Contact, non-contact, in-machine, off-line—these are the issues that
many machine tool users are debating as they strive for greater productivity and
efficiency through tool measurement and breakage detection systems. Here is one
viewpoint presented by Blum-Novotest Group.
Guenther Blum, founder and CEO of Blum-Novotest Group (Ravensburg,
Germany), has been inventing in and around the machine tool business all his
life. And after 33 years in business his company is not about to stop thinking
of ways that machine tools can be more productive and do more than remove metal
and shape metal parts. His position: That machine tools, specifically horizontal
and vertical machining centers and to some extent lathes and turning machines as
well, can incorporate systems for tool measurement and breakage detection all in
the machine work area and only negligibly interrupting the machining process.
“The technology to bring these things about,” Blum says, “has been around for a long time. As early as 30 years ago, when NC/CNC was in its infancy, we began looking at using the new control technology to effect measurement in the machine tool. Early on we worked with Fanuc and Siemens to convince them that NC could do more than control the axes of movement in machines tools; it could facilitate in-machine measuring as well.”
The
early days—
From the beginning, it was non-contact measurement in machine tools that
preoccupied Blum. However, there were other obstacles. For one, the control
community thought that the application was at best dubious, and one with perhaps
little hope for success. Most machine tool builders were equally unenthusiastic,
understandably, as their concern was making parts, not measuring. Then, too, the
technology Blum sought to employ in his non-contact system—laser
technology—was not advanced enough for use in the applications he foresaw. At
the time, lasers were bulky, expensive and they could not be switched on and off
repeatedly. Plus, the first affordable diodes operated only in the infrared
wavelength.
By 1982 Blum had introduced a mechanical spindle probe, which was soon
followed by other instruments, primarily mechanical tool setters and spindle
gages for measuring tightly fitting IDs. However, non-contact laser measurement
was still foremost in Blum’s mind, and three years later he tested a laser
system that could measure to 0.1 mm. He then improved the accuracy to 0.01 mm
and felt convinced that measurement as precise as 0.001 mm was within reach
(which was realized in 1992).
“We believe that the possibilities and opportunities for an optical
system were at least 10 times greater than those of mechanical systems,” Blum
says. “With mechanical systems you have to touch the tool to check its length
or check for breakage, but you cannot tell if the tool is just chipped. You
cannot measure length and diameter while the tool is in rotation, at full
operating speed. For this, as well as checking the contour of the tool, you need
an optical system. And then there are very sensitive tools, thin tools, which
are next to impossible to check with a mechanical system, especially ceramic or
diamond tools that are very easily damaged by contact.”
Trends
in measurement—
Early in his work with laser measurement systems, it became clear to Blum
that the system had to reside in the work area in order for it to have an impact
on affecting the process of the machine tool. Which raised two issues: Where to
locate the system and how to protect it?
The location part was solved fairly routinely. As long as the distance
between the transmitter and the receiver is not more than two to three meters or
so, the two can be aligned in almost any plane and achieve very precise
measurements. In fact, more than 200 configurations have been designed to
effectively place the laser in the working environment of a machine tool.
“Space is a premium in the working area of a machine,” Blum says.
“And while it may be sometimes difficult to arrange a contact probe in the
machine work envelope, the laser system can be mounted on both sides of the work
area to point the laser beam. An encouraging development is that many machine
tool builders are inviting us to participate in the design phase, designing in
our system as part of the overall machine design. This is ideal. You work with
the builder, you know the machine, how it’s going to be used, what it’s
going to make, and together you figure out where best to position the laser.”
Protecting the laser was another issue. Blum says that when he introduced
the instrument 15 years ago, conventional thought held that an optical system
could not survive the working conditions of a machine tool. The coolant, the
chips, the mist, the grit—these would destroy an optical device. “We spent a
long time looking at air expulsion, or blow-off, systems, which would basically
purge the optics and blow away the debris. But an air system alone wouldn’t
work.”
