Special Purpose Machine Tools: Types and Control | Industrial Engineering

In this article we will discuss about:- 1. Inroduction to Special Purpose Machine Tools 2. Index Mechanisms of Special Purpose Machine Tools 3. Types 4. Parts Handling Equipment 5. Control.

Inroduction to Special Purpose Machine Tools:

Special purpose machine tools are designed and manufactured for specific jobs and as such never produced in bulk. Such machines are finding increasing use in industries. The techniques for designing such machines would obviously be quite different from those used for mass produced machines. A very keen judgment is essential for success of such machines.

Broadly the special purpose machine tools could be classified as those in which job remains fixed in one position and those in which job moves from one station to other (transfer machines). In first case the machine may perform either only one operation or more.

In the second case, the product may be either moving continuously (as in the case of spraying, polishing, sanding etc.) or intermittently (the most usual case in machining operation). Rotary intermittent motion transfer machine is very popular production machine and is described in brief below.

Such a machine comprises a turret, on whose periphery several heads are mounted to receive and locate the components for working. The turret rotates intermittently about its central axis and is provided with fine and sophisticated mechanisms to control its motion so that before stopping, it is properly decelerated and desired positioning accuracy is attained.

At stationary positions around the turret, usually mounted on a table, are the several tools and units which perform the machining operation. It is essential that all movements be completely synchronised in order to obtain desired product.

All tools and units must have completed their operation and be withdrawn clear of the turret before it starts to index. Similarly the turret must index precisely and accurately and come to rest, before tools and units begin their work.

Index Mechanisms of Special Purpose Machine Tools:

There are a variety of index mechanisms and these need to be selected properly to suit the given requirements. A versatile indexing unit used in presses, drilling machines and other special purpose machines is described below. Number of indexes, speed of index and dwell time, etc. can be readily changed in this mechanism.

It operates by fluid power and uses ratchet and pawl mechanisms. One cylinder moves a gear rack assembly which is in mesh with a pinion gear keyed to a shaft on which the turret is mounted. Thus the linear movement of cylinder rotates the turret by a predetermined amount to the next indexing position.

The rack assembly is arranged in a pair, parallel to each other on either side of the pinion. Only one rack contacts the pinion. By another power cylinder mounted at right angle to the previous power cylinder, it is possible to make one of the two racks engage the pinion. Thus both forward and backward stroke of main cylinder can be utilised to rotate the turret in same direction. Arrangements are also made to govern the feed rate of the main cylinder.

Along with turret a dividing wheel having indexing holes on its periphery is also mounted. Second cylinder which changes position of lock, also operates a moving stop which engages the dividing wheel. During dwell, the turret is locked in position because first cylinder clamps the dividing wheal against the stop.

In a simple indexing mechanism used in lamp industry, a number of follower rollers (ground to close tolerances and to accurate concentricities) are mounted on the underside of the turret on a common pitch circle diameter.

A cross-over cam engages the follower rollers and is mounted on the main camshaft of the machine and driven by a motor through reduction gearing. The cam consists of a ridge perpendicular to the cam shaft and this ridge fits precisely in the space between two adjacent follower rollers, thus locking the turret to the cam.

The two ends of the ridge are curved outward and away from each other so that when the cam rotates the curved portions interact against the follower rollers causing a positive transference of movement to the turret.

The sides of the follower rollers are ground conical in order to give true running conditions without scuffing. The straight portion of the ridge does not interact against the rollers and this represents the dwell portion of the cam.

Types of Special Purpose Machine Tools:

In general, rotary index type of machine tools are used for smaller size of component. The components are usually located into jig against fixed stop. Components are clamped and un-damped hydraulically. Sometimes for safety reasons, machine can be started only by depressing two push buttons spaced well apart. The un-machining movements are designed to be quite fast, and feed during machining operation is judiciously selected with some dwell at the end.

Some manufacturers produce modular units of machine which can be assembled to produce a variety of machine tools as per requirement. Multi-spindle heads are used to perform parallel operations of similar size and depth.

Various standard attachments which can be used in construction of machine tools are coolant distributor, anti-friction rotary bushing centre for drilling unit to guide tools or tool holders, angular feed head, recessing head, milling head, planetary speed reducer, hydraulic slide for longer strokes, rotary index table, etc.

