NUMPON MAHAYOTSANUN DEPARTMENT OF MECHANICAL ENGINEERING KHON KAEN UNIVERSITY
Manufacturing consists of a large number of interdependent activities with distinct entities, such as, materials, tools, machines, controls, and people. Consequently, manufacturing should be regarded as a large and complex system, consisting of numerous diverse physical and human elements. In a manufacturing system, a change or disturbance anywhere in the system requires that it adjusts itself system-wide in order to continue functioning effectively or efficiently. The manufacturing system must also be capable of producing the modified product on a short lead time and, preferably, with relatively small major capital investment in machinery and tooling.
Automation is generally defined as the process of hav-
On the other hand, machinery in job shops generally requires skilled labor, and the production quantity and rate are low; as a result, the cost per part can be high. Quantities for small-batch production typically range from 10 to 100, using general-purpose machines and machining centers.
Production quantity or volume is crucial in determining the type of machinery and the level of automation required to produce parts economically. Total production quantity is defined as the total number of parts to be produced. Production rate is defined as the number of parts produced per unit time. Small quantities can be manufactured in job shops. These operations have high part variety, meaning that different parts can be produced in a short time without extensive changes in tooling and in operations. On the oth-
Batch production usually involves lot sizes between 100 and 5000 and utilizes machinery similar to that used for small-batch production, but with specially designed fixtures for higher production rates. Mass production generally involves quantities over 100,000 and requires special-purpose machinery and automated equipment for transferring materials and parts.
ing machines follow a predetermined sequence of operations with little or no human involvement, using specialized equipment and devices that perform and control manufacturing processes.
Schematic illustration of the components of (a) an open-loop, and (b) a closed-loop control system for a numerical control machine. (DAC is digital-to-analog converter.)
Numerical control (NC) is a method of controlling the
movements of machine components by directly inserting coded instructions, in the form of numbers and letters, into the system. The system automatically interprets these data and converts them to output signals. These signals, in turn,
In adaptive control (AC), the operating parameters automatically adapt themselves to conform to new circumstances, such as changes in the dynamics of the particular process and any disturbances that may arise. This approach is basically a feedback system. In manufacturing operations
LEFT: Schematic illustration of the major components of a numerical control machine tool. BOTTOM: Schematic illustration of the application of adaptive control (AC) for a turning operation.
control various machine components. In computer numerical control (CNC), the control hardware follows directions received from local computer software. Computer numerical control is a system in which a control microcomputer (onboard computer) is an integral part of a machine or piece of equipment. The machine operator can easily and manually program the onboard computer, can modify the programs directly, prepare programs for different parts, and store the programs.
adaptive control helps (a) optimize production rate, (b) optimize product quality, and (c) minimize production costs. Application of AC in manufacturing is particularly important in situations where workpiece dimensions and quality are not uniform, such as a poor casting or an improperly heattreated part.
(a) A self-guided vehicle (Tugger type). This vehicle can be arranged in a variety of configurations to pull caster-mounted cars; it has a laser sensor to ensure that the vehicle operates safely around people and various obstructions. (b) A self-guided vehicle configured with forks for use in a warehouse.
Material handling is defined as the functions and sys-
tems associated with the transportation, storage, and control of materials and parts in the total manufacturing cycle of a product. During this cycle, raw materials and parts (workin-progress) are typically moved from storage to machines,
entation (c) path conditions (d) level of automation and control (e) operator skill (f) economic considerations. 2. Equipment. Several types of equipment can be used to move materials: conveyors, rollers, monorails, carts, forklift trucks, automated guided vehicles, and various mechanical, electrical, magnetic, pneumatic, and hydraulic devices and manipulators. 3. Automated guided vehicles (AGV). This transport system has high flexibility and is capable of efficient delivery to different workstations. AGVs are guided automatically along pathways with in-floor wiring or tapes or fluorescentpainted strips. Some systems may require additional operator guidance. Autonomous guidance involves no wiring or tapes and uses various optical, ultrasonic, and inertial techniques with onboard controllers. Routing of the AGV can be controlled and monitored from a central computer. 4. Coding systems. Various coding systems have been developed to locate and identify parts and subassemblies throughout the manufacturing system and to correctly transfer them to their appropriate stations: (a) bar coding (b) magnetic strips (c) radio frequency (RF) tags (d) Acoustic waves, optical character recognition, and machine vision.
from machine to machine, from inspection to assembly and to inventory, and finally shipment. Material handling operations must be repeatable and reliable. Important aspects of material handling are summarized below: 1. Methods of material handling. Several factors must be considered in selecting an appropriate material-handling method for a particular manufacturing operation: (a) shape, size, weight, and characteristics (b) Distance, position, ori-
LEFT: Automated guided vehicle (AGV)
(a) Schematic of a six-axis KR-30 KUKA robot; the payload at the wrist is 30 kg and repeatability is ±0.15 mm (±0.006 in.). The robot has mechanical brakes on all of its axes. (b) The work envelope of the KUKA robot, as viewed from the side.
