In this article we will discuss about:- 1. Meaning of Transfer Machine 2. Arrangements 3. Components and Accessories 4. Advantages of Transfer Machine 5. Disadvantages .
Meaning of Transfer Machine:
A transfer machine is an automatic machine which indexes or transfers the work piece and its fixture from station to station while many operations are performed on it. It could be also defined as a combined material processing and material handling machine. It could thus be considered as several machine tools linked together mechanically, electrically or hydraulically.
Usually two operators are required to run the machine. One man loads and controls it while the second man unloads the work and stacks it or pushes it on a conveyor to be processed further.
The controlling operator attends a large switch board which indicates him the conditions all along the line and generally a mimic diagram incorporating lights indicating the following is provided; when each fixture in each station has located satisfactorily, the operation has been performed and the tools withdrawn or any malfunctioning when the gauges fail to pass the finished operation etc.
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Each station could be considered as a simple work head, with its own motor mounted on a base which slides along its own ways and is actuated by another motor and trip dogs to control the motion.
The basic ways of work handling in transfer machines are:
(i) Clamping the work piece to a fixture which is moved from station to station and is located at each station while the operations are performed; the fixture may be guided on ways or moved ‘free’ on a roller conveyor.
(ii) In second method, the work piece is moved to separate fixture at each station, where it is located on the fixture and clamped before the operation is performed.
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Installation of a transfer line can only be economically justified if the continuous production of the product is equaled by the demand for it. Before setting up a transfer line it should be ensured that demand for the product will continue.
As the component travels down the line, its processing time at each station must be balanced to approximately the same time value to reduce idle time. The cycle time/piece is determined by the longest operation at any one of the stations. The principles of specialisation, standardisation and interchange-ability are followed in design of transfer machines.
Arrangements of Transfer Machines:
Transfer machines are of following three types:
(i) In Line Transfer Machines:
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In this arrangement, several machining heads are arranged across the straight line on the sides at a pitch of nearly 1 metre and components flow in the middle of two rows of machines along guide rails as shown in Fig. 34.18. If space does not permit then machines, instead of straight line could be arranged in L- shape, square or rectangular pattern. The conveyor for transferring components could either run over, under or around the transfer machine.
The work may be loaded either manually or automatically onto the machine, and it is transferred by equal pitch movements from station to station where it is located through dowels and clamped. The work can be presented to the machining head in any desired position by using turntables or turn-over devices at the appropriate points on the bed.
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All types of machining operations are carried out at various stations and the chips produced are removed so that these do not foul or dog the working parts. Coolants need to be supplied in huge quantities. In case of jam up of components, automatic safety device operates and all machines come to a stop. Such an arrangement is very popular in automobile industries.
(ii) Rotary Indexing Table Transfer Machines:
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In this case work is transferred around a circular line, thereby giving a more compact arrangement and saving the floor space. The machining heads are arranged around the periphery of circle at equal intervals (Fig. 34.19). A rotary indexing table capable of fast and precise indexing is used for transferring components from fixed station of machining heads.
This type of arrangement is usually small in size, there may be 4 to 16 stations depending on size of component. This is due to limitation of size of a table which can be held and rotated in a base, which will be sufficiently rigid to maintain the component accuracy required. This type of arrangement is best suited for automatic assembly of a product.
(iii) Drum Type Transfer Machines:
In this case work fixtures are fastened to the outside surface or periphery of the drum and work stations are positioned radially around the circular path at equal intervals. As the work hangs from the fixture, the clamping arrangement must be fool proof and efficient. Like circular indexing arrangement, this too can’t be big in size.
Components and Accessories of the Transfer Machines:
We will now study the various components, mechanisms and accessories employed in the transfer machines.
I. Machine Design:
It is very difficult to lay down any hard and fast rules regarding the design and layout of transfer machining lines because these will differ according to the component involved and the nature of operations to be carried out.
In general, the machines consist of simple heads incorporating minimum mechanism. These may also incorporate provision for multi-spindle attachments, may be provided with hand feed for setting purposes, automatic feed and withdrawal mechanism. They may also incorporate a lead screw for thread tapping purposes. The feed cycle often provides for fast approach and quick withdrawal.
Each head is generally a self-contained unit and can be operated as such when setting up, but is connected either electrically, mechanically etc., with other machines so that it operates in unison with them, i.e. the cycle of each machine commences simultaneously with those of others.
The machine is required to perform following functions:
(i) Transfer the work from station to station.
(ii) Locate the fixture at each station and clamp it in position.
(iii) Move the tools rapidly to the work for the approach portion of the machine time.
