Touch Probes: A Good
Choice
For Closed-Loop Process Control
story by Tom Stewart
photos by Scott Rathburn |
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Today’s CNC machining capabilities,
together with advanced insert cutting grades and materials, offer
metal-cutting manufacturers unprecedented new production potential.
That potential has allowed many
high-production facilities to begin replacing dedicated transfer
systems with more flexible CNC machining centers, and many small job
shops to compete on equal terms with far larger OEM and supplier
plants. At both levels, modern CNC machines offer the potential to
make parts faster and to better tolerances.
Still, potential is the key word.
Both large and small manufacturers are quickly learning that, even
though the CNC machine is capable of making parts faster and to
tighter tolerances, it often can’t do so consistently without the
help of some means of process control. This is because of the many
variables that affect the metal-cutting process. These commonly stem
from inconsistencies in the part material, insert quality or machine
stability. Another primary source is the machine operator himself,
who can easily over-control or under-control the process, and make
tool-compensation decisions that are necessarily subjective. Such
decisions will be different from those made by other operators
running the same operation on other shifts.
CLOSED-LOOP PROCESS CONTROL OFFERS
A SOLUTION
| An effective way to eliminate process variables is to invest in
a system of closed-loop process control. Such systems provide
continuous control of part size through automatic feedback of gauged
information directly to the machine tool’s CNC. When the gauged
information falls outside pre-set limits, the CNC activates tool
offsets needed to maintain in-tolerance parts production. An
important advantage of closed-loop process control is its
predictability. Unlike human machine operators, the system makes
compensation decisions the same way every time, allowing production
management to target the machining process to narrow tolerance
bands. If the process represents an early stage of manufacturing,
the narrow tolerances maintained can pay huge dividends down the
line by making it easier and far less costly to maintain quality in
later stages of manufacturing. |
Ricardo
Gutierrez loads a gear blank for the Haas two-speed gearbox onto a
hobber for machining. The use of post-process control ensures that
every blank is exactly to spec. |
Closed-loop process control can be
achieved through feedback of gauged information either before,
during or after the machining cycle. Pre-process control, in which
measurements are made to establish correct setup and/or depths of
cut before the part is machined, is most effectively used for cast
parts, gang fixtures and pallet machines. In-process control, which
is effective for all types of machines and parts, is achieved by
measuring the part prior to finishing cuts in order to determine the
tool offsets needed to keep finish dimensions within tolerance.
Post-process control, which is used most effectively on CNC lathes,
is achieved by measuring the part after it is machined to determine
where each dimension falls within programmed statistically-based
process limits. Based on these findings, the machine CNC calculates
and activates necessary tool-size off sets.
TOUCH PROBES CAN BE AN EXCELLENT
FIT
Although part measurements for closed-loop process control can
be made by a device as simple as a hand-held gauge, or as complex as
a multi-dimensional gauging fixture, the most cost-effective
solution can often be a machine-mounted touch probe system.
Unfortunately, this highly practical measurement technology is often
rejected out of hand by Quality Control managers, since it relies on
the coordinate feedback system of the machine to produce its
measurement reading. Many QC people just can’t buy the concept of
measuring a part on the same machine that is making it. And they
have a point – if you’re thinking along the lines of taking a
measurement, plotting the data on a chart and doing an SPC analysis.
The probe is not recommended for applications where size data must
be extremely precise, since its measurements can be no more accurate
than the machine tool’s positional accuracy.
However, if your purpose is simply to
control the machining process, the touch probe can meet your needs
at far lower cost than alternative hard gauging systems. With a
locational repeatability of better than 40 millionths, it will
successfully control any process where the dimensional tolerances to
be maintained are 0.001" or greater. What’s more, when
programmed correctly, the touch probe system offers as much
flexibility as the CNC machining center or lathe on which it is
used. Generally, the system will provide process control for any
parts you can cut within the machine tool’s work envelope.
APPLICATION VERSATILITY
In addition to cost-effectiveness and flexibility, touch probing
systems provide the versatility for a wide range of closed-loop
process control functions. This is especially evident in pre-process
applications. On pallet machines, for example, the probe will allow
you both to confirm the correct part, and confirm or correct for a
good part load. If your setup is a load of parts on a gang fixture,
you can use the probe to set an individual work shift for each part.
For cast parts, which often introduce the variables of foundry flash
and thermal drift, the touch probe will determine the right amount
of material to remove.
|
In
a high-production atmosphere like Haas Automation, accuracy is
critical, and consistency from part to part is essential. The use of
touch probes for closed-loop process control provides automatic
feedback of gauged information directly to the machine tool’s CNC
for continuous control of part size. In-process
control measures
the part prior to finishing cuts to determine the tool offsets
needed to keep finish dimensions within tolerance. |
The touch probe is no less effective
in in-process applications, where it is used to measure parts prior
to finishing cuts, or in
post-process applications, where it will measure a part after it is
machined to determine where each dimension falls within programmed
statistically-based process limits. |
For all parts that it can measure,
the touch probe system, if correctly programmed, will pay for itself
rapidly by maintaining continuous in-tolerance parts production.
There may be some dimensions on some parts that a probe cannot reach
to measure – for instance, a groove inside a casting. But such
cases are relatively rare. Overall, a probing system offers
outstanding return on investment. It may cover only 75% of all
possible application requirements, but it will probably solve up to
90% of any shop’s closed-loop gauging needs.
USER-FRIENDLY APPLICATION
SOFTWARE
One of the only real problems with today’s touch probe technology
is the extent to which acquired probes go unused. In some cases, a
customer will receive the probe in a box, discover he has trouble
getting it to work, and put it back in the box under a workbench.
Larger manufacturers will often purchase a machine equipped with a probe, but stop using it when the
work part changes and the probe
needs reprogramming.
| Closed-loop
probe systems, when used for pre-process control, can be used to
make measurements to establish correct setup and/or depths of cut
before the part is machined. This type of system is most effectively
used for cast parts, gang fixtures and pallet machines. |
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The solution to this problem is in
the application software – the component that makes the probe do
what it does. Correct application programming is essential to
productive p robe performance. User-friendly application
software and easy-to-follow programming instructions – such as
those provided with the Marposs Mida® Touch Probe – liberate the
potential of touch probes for cost-effective process control.
Tom Stewart is the Sales &
Marketing Manager, Standard Products Division
of Marposs Corp.
(Auburn Hills, Ml).
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