The following points highlight the twelve main modules for automation of assembly process. The modules are: 1. Handling 2. Feeder Units 3. Handling Equipment 4. Transfer Systems 5. Indexing Mechanism 6. Transfer Equipment 7. Vibratory Bowl Feeder 8. Non-Vibratory Feeders 9. Parts Orienting Systems 10. Screw-Inserting Units 11. Rivetting Unit and a Few Others.
Module # 1. Handling:
Handling is a part function of material flow and may comprise of:
(i) Storage, stocking quantities
(ii) Variation of quantities
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(iii) Movement (realisation and change of a defined 3-D arrangement)
(iv) Securing (maintenance of a defined 3-D arrangement)
(v) Inspection
A knowledge of the work piece parameters (Work piece properties and behaviour) is important for correct selection. Work piece properties are subdivided into the behavioural type (form), geometrical work piece data, characteristic form elements and physical properties.
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The work piece behaviour is different when stationary and when moving. The resulting preferred axial positions of work pieces have an effect on automated handling. The reliability of automated handling is affected by the quality of the work pieces, cleanliness and the proportion of foreign bodies.
Module # 2. Feeder Units:
These principally fulfil the functions of storing, ranging, feeding or placing of individual parts in a magazine. The selection of correct feeder unit depending upon the part to be handled is very important. Further its success of operation depends on constant quality of the single part to be handled and its cleanliness.
Feeder units could be designed for:
(a) Parts with one arrangement feature. These include hopper with scoop segment, hopper with blade wheel, hopper with magnetic plate discharging system, inclined conveyors.
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(b) Parts with several arrangement criteria. These include vibratory spiral conveyor.
Module # 3. Handling Equipment:
Depending on the design, control and type of programming, the handling equipment could be classified as:
(i) Positioning units
(ii) Industrial robots.
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The positioning units consist of a drive, control, the kinematic structure, and the gripper. The drives could be operated by external drive (mechanical—like cam controlled units) or by pneumatic/electrical/hydraulic drive.
The kinematic structure of positioning unit is designed for obtaining three movements (all 3 translational, or 2 translational and 1 rotary) or one translational and 2 rotary, or all the three rotary) in order to move an object to any particular point.
The control of a positioning unit includes signal detection, signal processing, signal conversion, energy control and energy conversion. Grippers are required in order to manipulate work pieces with positioning units. The design of industrial robots is more complex than of positioning units since these are universally applicable and programmable.
The part systems and part functions of an industrial robot are shown below:
Module # 4. Transfer Systems:
The individual assembly operations are usually carried out at separate workstations. The partly completed assemblies have thus to move from one workstation to other. Since no relative motion should exist during this movement, the assembly is usually built up on some form of work carrier.
According to the system adopted for transferring the work carriers, the assembly machines can be classified as:
In continuous transfer machine the work carriers move continuously at constant speed and the work-heads index back and forth (Refer Fig. 35.1). Thus assembly starts and finishes when both work head and work carrier are moving simultaneously.
After finishing operation, work head returns back to original position and starts assembly operation over new work piece. Sufficiently accurate alignment between work-heads and work-carriers during the operation cycle are must.
In intermittent transfer, the work carriers are transferred intermittently and the work-heads remain stationary (Refer Fig. 35.2). These machines are called indexing machines and are more popular.
The transfer mechanism on in-line machines is generally one of three types:
(i) Walking beam
(ii) Shunting work carrier
(iii) Chain-Driven work carrier.
Module # 5. Indexing Mechanism:
Various indexing mechanisms available for use on automatic assembly machines are:
(a) Rack and pinion with:
(i) Unidirectional clutch
(ii) Ratchet and pawl
(b) Geneva Mechanism
(c) Crossover and cam indexing unit.
The various factors affecting the choice of indexing mechanisms are:
(i) Required life of machine
(ii) Static and dynamic torque capacity
(iii) Power source required to drive the mechanism
(iv) Acceleration pattern required
(v) Accuracy of positioning required from the indexing unit.