(iv) Move the tools slowly into the work for the cutting portion of the cycle.
(v) Return the tools rapidly to clear the work.
(vi) Unclamp the work from the fixture and indicate that the station is clear for operating on the next work piece.
The operations which it is usually required to perform are:
(a) Drilling,
(b) Boring,
(c) Counter-Boring,
(d) Reaming,
(e) Tapping,
(f) Counter-Sinking,
(g) Chamfering,
(h) Face milling,
(i) Spotting,
(j) Hollow milling,
(k) Trepanning,
(l) Gauging,
(m) Air pressure testing,
(n) Blowing out or dumping chips,
(o) Rolling over or revolving work.
II. Table:
The heads are mounted on one or more sides of a long table on which the work is carried. The table usually consists of rails, a static roller conveyor track, or a combination of both and its frame carries the mechanism for transferring the work from one station to other. The table is generally straight, although in some cases it may be circular, U-shape, curve forward and backwards or of any other shape to suit local conditions.
Generally roll-overs are also provided in the table line so that components could be rotated through 90 to 180° during its progress from station to station, if need be. Arrangements may also be provided for transferring the components to an auxiliary line or to a machine not built into the table. For fully automatic installations use of shuttle operated either pneumatically or hydraulically could be made for moving the work from one position to another.
III. Work Holding:
Although large and flat components can often rest directly on the surface of the table, but it is generally more convenient to mount the work on platen, pallet or jig plate etc. which are specially designed to suit the components they carry.
These are provided with some form of clamping device for securing the work to be carried and also incorporate means for locating the components on its carrier and often for locating the carrier under the various machines. The underside is designed to suit the type of table and may be flat for riding on rollers or recessed to suit vee or flat rails.
IV. Transfer Mechanism:
A wide variety of transfer mechanisms are available for moving the platen from station to station. It is to be noted that the distance between the centres of all the machine heads must be identical, and that the stroke and position of the transfer pawls must be arranged to deliver every platen to a precise position under each machine.
The system should also be inter-connected with the time cycle of the machines and interlocks or other means provided to ensure that the platens are not moved until the last tool is clear of the work, or that the spindles cannot descend until the transfer movement is completed.
One of the most common types of the transfer mechanisms consists of long circular-section ‘transfer bar’ mounted just below the table surface and provided with projecting spring-loaded fingers or pawls. The bar is given a lengthways translational movement, during the return stroke of which the pawls move all the platens on to the next station.
An alternative to the transfer bar is to carry the pawls on a chain which is caused to move backwards and forwards in the manner of a shuttle. Other ways of obtaining the transfer movements are by mechanical, hydraulic and pneumatic operation.
V. Swarf Removal:
A huge amount of swarf is produced due to high rate of production, therefore, some means must be provided for regular removal of the swarf while the line is in operation. It is common practice to provide gaps in the ways so that swarf falls below into some form of conveyor and sliding ways of the tables are kept clean. To facilitate the disposal of the chips on the conveyor, the bed of the table may be provided with slides which slope inwards, in hopper fashion.
VI. Sequence of Operation:
The whole of the line is set into operation by a single switch. The various movements are all automatic and controlled by limit switches.
The general lines along which the sequence of operation generally follows are described below:
(i) All the components are moved to the next machine simultaneously by the transfer mechanism.
(ii) Clamping devices automatically come into operation to locate the work or platen accurately under the machine and then clamp it to the table.
(iii) The machine head incorporates means of locating and securing the platen.
(iv) The head commences to descend and when it has come into contact with the work, it moves at cutting feed.
(v) After completion of the operation, the tools withdraw at a fast traverse. The other operations could be dwell period, a reciprocating motion for clearing swarf when drilling deep holes etc.
(vi) When the tools of the head employing the longest time cycle have withdrawn, the clamps are released and transfer mechanism comes into action again to repeat the cycle.
VII. Automatic Inspection:
Automatic inspection equipment is essential for transfer machines to keep up with the continuous production coming off the machine. A probe is fed by the hydraulic power to work piece. Usually air gauging is used for inspection.
VIII. Tool Servicing:
Efficiency of a transfer machine is enhanced by using automatic means to detect worn and broken tools and release them automatically.
Solution to the Problem of Transfer Machine:
The aim of the production engineer has always been to see that the output increases somehow or other; particularly in mass production. The technique of transfer machine has also been introduced to achieve the same motive.
In order to fully understand the technique of transfer machining it is better to discuss the problems faced by production engineer from time to time and to study as to how the solutions were found to overcome these problems.
