One safeguard (not) for all

2012-10-18 19:40:18

Press brake safeguarding requires a holistic approach

No one press brake safeguard works fore all applications.

press brake safety evaluation

Figure 1
Any press brake safety evaluation should consider factors such as the application, part geometry, tooling, and brake type.

Press brakes are one of the most versatile machine tools on the fabrication floor. Shops continually add new tools, from specialized die sets, hemming tools, and flattening dies, to tools for piercing, notching, and punching. This has done wonders for the machine tool's flexibility and efficiency, but it also has made the press brake one of the most challenging machine tools to safeguard properly.

For years safety professionals, insurance representatives, and various other inspectors realized that the technology available for safeguarding devices was either not practical for press brakes or created its own hazards. For instance, operators can become tangled in restraints, harnesses, and pullbacks, and even lose control of the part. Other times those evaluating press brakes didn't have a clear understanding of the technology. In the past the inspectors saw how people operated the machines, witnessed the great variety of parts going into them, and recognized there was no practical way to apply general-purpose safeguards and protect operators for everything they needed to do at the brake.

New safeguarding technology has changed matters for the better. Between light curtains, laser-based systems that travel with the ram, two-button control, and other guards, almost any brake situation can be effectively and reliably safeguarded with minimal reduction in productivity.

Still, no one safeguard does it all. OSHA inspectors look at press brake safety holistically, and companies should do the same (see Figure 1). It is not about this light curtain or that laser-based traveling system, foot pedal usage and placement, two-hand control, restraining systems, or other guarding devices; it's about choosing the best elements for the application and designing a system so the safeguards work together to create a safe environment for the operator.

Wrong Guard, Unsafe Operation

Using the wrong safeguard can be just as bad as no safeguard at all. For instance, safety distance requirements have meant light curtainswhich emit infrared beams in front of the work areamust be installed a significant distance from the pinch point, a measurement that varies depending on the reaction time of the machine. To bend small parts, operators may have to close the tooling to a-inch opening or less, when the curtain mutes, and insert the part. Or, more commonly, they just bypass the curtain altogether.

Odd-shaped parts also can cause light curtain issues, because certain flanges may block the light curtain at the wrong times, so the curtain must be deliberately ignored to complete the job. This can lead to "muting abuse" in which the operator, for convenience and efficiency, tells the system to mute high in the stroke, so high that it ignores most obstructions and ceases to function as a safeguard. "Blanking abuse" happens when operators tape off a series of optics on the curtain so it ignores side flanges, multiple bend progressions, and other procedures. Finally, many operators just turn off the curtain. Typical practice has been to allow the operator to turn off the light curtain as long as he uses another safeguard, be it a two-hand control, pullback device, physical guard, or something else. But quite often the operator doesn't use any guard at all.

This doesn't point blame at the operator. The reality has been that, until relatively recently, many press brake safeguards have been intrusive, and often they've significantly slowed production.

Laser-based Protection

This changed with the advent of laser-based safety systems. Certain systems (see Figure 2 and Figure 3) mount directly on either side of the ram; emit a beam just below the punch tip; and monitor the machine's performance, including its stopping distance and travel speed. If an operator's finger crosses the beam while a pinch-point hazard exists, the machine stops.

These beams also can be set up to mute certain portions of the beam automatically. In a box bend, for instance, the operator positions the flanges to the side of or behind the tooling. The laser system, drawing from information in the part program, knows which areas to mute to account for those flanges, and which areas to keep active throughout the brake cycle.

Certain laser-based, close-proximity safeguards watch the operation at a much higher level than light curtains or two-hand-control devices can. This is why laser-based systems affected the market in a big way when they were introduced during the last decade. For the first time, the market had a practical, efficient, and productive way to safeguard the press brake operator when working with small parts.

Of course, such laser-based safety systems don't work in every circumstance. During the operation, the beam, moving with the ram, travels directly below the descending punch, covering only just behind, below, and in front of the tooling. Wide tools including punching and notching die sets, wide-radius dies, and flattening diescreate problems. The laser beam may not be wide enough to protect the operator. For instance, a 6-in.-radius die set would not work with a traveling safeguard system emitting a beam just 2 in. wide. When the punch descends, there would be 4 in. of pinch-point area that the laser couldn't see.

Another issue is that a laser system aligns with one punch height at a time. A laser system is programmed to mute just before the punch comes in contact with the dieat in., too narrow for someone's finger. (If the system didn't mute just before the punch contacts the part, the safeguard would see the part being bent and tooling as "obstructions" and shut down the machine.) This means the laser beam aligns with just one punch height. If the application uses staged tooling without common punch heights, the laser beam would ignore obstructions at a point where the operator could still mistakenly reach in during the hazardous portion of the stroke. To use the laser-based system requires common passline height across all the punches.

Finding the Right Combination

More facilities today have applications that require more than one type of press brake safeguard (see Figure 4). Every application is different, and each requires, again, a holistic approach.

