Cobot Installs Screws in Electronic Assemblies

Danfoss’ assembly plant in Gråsten, Denmark, manufactures AC drives, frequency converters and other electronic products. Although circuit board assemblies are the heart of these products, they also require numerous threaded fasteners. Screws secure the boards, connectors and other subassemblies to the housing, and they assemble the housing itself.

As is often the case with assemblies that are joined with threaded fasteners, the cost of the fasteners is not as important as the cost of installing them. Engineers at Danfoss wanted to automate the fastening process to decrease costs, increase quality, and improve ergonomics. Given the high-mix nature of production at Danfoss, any automation system they considered would have to be flexible, reliable and precise.

A cobot automatically installs screws to screw a circuit board into a housing for a frequency converter. Photo courtesy Spin Robotics

“We use a lot of screws,” says Kristian Thorsen, platform architect for assembly at Danfoss. “We want to automate screwdriving to ensure quality, improve ergonomics, and make sure that screws are installed in the correct sequence.

“We want to automate all the ‘stupid’ things—the tasks that don’t necessarily require people, so that they can do tasks that are more fun, that require them to use their brains, and that are easier from an ergonomic standpoint.”

For Danfoss, the difficulty with manual screwdriving stems from employee fatigue. As assemblers get tired, they invariably miss screws or install them in the wrong order. Frequency converters also require horizontal screwdriving, which is particularly problematic in terms of quality. That is because the tightening torque can seem correct, but if it is applied at a wrong angle during installation, the screw will not be fastened correctly.
 

The system can install up to six screws in less than a minute, even in narrow and hard-to-reach spaces. Photo courtesy Spin Robotics

Here Come the Cobots

Danfoss engineers didn’t have to look far for help. They contacted Spin Robotics in Odense, Denmark—less than 100 miles away. 

The startup’s SD automatic screwdriving system is designed for use with collaborative robots. A shield covers the bit and screw when the cobot is moving, which protects workers from rotating and sharp elements. A spring-loaded slide on the back of the tool is a last resort. If a hand comes between the driver and the part, the spring triggers a sensor to stop the cobot immediately and enable the worker to move the arm away.

This was important for Danfoss, which places a high priority on employee safety. Production workers who install screws manually for long hours often feel strain in their shoulders, elbows, and wrists. Automating the process takes the strain from workers without compromising safety. The robot is equipped with a safety sensor with Level D performance and a safety shield that protects employees from piercing injuries.

The collaborative aspect of the technology was important to Danfoss, as well, since the company plans to deploy the robots on the assembly line working in parallel and in sequence with people.

Three models are available: the SD35, SD70 and SD120. The SD35 can handle a torque range of 0.15 to 3.5 newton-meters. The SD70 can handle a torque range of 0.2 to 7 newton-meters. And, the SD35 can handle a torque range of 2 to 12 newton-meters. Torque accuracy is ±10 percent.

The robots can install machine screws, thread-cutting screws and thread-forming screws with a variety of drive styles and head styles, including countersunk heads.

If different screws are needed in the same assembly, the system can be equipped with a bit changer. Photo courtesy Spin Robotics

The SD35 can handle fastener sizes M2 to M5; the SD70 can handle fastener sizes M3 to M6; and the SD120 can handle fastener sizes M5 to M8. All three can handle a maximum fastener length of 35 millimeters. The system can even handle socket-head screws with a head length of up to 15 millimeters.

Fasteners are picked up from a screw presenter using magnetic or vacuum bits. The presenter can feed one screw per second. If different screws are needed in the same assembly, the system can be equipped with a bit changer. The system can install up to six screws in less than a minute, even in narrow and hard-to-reach spaces.

The system is compatible with cobots from Fanuc, Universal and Kassow. The cobot is programmed in free-drive mode. A teach pen lets engineers pinpoint the exact position of each fastening spot.

Once a screwdriving sequence has been programmed, it is stored and ready to use whenever needed. The Spin Bridge control system can store 300 screwdriving sequences. In addition, Spin Bridge gathers and stores data from the assembly process. Via OPC-UA, engineers can view the data in real time from a web browser, and data can be exported to an ERP system as needed.

“My experience, so far, with the SD35 collaborative screwdriver is very promising,” says Thorsen, who has so far just evaluated the technology in the company’s production lab. “I’m looking forward to seeing it in a real working area.”

For more information on robotic screwdriving, click https://spin-robotics.com.

Danfoss has a long history of deploying state-of-the-art assembly technologies. Photo courtesy Danfoss

For more information on robotic screwdriving, read these articles:
Robotic Screwdriving Mistakes to Avoid
Is Robotic Screwdriving Right for You?
New Technologies for Robotic Screwdriving