I originally published this article under the title, “Changes in Assembly Work Environments” in the book “Programmable Assembly”, ISBN 0.903608.65.0.
 
The role of the modern assembly worker is very different now from that of 3 generations ago. Improvements in parts quality consistency has eliminated the previously required skill of the apprentice trained fitter. A new breed of unskilled assembly workers has been created, through the division of labour, to carry out repetitive and mundane tasks.  However, many companies use assembly automation if it can be economically justified, and after the product has been re-designed for automatic assembly.

The development of modern assembly techniques is discussed, together with future trends in manual and automatic assembly. Emphasis is given to the changing needs of the people directly involved in these assembly operations.

Introduction

There has been a rapid increase in living standards in the developed nations throughout the previous century, mostly due to the application of technology to manufacturing. The mass production of goods has made many items available at economic prices. Homemakers now have a multitude of labour saving devices to reduce the amount of time spent on household chores. This has enabled many homemakers to work in factories which produce these goods. Assembly workers can master a simple assembly task and repeat it for more than 1000 times per day; every day. Working with other people on an assembly line can create a sense of cooperation within a joint effort.

However, there has been criticism of the assembly line technique. It is argued that the repetitive work is boring and tedious and that workers no longer gain satisfaction from doing their job.  Workers never see the finished product and the continual repetition of movements creates boredom. Industrial unrest in high volume manufacturing companies has been associated with the job dissatisfaction of assembly line workers. Manufacturers now realise that the economic benefits of the division of labour have to be judged alongside the sociological and psychological disadvantages.

The use of assembly automation during product manufacture eliminates worker dissatisfaction with repetitive work, since most of the mundane tasks are done by machines. Workers are then used to fill magazines/feeders and to maintain the equipment. The reduced labour content often creates a cost reduction in the finished goods. The culmination of this desirable process is an increase in leisure time, through a reduction in the working week.  Emphasis must then be placed on how people are to spend their leisure time. This should be the subject of major reform in our training establishments.

Technology

Technology is the systematic knowledge of the industrial arts. Industrial engineers have been applying technology to the workplace for over two centuries. Manufacturing systems analysed by method and time studies have been improved by the division of labour, automation and robotics. Large productivity improvements have been achieved by applying technology to manufacturing processes. From the mechanisation of flour production to the robotic assembly of vehicles, process costs have been reduced. The application of technology to the motor industry has resulted in vast increases in productivity.

Method study is concerned with the dissection of a complex operation into it’s single constituent parts, which are then systematically analysed. The method study engineer synthesises the complete operation using components which optimise factors such as symmetry and the rhythm of movement.

The time study engineer measures the time taken to carry out an operation. The analysis is carried out in a systematic manner and it makes this form of study suitable only for simple and repetitive tasks. Often, time study exposes inefficient operations and these can then be analysed using method study.

It was the use of both method and time studies that led to the wide-scale use of the division of labour and the creation of the assembly line concept. Workers grouped on lines achieve productivity levels many times greater than single operatives making the entire product.

Automation has also produced large productivity increases by replacing men with machines. In highly automated manufacturing plants, the operator controls and supervises the process. The main power olders in future societies will not be capitalists or socialists, but people who possess expert technological skills. In this way, power will be passed to the techno-structure.

Automation

Automation in the manufacturing industries covers a whole range of electrical and mechanical equipment. In the field of automatic assembly, devices are used for automatic feeding and insertion. In addition, work transfer is by conveyor or rotating table. The type of system used for the assembly of a product is dependent upon many factors. The local cost of labour affects the economic justification of using automation to replace that labour. The frequency of design changes and the number of product styles dictate how flexible the equipment needs to be. The market life of the product influences the amortisation period of the capital investment. Finally, the annual product volume determines the required cycle time.

In addition to the above economic considerations, another reason for employing automatic assembly may be one of necessity. In certain areas, where labour is scarce, the use of automatic assembly is imperative. Certain operations may be hazardous or they must take place in dangerous working conditions. For example, the handling of toxic chemicals or working in extreme temperature conditions may exclude the use of manual workers. A further reason may be associated with the scheduling of the assembly operations: better control over production can be achieved with automation and product quality will be more consistent.

