In precision metal stamping, every stroke of the press exerts tremendous pressure on the metal being formed, which can lead to unpredictable problems over time, unless sensors are in place. Parts can be deformed or do not eject correctly, metal can feed improperly, and the die or part can be damaged. This, in turn, may result in parts being scrapped, material waste, production delays, damage to the tooling, and overall higher costs. To help catch any problems before they occur, tool and die manufacturers are turning to new metal stamping sensor technology, also known as in-die protection systems, rather than the typical mechanical monitoring techniques that have long prevailed.
Metal stamping sensors vs. mechanical devices
Over the years, metal stamping firms have used many techniques to monitor production in order to identify defects and quality issues as quickly as possible. However, older, mechanical technology has its limits. For example, electric pilots and micro-switches used on presses can detect the location of the metal strip inside the tool and identify misfeeds, but they do not stop the press before making the hit, which can result in damage to the tool, the part and the press.
Indications that it is time for a metal stamper to replace outdated mechanical monitors with metal stamping sensor technology include:
- Die crashes
- High reject rates
- High levels of scrap
- Cosmetic defects on parts
Not only do problems with press runs affect the metal stamper, but the client also may be negatively affected by the expense of repairing or rebuilding a tool or line interruptions.
Electronic sensor technology addresses these long-standing issues by detecting problems in production before they occur. Unlike mechanical devices, in-die sensors will stop the press as soon as a problem is detected within the cycle of the stroke of the press. In addition, if the sensor itself is not working properly, the press will not run.
Advantages of metal stamping sensors
- Improved overall quality (zero defects)
- Improved on-time deliveries
- Reduced sorting cost
- Reduced tool maintenance costs
- Improved focus on the root cause
- Reduced set-up time and downtime
How electronic in-die sensors work
Given the complexity of precision metal stamping, in-die sensors must be customized and programmed for each press and each process. The size and intricacy of the part being stamped will dictate the type of sensor required. A metal stamping firm that is concerned with quality should have customized electronic sensors installed in every die.
Metal stamping sensor technology monitors the entire process and the critical dimensions of each part as it goes through the progressive steps of metal forming. The sensors are used in combination with press controls, which can automatically stop the presses when something goes wrong and shut down the system before a die crashes.
Controls are programmed to monitor either static or cyclic events. According to Demystifying Die Protection from Stamping Journal, “A cyclic event occurs during the press cycle. Examples are stock strip feeding forward in a progressive die or a finished piece part exiting the die in the parting station. Anything that must happen at a specific time for a die to function properly would use a cyclic program. A static event is something that must always happen—or should never happen—regardless of where the press is in the press cycle, such as the material feeding into the die from the decoiler and straightener. If the material runs completely out, a short feed can occur as the material is pulled back after it has been fed to the correct length… Controls can be programmed to detect not only misfeeds, but also part ejection, slug ejection, or a broken punch that is no longer piercing a hole in the finished part.”
Types of metal stamping sensors
- Analog sensors
- Digital sensors
- Photoelectric and fiber optic sensors
- Inductive proximity sensors
Rockwell Automation, which produces sensors, describes the basic analog and digital sensor types in its Sensor Application Basics as follows: “A sensor is a device for detecting and signaling a changing condition. And what is this ‘changing condition’? Often this is simply the presence or absence of an object or material (discrete sensing). It can also be a measurable quantity like a change in distance, size or color (analog sensing). This information, or the sensor’s output, is the basis for the monitoring and control of a manufacturing process.”
The sensor types best suited for metal stamping applications are photoelectric and inductive proximity. Photoelectric sensors use a beam of light or fiber optics to detect an object, which has the advantage of not coming in contact with the part being measured. By converting electricity into a magnetic field, inductive proximity sensors identify metal objects. This type is also non-contact but can only be used at close range to the object.
Testing metal stamping sensors to ensure accuracy
With the quality of the part being produced and the integrity of both the die and the press at stake, metal stamping sensors must be tested regularly to ensure their continued, reliable operation. The metal stamper will test all sensors before installation to evaluate each sensor’s accuracy and consistency, as well as its ability to withstand the repeated shock of press operations without damage. The sensor then will be tested at different points of the production process, including during press runs to troubleshoot any issues and to help determine the cause of the problem.
As manufacturers in a wide range of industries develop new products with more complex parts, sensor technology is enabling metal stampers to produce extremely intricate parts without a glitch. And as sensors measure critical angles and features on every part that goes through the press, metal stampers can better ensure 100% part quality, while putting a stop to die crashes and minimizing downtime.
To read about Kenmode’s unique metal stamping sensor program, please see the article in MetalForming magazine: Precision Tooling, Superb Sensor Applications.