Before the advent of NC machines, operator had to manipulate the hand-wheels, levers, cams to make parts. This way the ability of the operator to make the parts with required narrow tolerance was limited. There were all possibilities of some variation resulting into variation of the axis dimensions resulting into poor fittings or wastages. The productivity of the worker was low so it was a dire need to operate the machine automatically. Earlier a series of cams were put to use to move the tools as an attempt to automate the process. Though they were tedious to set but once set it gave good precision, which were what later on known as Swiss machines or precision machines. (Wildes 1985)
MIT Servomechanisms Laboratory gives a good account of pioneering effort on development of numerical control of machine tools, which is as under (History of the MIT Servomechanisms Laboratory).
“A significant postwar project that began in 1949 and continued and evolved through the 1950s was the work that led to numerical control of machine tools. Under a contract with the Parsons Company of Michigan, William M. Pease and James O. McDonough designed an experimental numerically-controlled milling machine which received directions through data on punched paper tape. The first working model of a continuous-path numerically-controlled milling machine was demonstrated in 1952. Further research was then carried out under the sponsorship of the U.S. Air Force. Subsequently, the laboratory’s Computer Application Group, led by Douglas T. Ross, developed the Automatically Programmed Tool Language (APT), an easy-to-use, special purpose programming language. Eventually, APT became the world standard for programming computer-controlled machine tools.”
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Parsons NC Milling Machine
After World War II, Parsons was busy in developing rotor blades for aviation industry. For its complexity involved in shape, it was an uphill task to achieve precision, which was an utmost requirement. With his connections with an IBM computer, he realized that it is quite possible to produce accurate contour guides, which was hitherto difficult with manual calculations. Soon he received a contract with Air force to supply an “automatic contour cutting machine” for production of large wing section pieces for aircraft. He successfully developed a machine, which produced parts with accuracy and precision required by the aviation industry.
Meanwhile, MIT researchers were doing developmental work with various kinds of control processes. They had already done some work with Air Force Projects during World War II. With the active association of MIT, Parsons experimented with servomotors to the x and y axis and successfully controlled them using computer that read punch-cards to process instructions. This made possible to machine complex shapes required for aviation industry. With a manual milling machine it was never possible and that is how he developed the NC milling machine. (Olivo 1987)
After most pioneering efforts by Parsons and MIT researchers the ball set rolling for further growth and development of NC/CNC in the later years to come.
Broad CNC generations classification can be done with regard to the development of CNC machine as under (CNC-Web-handout):
1952 – 1st generation NC, The first NC controlled machine for metal processing (relays and electronic tubes)
1960 – 2nd generation NC, relays and electronic tubes was replaced with transistors.
1965 – 3rd generation, integrated circuits
4th generation NC – CNC (computerized numerical Control CNC Machines)
In the first and second generation NC machines, controller received a set of instructions known as programme consisted of alphanumeric characters. Through these instructions, controller regulated the motions of a machine tool such as a lathe, milling machine, cutter.
Preparation of programme was done with the very basic computers and then made available to the controller via a tape.
At that time, Magnetic tape recorders and floppy disk drives were used for storage and recording purposes for the programme and as such no direct links were provided between the computer and controller. Controller’s tape reader used to read these recorded programmes.
For the purpose of debugging the programme and correcting the errors, it was necessary to have a new tape and several of such tapes were needed before an error-free programme was made. Further, for any modification or engineering changes one was forced to prepare a new tape and so on. (Siegel 1956)
Historical Events in the Evolution of NC/CNC Machines
Let us peep into the past for more specific developments in reference to NC/CNC machines, its components and important accessories, which culminated into the modern day CNCs.
1950- MIT servo mechanism lab developed Numerical Control (NC) milling machine
1952-Parsons filed for a patent on “Motor Controlled Apparatus for Positioning Machine Tool”. The date was 5 May, 1952
1953- Using a magnetic-tape playback system, a digital control system named Numericord was developed in April 1953 by joint effort of G&L, MIT and General Electric Co. (Cuttingtoolengineering 2005)
1955- NC machine’s commercial version was on display at Chicago Machine Tool Show. In the show, several NC machines were on display, which were punched cards or punched paper tapes driven. (Cuttingtoolengineering 2005)
1955- IBM developed automatic tool changer. (Cuttingtoolengineering 2005)
1955-Numericord “NC5” was found into operation at G&L’s plant at Fond du Lac, WI. (History of Computer Aided Manufacturing)
1956- A year of automatic programming of NC machining.