The assembly operation consists of the two basic activities of handling and insertion.  If a product is to be assembled automatically then thought has to be given to the economics of these activities. The automatic feeding of simple parts is usually carried out using a vibratory bowl feeder. Components in bulk random orientation are placed into the feeder and the parts are presented to the workhead in an ordered manner. Difficult parts may be fed by special feeders, hoppers or by magazines. The insertion process is defined as being the action where one part is assembled to another part, or group of parts. High speed operations, where the same parts are inserted for long periods of time, are normally effected by standard pick-and place units. Difficult operations, involving the assembly of a number of different parts with different operations may require assembly robots.  The flexibility of the robot is created by using computer programs to control the robot arm movements. The difference between a robot and a pick-and-place is that the path of the robot arm is not restricted by mechanical means, whereas pick-and-place units rely upon mechanical stops to determine the path they follow.

Division of labour

The division of labour is the process whereby one complex operation is broken down into a number of simpler tasks. These single tasks are carried out using a series of people, each doing one task. In this manner, a complex task performed by one worker is replaced by a number of workers operating in series. This allows operations to be carried out simultaneously, instead of the single operator having to complete one task before commencing another, different task. Unskilled workers can then be used to carry out these simple operations and they soon become efficient at the particular task.

Assembly systems

An assembly method can be classified into one of six types, and most systems may contain a number of different methods.

The traditional form of assembly is manual and, for high volume production, the workers are arranged on an assembly line. Other forms of manual assembly include a single worker assembling a complete product and groups of workers assembling a portion of the product.

When the range of products is more limited, a manual assisted method can be used, whereby workers are assisted by mechanical devices, such as parts feeders. The feeders present the parts to the assembly worker in an ordered manner.  The assembly time is reduced by eliminating the time taken to separate the parts from bulk random orientation.

The third form of assembly uses automatic indexing assembly machines.  A rotary or in-line machine has a number of workstations with automatic feeders which supply components to workheads for assembly of the part to the fixture, or part-built assembly. The workstations are 'special-purpose' and are dedicated to the assembly of one product only. Production volumes need to be high for the economic justification of these machines. Component quality must also be high to avoid excessive workstation downtime, caused by jamming, etc.

The efficiency of an automatic free-flow assembly machine is less dependent on parts quality. The transfer of work pieces between each workstation is non-synchronous. Small buffer stocks are held between each workstation and the other workstations still operate even if one is stopped because of a fault, e.g. a defective part jammed in the escapement mechanism.

The programmable automatic assembly machine has a non-synchronous transfer line with a series of programmable or robotic workstations to assemble components. Parts are presented to the workheads by automatic feeders or, in the case of difficult components, magazines may be used. The workheads can execute one or a number of operations. Flexibility is acheived by using different programs for each product to be assembled.

The final type of assembly system is robotic assembly and it is used for the assembly of products manufactured in low production volumes. This method can also be used when there is large product variety. Work transfer is not by conveyor, as all the assembly operations are carried out by a single robot. Transfer of the completed sub-assembly onto the next operation may also be done by the same robot.

The direct labour content in assembly is reduced in the progression from manual assembly to robotic assembly. However, the complexity of the equipment increases as workers are replaced by machines. Indirect labour also increases for the maintenance and computer control of the equipment.

Economic aspects

The application of technology to manufacturing is used to increase productivity and the selection of a system for the economic assembly of a product depends upon a number of factors. The final selection must take into account the following:

- Market life of product - influences the decision of the company on investing in capital equipment. Products with short market lives are usually assembled manually.

- Variations in demand - Automatic assembly machines are designed to operate with fixed cycle times. Low demand leads to increasing stock levels or the machine has to be stopped. Both of these actions are expensive. Flexibility to assemble different types of products is needed if there are large demand variations. This flexibility can only be provided by manual assembly or programmable machines.

- Parts quality- Automatic assembly machines are intolerant of defective parts and they can cause a station to breakdown. Whilst inter-station buffers will reduce the effect on efficiency, manual assembly is necessary for products that use low quality parts.

- Number of products - to be assembled by a system determines how flexible it needs to be. Different products manufactured in high volumes can be assembled using programmable workheads. Smaller volumes require manual assembly.

- Major design changes - Products subject to frequent design changes need flexible assembly systems, in a similar way to systems used to assemble a variety of products.