For larger components, in line transfer machine is used. Such a machine consists of a series of machining units with their respective work-holding fixtures linked with work piece skid rails. All units are designed to be electrically interlocked to perform their functions in a predetermined sequence.

All machine motions and fixture clamps and locators are operated hydraulically. Machining operations are carried out at several working stations and several idle stations are provided to give adequate access for tool changing and maintenance.

Parts Handling Equipment for Special Purpose Machine Tools:

Handling and storage of parts have become a functioning part of the manufacturing system. There is no universal parts handling equipment which can be used for all feeding jobs at all rates of feed. A variety of feeders and orienting devices are available for handling parts.

It is important to understand how the performance of these devices is affected by configuration of the parts to be handled. Every part to be fed mechanically demands special consideration. Automatically feeding/orienting individual parts from a bulk supply depends upon some type of part agitation or mass part movement.

In the process, some parts assume the desired orientation and are separated from the others. Automatic feeder is not always the solution, particularly when the parts are not rugged enough to withstand agitation, or have a surface finish that can be marred, or tend to tangle with each other, or do not have some shape or weight characteristic that allows those with a desired orientation to be selected. In some cases it may be desirable to use magazine or a container to hold parts in a desired orientation for feeding into processing equipment.

A brief description of several types of feeders with their characteristics is given below:

(i) Rotary Disc Hopper Feeder:

It consists of disc which can rotate within a stationary hopper. Multiple profiles are machined or cast into face of this disc (to suit the component fed) so that parts can be picked up by these profiles and these parts are then guided by a cam into a discharge chute. Such feeders are especially suitable for headed parts and cylindrical parts requiring no end selection.

(ii) Centre Board Hopper Feeder:

In this feeder, a blade made of hardened steel, with a shaped top/groove is oscillated up and down (by a crank mechanism and a geared reduction unit) through a mass of parts. In the process, some properly oriented parts are picked up by the blade and discharged by gravity into a track. The form of the groove and the shape of the top edge are evolved by trial.

This type of feeder is suitable for parts having simple shape like balls, cylinders, short lengths of tube headed parts, nuts and bolts, rivets etc. where high feeding rate is desirable. They are very robust and have long working life. They can’t be used for fragile components and the degree to which they can orient is rather limited. The capacity of centre board hopper is large.

(iii) Tumbling Barrel Hopper Feeder:

In this hopper the parts are continuously agitated by the vanes in the rotating barrel. As a result some parts drop into a shaped track and are captured for discharge into a track which can move parts by gravity or vibration force. This type of feeder is suitable for handling irregular shapes and a wide variety of part sizes.

(iv) Horizontal Belt Feeder:

In this type of feeder, horizontal belts move side-by-side in opposite directions. Several deflector blocks are mounted to orient the parts and to circulate parts between belts or guide parts to a discharge track, and one blade contains a profile to pass correctly oriented parts. It is suitable for delicate parts which are likely to be chipped or scratched.

(v) Reciprocating Tube Hopper Feeder:

In this feeder, parts are picked up as a tube reciprocates to present the tube opening to a mass of products. The parts picked up are discharged into a track for further feeding. It is suitable for simple forms like balls or other regular shapes where random orientation is satisfactory.

(vi) Oscillating Box-Feeder:

In this feeder, the parts are tumbled as the box hopper oscillates about a pivot at one end. The properly oriented parts pass through a selection gate at the pivot end onto holding tracks. It is possible to incorporate dividers into the box so that several types and sizes of parts can be sorted and fed simultaneously.

(vii) Elevator Hopper Feeder:

In this type of feeder a blade flights on an elevating mechanism to pick up some parts from a mass in a storage hopper. Parts picked up are discharged into a delivery chute where these are further oriented and selected. It can handle large part size and the feed rate is very high. It is used for large hopper capacities.

(viii) Vibratory Feeder:

In this feeder a supporting surface is vibrated at a predetermined rate (to control feed) to make the parts move. The parts are then oriented by a series of devices mounted along the line of travel. In it, different parts can be handled simultaneously. Feeders may be circular bowls, or straight line, horizontal type.