5.1 APT Developed
Douglas Ross made pioneering efforts to put automatic programming of NC machining. Being a mathematician at MIT, Ross had gone to the ServoMechanism Laboratory to work with computer systems for high-speed data processing. His research converged to the development of the Automatically Programmed Tool (APT) system. (Upping Input Speed: automating NC. Cuttingtoolengineering 2005,)
Ross believed that programmer should be able to convey his or her machining instructions in a simple English kind language, which is at times rational and can be modified based on experience.
“Thus, Ross had removed the last significant technological impediment to utilization of NC by the manufacturing industry on a broad scale. “The development of APT was a major turning point in the evolution of NC, because it settled once and for all the issue of whether or not NC could be made economically viable in the light of programming costs,” Reintjes said. Justifiably then, the APT language became the U.S. standard for programming NC machine tools in 1974, and became the international standard in 1978.” (Upping Input Speed: automating NC. Cuttingtoolengineering 2005)
1957- G&L introduced its first 5-axis Variax NC profilers. (History of Computer Aided Manufacturing)
1957- G&L’s (Giddings and Lewis Machine Tool Co) produced first miller (History of Computer Aided Manufacturing)
1957- A year of first CAM software system, an NC programming tool named PRONTO.
Farnum, Gregory (2005) distinctly quotes contribution of Dr. Patrick Hanratty and term him as a father of CAD/CAM and categorically mentions,
“Then there’s the guy who is often called the father of CAD/CAM, Dr.Patrick Hanratty. Among other contributions to the field, Hanratty, in 1957, developed the first commercial CAM software system, an NC programming tool named PRONTO. Of course, if one can digitally create patterns in space to guide a machine tool, one can do the same thing for other machinery as well, a fact that wasn’t lost on the fledgling robotics industry and builders of other types of industrial equipment. Thus, the direct link between CNC and CAM.”
1958- The first commercial NC machining centre with an automatic toolchanger and automatic work positioning. The company was Kearney & Trecker Corp., Milwaukee and product was Milwaukee-Matic II. (Makely 2005)
1960- First controller with transistor technology introduced. (Groover 2007)
1960- Direct Numerical Control (DNC) eliminated paper tape punch programmes and allowed programmers to send files directly from computer to machine tool controller. (Groover 2007)
1963- A true CAD software, namely sketchpad, evolved. (Cuttingtoolengineering 2005)
1965- CAD drafting and the sculptured surfaces developed (Cuttingtoolengineering 2005)
1967- Use of integrated circuits (ICs) in NC reduced 90 percent components and 80 percent wiring connections. (Groover, 2007)
The use of integrated circuits can be considered as a major breakthrough in the evolution of CNC machines as mentioned by Makely (2005) as per the following.
“But true maturity in NC development, according to Paul Warndorf, vice president of
technology at AMT-the Association for Manufacturing Technology, didn’t come until
the development of integrated circuits replaced vacuum tubes with more efficient,
more reliable.” (Numbers Take Control: NC machines Cuttingtoolengineering 2005)
1968- First machining centre by Kearney and Trecker (machine tool builders) marketed. (Groover, 2007)
1970s- CNC machine tool developed. (Groover, 2007)
Farnum (2005) describes how CAD could make the application of CNC machine tools for variety of applications. He states:
“As the ’70s progressed, the increasing power of computers, and the introduction of lower-cost minicomputers, made CAD accessible to a wider array of users. A host of CAD companies, many of them still in existence today, arose to meet the growing demand. This trend was furthered by the emergence of powerful UNIX workstations and PCs in the early 1980s, along with the growing power of the CAD systems themselves.