1. The first thing thought of, for increasing the output was to increase the speeds and feeds to their maximum and the introduction of jigging; both aiming at reducing the machining time. The maximum speed was achieved by introduction of cemented carbides and further reduction of machining time after this was possible only by considering other factors.
2. Then the attention was drawn towards the reduction of the time in handling of the components during their passing from one operation to another. This aspect became more important when it was realised that the unnecessary movements lead to operator fatigue which ultimately resulted in still further lowering of the output.
To achieve this, means were introduced to eliminate the need for lifting heavy work and fixtures, these including the simplification of clamping by the large-scale introduction of air operated clamps, quick action and cam type clamps etc. Extensive use was also made of conveyors and lifting trucks to transport the heavy work from one station to other at the level of the machine.
3. Other important problem faced was the conservation of floor space as considerable area is generally occupied by work between the operations from machine to machine. This wastage was eliminated by the use of conveyors and using long belts running upto roof so that these carried a large enough ‘float’ of components to ensure that no machine ever ‘starved’.
Another obvious remedy was to place machines more closely together, but maximum saving in this direction was not achieved until the introduction of the transfer system.
Another solution to this problem was given by using ‘flow production’ laying down lines of machines and equipment arranged in sequence of operations so that the work flowed straight on from one machine to the other.
4. Realising the labour problem, it was thought of eliminating it completely by introduction of such arrangements as automatic feeding of work to the machine, automatically operating the machine and automatically unloading it. This ideal was achieved in several stages.
The first was introduction of hand transfer machining’, in which number of machine heads are mounted along or around a common table and a work is manually slided along the table from station to station and loaded and unloaded into the fixtures, often with the aid of lifting devices.
The system of hand transfer machining’ was later improved by making the provisions of automatically moving, loading and unloading ‘the work’. This system is known as ‘automatic transfer machining’. It will thus be noted that the basic difference between the automatic transfer machining and the ordinary automatic machines is that the former is capable of automatically transferring work from station to station and of dealing with large and heavy components of awkward shapes.
The next logical development in this direction was to build a number of such automatic transfer machines into a single line to perform a complete sequence of operations, or even completely machine the components (automation).
Although this system is largely employed for lighter operation such as drilling, tapping, reaming, spot facing etc. but there is no limit and it can be easily linked with the heavy machines (broaching, boring processes etc.) and also the means of automatically inspecting the work may be incorporated. The system is by no means limited to machining; extremely efficient lines have been developed for assembly purposes, and for press shop use.
Advantages of Transfer Machine:
(1) It can handle very heavy components and components of extremely awkward size and shape. No manual handling of work is involved except loading and unloading.
(2) Operator fatigue is practically eliminated and the need for operators can also be reduced.
(3) Output is considerably increased, the speed of output can be easily varied to ensure balanced production with other departments.
(4) Considerable floor space is saved by the elimination of inter-operation sacking and the close grouping of machines.
(5) Control of the work passing through the shop is simplified.
(The output from the line can be increased or reduced merely by altering the time cycle of the longest operation. If output is to be increased, the extra operators may be employed and vice versa).
(6) It is flexible and can be arranged to suit modifications in the design of the components.
(Generally some idle space is left in the line for inserting extra machines required for meeting some design modifications in the component).
(7) The plant can be disassembled and rebuilt to suit other work piece if the component for which it is already designed becomes obsolete.
(It will be realised, however, that the line is specially laid out for the production of one particular component and can’t be used for other types of work).
(8) The life of the cutting tools may be considerably extended thus reducing replacement costs and hold up due to resetting.
(In transfer machining, it is an essential requirement that all operations, occupy the same ‘time cycle’. Consequently, the same cycle of all machines is controlled by the longest operation on the line. Thus in order that the machines meant for shorter operations do not remain idle for any time, such machines have to be operated at much lower speeds and feeds. Thus due to use of lower speeds and feeds, the tool life is considerably increased between two regrinding).
(9) Alignment of work at each station is simplified and automatized.
(10) An overall economy is obtained if the production warrants the use of this type of equipment.
(11) Greater overall accuracy is obtained because the work is integral with the fixture throughout.
Disadvantages of Transfer Machines:
(1) Initial cost is very high and electrically things are very complex.
(2) A breakdown of one machine means stoppage of the whole of line and huge loss has to be tolerated in that case as other perfectly all right machines in the line also can’t be worked.
(3) It is limited to high production industries.
(4) Transfer machine is inflexible and can produce usually one product and one product only in large quantities.
(5) Generally when product is changed, the machine must be scrapped or costly reshuffling is required which tends to freeze the design.