Machine Safety Glass, Inc. Safety Article

Imagine this scenario, you go to your job as a machinist and arrive at your workstation. The machine that you're operating is a large vertical mill. You have to climb up four stairs to the control panel because the motor on this particular machine is near the floor and spins the parts horizontally from underneath. You load a 500 lb. part that is approximately the size of a car's tire rim. The cycle begins but something is wrong. The part that you're working on is spinning at approx. 2,000 RPMs but comes loose from the work holding device. It hits the safety window (vision panel) of the machine and goes right through without even slowing down. It hits you in the chest and sends you hurtling off of the platform, through the air 27 feet and onto the concrete floor. You don't survive the accident.

Unfortunately, this isn't a fictional story. It happened about ten years ago in Oregon to a 30-year old, father of two. His widow sued the machine builder and was awarded over twelve million dollars. This accident left one family decimated, a small, family-owned machine shop in shock and a major machine tool builder reeling. All over something as simple as one single piece of safety glass.

Back in 2006, I was asked by the plaintiff's attorney to testify as an expert witness in this case and have done so in two cases since. I've been involved with machine tool safety windows for almost twenty years. Now, I'm not exactly sure about the "expert" part but I believe that I've probably spent more time on this specific topic than maybe anyone else in the industry.

Believe it or not, early forms of polycarbonates were starting to be developed in the late 1890's. Lexan (a brand name for polycarbonate) was patented in 1953. For many decades since, polycarbonates have been used as guarding in machine tools. Even today, there are some brand new machine tools costing hundreds of thousands of dollars that still install just a single, thin layer of polycarbonate as their primary vision panel even though much stronger, multi-layered safety windows have been available for years. Machines have evolved greatly, especially over the past few years, shouldn't the safety windows have also evolved? Why are brand new, cutting-edge machines still being manufactured with an antiquated "safety" window that was invented over 63 years ago?

This isn't an attack on polycarbonates. Polycarbonates are the strongest, clear material made but the minute you start using your machine, the polycarbonate starts to break down. They are an integral component of mufti-layered safety windows, however, they are just one of the components that make a machine tool vision panel a true, ballistic quality safety window.

Polycarbonates seem smooth to the touch and to the naked eye but under a microscope, they are very porous. They soak up everything from oil, moist dust particles, natural fluids, caustic, synthetic cutting fluids and even cutting fluids that mist in the air. They are also being pummeled with hot metal chips that are continuously scraping the surface of the polycarbonate away. If you have a window in your machine that is cloudy, foggy or opaque, this means that your polycarbonate "safety" window has been scratched, etched and impregnated with fluids resulting in a brittle and unsafe window.

I have been in hundreds if not thousands of machining companies over the past two decades and they all have one thing in common, there is at least one (if not many) machines operating every day with an unsafe window. Just take a look around your shop and you'll see it for yourself. Maybe it's even the very machine that you're operating right now. Just because a safety window isn't cracked, broken and visibly compromised, doesn't mean it's protecting you.

The German Machine Tool Builders’ Association did a study on this very issue (see Figure 1 below).

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.

We've all seen thick, multi-layered, bullet-proof windows before at banks, convenient stores, armored cars, etc. These multi-layered windows are the same concept behind the latest machine tool safety windows. With spindle speeds of machines reaching tens of thousands of RPMs and coolant pressures exceeding 1,000 PSI, the need for stronger safety windows is obvious.

What I have found over the years to be the strongest, simplest and most economical machine safety window is a glass / polycarbonate sandwich. Glass is an amazing component. It isn't porous like polycarbonates which means it doesn't allow any fluids to soak into it. It's also extremely hard and resistant to heat which means it can stand up to hot metal chips generated by the machine. But glass isn't very good against impacts because it has a low tensile strength. So, if we take a piece of glass and bond it to a piece of polycarbonate, we have the best of both worlds. Extreme resistance to heat, coolants and hot chip loads while offering incredible impact resistance to protect the operator. We're also seeing a new trend in the industry. There are now safety windows being manufactured with wire mesh embedded right in the layers resulting in an even stronger window with unsurpassed impact qualities.

Another important factor in a safety window is the bonding agent that is used. You need a bonding agent that will also stand up to the hostile machining environment. If the interior glass layer sustains an impact hard enough to crack it, the coolant will start leaking into that crack and attack the bonding agent. If the bonding agent then also breaks down, the coolants will start attacking the polycarbonate and you're back to square one. You need the bonding agent to fuse the layers together forever and be impervious to caustic cutting fluids. The individual components of a safety window aren't very strong by themselves, they need to be bonded together in a specific order so they can act as one strong, cohesive window.

A third important variable is the frame holding the safety window into place. If you have a thick, multi-layered safety window in your machine but the frame isn't deep enough to adequately secure the window into place, the windows can actually become dislodged from the machine's door during an impact. Many safety windows on new machines are still being held into place using a thin, rubber gasket. In some situations when I have inspected machines, I was actually able to push the window out of the machine using only the force of my hand. Just think of the damage a rapidly traveling, chunk of metal would do? The rubber gaskets lose their durometer (hardness) after being subjected to coolants and become very weak.

Ensuring operator safety is everyone’s 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.

I keep fielding the same question over and over, "Do your windows meet the standards?" Well, I'm sorry to say that even after decades of empirical data, there are still no mandatory American standards that I know of regarding machine tool safety glass. There are a couple of loosely worded, voluntary standards to guide the industry but still no mandatory standards regarding specific components, products or manufacturing guidelines. This means that there is an entire industry who's primary (and in my opinion the most important) safety component is completely unregulated. Most of us aren't engineers, how are we supposed to know that the machine that we're purchasing and operating has a safe window? Would you purchase a brand new car with a substandard windshield?

I was involved on an ASTM (Association for Standard Testing Materials) Committee about fourteen years ago that wanted to address this very topic. After three years of traveling, meetings with OEMs, etc. (there wasn't such a thing as a virtual meeting back then) I personally wrote their final draft of the proposed new standard. After years of time and hard work, the committee was disbanded due to lack of interest. That's right, I said lack of interest. I was floored to the level of apathy industry-wide regarding this highly important machining safety aspect.

One thing you can do immediately is start asking questions. If you're purchasing a new or used machine, are an engineer or an operator, make sure you know that the machine's window is safe. Is it strong enough to contain the maximum amount of joules of energy that your machine can generate? Ask how thick the window is. Find out what materials it's made from. I understand companies want a machine that will help maximize profits for their businesses, but profits should always take a back-seat to an operator's overall safety.

In the whole realm of things, a safe operator window is a relatively easy and inexpensive safeguard against a potentially dangerous and vastly more costly accident. Many thick, laminated safety windows are available for under $300. If you spend hundreds of thousands of dollars on a new machine and only $300 to ensure your machinist's safety, it really seems like a no-brainer.

The other change that I would like to see is some effort to finally make mandatory standards for machining windows. My overall goal is to change the entire machine tool industry's view on safety glass. It shouldn't be a design after-thought. It should be the number one safety concern over all others. With just a few simple design changes to doors as well as a shift in our perception of what "safety glass" should really mean, any and all injuries from ejected parts could easily be a thing of the past. You don't think twice about wearing a hardhat, safety glasses, hearing protection or steel-toed boots because OSHA requires it and it just makes good, logical sense. Shouldn't we start thinking about machine tool safety glass in that same light? It could literally mean the difference between life and death.