- Company investment potential - Assembly system selection is influenced by the   company's policy towards investing in automation. If the company requires a payback period of less than one year then it is unlikely that any form of automated assembly system can be economically justified.
- Annual production volume – This determines the cycle time of the system and automatic systems must run continuously to be justified. If the annual volume is low then the product must be assembled manually.
- Number of parts – This dictates whether the product should be assembled in a series of simple operations or in a single, complex operation. Automatic indexing machines cannot be used for the assembly of more than 8 parts on a single machine. The downtime caused by defective parts rapidly increases for every part above this value. Free-flow transfer should be used for products containing a large number of parts.

Social aspects

The application of technology to the assembly environment has sociological and psychological effects. The economic advantages of certain assembly systems can produce serious social disadvantages. These social effects are not limited to the confines of the factory and they affect the whole of society.

Assembly line work can provide jobs for people challenged with limited abilities. They can soon acquire a skill for a specific task and take pride in doing a job that may seem uninteresting to other people. Working with others on an assembly line often brings a worthwhile feeling of cooperation in producing goods required by society. Some people enjoy the fact that they can start a job and, with minimal training, soon be earning a bonus on piece-rate assembly lines.  A highly specialised assembly task, requiring little dexterity, gives this opportunity. The correct candidate can be selected for an assembly line job by using aptitude and vocational tests. There is scope for job rotation and managers can circulate workers so that they don’t have to do the same operation for long periods. Job rotation also gives the manager with a labour force able to do many operations. This is beneficial to the company when there is a high rate of absenteeism. The assembly line workers soon adopt a rhythm of working, as they do not have to set aside one tool to pick up another.

Many assembly line workers don’t want to use mental effort and choose not to accept responsibility in a job. They prefer to execute a task that allows them to simultaneously talk with their colleagues and listen to music. The workers are also able to take advantage of the reduced selling price of goods assembled by the flow-line method, available in high street stores. They can buy goods that would normally be outside their budget, were it not for the division of labour. Low priced home appliances like washing machines and vacuum cleaners reduce the amount of time required to do work around the household. Homemakers find that they are more available to work on an assembly line, earn money and to gain companionship in a work environment.

The social advantages of assembly line work must be considered alongside the often serious psychological disadvantages. There can be a loss of job satisfaction when the worker is not involved in all of the assembly processes that lead to the finished product. The job is repetitive and some workers are unable to take much pride in the task itself, as they don’t have the opportunity of seeing how important their operation is to the successful completion of the product. Boring work may suppress the creative ability of the worker and their time out of work may be spent so passively that life goals may disappear. The effect of carrying out monotonous work is often excessive fatigue. With the decline in individual craftsmanship, many unskilled operatives have no opportunity to display their creative talent at work. Goods built on an assembly line lack the variety that can be created by craftsmen. This dull product uniformity can have an adverse effect on some workers who see the same product every 20 seconds or 1350 times a day.

Assembly lines are usually installed in factories with a large workforce. Each group within the factory is dependent upon the other for the manufacture of the product. Strike action by one group of workers may affect the production of the whole factory. The assembly worker output is effective only during the time spent doing tasks. The cycle time is fixed by the conveyor speed and so it is the periods of time spent off the job that reduce the output.

These psychological problems often cause the assembly worker to create avoidable delays in which they try to gain control of the rate of work.

The social effects of automation are different from those of the division of labour. Many of the simple operations carried out by assembly workers can be substituted with automatic workstations. By replacing workers with automation, these repetitive tasks are executed by machines. The displaced workers are then available to carry out other, less tedious, tasks like supervision and inspection. The automatic assembly machines must be fully utilised to be economically justified. Dedicated automatic assembly machines are less flexible than manual assembly lines. The products must be assembled in large batch sizes.  Overproduction and under-consumption lead to inefficiency.  Severe demand fluctuations and gross lack of demand can’t be accommodated with assembly automation.

Behavioural scientists say that technology can be applied to assembly without employing automation. They believe in job enlargement/enrichment and argue that the division of labour has been taken too far, to produce boring and repetitive assembly line jobs.

