CNC arrives
Many of the commands for the experimental parts were programmed "by
hand" to produce the punch tapes that were used as input. During the
development of Whirlwind,
MIT's real-time computer, John Runyon coded a number of subroutines to
produce these tapes under computer control. Users could enter a list of
points and speeds, and the program would calculate the points needed and
automatically generate the punch tape. In one instance, this process
reduced the time required to produce the instruction list and mill the
part from 8 hours to 15 minutes. This led to a proposal to the Air Force
to produce a generalized "programming" language for numerical control,
which was accepted in June 1956.
Starting in September, Ross and Pople outlined a language for machine
control that was based on points and lines, developing this over
several years into the APT programming language. In 1957 the Aircraft Industries Association (AIA) and Air Material Command at Wright-Patterson Air Force Base
joined with MIT to standardize this work and produce a fully
computer-controlled NC system. On 25 February 1959 the combined team
held a press conference showing the results, including a 3D machined
aluminum ash tray that was handed out in the press kit.
Meanwhile, Patrick Hanratty
was making similar developments at GE as part of their partnership with
G&L on the Numericord. His language, PRONTO, beat APT into
commercial use when it was released in 1958. Hanratty then went on to develop MICR magnetic ink characters that were used in cheque processing, before moving to General Motors to work on the groundbreaking DAC-1 CAD system.
APT was soon extended to include "real" curves in 2D-APT-II. With its
release, MIT reduced its focus on NC as it moved into CAD experiments.
APT development was picked up with the AIA in San Diego, and in 1962, by
Illinois Institute of Technology Research. Work on making APT an
international standard started in 1963 under USASI X3.4.7, but many
manufacturers of NC machines had their own one-off additions (like
PRONTO), so standardization was not completed until 1968, when there
were 25 optional add-ins to the basic system.
Just as APT was being released in the early 1960s, a second
generation of lower-cost transistorized computers was hitting the market
that were able to process much larger volumes of information in
production settings. This reduced the cost of programming for NC
machines and by the mid 1960s, APT runs accounted for a third of all
computer time at large aviation firms.
CAD meets CNC
While the Servomechanisms Lab was in the process of developing their
first mill, in 1953, MIT's Mechanical Engineering Department dropped the
requirement that undergraduates take courses in drawing. The
instructors formerly teaching these programs were merged into the Design
Division, where an informal discussion of computerized design started.
Meanwhile the Electronic Systems Laboratory, the newly rechristened
Servomechanisms Laboratory, had been discussing whether or not design
would ever start with paper diagrams in the future.
In January 1959, an informal meeting was held involving individuals
from both the Electronic Systems Laboratory and the Mechanical
Engineering Department's Design Division. Formal meetings followed in
April and May, which resulted in the "Computer-Aided Design Project". In
December 1959, the Air Force issued a one year contract to ESL for
$223,000 to fund the project, including $20,800 earmarked for 104 hours
of computer time at $200 per hour. This proved to be far too little for the ambitious program they had in mind, although their engineering calculation system, AED, was released in March 1965.
In 1959, General Motors started an experimental project to digitize,
store and print the many design sketches being generated in the various
GM design departments. When the basic concept demonstrated that it could
work, they started the DAC-1 project with IBM to develop a production
version. One part of the DAC project was the direct conversion of paper
diagrams into 3D models, which were then converted into APT commands and
cut on milling machines. In November 1963 a design for the lid of a
trunk moved from 2D paper sketch to 3D clay prototype for the first
time. With the exception of the initial sketch, the design-to-production loop had been closed.
Meanwhile, MIT's offsite Lincoln Labs
was building computers to test new transistorized designs. The ultimate
goal was essentially a transistorized Whirlwind known as TX-2, but in order to test various circuit designs a smaller version known as TX-0
was built first. When construction of TX-2 started, time in TX-0 freed
up and this led to a number of experiments involving interactive input
and use of the machine's CRT display for graphics. Further development of these concepts led to Ivan Sutherland's groundbreaking Sketchpad program on the TX-2.
Sutherland moved to the University of Utah after his Sketchpad work, but it inspired other MIT graduates to attempt the first true CAD system. It was Electronic Drafting Machine (EDM), sold to Control Data and known as "Digigraphics", which Lockheed used to build production parts for the C-5 Galaxy, the first example of an end-to-end CAD/CNC production system.
