Transfer machining is a system of automatic machining wherein a number of different types of machine tools are arranged across the transfer line (conveyors) in a series in the sequence of operations to be performed on a workpiece. Machine tools are linked together by means of mechanized handling and transfer equipment, by means of which the workpieces are automatically transferred from one machine to another at proper interval. Each particular machine tool in series, called a station or machining head, performs the same operation. repeated on every part and all the stations operate simultaneously on different workpieces. The number of workpieces being machined at a time equals the number of stations. Loading of the blanks is done at one end of the transfer line (or system) and machined components are collected at the other end of the transfer line.
Each station is a simple work head (or) machine head) which is a self-contained unit, each performing a set of different operations such as drilling unit, milling unit, etc. Each such unit is mounted on columns whose base is. capable of sliding over the guide ways (made radial to the transfer line) to move the work head backwards or forwards feed the tool to the work according to the setting of the tool with respect to the work. Rollovers are provided on the transfer line or system for rotating the work through 90° or 180° during their passage through the line from station to station. Thus, the complete unit, comprising a number of transfer mechanism and controls, etc. is termed transfer machine.
Transfer machining ideally suits to machining of identical parts which require the same sequence of operations to be performed repeatedly. The initial cost of installation is very high. Transfer machining is largely employed for lighter operations. such as drilling, boring, spot facing, reaming, lapping, etc. The method is used by motor industries, farm machinery, aircraft industry and many other heavy industries. Transfer machining has registered increase in production by over 30% besides bringing saving in floor area and the number of machine tools.
In brief, the transfer machines, often called automated machines, complete a series of machining operations at successive stations and transfer the work from one station to the next. In effect, they are the production line of connected machine tools that are synchronized in their operation such that the work after being loaded at the station progresses automatically through various machining heads (or stations) to its completion. These machines perform operations such as machining, inspecting, quality control functions, etc. Transfer machines. are also called automated flow line or automated transfer line.
A transfer machine consists of (i) central bed, (ii) machining heads, (iii) automatic work loading/unloading and transfer mechanism, (iv) locating and clamping devices, and (v) chip disposal device.
Central bed is the basic unit of the transfer line. It may be a fabricated or cast structure or platform which may be straight, circular or curved. Guide ways are provided on the upper surface of the bed for guiding and supporting the workpieces (in their pallets or fixtures) while they are transferred from station to station. The bed also carries turnover devices to rotate the workpiece, during transfer, whenever needed, for the next operation. A swarf conveyer is also fitted in the bed. Another conveyer may be installed for collection of empty pallets or fixtures after unloading the completed work.
Machining heads are specially designed machine tools arranged by the side central bed. These are self-contained working units and can be moved forwards for backwards to feed the tool to the work.
Work loading and transfer devices of great variety are used on the transfer machines. Various lifting and loading devices are used for handling the workpieces. Similarly for transferring the workpiece from one station to another station, pawl type transfer system, rotating bar and fingers system, and walking beam system are among the prominent types of transfer devices. Large and flat parts can often rest directly on the surface of the table; the job is usually mounted on pallets or holding fixtures with clamps.
Locating and clamping devices of large variety are used according to the type of transfer machine. In rotary table type and drum type machines, workpieces are held in stationary fixtures mounted on rotary table or periphery of drum. In case of in-line transfer machines, workpieces are slided directly on roller conveyor or held in stationary fixtures or pallets which are transferred from station to station.
Spring loaded a hydraulically or pneumatically operated dowels or plungers are used at the work station for locating the workpiece (or fixtures or carrying workpieces). A positive clamping of workpiece is ensured through hydraulic pneumatic devices.
Chip or swarf disposal is achieved through a conveyor (fitted under thin bed) which continues to run through out to carry fallen swarf away.
Types of Transfer Machines
The three main types of transfer machines are given in the following:
(a) In-line transfer machine
(b) Rotary indexing table transfer machine
(c) Drum type transfer machine
In-line transfer machine is schematically illustrated. In this system, workpieces held in pallets (which are special fixtures) are transferred or conveyed from station to station. The machine locates and clamps the workpiece in fixture at each station which is followed by the rapid approach of the tools to the work. Tools are automatically fed to the work throughout the cutting cycle. After the machining is over, the tool retracts and the work is unloaded and shifted to the next station.
Rotary indexing table transfer machine is a more compact machine. Workpieces are held in fixtures on a rotating circular table which brings the workpieces under different machining heads or stations. The table is horizontal and moves about a vertical axis. An indexing mechanism is provided under the table to index the table equally each time at fixed intervals. The system is compact and allows the workpieces to be loaded and unloaded at a single location without having to interrupt the machining.
Drum type transfer machine consists of work fixtures fastened to the outside surface of a rotating drum and the workstations are positioned radially around the drum at equal intervals.
Advantages and Disadvantages
The following are the advantages and disadvantages of using transfer machines:
- Higher production rates in comparatively lesser cost
- Better accuracy in dimensions of manufactured parts
- Require less floor area (using particularly rotary indexing type machine)
- Heavy jobs with awkward size and shape are easily handled and machined
- Reduced operator’s number
- Tool life increased
- Reduced cycle time as the machine is fully automatic
- High initial investment
- Complex electrical controls
- A breakdown of one machine means stoppage of the whole line
- Limited only to high production jobs
- High reshuffling and overhauling cost
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