If you’re searching for an application where a robust ROI must be realized, look no further than press tending. Whether making automotive, appliance or agricultural components, press tending applications remain a prime target for many manufacturers looking to automate production. No matter the size or style (hydraulic or servo driven) of the press, modern day press transfer robots and control systems can optimize throughput while maximizing safety and profit – as the same rules apply.
The Need for Automation
Press tending is a very fast application in a highly demanding environment. With high throughput demands and the risk for repetitive injury, finding workers for this labor-intensive application can be difficult. Moreover, most people do not find it appealing to stand in front of a press all day, transferring parts back and forth. The daily reconciliation of press tonnage, combined with the presence of metal parts with sharp edges, is also a deterrent to finding workers to fulfill the tasks available.
The Importance of Cycle Time
When it comes to productivity for press tending (or any application for that matter) speed and cycle time are two driving factors. For press tending, cycle time is definitely a key aspect in the success of an application, and it is important to keep dies stroking at all times. To determine a complete cycle on a press, a 360 degree cycle time is calculated from the point where the press is completely open (top dead center at zero degrees) until it has closed at 180 degrees, before once again being fully open at 360 degrees.
As a rule of thumb, when you’re doing a tandem press operation – where one robot is loading from the front and one robot is unloading from the back – you should expect an average cycle time of one part every 6.5 to 8 seconds, which translates to about 450 to 550 parts per hour. As a secondary rule, when a single robot is being used to perform the same operation, you should anticipate an average cycle time for one part to be 7.5 to 9 seconds, or about 400 to 480 parts per hour.
Additionally, as it relates to cycle time, one of the things that a robot can do is release a part anywhere from 0.5 to 1 inch above the die, assuming the die is constructed so that the part can nest itself in the die. This ability to release the part can improve overall throughput of the line.
The 411 on Robot Orientation
Robot orientation, as it relates to the press line, has a lot to do with robot selection. When deciding on the right robot(s) for your particular task, it is important to pay close attention to the following factors:
– Mounting Options: mounting options are basically determined by the operation that the robot is performing during the material handing cycle. Keep in mind, however, that robots in press room applications should always be mounted in a way that optimizes the flexibility and floorspace requirements of the shop. You still have to be able to get dies in and out of the workcell with ease, and you still might need manual access to the dies for manual operation on occasion. To accomplish the latter, robots can be mounted in various orientations to the press, including swing-out and roll-away bases to accommodate die changes and manual intervention.
– Flexibility: when it comes to flexibility, choosing the correct robot(s) greatly depends on if the part must be reoriented as it is transferred. The ability to tilt a part back and forth, or to rotate it between processes, often requires the dexterity of a six-axis robot. Less complex operations may allow for the utilization of a less costly 4- or 5-axis robot.
– Size and Speed: when selecting the proper robot, size does matter, and there are multiple things that must be considered. You should be mindful of the distance for the robot reach into the press, as well as the part weight (including the gripper). Moment and inertia also come into play with press tending applications, and you never want to undersize the robot from a payload perspective. And, finally, it is important to consider a robot that will work within the cycle time demands of the press itself, as you want to avoid slowing down the strokes per minute to accommodate the speed of the robot.
– Floor Mount Application: one of the most common and simple applications for press tending, a floor mount application typically entails a robot being placed on a riser, positioning the “sweet spot” of the work envelope of the robot near the center line of the die opening. This will optimize the ability to place tool racks, blanking stations or centering stations more freely within the robot workspace, and it will also allow for very optimized robot motion efficiency.
– Invert Mount Application: another common way of mounting robots is in an invert mounting application. In this case, a robot is mounted to a structure or the crown of the press. This type of application allows the robot(s) to reach much further into the press, enabling the service of deeper dies.
– Shelf Mount Application: a shelf mount orientation allows for the robot’s work envelope to extend far below its base. An ideal application for this is where the robot(s) would have to reach down, or in de-stacking applications. This type of mount is also very common for racking and palletizing tasks.
Tips for End-of-Arm Tooling
Very important in the automation of a press is the end-of-arm tooling, as it is what enables gripping of the part. This tooling or gripper allows the robot to be customized for specific applications. In press transfer applications, these tools are often placed on an extension, with the tool normally being the only part of the robot that reaches into the die. When choosing end-of-arm tooling, it’s helpful to weigh the following options:
– Vacuum Grippers: these are the simplest type of grippers, which are typically constructed of a set of vacuum cups that are placed on an extrusion or extrusion extensions. The vacuum cups, themselves, are usually controlled with one- or two-zone solar lights, and are known to be very forgiving, as the cups will always conform to the part geometry. One of the more flexible aspects of a vacuum solution is that you can adjust the center line or spacing on the cups to use them for a variety of applications.
