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BMFS 3373 CNC TECHNOLOGY Lecture 1. Lecture Objectives. At the end of the lecture, you will be able to: Differentiate between numerical control (NC) and computer numerical control (CNC). Explain what CNC is and what are the basic component surrounding it.
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Lecture Objectives • At the end of the lecture, you will be able to: • Differentiate between numerical control (NC) and computer numerical control (CNC). • Explain what CNC is and what are the basic component surrounding it. • Identify the different type of CNC machines and their various products. • State the advantage and special requirements concerning CNC use.
Trends in Industries • The objective: TO BE COMPETITIVETHROUGH INCREASING PRODUCTIVITY AND TOTAL QUALITY ASSURANCE • Efficiency of manufacturing: • COST = COST OF MANUFACTURING AND COST OF MATERIAL HANDLING • PRODUCTIVITY = AVERAGE OUTPUT PER MAN-HOUR • PROFIT = INCOME – COST • PROFIT increases as COST decreases and as PRODUCTIVITY increases.
Productivity Through Automation • Automation brings: • any means of helping the workers to perform their tasks more efficiently • transfer of the skill of the operator to the machine
Computerization of Industries • Utilization of computers in manufacturing applications has proved to be one of the most significant developments over the last couple of decades in helping to improve the productivity and efficiency of manufacturing systems. • Softwares, offline & online computer and development and progress of electronic components have all increase the possibility of more reliable, robust and capable manufacturing system. • If the system uses a computer to create a program, the process is known as computer-aided programming.
Numerical Control • Numerical control is a method of automatically operating a manufacturing machine based on a code of letters, numbers and special characters. • An NC machine tool is functioning the same as a conventional machine tool with an additional interface that translate the data input/program into the appropriate electrical signals for input to motors that run the machine. • The numerical data required to produce a part is provided to a machine in the form of a program, called part program or NC program.
Numerical Control - History • The concept is credited to John Parson (1947). Using punched cards he was able to control the position of a machine in an attempt to machine helicopter blade. • US Air Force teamed up with MIT to develop a programmable milling machine (1949). • In 1952, a three-axis Cincinnati Hydrotel milling machine was demonstrated. The term Numerical Control (NC) originated. The machine had an electrochemical controller and used punched cards. • Modern NC machine has a computer on board, Computer Numerical Control (CNC). They can run unattended at over 20,000 rpm (spindler speed) with a feed rate of over 600 ipm and an accuracy of 0.0001. • A new class of machines called machining centers and turning centers that could perform multiple machining processes was developed.
Difference of NC vs CNC • CNC machine comprise of an NC machine with the added feature of an onboard computer. • The onboard computer is often referred to as the machine control unit (MCU). • Machine Control Unit: generates, stores and processes CNC programs. The MCU also contains the machine motion controller in the form of an executive software program. • NC machine: responds to programmed signals from the machine control unit and manufacture the part.
CNC Machines • CNC machine has more programmable features than older NC tape machinery and may be used as stand-alone units, in a network of machines such as flexible machining centers or machining cells. • They are easier to program, and most CNC machines may be programmed by more than one method. • All machines can be programmed via on-board computer keyboard (MDI). • A CNC machine is a soft-wired controller, that is, once the NC program is loaded into the computer’s memory, no hardware is necessary to transfer the numerical control codes to the controller.
CNC Machines • The controller uses a permanent resident program called an executive program to process the codes into the electrical pulses that control the machine. • In any CNC machine, the executive program resides in ROM (read only memory), and the NC code resides in RAM memory. • The information in ROM memory is written into the electronic chip and cannot be erased without special equipment. • Rom can be accessed by the computer, but cannot be altered. This is why the executive program cannot be erased and is always active when the machine is on. • RAM (random access memory) can be accessed and altered by the computer. • The NC code is written into RAM by either the keyboard or an outside source. • The contents of RAM are lost when the controller is turned off. • Many CNC controllers utilize a battery backup system that powers the computer long enough for the program to be transferred to some storage media in the event of a power lost.
Input Media • Input media are used to electronically or mechanically store the NC programs until they are needed. • A program is simply read from the input medium when it is loaded into the machine. • The oldest medium for program storage is punched tape made of paper or mylar plastic. • The NC program code is entered into the tape by use of a tape puncher which punches a series of holes that represent the NC codes.