First, consider the brake's age and its features. Press brakes should have dual-channel, or redundant, controls, so that if one channel fails, another still can carry out a command to stop the machine. On older machines a control upgrade may be a worthwhile investment. Given that such machines expose operators to crushing injuries, their control reliability is incredibly important to back up any type of nonintelligent safeguarding device. (Note that certain intelligent laser-based systems can monitor speed and stopping performance to provide an extra layer of control reliability.)

Older friction-clutch mechanical brakes, air clutch brakes, and even some hydromechanical brakes usually cannot stop fast enough for close-proximity safeguarding. These safeguards work only for hydraulic brakes and similar machines that can stop quickly, such as many modern servo-drive machines.

In fact, older brakes in general are less safe. While it's true they may work well for their purpose from a productivity standpoint, the fact remains that modern press brakes, which generally have shorter stopping distance, make an operator's job safer. Regardless of the machine's age, though, proper preventive maintenance is essential, not only for optimal safety but for productivity as well.

Tooling design should also enter the safety evaluation. Using the wrong tool for the job can create an unsafe environment that no safeguard can fix. Say the operator is forming a small ring made of a thin strip of steel about 2 feet long, requiring about a dozen bumps. If he doesn't have the right tool for the job, the operator must reach around and hold the back of the material as it is formed. Because the operator can't see his fingers, he can easily get them caught in the backgauge or possibly against the ram. In this case, the proper-radius tool may eliminate the problem.

A shop needs to match the safeguard for the application by considering the part geometry, tooling, press brake type, and control. A laser system won't work for an application with extra-wide tooling, just as a light curtain won't work for small parts. A barrier guard won't work if the part's flange will bend up and hit it; in this case, the operator would be forced to open the barrier guard gap wider, exposing pinch points, and again defeating the purpose of the safeguard. Two-hand control may work in certain instances, but in many cases operators need to hold the material before and during the bend cycle.

In a machine that accepts a variety of tools from punching tools to hemming die sets, conventional V punches and dies, and gooseneck tooling using both a laser-based system and a light curtain may work best, with the operator able to switch between the two. Alternatively, some companies may group tooling and applications that require a light curtain on one machine, while relegating the remaining applications to a brake with a laser-based, close-proximity guard.

Consider one brake application that requires conventional tooling with a single passline height, good for a laser-based safeguard, but occasionally runs a specialized-radius tool curved like an S to bend a specific contour. In this instance, the laser safeguard wouldn't work. Just before the tool would contact the material, the laser would mute; meanwhile the tool would continue descending, exposing the operator to multiple pinch points as the S contour took shape in the metal. Here the company uses a two-hand control and installs a magnet on the backgauge, so the operator doesn't need to hold the part during the bend cycle. The next operation involves gooseneck tooling with a common passline height, and the two-hand control doesn't work, because the operator has to hold the parts. So, the operator simply switches back to the laser-based safeguard.

Safety-conscious Culture

Workers don't maliciously cheat safeguards. Often it's a matter of supervision and company culture. Workers usually are rewarded by how productive they are, not how safely they perform their jobs. They even may be paid on a piece-rate system, in which their paycheck depends on the number of parts they produce.

A brake with the wrong safeguards can seriously hinder productivity and cause serious hazards. If an operator needs to hold the part during bending and deal with a two-hand palm-button control at the same time, the operator will find a workaround. The potential avenues for safeguard abuse are endless, but tackling the problem with disciplinary action won't change things over the long haul. Instead, proper safeguard selection and a safety-conscious company culture, with periodic training and proper enforcement, are the only long-term solutions.

Through it all, input from operators and setup personnel is vital. They know the parts being produced, machine setup requirements, and the tooling involved. These people, along with engineers who know part design and bend requirements, as well as safety professionals who know all safeguarding options, need to be at the table before going forward with an evaluation.

With everyone at the table, the company can adopt the best safeguarding strategy for the applications at hand.

Ultimately, it's not unusual for a shop to have three or more types of brake safeguards for various press brakes. But it's incredibly rare to have one kind of safeguard work for everything.

4 Inches: Only a Last Resort

Buried in the pages of OSHA's press brake safeguarding directive is a statement that has caused much confusion. The document says that for certain jobs, 4 inches is an acceptable safety distance from the nearest pinch pointbut with three big caveats: only if no other safeguarding device can effectively and reliably protect the operator from accomplishing the task; only if there is a record of periodic, formalized, and comprehensive training programs; and only if the application is active four hours a month or less for one specific job.

Also, 4 in. is not a generic safety distance to use when installing safeguards. The safe distance for applying a point-of-operation safeguarding device, such as two-hand control or a light curtain, is a set formula. The machine's reaction time, control system or brake monitor reaction time, and other variables are used to come up with a value.

The 4-in. rule is a compromise to be used only when no other safeguarding solution will workand this doesn't happen often. Today, with more safeguarding options than ever, including close-proximity guards positioned just below the punch tip, almost every application on a modern press brake can be effectively and reliably guarded.