Module # 6. Transfer Equipment:
These are mechanical units by which the assembly part is transferred from one work point to other in the sequence of the assembly operations.
Transfer equipment could be either—cycled type (in which the work piece carrier or assembly fixture is rigidly connected to the transfer unit)—or un-cycled type (in which there is no rigid connection)
Cycled transfer units could be either circular or longitudinal transfer type. Circular cyclic transfer equipment could use ratchet drive, or cam drive. On non-cycled transfer equipment, the work piece carriers or assembly fixtures have no rigid connection with the transfer equipment, but are moved by friction against variable stops.
Module # 7. Vibratory Bowl Feeder:
It is the most versatile feeding device for small parts because it is able to feed and orient most of small parts. The parts travel along a helical track. The track passes around the inside wall of a shallow cylindrical hopper or bowl. The bowl is usually supported on sets of inclined leaf springs secured to a heavy base. An electromagnet mounted at base vibrates the bowl.
The support system constraints the movement of the bowl so that it has a torsional vibration about its vertical axis coupled with a linear vertical vibration. The motion is such that any part on the inclined track vibrates along a short approximately straight path, which is inclined to the horizontal at an angle greater than that of the track.
The effect of vibratory motion is to cause parts to climb up the track to the outlet at the top of the bowl. For maximum conveying velocity, the track angle should be as small as possible. Feeding around the flat bowl bottom is always faster than the inclined track. For any given conditions, there is an optimum vibration angle.
Module # 8. Non-Vibratory Feeders:
These comprise:
(i) Reciprocating feeders,
(ii) Rotary feeders, and
(iii) Belt feeders.
Following points must be kept in mind:
(i) The output of parts from the feeder is always governed by the machine being fed.
(ii) All parts should be presented to the machine in the same orientation (fed correctly oriented).
(iii) The reliability of parts feeder should be high, i.e., there should be no possibility of parts jamming in the feeder or in the orienting devices.
(iv) Parts feeder may also act as inspection devices.
(v) Operation should not cause un-tolerable noise.
Module # 9. Parts Orienting Systems:
In automatic assembly machine, it is necessary that the parts be fed to the work-heads correctly oriented. Part orienting devices may be incorporated either in the feeder itself (in-bowl tooling) or be fitted to the Chute between the feeder and the work-head (out-of-bowl-tooling).
The devices for in-bowl tooling work on the principle of orienting by rejection (passive orienting devices). In such devices only those parts which, by chance, are being fed correctly oriented pass through the device, while the other parts fall back into the hopper. In some cases, devices are fitted which reorient parts (active orienting devices).
Module # 10. Screw-Inserting Units:
This is the most commonly occurring assembly operation.
It must fulfil the following functions:
(i) Storage of unarranged screws
(ii) Arrangement of the screws
(iii) Placing the arranged screws in a magazine
(iv) Distributing the screws
(v) Positioning of a screw in the assembly position
(vi) Screwing-in of a screw
(vii) Infinitely variable tightening torque.
Module # 11. Rivetting Unit:
Rivetted connections are preferred because these result in saving of space and are cheaper. Press rivetting or rotating—mandrel riveting methods are commonly used. Other methods of joining available are welding (resistance, or laser-beam welding), soldering, and bonding.
Module # 12. Feed Tracks, Escapements, Parts Placing Mechanisms:
The part feeder is usually placed a little distance away from the work-head in order to provide easy access to automatic work-heads and the assembly machine. The parts between the feeder and the work-head have therefore to be transferred and maintained in an orientation by use of a feed track. Feed tracks may be either gravity tracks or powered tracks.
The automatic work-head usually requires parts at discrete intervals, whereas the parts feeders do not supply parts at desired discrete intervals. A metering device (also called escapement) is thus required to ensure that parts arrive at the automatic work-head at correct intervals. The parts-placing mechanism, after the parts have left escapement, place the parts in the assembly.