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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. It’s early Monday morning. You go to work at your machine shop. You’re a little sleepy but you make your way to an eight-year-old computer numerical control machine—a large, self-contained and complex contraption that forms precision machine parts—and 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 injured.

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 haven’t 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 to impacts.

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 strength.

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 down.

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 operator’s side eliminates spalling completely.

The Next Step

ASTM Committee F35 is getting the input of three groups for the new vision panel standard—original 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 dealer’s 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 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. //

Copyright 2002, ASTM

Raymond LeFavor is product manager for Mach Glass International in Big Lake, Minn. He is secretary of Committee F35.