Copyright © 1996,1997 Creative Technology Corporation
As Presented for the SME "High Speed Machining" Conference
at
Chicago, IL May 7-8, 1996
Advanced Controls
for
High
Speed Milling
By Todd J. Schuett, President
Creative Technology
Corp.
Arlington Heights, Illinois
Advanced high speed milling controls can impact 3-D milling productivity by
10 times or more! Not just 10 percent, but 10 times!
Imagine that over the next hour, your milling machines or machining centers
could already be ahead of schedule, finished with this entire day's work! The
latest of high speed controls can make that possible.
Although my company, Creative Technology Corporation, now makes CNC milling
controllers for new machines, our market for the past few years has been solely
used machine retrofits. Installing new controls on old machines has helped us to
understand the pivotal importance that the controller has in the overall
performance to achieve high speed. Simply changing the control on an existing
machine with its existing drive system can actually impact productivity by 10
times or more!
Overview
Over the following 40 minutes, we will first cover some of the benefits which high speed milling has to offer. This will be based largely on 3-D surface milling as seen in molds, dies, patterns, prototypes, etc., as is my main area of expertise. We will then get into a more technical study about some of the key factors in high speed:
Benefits
In simple terms, high speed gives you and your control the ability to finish
one task faster and move along to the next sooner. In drilling and tapping, this
can result in faster hole-to-hole times, quicker spindle reversals for tapping,
and moreover, when combined with higher power and improved cutter and holder
technologies, can result in substantial cycle-time reductions.
The most dramatic demonstrations of high speed's benefits, though, come in
3-D contouring. Few, if any 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 in cases as shown in figure 1.
Fig. 1: Mo-Tech sample part
Here's an example of a part which formerly took 3 hours, 43 minutes to mill
accurately on a Leblond-Makino MH65 machining center with Fanuc
11M CNC controller. The part shown is being cut in wax, but is actually commonly
milled in carbon as an electrode to burn cashew-shaped gates for plastic
injection molds. A control retrofit enabled that same machine to mill a more
accurate part in 17 minutes! The user enjoys greater productivity from his
people and machinery. He also enjoys a distinct competitive advantage when
selling his work, because he can deliver an equally good or better job for less
money, in less time! I probably needn't emphasize that your customers' first
consideration today is delivery.
Fig. 2: Kirby vacuum cleaner gearbox
In a broader sense, high speed creates many other benefits. Improved
accuracy, fit, finish, and cutter life are the most commonly reported peripheral
benefits. Customers share the benefits of high speed through the entire
manufacturing process, not just to produce more work in less time, but also
improving the accuracy and finish and reducing polishing and fitting time. They
do this by using the high speed to reduce the stepovers and the tolerances.
Tools simply last longer because their chipload is more consistent.
Moreover, high speed milling can help you utilize your machinery and people better by wasting less time between machine commands. This doesn't just make more parts faster, but improves the entire process with less fitting and finishing time, better accuracy, and improved tool life.
What has actually brought us together here? "High Speed Machining" lured us
with a promise of going faster. But faster than what? Each of us has a different
perspective on what high speed is, depending on our experience, applications and
our needs.
Fig. 3: Einstein's relativity
In one of Albert Einstein's two major papers on relativity, he stated that
"all motion is relative". Speed is the velocity of motion. Speed is the way we
measure how fast an object moves. Since all motion is relative, and speed is
part of motion and thus is relative, the term "high speed" is also relative.
"High speed's" relative connotation is evidenced by talking with different
people who utilize high speed milling on a daily basis. Going from 10
inches-per-minute roughing pre-hardened tool-steel to 30 inches-per-minute is
definitely high speed in the mold business. Increasing milling speeds from 15
inches-per-minute up to 105 inches-per-minute while cutting aluminum or
automotive head liner molds is really high speed. Still, if your application is
the milling of foam patterns for automotive stamping dies, 800 inches-per-minute
may not seem like it's fast enough!
There is no clear threshold where plain old milling becomes high speed milling. High speed is relative, based on your perspective, your materials, and your needs.
The evolution of CAD/CAM into a powerful tool for 3-D surface creation is the
main reason we talk about high speed milling for molds and dies.
