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About CNC Controllers
CNC controllers are devices that control machines and processes.
They range in capability from simple point-to-point linear control
to highly complex algorithms that involve multiple axes of control.
CNC controllers can be used to control various types of machine
shop equipment. These include horizontal mills, vertical mills,
lathes and turning centers, grinders, electro discharge machines
(EDM), welding machines, and inspection machines. The number of
axes controlled by CNC controllers can range anywhere from one
to five, with some CNC controllers configured to control greater
than six axes. Mounting types for CNC controllers include board,
stand alone, desktop, pendant, pedestal, and rack mount. Some
units may have integral displays, touch screen displays, and keypads
for control and programming.
Industrial
communications options for CNC controllers include ARCNet, CANBus,
ControlNET, Data Highway Plus, DeviceNet, Ethernet 10/100 Base-T,
parallel, PROFIBUS, SERCOS, Universal Serial Bus (USB), serial
(RS232, RS422, RS485), and web-enabled. Communications language
choices include bitmap, conversational, DXF file, G/M codes, Hewlett
Packard graphics language, and ladder logic. A bit map (often
spelled "bitmap") defines a display space and the color
for each pixel or "bit" in the display space. Conversational
language is a higher level, easy to learn programming tool. It
performs the same functions as the standard G-code commands. Drawing
eXchange Format (DXF) file that was created as a standard to freely
exchange 2 and 3 dimensional drawings between different CAD programs.
It basically represents a shape as a wire frame mesh of x, y,
z coordinates. G-code is the programming language for the Computer
Numerically Controlled (CNC) machine tools that can be downloaded
to the controller to operate the machine. M-code is the standard
machine tool codes that are normally used to switch on the spindle,
coolant or auxiliary devices. Hewlett Packard Graphical Language
(HPGL) was originally created to send 2 dimensional drawing information
to pen plotters, but has since become a good standard for the
exchange of 2 dimensional drawing information between CAD programs.
Ladder logic is a programming language used to program programmable
logic controllers (PLC). This graphical language closely resembles
electrical relay logic diagrams.
CNC
controllers have several choices for operation. These include
polar coordinate command, cutter compensation, linear and circular
interpolation, stored pitch error, helical interpolation, canned
cycles, rigid tapping, and auto-scaling. Polar coordinate command
is a numerical control system in which all the coordinates are
referred to a certain pole. The position is defined by the polar
radius and polar angle. Cutter compensation is the distance you
want the CNC control to offset for the tool radius away from the
programmed path. Linear and circular interpolation is the programmed
path of the machine, which appears to be straight or curved, but
is actually a series of very small steps along that path. Machine
precision can be remarkably improved through such features as
stored pitch error compensation, which corrects for lead screw
pitch error and other mechanical positioning errors. Helical interpolation
is a technique used to make large diameter holes in workpieces.
It allows for high metal removal rates with a minimum of tool
wear. There are machine routines like drilling, deep drilling,
reaming, tapping, boring, etc. that involve a series of machine
operations but are specified by a single G-code with appropriate
parameters. Rigid tapping is a CNC tapping feature where the tap
is fed into the work piece at the precise rate needed for a perfect
tapped hole. It also needs to retract at the same precise rate
otherwise it will shave the hole and create an out of spec tapped
hole. Auto scaling translates the parameters of the CNC program
to fit the work piece.
Features
common to CNC controllers include alarm and event monitoring,
behind tape reader, diskette floppy storage, tape storage, zip
disk storage, multi-program storage, self diagnostics, simultaneous
control, tape reader, and teach mode.
Encoder
interface closes the loop on control
The EnDat interface can transmit position
values from incremental and absolute encoders as well as transmitting
or updating information stored in the encoder, or saving new information.
Note: A free brochure or catalogue is available from
http://www.heidenhain.co.uk on the products in this news release.
Digital drive systems and feedback loops with position encoders
for measured value acquisition require fast data transfer with
high transmission reliability from the encoders. Further data,
such as drive-specific parameters, compensation tables, etc must
also be made available. For high system reliability, the encoders
must be integrated in routines for error detection and have diagnostic
capabilities.
The EnDat interface from Heidenhain is a digital, bidirectional
interface for encoders.
It is capable both of transmitting position values from incremental
and absolute encoders as well as transmitting or updating information
stored in the encoder, or saving new information.
