he press brake’s (or 'brake press') versatility means the machine can be found in almost every Australian fabrication shop but that versatility also makes guarding the unforgiving press brake so challenging. The guarding has to accommodate metal of all sorts of shapes and sizes and there is often no way of avoiding frequent human interaction with the massive upper tool or 'ram'.
Worst of all, there is absolutely no margin for error: most press brake injuries are amputations. Such severe injuries, the frequent interaction and the fact that injuries are scarcely avoidable mean most press brake operations come under Category 4, the most stringent category of AS4024.1, Australia's principal machine guarding standard.
The controls required by AS4024.1 for Category 4 hazards are detailed.
The standard says that: "…a single fault in the control does not lead to a loss of safety function(s)," which means there must be redundant circuits and devices, and later adds that: "the single fault is detected at or before the next demand upon the safety function. If this is not possible, then an accumulation of faults shall not lead to a loss of safety function."
All faults must be detected in time to prevent the loss of the safety function, so safe monitoring systems used to check the input and output safety circuits must detect more than the loss of redundancy. They also need to detect faults such as cross shorts between channels, which could hide a switch failure, leading to an accumulation of faults and a loss of safety.
What does this mean for the employer and the engineer? In short, safety gates, foot switches, two-hand controls, emergency stop buttons and light curtains or lasers are not enough. All the safety devices must be monitored and controlled by a doubly redundant safety system.
Press brakes also fall under Australian standard AS 1219 – 1994, Power Presses. While currently under revision, AS 1219 stipulates that when the ESPE is muted to allow the press to close while an operator holds the workpiece, the tool must not move faster than 10mm per second.
WorkSafe Western Australia summarises this area of the standard well on its website at http://www.safetyline.wa.gov.au/pagebin/edcntool0002.htm:
"Sometimes it may be necessary for the operator's hands to be close to the blade while the brake press is in operation. Access to the hazardous area via the front of the machine, by removal of a physical guard or the muting of a light guard, is only then permitted where it cannot be avoided and the press is set so that it will only operate in the following modes of operation:
(a) The press is operated with the maximum opening between the blade and the press block always being 6mm or less.
(b) The press is operated in the pulse mode and ALL of the following apply:
• the blade descends in steps of 10 mm or less, and
• the blade stops not less than 6 mm, nor more than 7 mm, above the work piece before the closing movement, and
• there is a delay of at least 0.3 seconds between each step.
(c) The press is operated in the slow descent mode and ALL of the following apply:
• the blade descent speed shall be constant and not exceed 10 mm/second, and
• pressure has to be maintained on the "deadman" foot pedal to keep the blade descending, and
• the blade stops not less than 6 mm, nor more than 7 mm, above the work piece before the closing movement can be made."
Achieving this level of control has several safety implications and a properly guarded press brake is likely to have many more safety devices aside from the obvious foot pedal and/or two-hand control.
First, electro-sensitive protective equipment (ESPE), such as light curtains or lasers, is essential even when two-handed controls are in use. Used in isolation, two-hand controls cannot protect second operators and many workers using foot pedals have amputated their own fingers in presses. In fact, two-handed controls are now not permitted to be the sole safety function according to world-leading European standard, EN 12622.
Secondly, rear and side gates, emergency stops, mute point and speed control are often integral to the controls demanded by the standards and all must be dual channel, properly monitored devices. The press's operating mode, shear pins, safe speed control and redundant hydraulic valves also need to be continuously checked for performance.
Multiply these devices by several access points and there is potentially a myriad of safety inputs and outputs to control and monitor. Fortunately, automation safety manufacturers have kept pace with the explosion of safety requirements and developed standard function blocks specifically for presses. This software, used in combination with controllers, means the design and commissioning of a Category 4 compliant safety system is not difficult, costly or tedious.
The safeguarding of press brakes will also take another step forward with the introduction of fully electronic press brakes. The e-brake by Safan is built around a new servo-electronic drive based on the pulley principle.
*Frank Schrever has 28 years’ experience in the instrumentation and automation markets and established Pilz Safe Automation .