Nine years ago, Blum developed a mechanical shutter system that seals the
optics from the harsh environment of the machining area. According to Blum, the
system is simple: the shutter opens, air is released in a ‘spitting effect’,
clearing away coolant and chips, the laser beam is transmitted, and then the
shutter again closes. “Other manufacturers offer laser systems without
shutters,” Blum reports, “but I don’t think they have the repeatability of
a shuttered system. We still offer a system without a shutter, but never for
machine tool applications. Wood carving, perhaps, or other special applications.
In machine tool applications, the development of the shutter system was almost
as important as the development of the laser.”
Contract,
non-contract, mechanical, optical—
Whether to use non-contact or contact measurement systems is often
dictated by the size of operation, the type of application, the condition of the
machine tool and, of course, the time involved in taking measurements.
“It’s absolutely clear,” Blum says, “that nobody installs
anything in a machine tool if it does not enhance the process and provide a
benefit. Whether it’s a laser system or a probe, the person buying the
technology has to determine whether invading the machining process is worth
it.”
Blum points out that all measuring takes time, whether it’s done in the
machine or off-line on a gage or CMM, measurement cannot be done in a vacuum,
outside of time. So, if one invests the time, there must be an offsetting value
in return. A significant value in measuring in the machine is the detection of
broken tools and, thus, the prevention of machine crashes. “If you crash a
high-speed spindle,” Blum says, “the result can be a two-to-three week wait
for a replacement. And that’s lost time. Further, if you run a machine
unattended, you need a system to detect worn or broken tools, so that a new or
redundant tool can be loaded automatically, permitting you to continue
uninterrupted operation.”
Blum points out that even the most sophisticated, highly accurate
machines require some degree of measurement. “Machine tool accuracy and
in-machine measurement are independent of one another,” says Blum. “Sure,
some functions of measurement—spindle probing, for example—will be easier in
highly accurate, very precise machines. But you still have to do tool setting,
which has no relation to machine accuracy. You’ll still have to measure
spindle growth and detect broken tooling, even in the most accurate machines. As
long as tools break and tools must be adjusted and the temperature has an impact
on the geometry of the machine and spindle, laser measurement, and probing in
general, will be necessary.”
Blum suggests, however, that for the customer the decision often involves
more than just getting a solid return on his investment. A number of factors
need to be considered, the nature and cost of the workpiece, for example. If a
shop is machining an aircraft component out of inconel or titanium, and the
workpiece costs $40,000, one crash due to undetected broken tooling can result
in a total loss. So in this case, a single avoided crash can pay for a laser
system many times over.
“Even in high-volume production of less-expensive workpieces,” Blum
says. “when you have a broken tool that’s not detected, you’ll continue to
produce bad workpieces until the problem is discovered. These related costs are
always higher than the cost of the laser.”
Blum suggests that golf club manufacturing is a good example. Here, you
have an inexpensive machine producing an inexpensive product from inexpensive
material. Now, when something goes wrong, the concern isn’t so much the
material costs as it is the time that’s lost. The time invested in developing
the process, the hourly cost of the machine and the salary of the
operator—these are unrecoverable.
Measurement
and the small shop—
“The dilemma for many small shops,” says Blum, “is whether they
should invest in tool measuring and breakage detection. Can they justify these
systems? If so, do they use contract of non-contact systems?”
Blum suggests that the economy is forcing the decision for many of these
small operations. He points out that it’s a mistake to make assumptions about
smaller shops—that they produce less-expensive components, utilizing
inexpensive machines and that they don’t suffer the same pressures as larger
shops. On the contrary, many small shops, faced with tight pricing and delivery
pressures, find their competitive advantage in being able to run 16-24 hours a
day—and running the second or third shift unmanned. “This allows the smaller
shop to be a third more productive,” Blum says, “without increasing his
personnel costs. A person is always more expensive than a tool; hiring someone
to stand at the machine during that second or third shift is more expensive than
the cost of a laser for tool measurement and breakage detection.”