Input Media • A tape reader employing electrical, optical, or mechanical means senses the holes in the tape and transfers the coded information into the machine computer. • A tape puncher may be attach to a teletype machine or a Flexowriter .With either of these two pieces of equipment, a code character is punched into the tape as it is being typed onto a sheet of paper. • The NC code is entered into either a CAM or word processor type of program and punched into the tape after all editing of the program is completed.
Binary Numbers • Within the CNC controller, each binary digit “one” may represent a positive and “zero” a negative charge, or a “one” may be the presence of charge and a “zero” the absence of charge. • The method used depends on the particular controller. In either case, the CNC program code in binary form must be loaded into the computer. • Programming formats and languages allow the NC code to be written using alphabetic characters and base ten decimal numbers. • When the NC program is punched or recorded on tape or other storage media, the information is translated into binary form.
Types of CNC Machine Tools • Mills and Machining centers • Lathes and turning centers • Drilling Machines • Boring Mills and Profilers • EDM Machines • Punch Presses and Shears • Flame/Plasma/Laser Cutting Machine • Routers • Water jet and Laser Profilers • Cylindrical Grinders • Welding Machines • Benders, Winding and Spinning machines, etc.
INDUSTRIESINVOLVED • Aerospace • Machinery • Electrical • Fabrication • Automotive • Instrumentation • Mold & Die making
Objectives of Numerical Control • To increase production • To reduce labor costs. • To make production more economical. • To do jobs that would be impossible or impractical without NC • To increase the accuracy of duplicated parts.
1. Quality Parts are more accurate • the high accuracy of CNC machines and their repeatability allows high quality parts to be produced consistently time after time. Parts are more repeatable • produced consistently time after time. • CNC ensures all the parts produced will be of uniform quality. • parts produced by CNC assures the interchangeability of parts.
2. Reduction Setup Time • modular fixturing, standard tooling, fixed locators, automatic tool changing, pallets and other advanced features, make the setup time more efficient than a comparable setup of a conventional machine. • the number of parts can be machined under one setup. • several operations can be grouping into a single setup.
3. Reduce Lead Time • once a part program is written and proven, it is ready to be used again in the future, even at a short notice. • change of the part design requires the program to be modified, it can be done usually quickly. • many jigs and fixtures formerly required are not necessary.
4. Accuracy And Repeatability • the high accuracy of CNC machines and their repeatability allows high quality parts to be produced consistently time after time. • CNC ensures all the parts produced will be of uniform quality. • parts produced by CNC assures the interchangeability of parts.
Simplified Tooling And Work Holding • multi-step tools such as pilot drills, step drills, combination tools, counter borers and others are replaces with several individual standard tools (cheaper and easier to replace than special and nonstandard tools). • fixturing and work holding for CNC machines have only one major purpose to hold the part rigidly and in the same position for all parts within a batch. • normally do not require jigs, pilot holes and other hole locating aids. • the cost of fixture design and manufacture may be reduced by as much as 70%.
6. Consistent Cutting Time • cutting time on the CNC machines is always consistent. • the main benefit of a consistent cutting time is for repetitive jobs, where the production scheduling and work allocation to individual machine tools can be done very accurately.
7. Increase Productivity • parts can be produced faster, with less setup and lead time. • because less time is required for setup and operator adjustments, production rates can increase up to 80%. • CNC produces parts of uniform quality, less inspection time is required.
8. Reduced Inventory • reduced setup time permits smaller economic batch quantities. • lower lead time allows lower stock levels. • lower stock levels reduce interest charges and working capital requirements.
9. Contouring of Complex Shapes • machining complex shapes slide movements under computer control. computer controller can calculate steps. • complex shapes, such as molds, can be manufactured without additional expenses of making a model for tracing. • mirrored parts can be achieved literally at the switch of a button. • complex operation can be done more quickly and accurately.
10. Better Management Control • CNC leads to CAD • Process planning • Production planning
Disadvantages of CNC • Increase in electrical maintenance. • High initial investment. • Higher per-hour operating cost than traditional machine tools. • Retraining of existing personnel.
Advantage of CNC over NC machine • Reduction in hardware necessary to add a machine function • The CNC program can be written, stored and executed directly at the CNC machine. • Any portion of an entered CNC program can be played back and edited at will. Tool motion can be electronically displayed upon playback. • Many different CNC programs can be stored in the MCU. • Several CNC machine can be linked together to a main computer known as direct numerical control (DNC). • Several DNC systems can also be networked to form a large distributive numerical control system. • The CNC program can be input from zip of floppy disks or downloaded from local area networks.
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