Today, it’s hard to imagine a manufacturing firm without a CAD/CAM system or the ability to transfer digital data to CNC machine tools.” (Farnum 2005)
1972- Major development of the CAD/CAM machines evolved (Minimizing Movement:
multitasking. Controltoolengineering 2005)
The development on CAD/CAM made possible multitasking on CNCs, thus improving the productivity to manifold.
“Jim Cordier, a veteran of 48 years in engineering and customer service at Hardinge Inc., Elmira, N.Y., said multitasking evolved “because you wanted to do more and more with one setup. If you do a part complete in one setup, you made a more accurate part and did it quicker.” (Numbers Take Control: NC machines. Cuttingtoolengineering 2005)
1976- 3D CAM/CAD systems were introduced (History of Computer Aided Manufacturing)
1980s- Graphics based computer application developed. (Groover, Mikell P.2007)
1989- Expert CAM/CAD systems were developed (History of Computer Aided Manufacturing)
1997- PC window based Open Modular Architecture Control (OMAC) systems introduced to replace “firmware” controllers. (Groover, 2007)
A History of 5-Axis CNC Machines
When someone tries to trace the history of 5-Axis Machine, it goes to even before NC as Herrin (1995) tries to convey in his article History of 5-axis machining. The some of the excerpts is mentioned from his above article.
“The history of 5-axis machining goes back even before NC. My first exposure to it was in 1958 on a project funded by the U.S. Air Force for the purpose of evaluating the feasibility of 5-axis machining. Cincinnati Milacron, then Cincinnati Milling Machine Co., was awarded a contract to build and test an electronic tracing version of a 5-axis vertical mill.”
He further describes that four major technologies that were critical to the success of 5-axis machines are: “machine, control hardware, control software, and part programming software.”
He further opines that considerable improvement in the computer hardware is the reason that has made CNC designers to provide for advanced capability to meet the requirements of 5-axis applications at a reasonable cost. (Herrin 1995)
6.1 Why 5-axis Machines
Three axis CNC machines define its ability to perform the task and movement along the three different axes simultaneously. Those are X, Y, and Z. Axis parallel to the tool spindle is known as Z axis. Three axis CNC machines work on sculptured or tapered surfaces. It is not possible to work on the complex jobs like tapping, internal holes on complex surfaces with variable curvatures.
Certain limitations of three axis machine forced the developer to think about the five-axis machine which has varied application in myriad of industries.
Five-axis machines are built by adding two axes that rotate around either Y or Z axis.
There are mainly three types of 5-axis machines.
(A). A Dedicated 5-axis Machining Centre.
The biggest drawback with such machines is that their range of motion is limited to +/- 30 degrees. When it is needed to have steeper angle of cut, manual intervention is required. They are not as rugged as three-axis machines
(B). A Tilting/Rotary Table Type
A tilting-rotary table is mounted to the bed of a (three-axis) CNC machine. They are available in the market for several years. Advantage is that they can be used by small and big shops alike for many machining jobs. The good thing is that it provides possibility to tilt the part at various angles and thus machining is possible at various sides of the part resulting to the real five-axis machining.
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However, tilting-rotary table machines exert limit on workpiece with regard to its width, weight, length, height. Besides, tilting tables are very large and cover up host machine’s working space in large amount. At times, it reduces the work space by as much as 75 percent.
They are good for smaller parts. They pose a difficulty in holding the part when heavy cutting operations are to be carried out. Size of the workpiece and its weight pose a limitation for machining in these types of machines.
(C). Spindle Head Attachments Type
The major benefit of a spindle head attachment is that it has full access of the machining centre’s working envelope. There is no restriction on the size of the workpiece required to be machined.
They are fully programmable and any three-axis machine can be converted to a real five-axis machining centre. It is possible to add tilting capability of +/- 90 degrees. It can also have full rotary motion of 360-degree.
The attachment can convert any 3-axis machine to 5-axis machine in less than 30 minutes. Since this head attachment is portable, it provides flexibility and rigidity of three-axis machine and work well for machining of variety of jobs.
Because this unique head is portable and can be mounted in less than 30 minutes, it allows the owner to benefit from the accuracy and rigidity of the three-axis mill for significant metal removal then mount the attachment to complete five-axis profiling, holes, pockets, etc. The cost is similar to that of the larger tilt-rotary tables.
