and long manufacturing schedules.
In order to ensure accurate production (consistency in quality of production) of small batch complex components within short scheduled time at lesser cost, it was essential that the human intervention be eliminated and at the same time the machine be automatic and flexible so as to be able to deal with a variety of products. Thus the concept of programmed automation came into being. In this system, the machine tool works automatically by instructions which can be changed as desired.
It may be noted that profiled cams also act as programs but these are fixed in nature and to change them requires lot of time. The changing of programs on paper tape of cassettes is extremely simple and least time consuming.
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Programs for various jobs are prepared well ahead and thus machines can be kept continuously loaded. Since the production does not depend upon the skill of the operator, but on a single program used again and again, very high consistency in quality in batches can be produced at different times.
The loss of time due to setting of works and tools, in reading of drawings and seeking instructions, in stopping the machine for noting down the dimensions in the conventional system are greatly eliminated in programmed automation system, thus resulting in high productivity even in batch manufacturing.
Initially problems were experienced in development of perfect workable programs but with the present development in technology, (computer assisted programming using high level languages, manual data inputs system, user friendly terminals, self-diagnostics facilities), numerical control systems are becoming more and more popular. These systems are very easy to learn, operate and maintain. With revolution in electronic industry, the cost is coming down, reliability increasing and overall size decreasing.
Advantages of Numerical Control:
The principal advantages of a numerical control are the substantial reduction in the lead time i.e. the time required to set up the machine in preparation for doing the job, (the non-machining time is eliminated to a great extent by incorporating facility of automatic tool changing, pallet changer, clamping and unclamping arrangement), the reduction of the number of special jigs and fixtures required to machine a part, and the elimination or drastic reduction of time used to take trial cuts in order to obtain the required size.
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The machine utilisation is very high due to flexibility of machine. Due to automation, consistency in quality is ensured. The amount of human error is also greatly reduced, thereby decreasing the scrap rate and rework. The machining function of the N/C tool can easily be changed by inserting a new tape. This facility is of great value for production of prototype and small batches of components.
Thus design changes can be easily incorporated. Ditto parts are produced again and again. Inspection costs are reduced. The N/C machine tool can thus produce 1, 10 or 10000 parts with unvarying accuracy. N/C machine can operate 24 hours a day, year round, if necessary.
In conventional machine shop, large number of machines are required to carry out a variety of varied operations on a part, but a single machining centre often used with NC machining can perform a large number of operations.
Thus the space to house large number of machines in conventional shop is greatly reduced. Accuracy of work is dependent on the program and not human skill. Due to built-in flexibility in the system, the sudden change in demand can be easily handled.
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Numerical control is thus equally suited for large batch production of parts, particularly complex parts requiring great skill on the part of operator. Tapes can be stored so that the machine can be quickly set up at any time and the same program reused. The greatest capability of NC machine tool is contouring or continuous path machining, like circles, angles, radius cuts, irregular shapes in two or three dimensions.
The use of microprocessors has increased the reliability and versatility of the control systems considerably. Interactive type systems have been developed in which the operator can carry on a dialogue with the system from the keyboard and CRT screen. The operator can enter and edit programs on-line and deal with a variety of production problems, modifications in product design in case of prototypes, thus making the system flexible in true sense.
The demerits of numerical control system include high initial cost, special skills in programming and maintenance. Proper training of operation and maintenance staff is absolutely essential.
Features of Numerical Control Machine Tools:
The following areas deserve greater attention in numerically controlled machines in comparison to other machine tools:
(i) Backlash in lead screws and gearing:
The backlash needs to be eliminated and friction reduced considerably. One of the ways is to use preloaded recirculating ball lead screws and nuts. Backlash errors can also be avoided by ensuring that the tool always approaches the required position from the same direction.
(ii) Friction between moving parts needs to be reduced, particularly the high starting friction. The starting friction which varies rapidly and non-uniformly imposes a lower limit to the distance a table can be moved between starting and stopping. One of the ways to counter this problem is to replace sliding friction with rolling friction by moving the table on recirculating balls.
Several types of linear motion bearings, providing friction free movement of slides are used. Friction can be practically eliminated by using hydrostatic or aerostatic bearings in which the table and slide ways are separated by a thin fluid film provided by forcing oil or air under high pressure through small orifices in the bearing surfaces.
Recirculating ball screw unit overcomes backlash problems (by preloading). These also provide high efficiencies (90% or more). The problem of backlash is further reduced by using rigid mounting techniques for bearings and screws.
(iii) High static stiffness of the table and drive unit are essential to minimise the amount of deflection. High natural frequency (much above 50 Hz) is desirable for fast response to changing motion demand by having high stiffness and keeping the mass of the moving structure as low as possible.
(iv) Drive system:
Hydraulic motors (which can respond rapidly to changes in demand due to their high ratio of power and inertia characteristics) are used to drive rams on small machine tools. Lead screws are used for medium machines, and rack and pinion for long beds.
Variable speed a.c. induction motors or rectifier controlled d.c. motors are also used but these are slower to respond to variable demands. Stepping motors, which rotate in discrete intervals in response to electrical pulses provide another cheap alternative but are not suitable for high traverse speeds. These are ideally suited to open loop NC systems.
