Robot grippers optimised for metal AM enable custom handling solutions for sheet metalworking

May 24, 2021

Additive Italia’s bespoke gripper arm, made using nTopology optimisation technology, is in use at Preziosa Francesca (Courtesy nTopology)

Manufacturer of sheet metal cabinets, Preziosa Francesca SRL, Bergamo, Italy, has turned to metal Additive Manufacturing to produce optimised robot grippers for handling metal sheets. Partnering with Additive Italia (Add-it), Bartolomeo, Italy, and nTopology, New York City, USA, the robot grippers were redesigned for the AM process and gripper production in-house, allowing the company to optimise its robotic systems for each job.

At Preziosa Francesca a Fanuc M710-iC robotic arm, equipped with two metal grippers, loads metal sheets onto the automatic press brake bending machine and maintains the sheet’s position during bending, after which it stores the sheet in an appropriate location.

In the original operation, three main issues arose:

  1. The standard flat grippers were too slippery, causing inefficient traction, which, in turn, affected the accuracy of the end product.
  2. The standard grippers were not agile enough, meaning the sides were at risk of colliding with the machine at certain bending angles. This would lead to a halt in operations.
  3. The manufacture of custom grippers was too slow, translating to wait times of at least two weeks to receive different grippers from CNC machine shops.

To overcome these challenges, Add-it redesigned the gripper for Additive Manufacturing by Desktop Metal’s Studio System. The company selected 17-4 PH stainless steel; it had considered polymers, initially, but quickly moved away from this option as the material would wear too quickly in an application where repeatability was essential. A primary goal for this project was to create an end-to-end digital manufacturing workflow.

“We immediately decided to start this project with nTopology due to the variety of grips that we could make,” stated Gianluigi Rossi, AM and Materials Engineer at Add-it and the lead design engineer of this project.

To increase the grip force, engineers experimented with two initial design variations. First, they consolidated the assembly of the gripper and the stop reference into a single component. Then, they applied two different patterns to the gripper: grooves created using a surface lattice and honeycomb-like perforation pattern.

Through testing, it was found that the honeycomb pattern provided better traction and was easier to additively manufacture. This design was used as a basis for the later iterations.

The pattern of honeycomb perforation proved the most successful traction for the gripper (Courtesy nTopology)

What stood out to the team during this project was the speed of iterations, from concept to application in less than four days. The team attributed this rapid momentum, to a large extent, to the design capabilities of nTopology.

“Shifting from classic CAD software to nTopology is a radical leap: I interfaced with another way of working and thinking,” Rossi added. “This software can open new paths, such as reusing the same workflow for other geometries and sharing it with colleagues. This is a unique time-saving in the working environment. In one afternoon, we managed to make the component we needed. In a few minutes, we could change the type of grip and produce a part that was ready to manufacture.”

The closer the gripper can reach the press brake, the safer and more repeatable the process can get. However, even small variations in sheet thickness can cause the gripper to collide with the machine tool at certain angles, resulting in stops. Replacing the standard gripper with a more slender design could therefore expand the robot’s safe zone and increases its agility.

This was the objective of the second design iteration. The engineers at Add-it utilised nTopology’s built-in topology optimisation capabilities to generate a gripper with optimised geometry – not for the purpose of lightweighting, but, rather, to create a design with a similar performance in stiffness, grip force and weight with a smaller footprint. The geometrically-optimised gripper enabled the system to perform more delicate robotic operations and handle smaller metal sheets.

“We estimated the space that we needed to save in the volume of the clamps. It was a matter of a few millimetres. Still, it was a few millimetres that counted in this application,” noted Rossi.

The team designed and additively manufactured two variations of the new gripper using two different levels for the density threshold. Using nTopology, Rossi automatically reconstructed and smoothened the topology optimisation results, bypassing time-consuming manual steps and allowing multiple variations to be quickly generated.

For Preziosa Francesco, the redesigned gripper greatly improved process reliability. By eliminating stops, it unlocked lights-out manufacturing and the full utilisation of their robotic manufacturing system. The ability to produce the parts needed in-house within a few days has also enabled the company to adapt its system on short notice and revisit processes, if necessary.

nTopology software enabled the Add-it engineers to compare density thresholds for the gripper (Courtesy nTopology)

“If you don’t have a very repeatable process, you are forced to run production only during the day,” stated Marco Preziosa, CEO, Preziosa Francesco. “It is very economically difficult to have these machines operate during day-shift only. Theoretically, you can have a very clever production system, but if it doesn’t work as intended because of a cheap part, an investment worth hundreds of thousands is at stake. nTopology allowed us to use robotic manufacturing our Desktop Metal Studio System to its full potential.”

With the new grippers in action, Preziosca Francesco and Add-it are now looking to replicate this process of other applications – for example, designs are being made for clamp replacements and spare parts for the company’s punching machine.

As for improvements on the existing workflow, the engineers of Add-it are now investigating how to create a new topology optimised version of the gripper that is twice as long while remaining durable and reliable. This would enable the manufacture of a customisable clamping tool that is even more manoeuvrable and flexible.

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