Fig 4: Gridwork of data, or point mesh
CAD (computer aided design) works with entities and surfaces. Points, lines,
arcs, cylinders, spheres, planes and more all join in CAD to create surfaces.
CAM (computer aided machining) then translates those surfaces into point meshes
or wire frames of data for machines. Once that data is passed to the CNC, it
executes one point at a time to reconstruct the surface. In order to do this
efficiently, points are not milled at random, but rather organized into a data
flow along slices or flow lines. These may be along any axis or across a
combination of axes or even along a constantly changing flowline detail on a
surface. In any case, the flow of data is from one point to another along the
flow line or slice, then typically to step over and repeat that slice either in
a single direction box-type cycle or in a zig-zag of back and forth movements
along the flow.
Point-to-point then is the process of creating surfaces by milling from one point to another in succession. As we apply point-to-point to high speed milling, that succession from one point to another should ideally be quite fast.
Although we have shown a uniform gridwork of points, CAM typically creates
points with various distances between them. This is done by sorting points based
on "chordal deviation". An example of a slice with points sorted by chordal
deviation is shown in the figure 5.
Fig. 5: Slice of part, with points sorted by chordal
deviation
We can see that the deviations to the surface vary. If we step back a moment
and just consider milling an arc by single point moves, we see that that arc
really becomes a series of line segments. Those line segments deviate from the
arc by a value commonly called the chordal deviation. This number is typically
set in your CAM system to define what the acceptable deviation from surface or
tolerance is. This deviation is set depending upon the accuracy required for
individual applications. The natural desire is to be as accurate as possible but
stating a deviation value which is too tight can result in enormous file sizes
and high data density which can be difficult to handle. Chordal deviation must
be properly set to lend balance to productivity and required job accuracy. As we
proceed, we will see that high speed milling enables smaller chordal deviations
and thus, higher accuracy, by executing the resulting mass of data more
efficiently.
Fig. 6: Arcs are milled by linear segments
The result of chordal deviation from CAM is a series of chord segments which are now becoming commonly referred to as point departures. This is the individual line segment lengths developed by the individual point-to-point moves which result from chordal deviation sorting (refer to figure 6). Point departures are the distances between successive point-to-point moves.
Look-ahead is a fairly new feature found in only a few controls. Look-ahead
has evolved from a need to prevent gouges while milling point-to-point in rapid
succession. When NC and CNC were first developed, data was executed one block at
a time. Typical uses were drilling and tapping of holes and linear milling.
Circular milling evolved over time. The big advantage to NC and CNC was that all
moves were planned in advance and could happen much faster than a manual
operator could execute the individual moves on the shop floor.
Fig. 7: Dangerous curves for overshoot
without
look-ahead
With the success of CAD/CAM, CNC has been used with increasing success to
develop 3-D surface contours. In this application, the cutter must flow through
the points without dwelling as older CNC and NC machines did. Most numerically
controlled milling machines take from 0.100" to 0.200" to stop from a move at
100" inches-per-minute. If a CNC control and machine are instructed to flow
through data at high feedrates, yet point departures are short, gouges can
result at points of abrupt changes in the contour. Look again at the shape shown
earlier in figure 5. There are several close data segments at the bottom of the
contour. This is an area of great danger for gouging. The longer line segments
going from left to right might easily permit high feedrates. As seen in figure
7, without look-ahead, the CNC might be surprised by the abrupt change in
direction over a short move of only 0.010". If the feedrate is too high to stop
in that distance, the result will be an overshoot. The centerline of the tool
will miss its projected path, resulting in a gouge in the part.
Look-ahead must evaluate data many blocks ahead to prevent gouges. In most
applications one or two or even ten or twenty block look-ahead is not enough.
The amount of look-ahead needed varies based on contours, feedrates, and machine
performance. In general, look-ahead can not be limited to any arbitrary value,
because conditions are constantly changing. Ideally, look-ahead should be
dynamic, varying the distance and number of program blocks based on the part
profile and the desired milling feedrate.
Offline Look-Ahead
Look-ahead is now offered by some companies either as a pre-processing step
or as a part of the DNC system. To achieve the effect of look-ahead, the system
must add data segments at varying slower feedrates. In this way, gouging and
overshoots can be prevented. The drawback is that by adding program lines, the
data throughput problem is exacerbated.