An industrial robot has been defined as a reprogrammable multifunctional manipulator designed to move materials, parts, tools, or other devices by means of variable programmed motions and to perform a variety of other tasks. Here are the basic components of an industrial robot:
used end effectors and are equipped with two or more fingers. 3. Power supply. Each motion of the manipulator, in linear and rotational axes, is controlled and regulated by indepen-
Various devices and tools that can be attached to end effectors to perform a variety of operations.
1. Manipulator (arm and wrist). The manipulator is a mechanical unit that provides motions (trajectories) similar to those of a human arm and hand, using various devices such as linkages, gears, and joints. 2. End effector. The end of the wrist in a robot is equipped with an end effector, also called end-of-arm tooling, end effectors can be custom made to meed special handling requirements. Mechanical grippers are the most commonly
dent actuators that use an electrical, pneumatic, or hydraulic power supply; each has its own characteristics, advantages, and limitations. 4. Control system. The control system is the brain of a robot. The control system is the communications and information processing system that gives commands for the movements of the robot; it stores data to initiate and terminate movements of the manipulators.
A toolholder equipped with thrust-force and torque sensors (\it smart tool holder), capable of continuously monitoring the machining operation.
A sensor is a device that produces a signal in response to detection or measurement of a specific quantity or a property. Analog sensors produce a signal, such as voltage, that is proportional to the measured quantity. Digital sensors have digital (numeric) outputs that can directly be trans-
and permeability. 4. Thermal sensors, which measure temperature, flux, conductivity, and specific heat. 5. Acoustic, ultrasonic, chemical, optical, radiation, laser, and fiber-optic sensors.
LEFT: A robot gripper with tactile sensors. BOTTOM: Applications of machine vision.
-fered to computers. Analog-to-digital converters (ADCs) are used for interfacing analog sensors with computers. Sensors may be classified as follows: 1. Mechanical sensors, which measure such quantities as position, shape, velocity, force, torque, pressure, vibration, strain, and mass. 2. Electrical sensors, which measure voltage, current, charge, and conductivity. 3. Magnetic sensors, which measure magnetic field, flux,
Sensors are also classified as follows: 1. Tactile sensing involves the continuous sensing of varying contact forces, commonly by an array of sensors. 2. Visual sensing (machine vision; computer vision) involves cameras that optically sense the presence and shape of an object. 3. Smart sensors have the capability to perform a logic function, conduct two-way communication, and make decisions and take appropriate actions.
PAGE 12: Components of a modular workholding system. PAGE 13: Schematic illustration of an adjustable-force clamping system. The clamping force is sensed by the strain gage, and the system automatically adjusts this force.
Fixtures are generally designed for specific purposes;
clamps are simple functional devices; jigs have various reference surfaces and points for accurate alignment of parts and tools and are widely used in mass production. These devices may be used for actual manufacturing operations, or they may be used to hold workpieces for purposes of measurement and inspection, where the part is not subjected to any forces. Workholding devices have certain ranges of capacity; for example, (a) a specific collet can accommodate rods or bars only within a certain range of diameters; (b) four-jaw chucks can accommodate square or prismatic workpieces of various sizes; and (c) other devices and fixrtures are designed and made for specific workpiece shapes and dimensions and for specific tasks, called dedicated fixtures. The emergence of flexible manufacturing systems has necessitated the design and use of workholding devices and fixtures that have built-in flexibility. 1. Modular fixturing. Modular fixturing is often used for small or moderate lot sizes. These modular fixtures are usually based on tooling plates or blocks configured with grid holes or T-slots upon which a fixture is constructed. A number of other standard components, such as locating pins, adjustable stops, workpiece supports, V-blocks, clamps, and springs, can be mounted onto the base plate or block to quickly produce a fixture. 2. Tombstone fixtures. Also referred to as pedestal-fixtures, tombstone fixtures have between two and six vertical faces onto which parts can be mounted. Tombstone fixtures are typically used in automated or robot-assisted manufacturing; the machine tool performs the desired operations on the part or parts on one face, then flips or rotates the tombstone to begin work on other parts. These fixtures allow feeding more than one part into a machine but are not as flexible as other fixturing approaches. 3. Bed-of-nails device. This fixture consists of a series of
air-actuated pins that conform to the shape of the external part surfaces. Each pin moves as necessary to conform the shape at its point of contact with the part; the pins are then mechanically locked against the part. The fixture is compact and has high stiffness and is reconfigurable. 4. Adjustable-force clamping. In this system, referred to as an adjustable-force clamping system, the strain gage attached to the clamp senses the magnitude of the clamping force; the system then adjusts this force to keep the workpiece securely clamped to the workpiece. 5. Phase-change materials. There are two basic methods to hold irregularly shaped or curved workpieces in a medium, other than hard tooling: (a). A low-melting-point metal is used as the clamping medium. For example, an irregularly shaped workpiece is partially dipped into molten lead and allowed to set. After setting, the assembly is clamped in a simple fixture. (b). The supporting medium is either a magnetorheological (MR) or electrorehological (ER) fluid. In the MR application, magnetic particles are suspended in a nonmagnetic fluid. Surfactants are added to maintain dispersal of powders. After the workpiece is immersed in the fluid, an external magnetic field is applied, whereby the particles are polarized and the behavior of the fluid changes from a liquid to a solid. After the part is processed, it is retrieved after removing the external magnetic field. In the ER application, the fluid is a suspension of fine dielectric particles in a liquid with a low dielectric constant. After applying an electrical field, the liquid becomes a solid.