By 1970 there were a wide variety of CAD firms including Intergraph, Applicon, Computervision, Auto-trol Technology, UGS Corp. and others, as well as large vendors like CDC and IBM.
Proliferation of CNC
The price of computer cycles fell drastically during the 1960s with the widespread introduction of useful minicomputers.
Eventually it became less expensive to handle the motor control and
feedback with a computer program than it was with dedicated servo
systems. Small computers were dedicated to a single mill, placing the
entire process in a small box. PDP-8's and Data General Nova computers were common in these roles. The introduction of the microprocessor
in the 1970s further reduced the cost of implementation, and today
almost all CNC machines use some form of microprocessor to handle all
operations.
The introduction of lower-cost CNC machines radically changed the
manufacturing industry. Curves are as easy to cut as straight lines,
complex 3-D structures are relatively easy to produce, and the number of
machining steps that required human action have been dramatically
reduced. With the increased automation of manufacturing processes with
CNC machining, considerable improvements in consistency and quality have
been achieved with no strain on the operator. CNC automation reduced
the frequency of errors and provided CNC operators with time to perform
additional tasks. CNC automation also allows for more flexibility in the
way parts are held in the manufacturing process and the time required
changing the machine to produce different components.
During the early 1970s the Western economies were mired in slow
economic growth and rising employment costs, and NC machines started to
become more attractive. The major U.S. vendors were slow to respond to
the demand for machines suitable for lower-cost NC systems, and into
this void stepped the Germans. In 1979, sales of German machines
surpassed the U.S. designs for the first time. This cycle quickly
repeated itself, and by 1980 Japan had taken a leadership position, U.S.
sales dropping all the time. Once sitting in the #1 position in terms
of sales on a top-ten chart consisting entirely of U.S. companies in
1971, by 1987 Cincinnati Milacron was in 8th place on a chart heavily
dominated by Japanese firms.
Many researchers have commented that the U.S. focus on high-end
applications left them in an uncompetitive situation when the economic
downturn in the early 1970s led to greatly increased demand for low-cost
NC systems. Unlike the U.S. companies, who had focused on the highly
profitable aerospace market, German and Japanese manufacturers targeted
lower-profit segments from the start and were able to enter the low-cost
markets much more easily.
As computing and networking evolved, so did direct numerical control
(DNC). Its long-term coexistence with less networked variants of NC and
CNC is explained by the fact that individual firms tend to stick with
whatever is profitable, and their time and money for trying out
alternatives is limited. This explains why machine tool models and tape
storage media persist in grandfathered fashion even as the state of the
art advances.
DIY, hobby, and personal CNC
Recent developments in small scale CNC have been enabled, in large part, by the Enhanced Machine Controller project from the National Institute of Standards and Technology (NIST), an agency of the US Government's Department of Commerce. EMC is a public domain program operating under the Linux operating system and working on PC based hardware. After the NIST project ended, development continued, leading to EMC2 which is licensed under the GNU General Public License
and Lesser GNU General Public License (GPL and LGPL). Derivations of
the original EMC software have also led to several proprietary PC based
programs notably TurboCNC, and Mach3, as well as embedded systems based
on proprietary hardware. The availability of these PC based control
programs has led to the development of DIY CNC, allowing hobbyists to build their own [18][19] using open source hardware
designs. The same basic architecture has allowed manufacturers, such as
Sherline and Taig, to produce turnkey lightweight desktop milling
machines for hobbyists.
The easy availability of PC based software and support information of
Mach3, written by Art Fenerty, lets anyone with some time and technical
expertise make complex parts for home and prototype use. Fenerty is
considered a principal founder of Windows-based PC CNC machining.
Eventually, the homebrew architecture was fully commercialized and
used to create larger machinery suitable for commercial and industrial
applications. This class of equipment has been referred to as Personal
CNC. Parallel to the evolution of personal computers, Personal CNC has
its roots in EMC and PC based control, but has evolved to the point
where it can replace larger conventional equipment in many instances. As
with the Personal Computer,
Personal CNC is characterized by equipment whose size, capabilities,
and original sales price make it useful for individuals, and which is
intended to be operated directly by an end user, often without
professional training in CNC technology.