– Mechanical Grippers: ideal for perforated, “oily” or wavy parts, mechanical or clamp-style grippers may be used where vacuum grippers are not as effective.
– Magnetic Grippers: occasionally used in tending applications, magnetic grippers are great for de-stacking applications. However, magnetic grippers are not as flexible as vacuum grippers and can add weight to the end-effector. This can be problematic when it comes to moment and inertia, because the more you weigh down the gripper, the less payload space you might have for processing the next part.
Methods of Tool Changeover
When you’re looking at changeover of dies, there are typically two ways to complete the task:
– Manual Changeover: traditionally, manual die changeover can take a long time, being completed over a period of hours. The good news, however, the gripper can be switched in a matter of seconds. This changeover is normally performed by using a lever-actuated clamping mechanism that allows you to easily mount the correct tool, engaging and dis-engaging it as needed.
– Automatic Changeover: more challenging, automatic die changeover can take two to ten minutes. During this process it is normal to use a tool changer in conjunction with an automatic die changeover system. Please note, the variety of parts being processed may make it difficult to store multiple grippers within the reach of the robot. In a situation like this, a robot track can be used to transfer the correct gripper into the workcell and to remove the previously used end-of-arm tool, extending the robot work envelope.
There is also a selection of sensors on the market that can be used to verify tool identity for use with the robot controller or PLC. These sensors are commonly used today with a Functional Safety Unit (FSU) in the robot controller, providing control-reliable change verification along with associated robot and gripper range limits.
Advice on De-stacking Accessories
For de-stacking, there are many designs of racks or turntables that can be used to feed raw materials into the workcell, allowing continuous operation of the stamping system. The robot controller’s high-speed search function, teamed with sensing devices, enables the quick location of the tops of stacked blanks. Adjustable stack handling equipment with fanner magnets will allow you to find a variety of blank sizes and shapes when performing a press transfer application. In fact, double blank detectors can be used to protect your dies – you never want to pick up more than one blank at a time, as this could risk damage to the die. Keep in mind, when needed, optional vision for detecting misaligned blanks is also available.
Best Practices for Racking and Palletizing
At the end of the line you have to be able to dispose of the parts correctly. Therefore, loading finished parts into engineered part-specific racks, bins or pallets is key. Normally, with modular vacuum grippers you are going to be able to unload parts from the final stamping process and place them into a rack. Sometimes, a part is not placed into a stationary rack immediately. Instead, the part is ejected from the last die, so a high-speed conveyor or shuttle can transport it to a racking or palletizing station where a robot will handle the part onto a pallet or into a rack.
When using a racking or palletizing station, it is important to remember the pallet or rack is most likely going to be removed by a lift truck operator, so it is a good idea to use robust locators with pallet- or rack-present sensors that are mounted to the shop floor, providing the utmost protection to the pallets or racks. Frequently, a vision system is used in applications like this to ensure parts are being placed in the correct location.
Ideas for Retrofitting Robots to Existing Presses
Often presses must be fitted with new controls including motors, encoders, motion controls and HMIs. It is common today to replace the existing programmable logic controllers or existing drives with more up-to-date variable frequency drives, as well as newer servos and PLCs – all of which can be placed into a convenient electrical enclosure.
Press control HMIs should also be considered, as there are many options available. When you consider press status, you should look for things such as die confirmation, cycle time and strikes per minute to verify there’s a part in the die using various switches and sensor technology.
With maintenance, it is helpful to use a master screen to guide you through the die change processes or to help you confirm things like I/O or robot readiness status. With a press line control package, it is very important to monitor variables such as I/O status to ensure that anything related to the RAM is functioning and interlocked properly for safe and efficient operation of the press.
Given today’s labor shortages and throughput rates, robots are a solid option for many press tending applications. Robots can reduce the number of potential injuries that result from prolonged repetitive lifting, and they can eliminate operator fatigue and downtime between shifts. Furthermore, as robots provide accurate, repeatable motion, they can contribute to reduced scrap rates and increased production rates. Most beneficial, perhaps, to making press tending applications a reality is the wealth of standard tools available to help manufacturers.
If you have questions about a shop floor challenge or need advice about retrofitting an existing press, reach out to our experts – we’re here to help.
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