Operating efficiency of vibratory feeders is sensitive to total weight of parts being vibrated, weight of individual parts, contact area and coefficient of friction between part and surface. Parts discharged from a feeder move via a track to an escapement mechanism that regulates parts flow according to the needs of processing equipment. Special measures are taken to avoid jamming of parts.

Escapements may be reciprocating, oscillating, or rotary mechanisms built around jaws, ratchets, gates, wheels shuttles or drum. Final part handing may require some type of placement mechanism—usually a push-and-guide or pick- and-place type.

The following points must be thoroughly considered in designing a parts handling problem:

(i) Part design should permit efficient feeding. Often it is found that part redesign might solve some of the problems associated with parts handling. For example, parts that tend to become interlocked with one another when agitated can be suitably redesigned to eliminate tangling tendency. The soft and thin parts require special consideration to avoid their damage and buckling.

(ii) Since feeder problems may be experienced with burrs on metal parts, or oil on them (which would collect dust and foul feeder tracks), or other foreign material along with parts, these should be properly inspected before feeding.

(iii) The feeder should have adequate feed rate to match with the cycle time of product on equipment. Time necessary to feed, orient and place an individual part must be properly evaluated. Other factors to be considered in hopper design are space requirement and availability of feeder, hopper load for optimum feeder performance, access to hopper, frequency of hopper refill.

(iv) The desired orientation of parts is an important condition which is dictated by machine served by the feeder. The proper feeder capable of providing preferred orientation is selected after studying the preferred orientation of part shape and profile. The preferred orientation is then changed to required orientation by track and escapement design or pick-and-place mechanism.

(v) Auxiliary controls, devices, mechanisms also deserve full attention. Arrangements to detect feeder malfunctions, for fall out of dirt or foreign material on track, removable track covers at points of likely jams etc. need to be provided.

Input/Output Control Equipment:

Input/output equipment for any machine tool provides communication mechanism. With increase in automation, electro-mechanical limit switches constitute most important communication mechanism. These switches usually have single-pole contact block with one normally open (NO) and one normally closed (NC) contacts. Time delayed contact limit switches are used for detecting machine jam-up by causing switch to remain actuated beyond a predetermined time interval.

Maintained contact switches are used where a second definite reset motion is desired. Push-in roller limit switch is used for linear or rotary cam actuation. Forked roller lever limit switch is used for reversing operations. Wobble stick limit switch is used for actuation in variety of directions. In addition, a very wide variety of limit switches are available.

Proximity Switches are also essential with increase in automation. These sense presence or absence of a target without touching it. The various types are magnetic, capacitive, ultrasonic, inductive and photoelectric. Magnetic switch is made of a magnetically-activated reed and a stationary contact. When a magnet is brought towards the switch an induced magnet field causes the movable reed to close upon the stationary contact.

It is the simplest device, low in cost, can be operated on AC or DC supply and can operate in dusty environment. However, it is sensitive to shocks and can’t be mounted on ferrous base. Capacitive type proximity switch consists of a sensing plate forming one side of a resonating capacitor which forms part of a tuned oscillator circuit.

When an object is brought close to the sensing plate, the oscillator produces a signal which is rectified to trigger output circuitry. It can sense metallic and non-metallic targets but is sensitive to temperature change and is affected by humidity changes.

In ultrasonic type of proximity switch, a transmitter directs an ultrasonic signal to a receiver mounted in a remote location. When an object interrupts the beam, the output device is tripped. Wide range of distance sensing is possible and this device can operate satisfactorily in dirty or wet environments.

However, targets must be non- porous. Its working is affected by rapid air movement and its accuracy is relatively low. Radio frequency inductive proximity switch consists of an oscillator that produces a radio frequency signal.

When a conductive object is introduced in the field, eddy current losses are induced on the target’s surface, absorbing energy and decreasing the amplitude of radio frequency signal. It is very fast in operation and can sense all conductive materials and has high repeatability.

Photoelectric proximity switches consist of light-emitting-diode (LED) emitting light in the infrared region, and a pulsed or modulated signal prevents most ambient-light interference. It can detect all materials in sizes upto fine wires. Dirty atmosphere can clog the lens and hence affect performance.