6.2 The Benefits of Machining with Five Axes
It eliminates a need to have multiple setups for refitting the workpiece at different angles. This definitely saves time and reduces errors. It also eliminates the need to have costly fixtures, tooling for holding the workpiece in place. The complex jobs like tapping, internal holes on a complex surfaces can be machined that are otherwise not possible.
Five-axis technology also eliminates multiple setups required to re-position the workpiece at complex angles. This not only saves time, but greatly reduces errors and costly tooling and fixture expenses required to hold the workpiece in place. It also provides the ability to machine complex parts that are not otherwise possible-including holes, pockets and tapping required to be normal to a complex surface.
To a mold maker, it provides following benefits.
Machining time greatly reduced by using a flat bottom end mill by using full diameter of the cutter
Side milling of the angled surfaces can be done with less number of passes on the complex surface.
Much better surface finish can be achieved eliminating ribbing from ball-nose end mills.
Cut down time and labour on manual millwork/handwork required to clean up convex or concave kellered surfaces.
Why CNC Became so Important?
Swamidass (2002) has distinctly defined the need of CNC machines stating:
“A CNC machine tool is a self-contained machine, where the tool-cutting movements, spindle speeds, tool exchange and other operations are controlled by a part programme executed by the computer controlled based at the machine tool. The machine design which holds the tool used to cut into the work piece. Conventional machine tools (lathes, drill presses, milling machines) are not computer controlled. The operation is done by skilled craftsman. There can be variations to dimensions on parts made on a conventional tool. The elimination of this variation is one objective (benefit) of automating the discreet part production process.”
The Historic Developments and Its Significance on CNC Machines
It is important to understand that why all these mile stone achievements were so significant in overall evolution of CNC machines. Let us take some of them.
8.1 CNC Control
A programme is interpreted by CNC Control. It activates the series of commands in given sequence and will initiate required function, set motion for axis and carry out necessary instructions mentioned in the programme. This amply proves that how important controller is in overall functioning of the CNC machines.
State-of-the-art present day CNC Controls carry out several other functions like editing the programme due to some error. It also has a dry run to check and verify the correctness of programme. Moreover, it also allows operator to provide certain important inputs separate from the programme such as tool length values. In short, it controls all the functions of the machine and keeps track of it as and when needed.
8.2 Changing the Tool Automatically
Centres have capacity to hold many tools in a tool magazine. As per the requirement tool can be automatically placed in the spindle for machining.
8.3 Spindle Speed and Activation
The spindle can be turned in a forward and reverse direction and speed can be notified in
RPM. When required, it can be turned off.
8.4 CAM System
For a simple application, the CNC programme can be developed manually by operator, which is also the best way to develop the programmes. When it comes to more complicate applications, it becomes tedious and difficult to write the programme. There comes a CAM (a computer aided manufacturing) system into picture. CAM is a software programme, which runs on a computer and helps CNC programmer to complete the programming process. It can also work with CAD developed by the company’s design and engineering department. Machine operator has to specify the machining operations and based on that CAM will have created the CNC programme as if it is written manually saving time, effort and complexities of the functions. (Momingstar 1993)
Once the programme is developed, it is required to be loaded into the CNC control. But when available memory on CNC controller is not sufficient to accept the programme then the need of DNC (distributive numerical control) comes into the picture. It was like a computer device that used to be connected to one or several CNC machines. However, with the advent of PCs during the 1990s ended the utility of DNC controls. (Momingstar 1993)
Current CNC Machines
Today’s CNC are much faster and offers automation features, which were never heard before. With modern machining centre axis-positioning requirements have specifically increased and tool management functions have become more complex. This forces to have far greater processing power to manage many complex functions.
Caussin (1999) has given a very good account of the controls that CNCs are capable for and they are described as under:
9.1 The Heart of CNC-Motion Control
CNC machines are known for their automation, precision and consistent motion controls. CNC machines use motion control in a way, which can be called as revolutionary. Motions could be either linear or rotary.