Thanks to the serial transmission method only four signal lines
are required.
The data are transmitted in synchronism with the clock signal
from the subsequent electronics.
The type of transmission (position values, parameters, diagnostics
etc) is selected by mode commands that the subsequent electronics
send to the encoder.
The EnDat interface provides everything needed to reduce system
cost per axis up to 50% - and at the same time improve the technical
standard.
The most significant benefits are: cost optimisation, improved
quality and higher availability.
Measurement
technology is key to automation
Advances in machine accuracy, on-machine touch probing technology
and noncontact tool setting provide powerful tools for automating
and speeding mould machining, says Barry Rogers.
Note: A free brochure or catalogue is available from http://www.renishaw.com
on the products in this news release.
Drives to faster, leaner, more flexible manufacturing are shifting
industry focus away from traditional post-process quality control.
The most expensive, non-value-added process in most shops is part
inspection. Inspecting good parts - parts that meet all print specifications
- is a waste of time, money and manpower.
Rather than back-end detection, attention is shifting to front-end
prevention.
The aim is to make 100% good parts, right the first time, to ever-tighter
tolerances in the lowest possible total processing time.
Under that mantra, a variety of practices and technologies are being
applied to machine tools to achieve greater process control.
Automated process checks can keep process and parts in control, while
minimising downtime for operator intervention.
These process control improvements can be particularly vital for mouldmaking.
The one-off nature of most mould/die work and the high accumulated
value that can go into a complex mould demand right-the-first time
processing.
At the same time, shorter lead times and global competition force
the need for faster mould processing.
By minimising need for operator intervention, these process controls
give mouldmakers an 'eye on the job' during long machining runs and
lightly staffed second and third shifts.
Front-end prevention takes three forms: identifying and maintaining
machine capability; in-process probing; and automated tool monitoring.
A technology leader in all three areas, Renishaw offers single-source
expertise and assistance in creating an integrated programme of mouldmaking
process control.
To move from defect prevention, you must be able to document your
process capability and the accuracy of your machine tools.
To do this, inspect them to a nationally recognised and accepted standard,
such as ISO230 or ASME B5.54.
Both call for a ballbar and laser interferometer to be used with a
recommended procedure for checking machine tool accuracy.
The purpose of these standards is not to specify an accuracy the machine
must meet, but to find out what accuracy level it can meet - its process
capability.
The part print dictates the accuracy your machine must have to make
good parts - where to set the accuracy bar.
Testing tells you how high your machine can jump.
As long as your machine can top the bar, you have process capability.
Test and calibration technology are now available - and affordable
- to enable shops to ensure the accuracy and health of their machine
tools.
Plants and large shops increasingly maintain their own laser interferometers
and electronic levels, while rental equipment and diagnostics services
are commercially available to small shops from various sources and
competitively priced.
Renishaw's QC10 ballbar system is readily affordable by virtually
any shop and provides a fast, 15-minute check-up for prevention and
diagnosis in maintaining machine accuracy.
The ballbar test allows precise assessment of machine geometry, circularity
and stick/slip error, servo gain mismatch, vibration, backlash, repeatability
and scale mismatch.
Renishaw's Ballbar5 software provides diagnosis of specific errors
in accordance with ISO230-4 and ASME B5.54 and B5.57 standards, then
provides a plain-English list of error sources rank-ordered according
to their overall effect on machine accuracy.
This allows maintenance people to target those factors which most
need attention.
Periodic ballbar testing enables trend tracking of machine performance.
Preventive maintenance can be scheduled before a machine drifts out
of process capability.
The industry trend is to calibrate the machine on need, not time.
There is no reason for maintenance to pull a perfectly good machine
out of production for calibration.
Let the ballbar and the accuracy of your parts determine when something
has gone awry.
Meantime, run production.
Today's standard machine tools can deliver accuracy and repeatability
approaching levels formerly available only on CMMs.
This enables the machine tool itself to be used for probing checks
of workpieces during critical stages of the machining process.
Once a machine tool's performance as a measuring instrument has been
established, the touch probe becomes the operator's CNC gauge.
Probing routines can be programmed as part of the machining process
and automatically run at various points to check feature dimensions
and locations and apply necessary compensations.