High Feedrates
How accurate can a CNC really be at high feedrates? We can answer this by
analyzing the entire machine system, starting at the CAD/CAM system and ending
with the machine iron. Since we are talking specifically about the controls for
this presentation, let's assume that everything else is in order, and that the
control is the only issue.
Servo cycle time is the amount of time a CNC control takes for
each measuring and command cycle. In other words, if the control's servo
cycle time is 20 ms (milliseconds or thousandths of a second), then the
axis positions are measured and a new direction commanded by the control 50
times a second. Though 20 ms servo cycles were thought to be good just 10 years
ago, servo cycle times over 4 ms. are now considered inadequate. At a fairly
common 3 ms. servo cycle time, positions are being measured and corrected 333
times per second. A machine moving at 100 inches-per-minute is moving 1.66
inches per second, so each time the axes are measured, the machine should be
moving 0.005". This might be alarming to you if you are trying to hold
tolerances to 0.0001" or so, since your machine is basically out of control for
0.005" increments at a time!
The accuracy problem gets worse when attempting to mill accurately at 400
inches-per-minute, where a 3 ms. servo cycle results in 0.020" moves between
measurement and correction commands from the control! Recent trade publications
have even touted high speed gantry-style mills and routers milling "accurately"
at speeds to 1200 inches-per-minute using controls with 3 ms. servo cycle times.
Each servo cycle is then commanding a 0.060" move. How can 0.060" moves be held
to close tolerances over contoured surfaces?
| Time
ms. |
Cycles/
Second |
100
IPM |
400
IPM |
1200
IPM | |
| 20 | 50 | .0333 | .1333 | .4000 | |
| 10 | 100 | .0166 | .0667 | .2000 | |
| 3 | 333 | .0050 | .0200 | .0601 | |
| 1 | 1000 | .0016 | .0066 | .0200 | |
| .4 | 2500 | .0007 | .0026 | .0080 | |
| .1 | 10000 | .0002 | .0007 | .0020 | |
Chart 1: Distances moved at given feedrates and servo cycles
Chart 1 shows a few sample servo cycle times, measuring speeds, and distances
at feedrates. This chart demonstrates that to mill as accurately at 1200
inches-per-minute as at 100 inches-per-minute, the control must indeed be very
fast.
The importance of fast servo cycle times becomes quite obvious as we look at
these numbers. The faster the desired milling speed, the faster the servo cycle
time must be. Years ago, rapid traverse rates were increased to 400 IPM, then
800 IPM, and now faster. We couldn't dream of milling accurately at those
speeds, though, because of the slower controls. The machines could withstand the
stresses of quickly moving around from one point to the next, but the accuracy
wasn't there for feedrates in those ranges.
Feedrates will continue to increase, and the need for faster servo cycle
speeds will continue to grow. Cutter technologies are proving capable of
supporting amazing speeds and feeds. The other supporting technologies like high
speed spindles, end mill holders, and so on are all enabling amazing speeds and
feeds. Machines with linear motors are now available with traverse rates to
3,000 IPM and more. They can accelerate to 3,000 IPM faster than most machines
today can get to 300!
Fast servo cycle times are one of the key considerations for milling fast
with accuracy.
Blocks-Per-Second
The number of blocks-per-second the control executes or the block transfer
time should not be confused with the servo cycle time. Ideally, the servo
cycle time should be faster than the block transfer time. Still, it is possible
for controls that execute a high number of blocks-per-second to execute at
slower servo cycle times. In these cases, the number of blocks-per-second is
misleading, indicating a higher speed than can really be achieved if each block
is an actual discrete motion block. The combination of fast block transfer time
with a still faster servo cycle time ensures high data throughput, with optimal
accuracy.
The DNC Bottleneck
Now that we've looked at the control's ability to mill accurately at high
speeds, we have a new dilemma, how to get the program information to the CNC
fast enough to avoid data starvation. Most anyone who has milled 3-D contours
has watched as their CNC has stopped and waited to fill the buffers again to
continue program execution. Loading the program into the control helps it run
faster, yet that can often be impractical with the small CNC memories, or slow
communications speeds.