Job enlargement increases the number of tasks completed by a single operator and this is intended to give more interest and variety to the job. The same grade of worker does more complex operations.  The net effect of job enlargement is a reduction in the number of operators per assembly line, an increase the cycle time and more flexibility, but an overall increase in assembly costs.

Job enrichment increases the responsibility of the assembly worker by giving more opportunity to make decisions. If the responsibility of the assembly worker is increased then it is anticipated that the feeling of job fulfilment will also be increased. Job enrichment, by the strictest definition, is more easily applied to skilled or semi-skilled employees.  Nevertheless, forms of job enrichment have been applied to assembly workers in car factories and electrical companies with some success.

There are many critics of the theories of job enrichment and job enlargement. The trade unionist view of these ideas is that the workers are misled into participation and into accepting leadership, whilst the 'conflict' between the workers and management remains unchanged. Others claim that the nature of the work is only one of the many factors contributing to the attitude of workers towards their jobs. It is the nature of the job itself, that enrichment theorists believe is the difference between satisfied and dissatisfied workers. By changing the nature of the job, social attitude will also change. Others claim that the nature of the work is not the top priority and other factors such as; pay, working conditions, job security and the attitude of the supervisors must also be considered. Assembly workers have individual preferences for the nature of the job. Some prefer routine work, whilst others enjoy performing complex tasks. Many workers purposefully don’t choose the job that they would most enjoy, in return for higher pay. Others, in periods of high unemployment, necessarily accept any paying job to financially support themselves and their families.

The future

Globalization and “offshoring” has transferred assembly work jobs from the developed nations to the developing nations to achieve lower assembly costs.  It was believed by most, a few decades ago, that the displacement of assembly workers would result from the domestic implementation of assembly automation.  It was not foreseen that developing nations’ infrastructure improvements, lowered trade barriers, foreign direct investment encouragement and lower logistics costs would cause the transfer of domestic assembly jobs to overseas locations.

However, the economic benefit of “offshoring” assembly work is now being eroded by higher wage demands in the developing nations, higher logistics prices, copyright infringements and the lower cost of assembly automation.

Assembly automation reduces the cost of producing goods domestically and the availability of goods at economic prices creates a higher standard of living.  The lower labour content in producing goods leads to a shorter working week.  The greater availability of human resources, if used wisely, should be available to achieve a better quality of life for all.

I re-design products so that they cost less to make.  Redundant parts are eliminated and design features are integrated.  The least number of components are used to meet functional requirements.  Parts handling is made easier and assembly operations are simplified.

Here's an article that I originally presented at the 6th Annual British Design Engineering Conference ...

Product design for ease of assembly is a factor that should be considered alongside other parameters from the start of product creation. Considerations, such as minimizing the number of parts and reducing the difficulty levels of handling and assembly, are of major importance for both manual and automatic assembly. Additional areas of investigation are needed when employing assembly automation, due to the use of relatively 'sensor-less' human imitators.

Certain design features must be incorporated, for ease of orientation and feeding, and assembly processes must be kept simple and efficient. I describe, on the following 14 posts, how to design a product for automatic assembly and achieve the required production rate, with minimum rectification work of defective products.

A big part of a product's factory cost is dictated by the product designer. Established design goals, such as minimum material usage and the use of standard components, have always been given top priority by the designer to obtain low direct material costs. Manufacturability has played an important part when considering parts forming techniques and, to a lesser extent, assembly techniques.  It is now time to change the emphasis from "how parts are to be made" to "how they are to be put together".

Historically, industry's direct labour costs in the developed nations have been acceptably low in the manufacture of medium to high volume, low technology products. This situation has changed over the last decade .  Globalisation exposes the significant difference in labour costs between developed and developing nations.

Labour costs can form a major part of the factory cost of a product. In particular, assembly labour costs per product can account for well over half of the total labour costs.

Assembly is the final labour intensive manufacturing process to be conquered by the automation engineer.  The comparatively high factory cost caused by assembly labour points engineers to assembly automation as an economic alternative to 'offshoring'. Assembly automation not only gives tangible economic benefits.  It also gives other, less quantitative advantages. The added benefits are a greater control over production, lower floor space requirements and higher finished product quality levels. Exclusion of these less quantifiable benefits can have a critical effect upon the economic justification of an automatic assembly system.