Transfer systems for automated assembly: (a) rotary indexing machine, and (b) in-line indexing machine.
Some products are simple and have only two or three components to assemble. Most products, however, consist of many parts, and their assembly requires considerable care and planning. There are three basic methods of assembly: manual, high-speed automatic, and robotic.
matic mode. However, a breakdown of one station will shut down the whole assembly operation. (b) Nonsynchronous systems. Each station operates indepndently, and any imbalance in product flow is accommodated in storage (buffer) between stations. The station
Examples of guides to ensure that parts are properly oriented for automated assembly.
1. Manual assembly uses simple tools and is generally economical for relatively small lots. 2. High-speed automated assembly uses transfer mechanisms designed specifically for assembly operations. (a) Synchronous systems, also called indexing systems. Individual parts and components are supplied and assembled at a constant rate at fixed individual stations. These systems can operate in either a fully automatic mode or a semiauto-
continues operating until the next buffer is full or the previous buffer is empty. Also, if for some reason one station becomes inoperative, the assembly line continues to operate until all the parts in the buffer have been used up. (c) Continuous systems. The product is assembled while moving at a constant speed on pallets or similar workpiece carriers. The parts to be assembled are brought to the product by various workheads, and their movements are synchronized with the continuous movement of the product.
COMPUTER-INTEGRATED MANUFACTURING (CIM)
A schematic illustration of a computer-integrated manufacturing system.
Computer-integrated manufacturing is a broad term
used to describe the computerized integration of product design, planning, production, distribution, and management. Computer-integrated manufacturing systems consist of subsystems that are integrated into a whole. These sub-
as the input to another subsystem. An effective computerintegrated manufacturing system requires a single, large database that is shared by members of the entire organization. A database typically consists of the following information: 1. Product data (part shape, dimensions, tolerances, and
A schematic illustration of a computerintegrated manufacturing system.
systems consist of the following: 1. Business planning and support. 2. Product design. 3. Manufacturing process planning. 4. Process automation and control. 5. Factory-floor monitoring systems. The subsystems are designed, developed, and implemented in such a manner that the output of one subsystem serves
specifications). 2. Data management attributes (creator, revision level, and part number). 3. Production data (manufacturing processes used in making parts and products). 4. Operational data (scheduling, lot sizes, and assembly requirements). 5. Resources data (capital, machines, equipment, tooling, and personnel, and the capabilities of these resources).
COMPUTER-AIDED DESIGN AND ENGINEERING (CAD/CAE)
A schematic illustration of a computer-integrated manufacturing system.
Computer-aided design (CAD) involves the use of
computers to create design drawings and geometric models of products and components and is associated with interactive computer graphics, known as a CAD system. Computer-aided engineering (CAE) simplifies the creating
Computer-aided manufacturing (CAM) involves the
use of computers and computer technology to assist in all phases of manufacturing, including process and production planning, scheduling, manufacture, quality control, and management. Because of the obvious benefits, computer-
Various type of modeling for CAD
of the database by allowing several applications to share the information in the database. These applications include, for example, (a) finite-element analysis of stresses, strains, deflections, and temperature distribution in structures and load-bearing members, (b) the generation, storage, and retrieval of NC data, and (c) the design of integrated circuits and various electronic devices.