Today
Although modern data storage techniques have moved on from punch tape
in almost every other role, tapes are still relatively common in CNC
systems. Several reasons explain this. One is easy backward compatibility
of existing programs. Companies were spared the trouble of re-writing
existing tapes into a new format. Another is the principle, mentioned
earlier, that individual firms tend to stick with whatever is
profitable, and their time and money for trying out alternatives is
limited. A small firm that has found a profitable niche may keep older
equipment in service for years because "if it ain't broke [profitability-wise], don't fix it." Competition
places natural limits on that approach, as some amount of innovation
and continuous improvement eventually becomes necessary, lest
competitors be the ones who find the way to the "better mousetrap".
One change that was implemented fairly widely was the switch from paper to mylar tapes, which are much more mechanically robust. Floppy disks, USB flash drives and local area networking have replaced the tapes to some degree, especially in larger environments that are highly integrated.
The proliferation of CNC led to the need for new CNC standards that
were not encumbered by licensing or particular design concepts, like
APT. A number of different "standards" proliferated for a time, often
based around vector graphics markup languages supported by plotters. One such standard has since become very common, the "G-code" that was originally used on Gerber Scientific plotters and then adapted for CNC use. The file format became so widely used that it has been embodied in an EIA standard. In turn, while G-code is the predominant language used by CNC machines today, there is a push to supplant it with STEP-NC, a system that was deliberately designed for CNC, rather than grown from an existing plotter standard.[citation needed]
While G-code is the most common method of programming, some
machine-tool/control manufacturers also have invented their own
proprietary "conversational" methods of programming, trying to make it
easier to program simple parts and make set-up and modifications at the
machine easier (such as Mazak's Mazatrol and Hurco). These have met with
varying success.[citation needed]
A more recent advancement in CNC interpreters is support of logical
commands, known as parametric programming (also known as macro
programming). Parametric programs include both device commands as well
as a control language similar to BASIC.
The programmer can make if/then/else statements, loops, subprogram
calls, perform various arithmetic, and manipulate variables to create a
large degree of freedom within one program. An entire product line of
different sizes can be programmed using logic and simple math to create
and scale an entire range of parts, or create a stock part that can be
scaled to any size a customer demands.
Since about 2006, the idea has been suggested and pursued to foster
the convergence with CNC and DNC of several trends elsewhere in the
world of information technology that have not yet much affected CNC and
DNC. One of these trends is the combination of greater data collection
(more sensors), greater and more automated data exchange (via building new, open industry-standard XML schemas), and data mining to yield a new level of business intelligence and workflow automation in manufacturing. Another of these trends is the emergence of widely published APIs together with the aforementioned open data standards to encourage an ecosystem of user-generated apps and mashups,
which can be both open and commercial—in other words, taking the new IT
culture of app marketplaces that began in web development and
smartphone app development and spreading it to CNC, DNC, and the other
factory automation systems that are networked with the CNC/DNC. MTConnect is a leading effort to bring these ideas into successful implementation.
Description
Modern CNC mills differ little in concept from the original model
built at MIT in 1952. Mills typically consist of a table that moves in
the X and Y axes, and a tool spindle that moves in the Z (depth). The
position of the tool is driven by motors through a series of step-down
gears in order to provide highly accurate movements, or in modern
designs, direct-drive stepper motor
or servo motors. Open-loop control works as long as the forces are kept
small enough and speeds are not too great. On commercial metalworking
machines closed loop controls are standard and required in order to
provide the accuracy, speed, and repeatability demanded.
As the controller hardware evolved, the mills themselves also
evolved. One change has been to enclose the entire mechanism in a large
box as a safety measure, often with additional safety interlocks to
ensure the operator is far enough from the working piece for safe
operation. Most new CNC systems built today are completely
electronically controlled.
CNC-like systems are now used for any process that can be described as a series of movements and operations. These include laser cutting, welding, friction stir welding, ultrasonic welding, flame and plasma cutting,
bending, spinning, pinning, gluing, fabric cutting, sewing, tape and
fiber placement, routing, picking and placing (PnP), and sawing.
End
Sorce : wikipedia
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