In this article we will discuss about:- 1. Meaning of Assembly 2. Objectives of Assembly Process 3. Product Design to Facilitate Assembly 4. Manual Assembly 5. Design Rules for Product Design for Ease of Automatic Assembly 6. Assembly Cell.
Meaning of Assembly:
Most of the products are formed from a large number of single parts produced at different times and by various production processes. The object of assembly is to form a part of higher complexity with specified functions in a specific period of time from the individual parts.
The principal activities during assembly are shown:
It is generally observed that cost of assembly comprises significant portion of total cost of overall production cost. Thus rationalisation of assembly is essential, for which following obstacles often experienced should be carefully tackled.
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(i) Product service life is decreasing, batch sizes becoming smaller, and number of variants greater,
(ii) Assembly must continuously be adapted to changing market demands,
(iii) All the errors in planning, product development and initial production directly affect assembly,
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(iv) Products are usually not designed in terms of assembly considerations.
Objectives of Assembly Process:
The main objective of modern assembly process is to produce high quality and low cost products. It has been realised by most of the assembly-intensive companies that productivity can be increased by optimising, mechanising and automating the assembly process. The range of solutions may extend from the ergonomically designed manual assembly station up to highly flexible robot—equipped and computer controlled assembly cells and systems.
The assembly work can be facilitated if following points are remembered:
(i) Assembly parts must be standardised. Also parts of the same type must be subject to the same specifications, i.e. uniform parts quality must be ensured.
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(ii) Division of the assembly job—A finished item can be produced by progressively adding parts to an assembly. Breaking an assembly task into a number of smaller tasks results in speeding the assembly process and improved quality since each task has a relatively limited content, skill can be developed in a short time.
(iii) Operator productivity can be improved by introducing a dedicated material handling system and letting operators concentrate their efforts on assembly tasks. Automated material handling systems result in prompt material delivery, smaller space requirements, better inventory accountability, less handling damage, and less labour.
(iv) To improve the efficiency at the workstation level, the assembly job design should integrate human behaviour into workstation design by making use of time and motion study, analysis of human performance, and ergonomics.
(v) Introduction of line-integration concepts. The line designer must take a system view and a structural approach.
Product Design to Facilitate Assembly:
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As a general rule, the degree of difficulty of assembly increases with the complexity of the product. Products with a small number of parts are easier to design to be assembly— oriented. For complex parts, it is advisable to subdivide the assembly operations and a product must be designed so that sub-assemblies can be pre-assembled.
The following rules should be observed for the assembly—oriented design of subassemblies and final products:
(i) The whole assembly operation must be sub-divided into clearly discernible stages by suitable subassembly formation.
(ii) A sub-assembly must be completed as a unit so that it can continue to be handled and manipulated as a single part.
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(iii) It must be possible to test the subassemblies separately.
(iv) Every subassembly must have the minimum possible number of connections to other subassemblies.
(v) Base part (principal part to which further parts are attached in the course of assembly) must be designed so that it is self-centering, or be centered by a hole and a slot. The number of parts in a subassembly should be minimised.
Manual Assembly:
In manual assembly, a person performs assembly operations by using dexterity, sense organs and intelligence, and using tools, fixtures and gauges, etc. Manual assembly work points must be physiologically adapted to the human body dimensions.
Work benches for manual assembly must be robust and have the following features:
(i) Adequate leg room.
(ii) Foot rest adjustable in height and inclination.
(iii) Facilities for attaching adjustable arm rests.
(iv) Adjustable work rest and seat height of chairs.
Single Point Assembly:
It is mainly used for smaller products or assemblies with a low level of complexity and small batch size.
Line Assembly:
For complex or high volume production, it is not economical to assemble a product on a single assembly work point, but assembly must be distributed over several work points. The linking of several single assembly points to form a line is termed as line assembly.
Line assembly could involve either manual transfer of assembled part or the mechanical transfer in an unarranged/or arranged form.
Design Rules for Product Design for Ease of Automatic Assembly:
Usually in design of a part, little thought is given to aspects which will facilitate assembly. Thus design for automatic assembly should consider.
(a) Product design for ease of assembly:
(i) Use as less number of parts as possible
(ii) A suitable base be provided on which assembly can be built
(iii) Base part must have features that will enable it to be readily located in a stable position in the horizontal plane
(iv) The product may be designed so that it can be built up in layer fashion, each part being assembled from and positively located to avoid its movement during machine index period
(v) Try to facilitate assembly by providing chamfers or tapers which will help to guide and position the parts in the correct position
(vi) Time-Consuming fastening operations (screwing, soldering) should be avoided.
(b) Design of parts for feeding and orienting:
(i) Avoid projections, holes, or slots, that will cause tangling with identical parts when placed in bulk in the feeder (Holes and slots must be smaller than projections).
(ii) As far as possible, parts should be symmetrical to avoid the need for extra orienting devices. If symmetry is not possible, the asymmetrical features must be exaggerated to facilitate orienting.
Advantages of Automatic Assembly:
(i) Reduction in costs. This is often the main consideration.
(ii) Increased productivity. Automation of assembly lines generally reduces the number of operators required and hence increases productivity.
(iii) It ensures a product of consistently high quality.
(iv) Assembly by mechanical means is must in situations hazardous to operators (like high temperature and the presence of toxic substances).
Automatic assembly machine usually consists of a transfer system for moving the assemblies from one workstation to other, automatic work-heads to perform the simple assembly operations, vacant workstations for operations to carry out the more complicated assembly operations and inspection stations to check that the various operations have been successfully carried out.
The automatic work-heads are either fed manually with individual or magazine-stored component parts or are supplied with parts from an automatic feeder (like vibratory bowl feeder) through a feed track. The work-heads themselves usually consist of either a fastening device or a parts-placing mechanism.
Assembly Cell:
Assembly involves lot of work and cells for lot of skill. Automation of this part is desirable for higher reliability. Assembly work may include parts supply, assembly, inspection and transfer. Screws and nuts, usually arranged on a pallet, can be supplied by a unit based on vibrations (such as ball feeder). Relatively large parts can be supplied by using a mobile conveyor feeder, etc.
The supplied parts may undergo press fit, fitting, screwing, caulking, bonding, etc. These operations are conducted by an assembly cell comprising the assembly robot and assembly machine components such as a press, special screwing machine, caulking machine, etc., depending on the time and force required. The assembly cell is provided with sensors for the purpose of inspection.
The assembly cell has following features:
(i) Unmanned operation.
(ii) All processes can be automated either by using one assembly cell capable of performing several assembly processes, or by combining several assembly cells.
(iii) Assembly cell is highly flexible and can cope with a varied, small quantity parts production, and frequent set up changes.
(iv) Easy to operate and setup work is simple.
(v) Each to introduce and inexpensive to operate.
(vi) Assembly cell can automatically assemble various, small quantity parts, which could not be economically assembled with the conventional assembly machine.