As a greater demand for a larger variety of products grows, and the increase in global
competition for world markets increases, the efficiency in design, production, and delivery
becomes critical for the survival of manufacturing firms. Flexibility has become the key
ingredient for success and has been possible largely due to advances in computer
technology. In this module we will begin with a broad overview of the functions of CAD,
CAM, and Manufacturing followed by the comparison of production systems, and finally,
an introduction to programmable automation. This topic will be the focus for the
remaining class meetings and will deal primarily with control at the machine and
Production Systems Overview: In its basic form, a production process is simply a system
that converts raw materials into useful products. Usually the system approach is either
changing the properties of a raw material or combining different components to make a
final product that is acceptable to the customer. However the manufacturing cycle,
coordination of data, and system control can become complex across different types of
Two main factors must be established before a firm can gain significant ground in picking
up a share of the market and remaining competitive. Efficiency and Flexibility. In other
words, the raw materials must be available in time (theoretically no sooner or no later
than needed) and flexibility to rapidly change production from one product to another
must be in place. The greatest influence in how these factors come about in the integration
of computers for design (CAD) manufacturing, (CAM) and control (e.g. CNC and PLC).
CAD to CAM to Manufacturing. The basic functions are shown below:
Material Requirements & FEA
Testing and Simulation
Scheduling and Control
Production and System Control
Production systems are classified by the arrangement of departments and processing
within manufacturing facilities. While many variations can exist, typically there are three
major classifications including: 1) Job Shop, 2)Batch Production, and 3) Mass
Production. A brief overview along with advantages and disadvantages will be presented.
Job Shop. This type of system is highly customized and produces many different types of
products, with very low volume for a given product. A variety of general purpose
equipment is used in this environment. Generally speaking, a high degree of skill is
required of operators. Machines are typically grouped by type and is referred to as process
A job shop can produce a wide variety of products however, scheduling becomes very
difficult to manage from a flow stand point. To give an example of how things can get
complicated in a hurry, consider m number of parts being routed through n number of
machines. Suppose there are 4 different parts that must be routed through 8 machines.
The possible sequences of routing becomes mn or 4 8
= 65,536 possible sequences. It is
quite easy to see how this can be a scheduling nightmare.
Batch Production. When products are manufactured in limited quantities, it is referred to
as batch production. This type of system is more suited for intermediate size quantities,
but those that are not sufficient to warrant a dedicated production line per product.
Typically the production capacity is greater than the demand, and products are produced
then stored in inventory. Safety stock levels are generated to meet the current and future
demand, then production is switched to the next item to be produced to meet scheduling
demand. Production equipment and processing machines in a batch environment are
more specialized than the job shop. However; the skill level required is decreased. Varying
levels of automation exist, and typically machines are arranged in a manner to conform to
the products. This type of layout is called product layout.
Mass Production. The mass production system is strictly for high volume and virtually no
flexibility. A production line or even an entire plant is dedicated to producing only one
product. Hard automation is employed since no changeover is required. Mass production
is capital intensive and requires specialized tooling, jigs, and fixtures. A high and constant
demand is a must for mass production to pay for the capital invested. Work is moved
between stations, and the production line is balanced to maximize the rate of production.
Labor skills are reduced to a minimum, making work on an assembly line repetitious.
Thus the workstations are good candidates for automation. Two common terms associated
with mass production are assembly line and flow line. In assembly line production
systems workstations are sequential, and parts are usually moved using conveyors or high
speed material handling equipment. Flow line production is usually associated with
processes that are continuous such as paper, petro-chemicals or continuous casting steel
FMS. Across all manufactured goods, batch production is by far the most prevalent. It
has been estimated that 95% of all manufactured goods are produced in lot sizes less than
an average of 50 parts. Obviously processes layout cannot accommodate high volume, and
mass production is not capable of quick changeover. Therefore, cellular or flexible
manufacturing systems are require. In this type of system, machines are arranged in a
manner to accommodate a "family" of parts within a given group. This is called group
technology. By processing similar parts through a cell, set-up times are reduced and
throughput times improved. Duplication of equipment and tooling is also reduced.
The diagram shown below illustrates a comparison of job shop, FMS and transfer lines with
respect to volume and flexibility.
With increased competition and the need for flexibility, automation has become a major
factor in the success of many manufacturing firms. Two common types of automation at
the "workstation" level are CNC and PLC controls. We will look at the basics of how CNC
machines are controlled in this module, and continue with PLC in module 8.
CNC. Computer Numerical Control has it's roots all the way back to the Jacquard Loom.
However it was in the late 1950's that numerical controlled machines tools found their way
into American industry. The most prevalent application of CNC is in metal cutting in
processes such as the the following: