Manufacturing Production Systems

Purpose:

The purpose of this module is to provide an overview of various production systems and layouts commonly used in manufacturing  operations.  Specifically, the module will cover different types of layouts, production systems, and an introduction to automation and control at the shop floor level.  The intention is to provide a basic understanding of different systems of production and how production machines are controlled.

Objectives:  After completing this module, you should be able to do the following:

     1.  Develop a relational diagram of production systems;

     2.  Explain the differences between Transfer Lines, FMS, Job Shops;

     3.  Differentiate between process layout, product layout; and cellular layout;

     4.  Identify the different levels of control systems in manufacturing;

     5.  Identify the "components" of a CNC system;

     6.  Define CNC "words"

     7.  Define the coordinate system for CNC control;

     8.  Develop a basic CNC program;

Terms:

CAD
CAM
CAE
Transfer Line
Flexible Manufacturing Systems
Job Shop
Process Layout
Product Layout
Cellular Layout
CNC
APT
Word Address
EIA RS-274 D Standard
 

INTRODUCTION

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 workstation level.

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 production systems.

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:
 

CAD

 Engineering Documentation
 Process Requirements
 Material Requirements & FEA
 Testing and Simulation
                                                                                    CAM

Production Engineering
   Tool Design
   NC/Control
   Process Planning
   GT Planning

Manufacturing Engineering
    Scheduling and Control
    Production Planning

Quality Engineering
    Process Capability
    Reliability

                                                                                                                                                              MANUFACTURING
Fabrication
Assembly
Quality Assurance
Production and System Control
PRODUCTION SYSTEMS

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 layout.



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 mills.
 
 

 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.

PROGRAMMABLE AUTOMATION

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:

Milling
Drilling
Boring
Turning
Grinding
Sawing
EDM
However the same technology is used in a wide variety of processes such as:
 
Automated knitting machines in textile production
Glass cutting
Automated Riveting
Plotters and printers
Tube Bending
Welding
Blanking and Stamping
A CNC system consists of three essential parts:

                      1. Program

                      2. Controller

                      3.  Machine Tool

In the following section the theory of control, using an open loop stepper motor system will be explained.   The coordinate system and theory of control will be covered using vertical milling as an example.

A CNC machine tool has a dedicated controller for each axis of movement.  In milling, the part is fed into a rotating cutter typically with at least 3 axis of control.  A diagram showing the coordinate system for a 3 axis, vertical milling machine is shown below.



The program provides the instruction set for controlling the axis of movement and particular auxiliary or miscellaneous function.  While several languages are used, the most common over the years has been
EIA  RS-274 -D Standard.  This standard is based on word address format and "G" codes for prepatory functions, and "M" codes for miscellaneous functions such as coolant on/off, spindle on/off.   NC words are categorized as:

               "N" words (sequence numbers)
                "G" words (prepatory words  e.g.  G00 - Rapid,  G01 - Linear, G02 Circular CW,
                        G03 Circular  CCW, G90 Absolute Positioning)
                "X, Y, Z " words  - Addressing the axis of movement
                "F" words (feed rate)
                "S" words (cutting speed)
                "T" words (tool change)
                "M" words (miscellaneous auxiliary functions such as M03 - Spindle on CW, M05 Spindle Stop,
                         M30 End of Program

The following example will show a word address CNC program to machine the following tool piece part.
 

Before the tool path can be defined, offset dimensions must be determined.  The tool path will be located parallel to the part geometry, and offset the distance corresponding to the radius of the cutter.  For this example, a 1/2 inch diameter, 2 fluted end mill is assumed.    Manual calculations are tedious when angular cuts are required.  Fortunately, there are options available to make this process relatively easy.  This include calculated assist routines in CNC programming software,  cutter compensation routines built into the controller (by using specific G-Codes), or translation from the CAD drawing using a CAM software package.


Assuming the 1/2 inch diameter cutter is located 1 inch above the XY PLANE,  and the home position of the XY coordinate is -2,-2 relative to the lower left corner of the piece part,  the follow tool path would be required to produce the part.