Successful implementation of an automatic assembly system involves many disciplines. Harmonizing of a product design and it's assembly system is an iterative process.  It relies upon co-operation between the product designer, production engineer, cost accountant, and equipment supplier. Product design changes requested by the production engineer are created by the product designer and evaluated by the cost accountant, for the effect on factory cost.

The design for assembly process starts with an existing product already in production, a prototype ready for the product to go into production, or a set of product drawings - whilst the design is still being finalized.  An optimum assembly system will exist for any product, by considering the following points :

- market life of product
- company policy towards automation
- number of product styles
- number of anticipated design changes
- annual production volumes
- number of parts in the product
- individual component properties
- type of assembly operations

Once the most economical form of assembly system has been established, the product can be re-designed for manual or automatic assembly.

The product is assembled with a note of all handling and assembly operations required to complete the product. For manual handling; weight, shape, symmetry, and bulk properties are noted and recorded.

For manual assembly; the process, access, movement, resistance, and alignment of the parts are considered. Time penalties are given to those parts which have difficulties in excess of handling and assembling a standard part. A design efficiency can be calculated for the existing design. The task of the design team is to increase this efficiency and reduce the factory cost. The efficiency can be increased by similar methods to be described for automatic assembly.

Product design for automatic assembly can be implemented at any stage in the manufacturing process. It is best, however, to design for automatic assembly before the product goes into production. The problems associated with re-design, once the product has gone into production are many. Firstly, the cost of design changes, in terms of re-tooling, may outweigh the savings gained by greater productivity. Secondly, an unacceptable lead time may exist from company approval and customer approval to getting the re-designed product into production. Also, once a product has been re­designed for automatic assembly, good productivity gains can often be realized by incorporating the new product into an existing manual assembly line, without investment in an automatic assembly system. This highlights the fact that a product re-designed for automatic assembly always provides savings in manual assembly, when compared with an existing design.

The object of a design for automatic assembly investigation is to increase the design efficiency of the product.

The UMass method of representing the efficiency of a product design is :

Design efficiency, E    = N (H + A) / T

Where :
        E    = automatic assembly design efficiency
        N    = theoretical minimum number of parts
        H    = automatic handling cost per part
        A    = automatic assembly cost per part
        T    = total operation cost

A 100% efficient product design has the following qualities :

- It uses the theoretical minimum number of parts
- The number of insertion operations equals to the theoretical minimum number of parts
- The difficulty level in feeding is similar to feeding a 2.5 cm cube at 1 per second
- The difficulty level of insertion is similar to that of inserting a standard part at 1 per second

Well designed products have UMass design efficiencies between 20% and 30%.  Poorly designed products have efficiencies less than 5%. Alternative product designs can be compared, in terms of design efficiency, to obtain the most economic design.

Any detailed investigation into the design of a product results in a more productive method of assembly. The parts and assembly operations used to put together the parts can be viewed with either manual or automatic assembly in mind.  When designing for automatic assembly, remember that the intricate feedback loop that co-ordinates human motors is not present in economically justifiable automatic assembly systems. For example, parts that are manually picked up the wrong way round can have their orientation corrected. The human assembly worker detects this error through sight or touch and quickly corrects the orientation.    Similarly, defective parts can be detected and discarded by a human assembly worker.

Automatic workheads do not detect rejects without the aid of complex sensor systems.  A defective part arriving at the workhead causes a jam and the workstation is down for a period of time. These events are minimized by having high component quality levels and re­structuring the inspection routines. Most manual assembly systems have three inspection stages - goods inward, during assembly, and upon final assembly of the product.  Parts or assemblies which do not fall within quality bands, at each of these stages, are rejected. Automatic assembly equipment requires higher quality components and, therefore, greater quality control is required at the goods inward stage than for manual assembly. Automatic assembly equipment, fed with high quality parts, gives a higher quality finished product than manual assembly.  The consistency of an automatic system, aided by high quality parts creates a high quality product.

Dedicated assembly systems, compared with flexible assembly systems, are generally intolerable of defective parts.  It has proved effective to install a 100% inspection station prior to the workhead to maintain up-time in some installations.