-aided design and computur-aided manufacturing are often combined into CAD/CAM systems. This combination allows information transfer from the design stage to the planning stage for the manufacture of a product, without the need to manually reenter the data on part geometry.
(a) Two parts with identical geometries but with different manufacturing attributes. (b) Four parts with similar manufacturing attributes but with different geometries.
Many parts have certain similarities in their shape and in their method of manufacture. Group technology (GT) is a concept that seeks to take advantage of the design and processing similarities among the parts to be produced. In group technology, parts are identified and grouped into
d. Surface finish. e. Part function. 2. Manufacturing attributes pertain to similarities in the methods and the sequence of the operations performed on the part. The manufacturing attributes of a particular
(a) Functional layout of machine tools in a traditional plant; arrows indicate the flow of materials and parts in various stages of completion. (b) Group-technology (cellular) layout.
families by classification and coding (C/C) systems. This process is a critical and complex first step in GT, and it is done according to the partâ€™s design and manufacturing attributes: 1. Design attributes pertain to similarities in geometric features and consist of the following: a. External and internal shapes and dimensions. b. Aspect ratio.
component consist of the following: a. Primary processes. b. Secondary processes and finishing operations. c. Process capabilities, such as dimensional tolerances and surface finish. d. Sequence of operations performed. e. Tools, dies, fixtures, and machinery. f. Production volume and production rate.
Schematic view of an attended flexible manufacturing cell, showing various machine tools and an inspection station. Note the worker positions and the flow of parts in progress from machine to machine.
The concept of group technology can effectively be implemented in a cellular manufacturing, which consists of one or more manufacturing cells. A manufacturing cell is a small unit consisting of one or more workstations. A workstation typically contains either one machine or sev-
tomated system. FMS consists of a number manufacturing cells, each containing an industrial robot and an automated material-handling system, all interfaced with a central computer or fileserver. The flexibility of FMS is such that it can handle a variety of part configurations and produce them
A schematic illustration of a flexible manufacturing system, showing two machining centers, a coordinate measuring machine, and two automated guided vehicles.
eral machines, each of which performs a different operation on a part. The significant benefits of cellular manufacturing are (a) the economics of reduced work in progress, (b) improved productivity, and (c) the ability to readily detect product quality problems right away. In a flexible manufacturing system (FMS), all major elements of manufacturing are integrated into a highly au-
in any order. This highly automated system is capable of optimizing each step of the total manufacturing operation.
In traditional manufacturing operations, the parts are made in batches, placed in inventory, and used when necessary. This approach, known as a push system, means that parts are made according to a schedule and are in inventory to be used if and when they are needed. In contrast, just-in-
fective parts or process variation. Adjustments can then be made rapidly to make a uniform, high-quality product. The just-in-time production (JIT) concept has the following goals: 1. Receive supplies just in time to be used.
time manufacturing is a pull system, meaning that parts
2. Produce parts just in time to be made into subassemblies. 3. Produce subassemblies just in time to be assembled into finished products. 4. Produce and deliver finished products just in time to be sold.
are produced to order, and the production is matched with demand. Consequently, there are no stockpiles, and the extra motions and expenses involved in stockpiling parts and then retrieving them from storage are eliminated. Moreover, parts are inspected in real time either automatically or by the worker as they are being manufactured and are used within a short period of time. In this way, control is maintained continuously over production, immediately identifying de-
Lean manufacturing is a systematic approach to iden-
anced work loads, quality problems, and unplanned maintenance. 3. Maximizing the efficiency of workers at all times. 4. Eliminating unnecessary processes and steps, because they represent costs.
of customer, and optimizes processes to maximize added value. The focus of lean production is on the entire process flow and not just the improvement of one or more individual operations. Typical wastes to be considered and reduced or preferably eliminated include the following: 1. Eliminating inventory (using JIT methods) because inventory represents cost, leads to defects, and reduces responsiveness to changing market demands. 2. Eliminating waiting time, which may be caused by unbal-
5. Minimizing or eliminating product transportation, because it represents an activity that adds no value. 6. Performing time and motion studies to identify inefficient workers or unnecessary product movements. 7. Eliminating defects.
tifying and eliminating waste in every area of manufacturing, through continuous improvement, and emphasizing product flow in a pull system. Lean production requires that a manufacturer review all its activities from the viewpoint
- S. Kalpakjian, S. R. S. (2003). Manufacturing Engineering and Technology. New Jersey, Pearson Prentice Hall. - Groover, M. P. (2010). Principles of Modern Manufacturing: Materials, Processes, and Systems, John Wiley & Sons Ltd.
Published on Jan 21, 2012