  In word address format, the CNC program would be as follows:

   PROGRAM CODE     COMMENTS (OPTIONAL--NOT ALLOWED ON ALL MACHINES)

   N05  G91                    :  SET TO INCREMENTAL MODE
   N10  G00                     :  SET TO RAPID MOVEMENT
   N15  Y2.0                     :  MOVE RAPID TO X AXIS START
   N20  F15.0                   :  SET FEED RATE TO 15 INCHES PER MINUTE
   N25  M03                    :  SPINDLE ON
   N30  M08                    :  FLOOD COOLANT ON
   N35  X 1.75                  :  MOVE RAPID TO LEAD IN OF CUT
   N38  Z -.75                   :  MOVE RAPID APPROACH - CUTTER DOWN INTO POSITION
   N40  G01                      :  SET FOR LINEAR MOVE
   N45  Z-.25                    :  LEAD IN ON Z-AXIS
   N48  X8.75                   :  MACHINE X AXIS BOTTOM OF PART (NOTE FIRST .250 IS LEAD IN X AXIS)
   N50  Y2.5                    :  MACHINE Y AXIS RIGHT SIDE OF PART
   N55  X-3.1365              :  MACHINE LEFT ON X AXIS TO START OF ANGLE CUT
   N60  X-2.0083Y2.0     :  MACHINE ANGLE CUT
   N65  X-3.3552             :  MACHINE TOP OF PART
   N70  Y-4.50                 :  MACHINE LEFT SIDE OF PART
   N75  Z .25                    :  MOVE SPINDLE UP TO CLEAR PART
   N80  M09                     :  FLOOD COOLANT OFF
   N85  G00                      :  SET TO RAPID MODE
   N90  Z .75                     :  RETRACT SPINDLE TO HOME POSITION ON Z AXIS
   N95  X-2.0Y-2.0           :  RAPID TO HOME
   N100 M05                      :  SPINDLE OFF
   N105 M02                     :  END OF PROGRAM

Another form of programming is APT which uses move geometrical definitions.  This language was important in the development of CAM packages and formed the basis for creating modules for translating CAD geometry into machine code (referred to as post processing).   In APT geometry is first defined, then the program is written.  A partial listing for the previous word address is show below in APT language.

POINT (2,2) = PT1
   .....              = LINE 1
   ....               = LINE 2
    :
    :
    :
FEDRAT 15
GODELTA PT1
SPINDLE ON
LINEAR
DOWN
COOLANT ON
GOPAST LINE2
GOPAST LINE3
GOTO LINE4
GODELTA
GOPAST LINE5
LINEAR
GOPAST LINE6
GOPAST LINE1
UP
SPINDLE OFF
COOLANT OFF
GODELTA PT 0
FINI

Note:  This is intended as an conceptual example only and not an accurate representation of APT structured code.

Manual CNC programming is rapidly being replaced by CAM software packages that read part geometry and translate from files such as dxf or IGES.   A laboratory exercise will be conducted at our next Saturday session using SurfCam to allow you to gain a better understanding of the steps involved in "tooling" up a drawing, and preparing the CNC code.

Summary
The intent of this module was to provide an overview of various types of production systems form job shops to transfer lines, and process, product, and cellular layout.   Job shops are low volume but highly flexible production systems, while transfer lines are dedicated high volume systems with little if any flexibility.    Process layout (which is typical of job shops) is not suitable to higher volume production due to complexity of flow and scheduling.   Process layout simplifies the flow problems; however, flexibility is lost.  Increased competition, and product design variations have produced the need for adapting to flexibility in manufacturing.  This is accomplished by a FMS.   Often a remainder cell is added to act as a buffer.    Automation has become a vital part of modern manufacturing in CAD, CAM, and Production.   At the machine level, CNC and PLC controls are two common types employed.  The components of CNC are a Program, Control Unit, and machine tool.  Various methods of programming CNC equipment are used, however word address format has long been a common method.  APT is a more geometrical, high level language and is the basis for many CAM post processing modules.  In Module 8, we  will continue with automation and go deeper into the functions on a CNC controller and how positional resolution is achieved.  PLC controls will also be presented.
 
 

Assignment:  Write a CNC program in word address format to machine a .250 inch cut, .250 inches deep around the perimeter of the work piece shown below.  Make a diagonal cut at 45 degrees across the work piece.  The cutter to be used is a 1/2 inch, 2 fluted end mill.   Assume the cutter is located 1.0 inches above the XY plane.
ASSUME INCREMENTAL POSITIONING.  FOLLOW THE FORMAT SHOW AT THE END OF THE PAGE.
 


 

The sequence of cuts is shown in the diagram below.  Note: A premilled slot is used to clamp the workpiece in place.  This should not be included as part of your assignment.

YOUR GRAPHICAL PATH SHOULD LOOK SOMETHING LIKE THE ONE SHOWN BELOW:

 Follow the format as shown below:


BACK  TO MODULE INDEX PAGE