This saves operators from using dial indicators and shim stock, or
eliminates errors in manually entering fixture, part and tool offsets
into the control.
Probing on the machine makes it part of the process - a powerful process
improvement tool for making parts right the first time in the shortest
throughput time.
Used to locate the part automatically and establish a work co-ordinate
system, probing cuts setup time, increases spindle availability, lowers
fixture costs, and eliminates nonproductive machining passes.
On complex parts, 45 minutes of fixture alignment can be replaced
by 45 seconds of touch probing - performed automatically by the CNC.
When starting with a casting or forging, probing can determine workpiece
shape to avoid wasted time in air-cutting and help determine best
tool approach angle.
In-process control uses touch probing to monitor size and position
of machine features during the cutting process, as well as verify
precise dimensional relationships between various features at each
step to avoid problems.
A touch probe can be programmed to check actual machined results at
various stages against the program and automatically apply cutter
compensation - particularly after rough machining or semi-finish machining.
Reference probing - comparing part features to a dimensional master
or reference surface of know location or dimension - enables the CNC
to determine positioning discrepancies and generate an offset to make
up the difference.
By probing the artefact before a critical machining pass, the CNC
can check its own positioning against the master's known dimensions
and program an offset.
If the dimensional master is mounted on the machine and exposed to
the same environmental conditions, reference probing can used to monitor
and compensate for thermal growth.
What results is a closed-loop process requiring no operator intervention.
Every machine has its own set of numerous small errors in its motions
and structure.
As a result, there is always a slight discrepancy between a CNC's
programmed position and the true position of the tool tip, even after
laser compensation has brought the two into closer agreement.
Programmable artefact probing provides a way to further compensate
for remaining machine errors.
It gives process control feedback to enable positioning accuracy that
can approach the machine's repeatability specification.
Such closed-loop process control can allow a machining centre to achieve
accuracies comparable to boring mills and other high-precision machines.
Many probing operations are accomplished through the use of memory-
resident macro programs.
Work co-ordinate updates, tool geometry changes, part measurement
etc, are automatically determined by the CNC after the successful
completion of a probing cycle.
This eliminates costly errors resulting from miskeyed information
or incorrect calculations.
Used to inspect parts after machining, probing can reduce the length
and complexity of off-line inspection, and it some cases eliminate
it altogether.
Inspecting on the machine is particularly beneficial with large, expensive
workpieces, such as mould or dies, which can be especially difficult
and time-consuming to move.
Here, too, reference probing against a traceable artefact can be used
to compare final dimensions to the known dimensions for a metrology
master.
When making this comparison, the CNC can determine if the specific
machining tolerances were actually achieved.
Based on these results, an intelligent decision can be made on corrective
actions, while the workpiece is still on the machine tool.
Laser tool setters provide a fast, automated means to verify tool
dimensions, especially critical in checking for wear during the long
machining runs in mouldmaking.
A cost-effective solution to high-speed, high-precision tool setting
and broken tool detection, laser tool setters rapidly measure tool
length and diameter on-the-fly, while the tool is indexing through
the laser beam and rotating at normal speeds.
Laser checking at working spindle speeds identifies errors caused
by clamping inconsistencies and radial run-out of the spindle, tool
and toolholders - not feasible with static tool setting systems.
Renishaw's NC family tool setters can perform broken tool detection
at maximum traverse to further minimise out-of-cut time.
As the tool moves through the laser beam, system electronics detect
when the beam is broke and issues and output signal to the controller.
The NC systems can accurately measure tools as small as 0.2mm diameter
anywhere in the beam.
The system triggers when the laser beam is broken beyond a 50% threshold
by the tool being checked.
The noncontact tool setting system uses a visible-red diode laser
proven reliable in machining conditions.
Advanced electronics and simplified design makes noncontact tool setting
an affordable alternative to contact systems.
No moving parts make NC systems virtually maintenance free.
The design avoids the brackets and actuators with contact-based systems.
Housed in a rugged stainless steel unit, the NC laser tool setters
feature Renishaw's MicroHoleTM protection system.
This uses a continuous stream of compressed air to keep out contaminants
and provide uninterrupted protection from chips, graphite and coolant
ingress, even during measuring routines.
Three different Renishaw NC systems enable installation on nearly
any size and configuration of machine tool without impinging on the
work envelope.
