|Machine Tool Safety Glass
The Thin Line of Defense
by Raymond LeFavor
All components of a huge machine tool need to work together perfectly
to ensure not only a quality finished product, but operator safety.
One of those many components is the material of which vision panels
are made. Ray LeFavor of Mach Glass International describes the
safety issues currently of concern and how standardization will
benefit everyone from the OEM to the end user.
Imagine a scenario. Its early Monday morning. You go to work
at your machine shop. Youre a little sleepy but you make your
way to an eight-year-old computer numerical control machinea
large, self-contained and complex contraption that forms precision
machine partsand begin entering information into the control
panel. The machining cycle on a piece of stainless steel starts
but something is wrong. You made a mistake while entering the
program. Instead of having an eight-inches-per-minute feed rate,
you accidentally programmed 80-inches per minute. The next thing
you see is inserts that broke off from the tooling shoot straight
through the vision panel. The inserts fly past you like bullets
and luck was the only thing that saved you from being severely
Is this fiction? Unfortunately not. This happens hundreds, perhaps
thousands of times per year in the machining industry. In fact,
in the past six years alone, it was a fatal scenario for over
50 American machinists (the worldwide fatality total is unknown).
Parts flying through vision panels in machines can be the result
of many factors. It could have been a programming error, poor
work-holding devices or even defective tooling. Whatever the reason,
you only have one line of defense, the vision panels (glass) in
your machine. The vision panel is the only thing between you and
a deadly projectile traveling hundreds of miles per hour.
There havent been changes regarding vision panels in machine
tools since 1985. With spindle speeds getting faster, more caustic
cutting fluids, and the introduction of higher-pressure coolant
applications, changes need to be made immediately. The new ASTM
Committee F35 on Compatibility of Machine Tool Components with Industrial Lubricants
is addressing this very issue. Its subcommittee F35.20 on Vision
Panels is working to develop a standard on vision panels as guarding
materials, including their proper use and installation. The standard
will evaluate impact characteristics, degradation due to chemical
exposure, and compatibility of the interlayer,
The Current Situation
Vision panels are porous; the polycarbonates of which they are
made soak up everything from dirt to oil-based coolants and, most
dramatically, synthetic coolants. Once the coolants soak in, the
polycarbonates become very brittle and offer little or no resistance
The German Machine Tool Builders Association did a study on this
very issue (see Figure 1). It was a 12-year study exposing polycarbonates to coolants.
The findings were alarming. After just two years, polycarbonates
lose 40 percent of their impact characteristics and 90 percent
after only seven years. This means that a piece of polycarbonate
that started out being .375 inch [9.52 mm] thick now has the impact
resistance of a piece that is .0563 inch [1.43 mm] thick. That
is simply unacceptable. This study did not take into account any
chip-load or higher-pressure coolant applications. Either of these
factors would accelerate the degradation rate several times over.
The same study states that protected polycarbonates (polycarbonates
laminated to a piece of glass) retain 90 to 100 percent of their
impact characteristics for up to 12 years. If we can prevent the
cutting fluids from coming into contact with the polycarbonates,
we prevent them from breaking down.
This is not an attack on polycarbonates. But it is clear that
unprotected polycarbonates are not effective enough for the machine
tool industry. A brand new piece of polycarbonate in a brand new
machine is extremely safe. But, the very first time the machine
is turned on, the polycarbonate vision panel becomes exposed to
hot chips, high pressure coolants, and aggressive fluids and begins
to break down immediately.
The best combination is a polycarbonate-glass sandwich. Hardened
(not tempered) glass goes on the inside of the vision panel and
polycarbonate on the operator side. The glass sheds the coolant
and chip beating and the polycarbonate provides an unsurpassed
transparent safety barrier. This way, the polycarbonate is always
protected from the coolants, and will not break down or become
foggy. Thus, it will retain 90 to 100 percent of its original
One of the most important factors of a glass-polycarbonate makeup
is the bonding agent used. It is critical that the bonding agent
be a resin or adhesive film that is compatible to the cutting
fluids being used in the machine. It is also extremely important
that the glass and polycarbonate is laminated throughout the entire
surface which gives the product uniform strength. In the event
of an impact that results in the interior glass cracking or breaking,
the bonding agent becomes exposed to the cutting fluids. If the
bonding agent is not compatible with the cutting fluids, the two
pieces may delaminate. The cutting fluids would then come into
direct contact with the polycarbonates and begin to break them
Other products for vision panels in the machine tool industry
include laminated glass-glass products or glass-polycarbonate-glass
products. However, any time there is glass on the operator side
of the machine, there is a potentially serious risk for glass
to shatter toward the operator. Mach Glass International performed
dozens of impact tests for the various vision panel make-ups in
the industry. The test consisted of a two pound [0.91 kg] steel
projectile shot with a cannon at 115 mph [51 m/s] at 28-inch [710-mm]
square vision panels. In most cases, the glass-glass or glass-polycarbonate-glass
products contained the projectile just as it was designed to.
But we found that an alarming amount of glass broke loose from
the laminates on what would have been the operator side of the
vision panels. This is referred to in the glass industry as spalling.
There is a product called spall-shield that can be applied to
the glass surface to reduce spalling, but this is not 100 percent
effective. If the impact is great enough, the spall-shield has
been known to tear. If the spall-shield gets foggy, scratched,
or begins to separate from the glass surface, it can be peeled
off, again leaving the glass near the operator unprotected. Having
polycarbonate on the operators side eliminates spalling completely.
The Next Step
ASTM Committee F35 is getting the input of three groups for the
new vision panel standardoriginal equipment manufacturers, end-users,
and used machinery dealers.
The OEMs are critical to the development of the standard. An end-user
will usually replace a vision panel with whatever material the
OEM recommends. If the machine originally came with a thin sheet
of polycarbonate, the end-user perceives this as being safe. Some
OEMs realize that polycarbonates alone are not the answer. They
have already begun to build their machines with thicker, stronger
frames to accommodate thicker vision panels. Some are already
using laminated glass-polycarbonate products directly from the
factory. A new standard from ASTM would definitely speed this
process up. It would give OEMs a guide as to what product is right
for their machine. A thinner laminate could be used for vertical
machining centers and a thick make-up for grinders and lathes.
End-users are next on the list. For 40 years, the only option
machinists had for vision panels has been a thin sheet of polycarbonate.
That was the only product available. Educating machinists is a
tremendous first step. We, as an industry, need to let shop owners
know that there are myriad ways they can protect their operators.
First and foremost is providing the safest vision panel available
and not only having safety glass regulations, but enforcing them.
Remember, the vision panel is the only thing between the machined
part and the machinist.
Third, the used machinery dealer is important to the standards
development process. When reselling a used machine, it is the
used machinery dealers responsibility to make sure that it is
safe. Selling a machine with broken, discolored, or completely
opaque vision panels should never be acceptable.
Ensuring operator safety is everyones job in the industry. Taking
a pro-active stance through standardization is the best thing
we can do to limit catastrophic injuries. It is not a matter of
if an accident will occur but when. Vision panels are a great
place to start. Creating standards for them is an easy, immediate
solution that will provide lasting safety for operators. //
Copyright 2002, ASTM