Fig. 8: Simple DNC
First, let's consider DNC, the most common communications in use for CNCs
today. DNC stands for Distributed Numerical Control, the distribution of numeric
cutterpath data to CNC machinery, or Direct Numerical Control, the "drip
feeding" or dynamic downloading of numeric cutterpath data "on-the-fly" as the
CNC executes it.
DNC is typically performed through a serial communications link at data rates
of 110 to 38,400 baud or bits-per-second. Most common is 9600 baud, resulting in
potential throughput of up to 960 characters-per-second.
Program information for the CNC usually is in blocks or lines of program data
averaging about 20 characters per line. For example:
G1 X123.456 Z234.567 <Enter> <Linefeed>
Even spaces and invisible "control" characters like the carriage return and
the linefeed take time for transmission. The addition of 3, 4 or 5 axis
definitions, line numbers, and feedrates simply add to the overhead in data
transmission. Given a communications rate of just 960 characters-per-second, the
CNC is then limited by DNC to just 48 blocks-per-second! In reality, DNC
overhead commonly results in still lower performance, generally about half the
theoretical potential. At this rate of 24 blocks-per-second with 0.010" point
departures, the resulting DNC speed limit is just 14.4 inch-per-minute! That is
too slow for high speed milling no matter where the observer's relativity is
based!
Proponents of DNC are now promoting several techniques in order to optimize
DNC performance:
Moreover, techniques like those above all increase the operator workload,
increase the opportunities for errors, and reduce the operator's flexibility
with the machine. DNC is operating too close to the limit for high speed
milling.
Networking your DNC computer may help work flow, but does not solve the data
flow problem. Many DNC computers are networked to get the data from the CAD/CAM
to the DNC system fast, but the data flow is still restricted between the DNC PC
and the CNC control by the limited bandwidth of RS-232 communications.
Fig. 9: Networked DNC PC
Direct CNC Networking
The Communications
Solution
Direct CNC Networking or DCN offers a much better solution to the data flow
problems in high speed milling. DCN simply uses existing networking
architectures to provide a direct network link from the CAD/CAM to the CNC,
eliminating the DNC system entirely. DCN is normally 1,000 times faster than
DNC. This can best be illustrated by the fact that a 10 megabyte file which
takes about 3 hours to transfer by DNC at 9600 baud, takes less than a minute by
DCN!
While Ethernet is the most common network architecture in use today, Arcnet,
Token Ring, and Fast Ethernet are examples of other common networks in use.
Software protocols used include Novell's IPX/SPX specification, TCP/IP and NFS
as found in most Unix systems, Netbeui as used by Microsoft's LAN Manager, and
more.
The point is that Ethernet's performance of 10 Megabits per second translates
to 1 million characters-per-second, about 1,000 times faster than most DNC!
Networking architectures are already commonly available with data rates of 100
Megabits per second or more, even 10 times faster than standard
Ethernet!
Direct CNC Networking eliminates any data bottlenecks getting the numeric
program data into the CNC and has lots of room to grow to fill your changing
needs.
Fig. 10: DCN layout
DCN also provides infrastructure for growth, a foundation for expansion into
the future. On PC-based (personal computer) controls, interesting possibilities
include running CAD/CAM software right in the controller. Additionally, job
control and quality control programs can be run right within the CNC. The data
requirements for these types of peripheral programs demands high network speeds
rather than the outdated throughput of serial data communications as used with
DNC.
DSP (Digital Signal Processing)
The technology that allows "plain old PCs" to act as high-performance CNC
controls is known as DSP. Digital signal processing uses special dedicated
processors to convert and interpret digital signals at very high speeds. Using
DSP, a single board as pictured in figure 11 can control up to 8 axes at the
fastest servo cycles discussed earlier! Not so long ago, a single board, much
larger than this, would have been required for a single axis, operating at
speeds 200 times slower!
Fig. 11: Delta Tau PMAC DSP board
The incredible power of DSP for specific tasks is illustrated by the fact
that a DSP chip can execute a multiply-accumulate (MAC) instruction, a
fundamental operation, in a single clock cycle. This same operation on a current
Pentium processor chip takes 11 clock cycles! Obviously a 120 MHz Pentium will
still take nearly 4 times as long as a 40 MHz DSP processor!