These proven, affordable control technologies can allow greater automation
of mould machining with greater process control.
They can make it possible for mouldmakers to produce moulds faster,
with greater geometric and dimensional accuracy, and less operator
intervention, rework or manual finishing. Request a free brochure
from http://www.renishaw.com
High
Speed Milling Machines
High speed machining is characterized by low cutting forces and high
metal removal. High Speed Milling is a technique used in the CNC Machining
Industry that combines high spindle speeds with increased feed rates.
This results in a high chip-forming rate and lower milling forces,
producing an improved surface quality finish and closer tolerances.
In high speed milling, the electronics can make all the difference.
The right CNC coupled with other elements of the control system can
let a slower machine mill a given form faster than a machine with
a higher top feed rates.
1. High Speed Uses
High-speed CNC milling is used, for example, to machine the titanium
rotors of the first high-pressure compressor stages of the EJ200 engine.
High speed CNC milling allows cost-effective milling of the different
airfoil geometry from the solids. By subsequent finishing operations
the planned surface finish is achieved. The CNC milling which caters
to high speed must be structured with an axis movement system that
is suitable for CNC machining.
2. Axis Movement
The high-speed CNC milling machines required for the process must
be fitted with an axis movement system suitable for machining blisks,
which should be at least 5 axes simultaneously, depending on the milling
task involved and an efficiently high-speed control system.
3. 3D Surfaces
High Speed CNC milling machines working on 3D surfaces in any materials
produce a finer surface finish and higher accuracy in less time that
the traditional milling machine. Acceleration is the most critical
factor that affects the high speed machining. Since one or more axis
are always increasing or decreasing velocity in a 3-D cut, ultimate
feed rate is directly related to acceleration
4. What Can A High Speed Control Possibly Do?
A CNC milling machine which possesses a higher structural stiffness
has a greater potential acceleration rate. Box shaped high speed CNC
milling machine, like Bridge and Gantry is the mostly widely used
types of High speed CNC milling tools. The overhead type Gantry exudes
the highest stiffness, acceleration and accuracy among other high
speed CNC milling tools. Due to its scalability, this machine type
is available in sizes to match the work piece, from small to large.
In usual terms, it simply gives you the ability to finish one task
faster and move along to the next sooner, making work output higher.
In drilling and tapping, this can result in faster hole-to-hole times,
quicker spindle reversals for tapping, and substantial cycle-time
reductions. The most dramatic benefits, though, come in 3D designs
machining. Few, drilling and tapping jobs require a million lines
of machine codes. In molds, dies, patterns, and prototypes, complex
surfaces comprising a million or more line segments are not at all
uncommon. Saving just a fraction of a second per move can result in
substantial cycle-time improvements.
5. Downsides - When Is Fast Too Fast?
But despite all these benefits, in high milling, the tool path segments
can be so short that a machining center moving at a high feed rate
can't accelerate or decelerate fast enough to make direction changes
accurately. Corners may be rounded off and the work piece surface
may be gouged. Look-ahead is one answer. Look-ahead capability can
let the CNC read ahead a certain number of blocks in the program,
to anticipate sudden direction changes and slow the feed rate accordingly.
6. Additional Benefits:
- Improved accuracy - Better fit - Superior finish - Better life -
Produce more work in less time - Improving the accuracy and finish
- Reducing polishing and fitting time - Tools simply last longer because
their chip load is more consistent.
Part Program Selection
on a Fanuc Power Mate with a Selector Switch
Author: ControlOn
Synopsis:
Fanuc Power Mate (Models D, F) has an option called “Workpiece Number
Search”. Using this option, the PMC can initiate the execution of
a specified part program. This is especially useful in cases where
the Power Mate is used to control special purpose machines that
produce a known set of components. Such applications are found in
batch production factories.
How it Works:
To synchronize the PMC and CNC functions, there is a communication
area in the Fanuc Power Mate where the PMC & CNC exchange signals.
These are a set of bytes and there are two areas namely:
a)
The “G” area - signals from the PMC to the CNC
b) The “F” area - signals from the CNC to the PMC
By selectively loading G009 with a value in the range 1-255, the
CNC will execute a program between O001 to O255.
To read the complete article please visit http://www.controlon.com/resources/default.asp
About
the Author
ControlOn
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