It’s always critical to work to improve the efficiency of the stamping process, doing everything possible to avoid unexpected downtime or irregular parts, and to protect the die from damage. Proper implementation of die protection prevents die crashes, which ultimately helps fabricators increase press time and productivity and, of course, avoid wasting time and resources on repairs.
The goal of die protection is to stop the press before the die crashes and is damaged. The best way to protect a die is to make sure that nothing is physically out of place during a press cycle. And the best way to do that is to mount sensors in the tooling where the press is equipped with a controller to interpret the signals from the sensors. By detecting target orientation and position, including part ejection and hole placement, the sensors verify processes and reduce the potential for damaging the die.
To learn how to better use die protection to keep a press running smoothly, here are 10 tips for better die protection gleaned from Jim Finnerty, product manager, Wintriss Controls Group.
Know the critical angle
The goal of die protection is to stop the press before a bad hit can occur. To be able to do this, knowing the critical angle for every die is a must.
When a die protection sensor detects a potentially die damaging event, it’s necessary to emergency stop the press. The critical angle is the last point in the press cycle where an emergency stop signal will cause the ram to stop before the die closes. When setting up the controller, it is imperative that it evaluates each sensor with enough time to stop the press. Therefore, the critical angle is the first piece of information needed.
Press stopping time
The faster the press stops, the more time a die protection system has to detect and react to problems. In addition to regular brake maintenance, there are several approaches to improve press stopping time. Here are a few tips:
- If the press is not equipped with one already, install a quick dump valve on the clutch/brake assembly. This auxiliary valve can exhaust the air from the clutch more quickly than the main clutch/brake valve alone.
- Ensure the counterbalance pressure is correctly set for each upper die. Heavier upper dies need a higher counterbalance pressure. If the counterbalance pressure is set too low, it can cause an increased stopping time on the downstroke of the press. If it is set too high, the press takes longer to stop on the upstroke.
- Make sure that the dual safety valve is properly sized to the press. OSHA requires that a part-revolution mechanical power press be equipped with a cross-checked control valve commonly called a dual safety valve. To save money while gaining OSHA compliance, older presses were sometimes retrofit with a too-small dual safety valve.
Sensors that require adjustment during setup can lead to “nuisance stops,” which are stoppages initiated by a sensor when no real problem exists. The only time a sensor should stop the press is when there’s a real problem. If there are too many nuisance stops, the system typically ends up being bypassed or disabled.
Problems arise when sensors are installed in the “general area” with the expectation that the operator or setup person will do the final adjustment when the die is in the press. The practice often means the sensor location is less than ideal, which can result in frequent nuisance stops.
The goal of any installation should be to select a sensor that never needs adjustment, as well as a location for the sensor where it only stops the machine when a real problem is present. Selecting specific sensors for specific dies and leaving them in place reduces setup time as well as the pitfalls of adjustability.
When a die protection program is well established and the tools are fully protected with sensors, die crashes can still occur. And they are most likely to occur when presses are run with the sensors disabled.
Although rare, a sensor could be disabled due to operator error. An operator could forget to re-enable the sensors after a temporary disable, and it could also happen because someone decided that it would be “easier” to run the die without protection.
Fortunately, many control systems offer “auto enable” based on a stroke count. When auto-enable is initiated, sensors are automatically enabled after X number of strokes. This setting can be accessed through the die protection programing menu.
The efficiency and longevity of a sensor is based on several factors, including the location of the sensor. A good rule of thumb is to avoid putting sensors in the upper die.
When a sensor is installed in the upper die of a stamping press, its cable flexes every time the press cycles. This repeated flexing causes the sensor cable to eventually fail. Also, sensors in the upper die are far more likely to be damaged by excessive shock.
There are some cases, however, where it is absolutely necessary to install a sensor in the upper die. In those cases, use an upper die-mounted junction box and run a cable from the upper die to a press-mounted interface. Like the sensor cable, the interface cable will fail from repeated flexing, but it is easier to replace.
Surprisingly, feed detection can be too precise. It is easy to install sensors to detect events such as misfeeds as small as 0.0005 in. However, doing so almost guarantees ongoing nuisance stops because this level of precision is unobtainable in most production environments. If the feeder can hold a tolerance of ±0.005 in., but the sensors are installed to detect a misfeed of ±0.003 in., it will inevitably cause nuisance stops.
If a feed isn’t perfect, the pilots have the ability to align the strip. If the die is designed so that the pilots can align the strip if it is misfed by ±0.020 in., any feed progression within that 0.040-in. window is not die threatening and should not be detected by the feed sensors.
When installing the die protection sensors, be sure to use up all of the available “slop.” Doing so gives the setup person and the equipment enough wiggle room to keep the machine running while still protecting the die from damage due to misfeeds.
Shielded proximity sensors
Inductive proximity sensors are popular and easy to use. These sensors can detect the presence of metal being fed into a machine without needing to be in direct contact with it.
Proximity sensors can be shielded or unshielded. Both types emanate a field to detect the incoming metal. With a shielded proximity sensor, the field is confined to the area immediately in front of the face of the sensor, and the sensor may be mounted flush and embedded in metal, whereas an unshielded sensor may not be mounted flush and embedded in metal because the sensing field will constantly detect the surrounding material, and the sensor will always be on.
Use an unshielded sensor only when extra range from a smaller sensor is needed. Unshielded proximity sensors allow for greater sensing distances.
When using photoelectric sensors to detect air-ejected parts, there are a few rules to keep in mind. For starters, do not use visible mini light curtains or visible light diffuse reflective sensors. Visible light can incorrectly pick up on small droplets of lubricant picked up by the air blast, and confuse those droplets for actual parts.
As an example, the air blast used to eject the part will often pick up some of the lubricant on the die and send it through the sensing field along with the part. In these situations, the lubricant “fools” the visible light sensor into thinking a part came out. This can result in a die crash if the part sticks in the die because the lubricant was ejected and detected by the sensor.
Infrared sensors, therefore, are a good alternative as they minimize the possibility of this occurring because they are much less susceptible to detecting the lubricant in the sensing field. Infrared light is better able to “shine through” lubricant, whereas visible light reflects off of the lubricant.
A sensor’s IP rating indicates the type of environment that the sensor can tolerate. The rating standards are managed by the International Electrotechnical Commission and were set in place to avoid confusion with marketing terms, such as “waterproof.” They offer clear descriptions of the degree of protection that is offered by an electrical device.
Considering the harsh industrial environment of a stamping die, it is best to use sensors with a rating of IP-67. All IP ratings include a two digit number with the first digit indicating the solid particle protection and the second digit indicating the liquid ingress protection. The first number ranges up to 6 while the second number ranges up to 9; the higher the number, the higher the protection.
An IP-67-rated sensor, therefore, is dust-tight and can be submerged in liquid to a depth not exceeding 1 m. In the absence of an IP rating, any device for in-die use should have a NEMA 6 or NEMA 6P rating, which is managed by the National Electrical Manufacturers Association and follows similar conventions to the IP ratings.
Worst comes first
When just starting out with die protection, don’t pick an “easy” die to start. Instead, install sensors on the die that crashes the most because it provides the quickest return on investment. When the worst die stops crashing, the other dies should then be converted with sensors as well.
Manufacturers that implement a die protection system using sensors will help to decrease downtime, part defects and associated maintenance costs by preventing die crashes, reducing scrap, shortening setup times and allowing better labor utilization.