In conventional machine tools, these motions are caused by the use of cranks and handwheels. But the programmed commands initiate the motion in CNC machines. The motion type, motion rate (feed rate), motion type and which-axis-to-move all that are programmed with CNC machine tools.
A CNC command conveys the drive motor to rotate specific number of times, which causes the rotation of the ball screw. The ball screw, in turn, moves to the linear axis. A feedback device confirms that specified number of rotations have taken place.
9.2 CNC Speed and Feed Rates
Now a days, dual processors or even parallel processors are in demands to increase the processing speed. It can notify the broken tools and necessary inspection can be done at machine centre itself.
Parallel processors are employed to increase the speed of complex task. It is capable to change the feed rates based on the spindle load parameters. This reduces the required processing time. Further, depending upon the actual tool load, it can control and monitor the feed rate so as to avoid any damage to the tool. A feed rate will also determine when to change the tool when it reduces to the less than certain percentage of the normal.
9.3 Voltage Fluctuation and Loads
They can also monitor voltage fluctuations to decide if a tool has broken. CNC can locate if some catastrophic problem is there with the tool and it will shut down the process.
It can probe in a variety of ways to reduce setup time, as many controls have number of setup features like diameter offsets and tool length. That is to say that with an automated tool offset feature, it can set offsets for number of tools in a few minutes.
Probes also facilitate setting the Z fixture offset automatically for speedy processes. It also allows setting for the X-Y axes automatically by imparting an appropriate programmeme.
While machining a casting, it is required to verify the datum point and there also a probe will save considerable time. A feature known as rotation searching will probe the casting and work out the angle of difference and rotate the programme accordingly.
Off line inspection of parts is a time consuming process but here also CNC will allow the use of a probe and verify its measurements. By probing a part it can also ascertain tool wear for its replacement.
CNC can be programmed so that the tool will be probed as part of the machining cycle. It is so automatic that as the tool touches the probe, the next tool is loaded and the machining goes on. If it does not touch the probe then programme stops further machining.
9.5 Better Axis Synchronization
State-of-the-art technology of the present day CNCs offer better control and make possible the synchronization of the Z axis with the spindle and rest of the system. That results into the possibility of rigid tapping which delivers high level of accuracy.
Perfect synchronization between X-Y and Z axes makes helical interpolation possible so that very cost-effective threading on holes over 1 inch can be carried out. It also makes possible threads in blind holes, pipe threads and threads in holes in odd-shaped parts.
Present day CNCs offer possibility to communicate with other processors. This saves a lot of time of the operator to set communication parameters from his PC to the machining centre.
Nowadays CNCs offer the capacity to connect to the internet. Companies with global operations are greatly benefitted from this so that they can transfer their programmers to other locations.
So today’s CNCs provide a very high level of automation. Any skillful operator can use all the the features to increase his own and company’s productivity.
The Future of CNC
The future development of CNCs will always depend upon the other technologies, which are developing simultaneously. Even innovators like John Parsons and others never thought of the present day achievements of CNC machines. At that time, they could not have visualized in their dreams that we would have colour graphics, high speed mega hertz microprocessors, touch screen CRTs and so on. Similarly it is difficult to visualize for us now how new technologies will develop in the next 20-30 years. However, one common factor that can be described is a need based development and innovation, which will not only continue but will have an accelerated pace.
Vonasek (2009) interviewed many industry experts and described some of the possible future developments, which can be listed as under.
CNC machines with more integration toward loading and unloading systems.
Integration between CAD programmes and the CAM software for the machine, making connection between workpiece production and actual product development more feasible.
More energy efficient and increased productivity.
Faster and more powerful drive systems with even greater accuracy.
CNC routers with more application
As science will make inroads in artificial intelligence, it is quite likely that future CNC machines will turn more and more user friendly but that will come with a cost attached to it. The sophistication will bring many new features but affordability will be a big question for many small and medium sized companies.
However, in all likelihood basic CNC machines with 3-axis movements will be a preferred choice for common applications, which do not come under purview of high-tech areas. The future of CNCs is extremely exciting and rewarding. It can be said that CNCs have always been the back bone of engineering industries and will remain like that for future years to come.
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