Denesting and Sorting
Following is part two of a larger article originally printed in the July 2019 issue of The Fabricator.
Many punch/laser combo machines come with some type of in-process part removal, which is a big change from conventional laser cutting where denesting and sorting usually occur manually. Some combos have a chute, which can accept parts of a certain size; others have a mechanized suction device that lifts and removes cut parts. On the surface, these systems appear simple and logical, but from a programming perspective, they introduce lots of complexity.
Using a Chute
First of all, the program sequence matters. Consider a part that’s being sent down a chute. If the part goes down the chute in the middle of the cutting cycle, your sheet characteristics will change. A chute-evacuated part will leave an empty space in the nest that needs to be accounted for, lest skeletal integrity problems arise. If the skeleton becomes unstable, you might need to nest with thicker webs and accept a reduced material yield.
These parts need to be able to fall reliably too. If you use a form tool to place a high flange onto a part, that’s great—you’ve eliminated a secondary operation. But to truly save time, you need to make sure the part doesn’t catch on anything as it falls.
For large parts, gravity usually makes chute part evacuation pretty reliable, as long as the chute itself is big enough. But if you’re working with small parts, you might take a different approach. You might laser-cut and punch a group of small parts, leaving microjoints; you’d then use a punch or part ejector to push the small parts down the chute. Because you’re removing many parts consecutively down the chute, the machine spends less time moving a sparse skeleton, which could make the process more stable. After some experience, you might decide that the process is stable enough for a tighter nest (narrower webs) with higher material yield. That said, this is just one hypothetical situation. Your specific part evacuation strategy will depend on your application and company practices.
Suction-based part removal systems introduce additional factors. Configured well and accounted for correctly in the nest layout, these removal systems can reduce labor costs significantly. With no human intervention, parts can be cut, lifted out, and sorted for the next operation.
Parts need to be placed on the sheet in an orientation friendly to the unloading device. Depending on your mix of work, these orientation constraints might affect your material yield.
The part characteristics need to allow secure suctioning. In some cases it might not be possible to remove certain parts. Perforation is a prime example. Depending on the hole geometry and density, and the characteristics of the suctioning device, you might not be able to remove a perforated part automatically. One alternative is to redesign the part slightly or add some sacrificial area to create the surface required for secure suction. You’ll waste a little material, but the labor savings you achieve might make this a small price to pay.
The same thing goes for difficult part geometries, like a star shape with narrow points—especially problematic with a new fiber laser that cuts narrow kerfs. These pieces might not release easily from the skeleton. Additional laser cutting techniques might be necessary, like going beyond and looping back around a sharp point. This gives you a clean edge and creates some vacant space to ensure the part has enough clearance to be removed reliably.
The last operation before part removal is especially critical. For a smaller part, the last punch hit might vibrate the material and introduce some skeletal instability, not good for part removal automation. In this case, the punch might cut most of the part. The laser head (which doesn’t contact the part) then makes the final cuts before retracting and making space for the part removal system.
Still, that’s just one example. For other parts it might make sense for punching to be the final operation. It can change from case to case. As long as the part position is stable after the final cut, it’s oriented correctly, and the suctions can work with all the cut and formed features, part removal automation can do its job.