Technological advances in automation continue to pave the way for dramatic increases in productivity, as manufacturers are finding new ways to integrate more sophisticated systems on the plant floor. Machining and assembly companies are no exception to this trend, and Swiss-based Strausak is just one example. Its U-Grind models are designed for grinding and regrinding of special tools—especially for small batches that frequently require the production of complex geometries. Strausak recently automated the operation of its U-Grind machines by integrating Heidhain’s EQI 1100 rotary encoders on the six axes of an incorporated Staubli positioning arm. The robot is integrated into the machine enclosure but sits outside of the machine working area, giving operators unrestricted access for set up.
What makes Strausak’s approach interesting is that this is not a typical application for robotics. "Before this product, I would never have believed the pinpoint motion sequences could be carried out so successfully with an industrial robot," says Alexandre Condrau, managing director of Strausak. "Moving objects—such as slugs—from the tool holders and the tight loading of tool pallets seems to be no big deal until you consider the demands regarding accuracy of fit of the tool holders and the tight loading of the tool pallets.
Strausak’s legacy machine setup required slugs to be loaded manually or with a simple pick-up loader. Both approaches limited the equipment to the production of prototypes or small batches, and the machine's setup wouldn't allow for operator-free shifts.
Automating this process would free operators during the machines’ long run times. The carbide metal tool material and its complex geometries can translate into several hours when grinding even small series of special tools. Also, as with most manufacturing systems, limited floor space was a challenge, as was the integration of pallets of blanks into a limited surface area.
"The slim design of our robotic double gripper fulfilled the limited space requirement and another challenge was the wide range of blank dimensions," says Condrau.
Tight tolerances
While conventional machine tool collets have automation-friendly tolerances, the frequently used hydraulic expansion slugs were a challenge for Staubli's TX60L robot and end-of-arm gripper due to diameter tolerances of just a few hundredths of a millimeter.
Removing the tool from the tightly loaded pallet, moving it to the tool holder, and inserting it also posed complex motion sequences for the robot arm. This process is executed—in reverse order—after the grinding sequence.
"We depend on the encoders for positioning of the swivel head of our grinding machines and thus the sensitive positioning of the grinding disk," says Condrau. For the swivel head positioning, the machine designer chose Heidenhain’s ERA 400 angle encoders.
The grinding machine includes one encoder per motor, with one motor per robot axis. Stäubli uses Heidenhain’s absolute inductive rotary encoders with 18-bit resolution for the robot's total of six axis drives. All drives need to be interpolated to enable implementation of the straight-line insertion and deposition movements.
According to Strausak, the permissible deviations in each of the six axes are very limited in this context because that is the only way the process can be reliably controlled.
The industrial communications protocol between encoder and robot controller is Heidenhain’s EnDat 2.2, which allows positioning using a safe protocol. The design also allows for a SIL3 safe encoder position and memory functions that store relevant robot data. Strausak provides service integration and support in creating standard safe paths and also advises on changes to those robotic paths for customers.
The robot controller delivers robot and application information via OPC UA.
"Many of our customers are small companies or even one-person operations. They need functional technology that's easy to operate and supports them as much as possible in their tasks," says Condrau.
Strausak also sees further development possibilities that could help its customers even more. "We're currently working on the robot’s [process of] following up and regripping when it inserts very long tools into the tool holders," adds Condrau. "[We realize that] it would also be highly interesting for customers if the robot could not only change the tools, but the holders as well."