Fig. 12: Straight-line acceleration command
Fig. 13: Optimized acceleration command with DSP
One of the big benefits of DSP is its speed. Earlier, we saw how dramatically
the control's servo cycle time can impact the accuracy of the control. DSP is
the key to fast servo cycles. DSP also influences the acceleration and
deceleration "ramps" of the CNC control. Traditional CNC controls simply set up
a ramp rate for accelerations. Because of machine dynamics and drive systems,
the machine suffered "following error". With so much power, DSP systems allow
tuning the acceleration for real conditions and specific machine
characteristics. Acceleration is no longer simply a straight-line ramp, but
rather is tuned as a sort of "bell" shape. This optimization minimizes following
error, reduces strains on the machine, yet provides greater acceleration
overall. DSP's speed allows better accuracy and speed, yet is gentler on the
machines it controls.
There are many varieties of DSP, and those choices give control builders
choices in their priorities for control functions. The variety of DSP available
means that you as a user can obtain different control performance, functions,
and capabilities to meet your specific needs.
DSP also has the ability to handle a variety of servo amplifier and measuring
scenarios. While most traditional CNC controls are limited to interfacing with
one specific drive type, many DSP boards allow the integrator the flexibility to
work with several, or most available interfaces. Given the concept that
technology will change over the life of a machine, this flexibility is
reassuring.
One of the other great benefits of DSP is that the main CPU (central
processing unit) in the PC is still free to perform other tasks. In reality, a
PC equipped with a DSP for machine control is using multiple processors, gaining
a tremendous performance advantage. The DSP can be measuring and commanding axis
positions while the main CPU is handling the receipt of network data and
pre-processing that data for look-ahead to optimize milling speeds and part
accuracies at the same time. A Pentium PC using the 64-bit Intel Pentium
processor, coupled with a Delta Tau PMAC using the 48-bit Motorola 56001 DSP
chip offers amazing speed and processing power for tasks of all sorts, well
beyond the dual 32-bit processors touted by many high speed CNCs today.
Fig. 14: PC Enclosure
Digital signal processing allows your modern CNC to be physically small with
high-performance, while giving you a hedge on the future developments and
changes.
Open-Systems Architectures
The use of an Intel x86 series processor in itself does not mean that a
control is open architecture. Even using a PC-based design does not always mean
that the architecture is really open. Open architecture can have many
interpretations, none necessarily more right or wrong than another.
To me, open architecture implies two key components; an ability to be
serviced and an ability to be changed without proprietary parts and/or
knowledge. This means that open architecture should empower the owner to
have choices in service and in expansion and updates.
Those neat looking hand-held control pendants that do everything, and
special, custom high-performance processors may be claimed as today's best, but
if they're proprietary in design, you can expect high maintenance costs and
limitations for the future. Traditionally, CNC builders have sold new controls
at or near cost to establish the lucrative, ongoing replacement parts business.
Likewise, CNC builders often find it easier to completely redesign a new
control, rather than designing improvements for existing controls. This helps
encourage control replacements, at higher costs, rather than enabling the
advantages of incremental advances in the technology as they are developed. Open
systems architecture can really enable you to keep your maintenance costs down
and to sustain capabilities current with the unfolding state-of-the-art.
There is a general convergence in the industry toward PC-based controls. This
gives everyone the widest possible choice of design options and expansion
flexibility. Personal computers, serviced by virtually anyone, in a variety of
environments, have arguably proven to be as reliable or more reliable than the
best of "hardened" CNC controls. The amazing development of PC features and
performance has left the industrial CNC control market reeling to stay remotely
in step with developments.
For use as a CNC, the PC may be used in a "shop hardened" configuration,
available from a limited number of specialty PC suppliers. Alternately,
virtually any common PC may be implemented within a variety of shop enclosures,
providing protection from the contaminants in the shop atmosphere. If the air we
breathe in a shop is worthy of our health risks, though, perhaps a PC need not
even be protected from the shop atmosphere to be reliable. Many users of PCs for
DNC would argue that plain old office-type PCs are perfectly fine for shop
use.