The ease with which a part can be presented to a workhead automatically at the required feed rate, and assembled within the specified cycle time, depends upon the; design of the part, design of the equipment, and the method of assembly. Each element of assembly automation has it's own maximum performance characteristic. For example, the cycle time of a pick and place unit depends upon the degrees of freedom, the actuator stroke and the actuating medium, i.e. compressed air, hydraulic oil, DC servo motor. A feeder  is limited by its maximum conveying velocity and the insertion process is affected by the positional accuracy of the rotary table, robot, or platen location.

The product designer must analyse the product and consider if it can be economically assembled in it's present state, or decide if design changes are necessary for automatic assembly.  In practice, the majority of products assembled manually require design changes to make automatic assembly viable.  The product designer must consider what further benefits can be gained from more re­designing of design features, assembly operations, or even the elimination of parts.  The process of design for automatic assembly is best effected by a systematic approach. A structured method for evaluating designs to identify inefficient features has been developed by UMass and other organisations.

An automatic feeding device and, at least, one workhead is required for each component to be automatically assembled into a part-built product.  A significant reduction in cost is achieved by eliminating a part from a product.  Designers should strive for the irreducible number of separate components per assembly, consistent with its performance and fitness for purpose.  An investigation into the function of the product exposes redundant parts and these should be eliminated.

Fasteners, which are separate from the component being secured, should be avoided.  Fastening technologies of the future are based on adhesives, ultrasonic welding, soldering, resistance welding, clip fastening, and twisted tab joining.  Fasteners can be classed as being permanent or semi-permanent. Permanent fasteners do not permit removal, e.g. adhesives.  Semi-permanent fasteners do permit removal, e.g. screws.

The range of plastic snap-in fasteners is classed as permanent or semi-permanent, as they can be removed with the aid of special tools.  It is not feasible to repair products with these permanent joints.  This leads to 'throw away' products, upon a fault occurring, unless self-contained sub-assemblies are used in standard modules.

Parts integration dictates that groups of components should, where possible, be manufactured as a single part by chip-less forming, e.g. precision die-casting, precision plastic moulding, powder metallurgy, investment casting, fine blanking, and high energy rate forming. These methods produce, in one single operation, the features of a number of parts without the use of fasteners.  Also, features for identification by the automatic bowl feeder tooling can be cast into the part.

Each part in an assembly serves a purpose with the aid of functional features.  The designer should group a number of parts together to form a single part with multi-functional features.

Create a precedence diagram for the assembly operations. Identify redundant areas of the operation and incorporate the functional parts of these operations into other operations.

Ensure that each part is fully and correctly specified for dimension, function, quality, material, and shape.  Don't accept supplier parts outside specification.  These parts may be incorrectly accepted on the grounds that they perform the same function, and yet they may not be acceptable to the automatic feeder tooling, workhead or fixturing.

Minimize the variation in component and product designs.  Minimizing the variation in part designs reduces the numbers of feeders used and enables standardisation of gripper and fixture design.  Minimizing variations reduces changeover times and enables standard magazines and packs to be used.

An existing product will be designed for production by manual assembly.  It is unlikely that an automatic assembly system will accept the part designs for a product currently assembled manually. The product needs to be re-designed to suit the machine principles, e.g. positional accuracy of a pick and place unit.

If a part-built assembly needs moving during assembly then problems arise if all parts are not located.  During manual assembly of a product, the operations are structured so that transportation only occurs with stable assemblies. This is achieved by assigning two or more parts to the assembly worker, or enough parts that are required, to create a stable structure. The operative, using two hands, holds the unstable part whilst assembling the part required to complete the operation.  An example of an operation such as this is where an assembly worker holds down a spring with one hand, prior to assembly of a spring retainer with the other hand.  This type of operation is difficult to perform automatically and should be re-designed so that each part is self-locating.

Design the product with many sub-assemblies.  Each sub-assembly should be common to all product styles.  Product variation can then be created in the final assembly of the product.  Sub-assembly work centres give a greater overall efficiency of the assembly system, in conjunction with buffer storage.  This is achieved by using a free transfer line or by intermediate storage systems.