Fibre Optic Scanners are also finding wide applications in machine tools. These are resistant to shock, vibration, moisture, corrosion, environment temperature, electrical noise. The sensing tips are extremely small and available in a number of shapes.

For counting parts, electric scales are used. Sophisticated input/output controls like voice data entry and machine vision technology are also finding application in machine tools.

Control Systems:

Control systems for machine tools range from relay panels, programmable controllers, computerised numerical controllers and drive systems to highly complex computers. An automated factory consists of any of these in combination with robots, distributed control, and data communications networks.

Programmable controllers have been very effective in improving production line uptime, lower maintenance cost, increasing production volume and product uniformity, optimum space utilisation etc. The programmable controllers used should be compatible with the communications network. In deciding the extent of automation, the cost/value ratios should be determined.

Electric Power Supply:

Most of the drives of machine tools and controls require electric power supply. Uninterrupted power supply obtained by batteries or back up engine generators is essential for computer systems. High production rates and high costs of idle time and materials demand that no part of a plant should be shut down unnecessarily.

Nuisance tripping of branch and feeder circuits needs to be avoided by using better protective devices such as breakers and ground fault protection and better co­ordination of such devices among main branch, and feeder circuits.

Microprocessor technology is being used to achieve circuit protective devices to obtain precise co-ordination. Power supply for microprocessor based equipment should be clean, i.e. contain no voltage spike from any disturbances in the system or by external influence.

Drives and Drive Controls:

The rotary shafts and linear motion members are most important for any machine tool. The use of high efficiency motors is desirable. Since drive efficiency is the product of all drive component efficiencies, all drive line components should be efficient.

This also reduces heat dissipation, thereby extending equipment life. Adjustable speed drives play a greater role in energy conservation. Both AC and DC motors are available with adjustable speeds. DC motors with no brushes and easily replaceable modular components are available.

Hydraulic and mechanical adjustable speed drives are also available. Electric motors with soft-start devices (fluid or mechanical, with coupling in the drive line, or solid state electrical soft start) devices and injection brakes (injecting DC into starter) are also available. Other good tips in improving drives are use of high ratio helical-gear drives for large speed reductions instead or worm-gear drive, efficient V-belts, high water based fluids for hydraulic systems etc.

Control of Special Purpose Machine Tools:

Special purpose machines are not produced in large quantities since these are specially tailored for each application. These are basically multi-way, multi-spindle machines. Many tools work on the same work piece from different directions. It is essential that each tool be controlled independently, and as the number of units to be controlled are many the control of these machines is different from other NC and CNC machines.

Slides are controlled by independent position and speed controllers which in turn interface with main controller. The main controller also provides interface between the user and machine. The control system for such machines must employ modular control and programming system so that these could be easily tailored for each application.

Provision for adjusting position commands and spindle and slide-speeds is made after the programming is done in order to optimize the machining. Programming can be done by push buttons on the control unit. Specially made fixed subprograms are used to make programming easier.

Work piece handling systems such as robots capable of being interfaced with the control system are used. A controller block diagram for a slide unit is shown in Fig. 34.17. The slide position is sensed by a rotating or linear incremental transducer like resolver.

The pulses from transducers are fed to counter. Every slide unit has its own counter. The counter is read by the microprocessor and slide position is compared with the command value and the error signal is used to calculate the output signal to D/A (Digital to analog) convertor. D/A converter also acts as a buffer memory.

When the controller is in auto mode, it gets new position and speed values from main controller. In accordance with speed value, a position increment is calculated and every time the position command value is executed that position command value is increased or decreased with this increment until the desired position is reached. After the command is executed, the main controller is informed and slide remains in last position till fresh command is received.

Special Purpose Machines range from dedicated production milling machines, through rotary multi­-station machines right upto full scale linear transfer machines.

The Transfer Line optimises the application of production engineering to achieve the lowest possible unit cost. Each line is a unique combination of machining stations which can encompass milling, drilling, reaming, tapping and boring operations. All these stations plus the transfer system between stations has to be integrated into one control system.

Special Purpose Machines and Transfer Lines are designed to satisfy specific production requirements, taking into account complexity of component and production rate required.