Fig. 15: Hardened PC
One advantage which some industrial PCs offer is a "watchdog timer". This is
an output which may be incorporated in the Emergency Stop circuit for a machine
to add to the CNCs safety. A dedicated circuit in the computer constantly checks
the integrity of the computer processes at very high speeds to ensure that the
system is operating normally. If the system does not check out, the output can
disable the machine motion by shutting off the drives as an Emergency Stop. This
PC watchdog timer does not replace the DSP's watchdog timer, but rather
compliments it with additional security.
As you can see, open-systems architecture with PC-based CNC controls can be a
real boon to your bottom line, through a reduction in maintenance costs,
up-time, and maintaining a highly competitive posture with ongoing up-to-date
features.
Multi-Processor Strategies
Earlier we talked about the fact that a PC teamed with DSP is actually a
multi-processing computer. This concept can be carried further in several
ways.
Multi-processors can give the CNC operator the ability to multi-task. He may
simply edit programs which are about to be run, or carry that further with
graphical verification of programs submitted by the CAD/CAM department for
operation. The operator may even use shop floor programming to prepare
cutterpaths from IGES surface data right in the CNC control.
Within the PC marketplace, there now exists a number of computers which are
equipped with or can be updated to multiple processors. For controls operating
under Windows NT or UNIX, the multiple processors may offer performance benefits
and the opportunity to use other programs effectively within the control while
cutting.
Fig. 16: Dual Pentium PC board
An alternative is the use of entirely separate computers, controlled by the
same keyboard and monitor. There are several CNCs in the marketplace today using
this concept to maintain a high level of CNC control integrity and security
while still performing other tasks.
Multiple computers may be implemented in a number of different ways. Several companies have long offered dual-processor options, using a physical switch to select the "MS-DOS" mode or the CNC. Others use keyboard commands to select between the computers. Still others obscure the second computer, using one computer with multi-tasking as the user interface, and having it pass commands to the machine control computer. The multiple computer strategy can be very simple to implement, gives the operator multi-tasking capabilities, and offers security for the CNC process.
Fig. 17: Dual processor CNC
in parallel
Moreover, multi-processors offer another level of performance for high speed milling, allowing the operators to perform multiple tasks quickly and efficiently at their machine control station.
Fig. 18: Dual processor CNC
in series
A Look At The Future
As we consider the future for CNC, the certainty is simply that CNC will continue to change. CNC will have to be able to accommodate the evolutionary changes in the industry without requiring the complete replacement of those controls as has been the norm with traditional CNC designs.
The example that I like to use to illustrate this is the word processor. When first introduced, it was an impressive contraption, consisting of a large work center with a green display screen, keyboard, printer, and built in logic, the firmware. When new features were introduced, the entire word processor required replacement at a cost of tens of thousands of dollars, and the old system was traded in for pennies on the dollar. Through evolution, today's word processor is now a shrink-wrapped piece of software that may be used on virtually any personal computer. Costs run in the tens or hundreds of dollars, and updates for new features are less than that. Installation of the update is performed by the user, generally in just minutes. Again, the evolution of the word processor parallels developments in many other computer industries, and is a good example of where CNC is likely to go.
Shrink wrapped CNC? It is possible? Imagine buying a fancy new CNC machine,
and selecting your CNC as a software package for it, not an entire custom
interface!
Change
is the one thing
we can count
on
in the future.
CNC
must be able
to accommodate
that
change.
Think about new features evolving. They have historically made your CNC
machine obsolete. Now imagine again getting updates at low cost, as with the
word processor, or with your CAD/CAM system. We call these incremental advances
in the technology. Simple, low-cost changes will keep your present equipment and
entire company competitive!
All this is not just a pipe dream, but the developments are taking place
right now to make it a reality! Change is the one thing we can count on in the
future. CNC must be able to accommodate that change.
Summary
In this study of high speed milling controls, we have seen that the controller itself plays a very major role in the overall performance of the high speed machine, as specifically evidenced in the retrofit of older machines with new CNCs. The controller enables the benefits of other high speed machine components to be maximized through:
Open systems architecture gives you the flexibility to configure features as
you want and need as you and the industry evolve.
Advanced controls for high speed milling can provide dramatic benefits for
both new machines and existing machines, even bringing impressive performance
gains to old technology drive systems and machine designs.
Copyright © 1996,1997 Creative Technology Corporation