The feeding of a part to an automatic workhead is by components in bulk random orientation or structured orientation.  Methods of feeding are usually determined by the part characteristics and required feed rate.  All feeders are classified as being; automatic, magazine, final parts forming stage, or manual.  It is uneconomic, or impossible, to feed certain parts automatically and these are not fed by automatic feeders. Flexible gaskets, open ended springs and acute angled cones are examples of such parts.  Large parts, parts having no symmetry, and delicate parts (e.g. with print face) cannot be fed automatically, but may be fed by magazines.  Relatively simple parts, with a degree of symmetry or definite asymmetry, can be fed by automatic feeders.

The most common form of automatic parts feeder is the vibratory bowl feeder. Other automatic feeders include the hopper feeder, centrifugal hopper, barrel hopper, magnetic feeder, and elevating hopper feeder. These parts feeders are specialized and the feeding mechanism is designed to handle unique parts. The device which converts the bulk random orientation of parts into flow of orientated parts in these automatic feeders often takes the form of a fork or blade.

The movement of parts in a vibratory bowl feeder is created by a drive unit which induces vertical and angular vibration to the bowl.  Parts are momentarily caused to leave the track surface during the vertical phase of vibration.  The angular vibration moves the track and the part falls onto a track portion beyond the initial position of the part.  Incorrectly orientated parts are rejected at the bowl tooling stage by passive devices and returned to the bowl base. These mechanisms of operation make the bowl feeder unsuitable for automatic feeding of parts dosed with viscous fluids because the vertical vibrational force is not sufficient to lift the part from the track, due to the adhesion of the viscous fluid.  The continuous rejection of parts at the tooling stage and the vibration of the track against the part tends to damage delicate parts. Parts with print faces and delicate projections are particularly affected.

Once it has been determined that a part can be fed by a bowl feeder, the remaining consideration is the maximum feed rate that can be obtained from the feeder.  The feed rate of the part must be within the cycle time of the complete assembly operation.  This rate, for a given conveying velocity, depends upon the physical size of the component and the features for orientation.  For parts symmetrical about all axes, e.g. cubes, spheres, every part will leave the feeder 'first time through', or 100% of the parts will leave the bowl because no tooling is required.  This 'first time through' rate is the measure of a part's efficiency at being fed automatically.  These parts will always be correctly orientated and ready for insertion.  Components having little symmetry have a much lower tooling efficiency and it can be difficult to achieve the required feed rate using one bowl feeder with passive tooling.

Parts feeding is the most difficult area of assembly automation.  If the part can be automatically fed and orientated to the workhead then it can usually be assembled.

The final parts forming stage can be used as a method for feeding difficult parts.  The parts are usually fed in bulk on strips and pressed or guillotined, before being fed to the workheads.  The strips are easy to handle and orientation can be better controlled.

Parts are orientated by tooling, inside or outside of the bowl feeder.  In-bowl tooling tends to be passive and relies on the probability of a correctly orientated part moving along the conveyor track.  Incorrectly orientated parts are detected by the bowl tooling and deflected back to the bowl base.  Correctly orientated parts are accepted by the tooling and presented to the workhead.  Active tooling accepts parts in more than one orientation and re-orientates them correctly for the workhead.  This method of tooling gives a 100'first time through\' rate and can be used outside the bowl.

It\'s important to make parts as symmetrical as possible.  Higher feed rates are obtained with parts having greater degrees of symmetry.  This is achieved by duplicating non-productive features.  If it is not feasible to make a part symmetrical then it must be designed to be definitely asymmetrical.  Features which are too small to be detected by bowl tooling must be exaggerated, for them to be detected.  The use of cylinders having a length to diameter ratio of unity and rectangles having slightly dis-similar sides should be re­designed to give greater asymmetry.

The most efficient insertion processes are those from vertically above, in a straight line movement.  Most assembly processes take this form.  If this is not the case with a part then it should be examined to see if the action can be simplified.  Simple insertion processes need low cost workheads.  This is because more complex operations require more degrees of freedom.  Each degree of freedom needs an individual pneumatic, hydraulic, or DC Servo motor and, therefore, the cost increases accordingly.  The cycle time of the operation also increases.

Summary

Increases in productivity can be realized by re-designing an existing product for automatic assembly.  Component re-design is more beneficial than assembly system re-design. All of the design considerations mentioned are related to the three main rules for design for automatic assembly :

1.Use the least number of parts
2.Obtain low feeding and orientation difficulty levels
3.Use simple insertion operations