(v) Measuring transducers:
Analogue or digital types have to be used to sense the linear displacement or rotations. Analogue transducers like synchros and inductosyns measure distance with reference to some datum. For larger distances, two transducers, one for coarse positioning accuracy over the entire traverse and other for a much finer accuracy over a short distance are used. Digital transducers may either be absolute (encoders) or incremental (pulse digitisers) type.
(vi) Automatic tool changers:
A machining centre requires several types of tools in order to be able to perform a variety of operations. Various types of tools set in their spindle adapter are stored in drum, chain or egg box type of magazines. The desired tool can be selected from the magazine by a tool transfer arm as per requirements of the part program. The drums and chains can be moved to the selected positions.
(vii) Automatic pallet changer:
By providing more than one table (pallet), it is possible to reduce or eliminate setting times. While machining is being performed on one work piece, the operator can unload the finished work piece and set up un-machined one on the other pallet. Two, three and four position versions of rotary pallets are commonly used.
(viii) Multi axis machining:
In addition to X, Y, Z, machining, rotation of table (4th axis) and tilting of table or head (fifth axis) can be provided to facilitate the presentation of work piece at the desired orientation and thus to machine several surfaces in one set up.
(ix) Multi-Spindle heads:
Multi-Spindle drilling and tapping heads designed to provide a number of pattern holes to be drilled repeatedly can be provided as one of the tools to be handled by the automatic tool changer.
(x) In process gauging:
This is essential to reduce the un-operational time so that gauging continues while the machining is in progress.
Machine Control Unit (MCU):
Every NC machine is fitted with a machine control unit which may either be housed in a separate panel, or be mounted in the machine itself, or be housed in a pendant which can be swung around the machine.
The MCU performs the following functions:
(a) Ancillary functions:
Concerning machine operations like machine tool spindle start/stop, vary spindle speed, change direction of rotation of spindle, coolant supply on/off, change the desired tool, lock/unlock the fixtures and work pieces, etc.
(b) Positional (dimensional data) control:
To guide the cutting tool tip along the desired path/positions by controlling the displacement of slide members and rotation of circular tables.
(c) Feed rate control of the tool tip:
The MCU is a special purpose computer which is operated by a program to execute above mentioned commands. The program is usually in the medium of paper tape. It is also possible to manually input the program to MCU from the control panel. It is possible to override the automatic control by manual intervention in case of emergency. The current status of the program is displayed directly on the control panel.
The MCU receives information from the tape program and sends command signals to the actual systems (which may be electro-mechanic, hydraulic, or electrohydraulic) provided for each of the control functions.
Design of NC Machine Tools:
The functions like displacement of machine slides, angular rotation of circular table, start/stop main spindle, changing spindle speeds, reversing spindle direction of rotation, changing feed rate of slides, rotating tool turret, changing tools, switching on/off cutting fluid, locking/ clamping table in position can be easily automated to relieve the operator.
The important design considerations for NC machines are:
(i) The machine structure should be able to withstand normal weight distributions, and the effect of changing the position of a heavy slide. Stresses set up by rapid acceleration or deceleration of moving masses and by cutting forces should not cause objectionable deflections. Inaccuracies due to change in temperature at various locations and different thermal expansions; alignment etc. should also be considered.
(ii) Slide ways should be designed to respond quickly to command signals, and offer constant frictional resistance. For rapid response rolling friction has to be substituted for sliding friction in slide ways. Roller and ball slide way elements achieve this objective but these provide less clamping and have a lesser load carrying capacity. These are good for point to point control where no machining takes place during traverse of slide.
Hydrostatic slide ways in which the surface of slide member is separated from the slide way by the continuous supply of a very thin film of lub oil at a pressure of upto 300 kg/cm2 are used where machining during slide movement is required. Frictional wear and stick-slip are totally eliminated, high degree of dynamic stiffness and good damping are obtained.
(iii) Usually variable speed drives are used to obtain different spindle speeds and feed rates. Hydrostatic hydraulic drives are used to maximum in NC machine. Hydraulic systems incorporating both variable delivery pumps and variable displacement motors are used.
These have the advantages of wide range of steplessly variable speeds, high torsional stiffness, achievement of repeated and sudden reversals, no backless, high power and torque, long life of moving parts, compact unit, and no damage due to overload.
(iv) Slides can be positioned by:
(a) Hydraulic ram,
(b) Recirculating ball lead screw and nut, or
(c) Rack and pinion.
The choice being influenced by length of stroke and mass to be displaced.
Part Program:
Part program contains the plan for machining a part in the form of instructions written on sheets keeping in view various standard words/codes and symbols. The programs written in high level language are converted into machine tool level program (low level language understood directly by MCU) by a processor.
The NC machine operator usually gets a roll containing a long strip of paper having punched holes, representing part program. The hole patterns on the tape follow the standard codes.
Each line of instruction is capable of specifying dimension and non-dimension data and generally follows the following order:
n g xyzab f s t m eob
where n = sequence number
s = speed function
g = preparatory function
t = tool function
xyzab = dimension data
m = miscellaneous function
f = feed function
eob = end-of-block
The part program may also be written on magnetic cassettes, floppy discs, or could be directly entered by manipulating the keyboard of the MCU.