But are there criteria a small shop can use to determine the type of
system it needs? Yes and no, says Blum. High-end applications, with complex
machines and complex workpieces and very complex shaped tools might well be the
criteria for a non-contact system, like the laser. On the other hand, simple
components like the golf clubs referred to above, with few, very simple tools,
and a less expensive machine might well be the criteria for a touch probe.
Blum advises us to remember, however, that there are many, many
applications where very simple workpieces are run on very low cost machines, but
the tools in these cases may be very thin and a mechanical contact system would
present the real danger of tool breakage.
Blum further points out that touch probes for tool length and diameter,
of which there are very many in use, make little sense. “Our research shows
that 90% of mechanical proves are used in only one direction, as in length
setting,” he says. “And while you can make a mechanical probe very precise,
such as a spindle probe, when you use a mechanical or contact probe to create a
measuring signal, there are often difficulties.”
The difficulties Blum cites are problems inherent in all mechanical
devices and switches. Mechanical switches wear due to repeated contact. And as
the switch wears over time, you begin to lose accuracy in measurement. “The
fact is,” says Blum, “mechanical devices have a limited life span. But which
is more reliable, mechanical or optical? This is really a function of the risk
of a mistake due to a wrong measurement. The risk is higher with a mechanical
probe—statistically at least 60% higher than that of an optical system. In the
case of the mechanical probe, you have to contend with contamination of the
tools, contamination of the measuring surface and the life of the mechanical
switch. None of this applies to the laser system.”
The
measurement misnomer—
One of the issues Blum is quick to point out is that although his laser
systems are used for tool measurement and breakage detection, they are not
measuring systems, per se, and have little or nothing to do with SPC or other
quality programs. “SPC,” Blum says, “is statistical process control. If I
measure five workpieces out of 100 and through mathematical models I calculate a
trend and analyze the trend, then I am using the machine tool like a CMM. If
I’m making engine blocks and I’ve got a bore gage in the tool magazine, and
after 94 engine blocks I measure all the bore diameters of the next five, then I
can predict the accuracy of the previous 94. This is SPC, and it’s not what
the laser system does.”
“Our laser system can provide statistical data about how a machine tool
behaves,” Blum says, “We can dynamically check what’s going on, spindle
expansion, for example, and thermal drift of the machine axes. Contact systems
cannot do these things because for them to work the spindle has to stop. But
rather than focus on these statistical features and issues, we should focus on
what the customer really wants and needs.”
The need is production supervision. “We call our systems measuring
systems, but really they are measuring to guarantee production. Not quality
inspection, our laser systems are for tool setting and breakage and detection
assures good workpieces, whereas a workpiece probe or post-process gage
identifies bad workpieces. Our system is designed to prevent things from going
wrong, not to detect them after the fact. We want to prevent scrap, not produce
it.”
And
the future?
Blum is optimistically realistic when it comes to looking ahead. He can
clearly see certain things happening—contact probes with wireless transmission
as standard for workpiece measurement. But on the larger issue of total systems
acceptance, he’s less clear. “Some customers will still be skeptical,” he
says. “They’ll see measuring as time-consuming and use it only when
necessary. Others will try to avoid it entirely because machines should make
chips and not be used for measuring.”
He conservatively estimates that of new machining centers, perhaps 30%
will be equipped with spindle probes. Tool probing, however, is trending toward
100% acceptance. Among these laser systems will increasingly find acceptance in
the checking of rotating tools, as in milling and horizontal machining centers,
and especially in higher-end applications and machines. Lower-end applications
and vertical machining centers will probably stick with the simpler, mechanical
approaches.
“One thing we can say with certainty,” Blum says, “is that these
systems will continue to evolve. What is state-of-the-art today will not be
tomorrow. Our products will continue to advance and be more robust and
flexible.”
For
more information contact:
Corporate
Headquarters
Blum
LMT, Inc.
250
Grandview Drive #10
Ft.
Mitchell, KY 41017
859-344-6789
Fax:
859-344-6799
www.blum-novotest.com
E-mail:
solutions@blumlmt.com