Making Robots More Multifunctional

Among robotics system developers and programmers, a new trend is emerging: High-volume finishing of engineered parts, including castings, with installations conceived for grinding or deburring, and lately for more value-added machining processes, such as milling, drilling, or thread-cutting. Robots are no longer systems for simple grinding or deburring tools.

A German developer, Kadia Produktion GmbH + Co., is a long-time designer of workcells for high-volume part deburring based on six-axis industrial robots. The company’s Deburr robot cells adopt two program concepts: Either the robot grips the workpiece and moves it to fixed tools — often brushes — or it guides the tools itself, such as milling tools. The latter case presents a greater challenge, for example in deburring of large gears.

Calling this process deburring is no longer quite accurate, according to Kadia developers: It is clearer to characterize it as “edge shaping”. With the robot fixed to hold a solid-carbide end mill, the gears are given chamfers of up to 5 mm. More complex programming is needed for this operation because the cutters follow the involute contour of the tooth flanks.

A further development in recent years has been the Deburr automation cell, in which a robot performs comprehensive handling tasks in addition to deburring. In this set-up, the robot works together with machining centers, placing the workpieces and removing them again, and if necessary it will act as an interface for auxiliary processes, such as quality inspection or part-washing stations. Again, the deburring process just one among several tasks to be programmed and carried out.

The number of tasks being appended to a robot’s functions continues to increase, and now Kadia is offering a third type of robot systems: machining robot cells. "More and more customers are asking whether it is possible with the robot, for example, to also apply a thread or a flat surface," explained Jannik Weiss of Kadia deburring machines sales.

The customers plan want to avoid reclamping operations. If the deburring robot, which often continues the process chain after mechanical processing, can take work away from the other processing machines, a lot of time can possibly be saved.

"We are repositioning ourselves a bit as a result," added Kadia managing director Henning Klein. "Since we have accumulated many years of know-how with our automation solutions with robots or with our Deburr robot cells, the step to becoming a supplier for robot-assisted mechanical processing is no longer a big one."

Cost-effective machining

In principle, a six-axis industrial robot is suitable for a wide range of machining technologies: Drilling, milling, thread cutting, etc., performing dry or wet machining or using minimum quantity lubrication. The main advantage is that the robot is a comparatively low-cost machining solution.

With a six-axis robot, all exposed sides of a three-dimensional workpiece can be reached easily. If the same number of degrees of freedom is to be achieved with a machining center, much more complex five-axis machine concepts are required, which results in high costs. In addition, a robot can alternately pick up grippers and tools and therefore it is suitable for multifunctional process set-ups.

The limitation of a robot for mechanical processing is its comparatively low rigidity. It cannot offer the repeatability of precision guides available on a machining center. The further the arm reaches out, the less accurate will be its performance. Therefore, a robot’s use is limited to applications with correspondingly large tolerances and small chip depths. However, there are certain adjustments and parameters that can influence the results.

Programming can compensate for deviations from the ideal path at the reversal points, within certain limits. "The Kadia process development department determined which parameters need to be optimized and how… We take the necessary time for this so that we can give the customer a capable process at the end," Weiss explained.

One application for which a robot is ideally suited is the machining of the parting surfaces on aluminium housing trays for holding vehicle batteries. These housings are effectively the successors to fuel tanks. The quantities required are increasing rapidly. Due to the required surface requirements and tolerances, a machining center would be oversized. A robot, on the other hand, meets the dimensional accuracy requirements and fully exploits its cost advantages as well as its flexibility.

Kadia recently developed a corresponding machining concept for an automotive manufacturer. In detail, the task is to mill the parting surfaces and then subsequently deburr those surfaces using a brush so that the frames can later be bolted and sealed with a steel cover. An important detail of the customer's requirement was flat-milled surfaces with low waviness. The customer specified the quality of the surfaces at Rz < 20 µm / Ra < 4 µm.

Complex process made simple

The solution involves one cell with three robots. To meet the cycle time, two robots are required on one side of the workpiece — where the machining volume is larger — and one is sufficient on the other side. The set-up requires less than 80 seconds for complete machining, i.e., milling, including brush deburring. If a different workpiece variation with further details would have to be machined, the cell offers space a fourth robot.

Advanced testing with milling tools showed that minimization of vibrations is the big issue when defining almost all machining parameters in robotic cutting. The cutting geometry, macro- and micro-geometry, for example, are important adjusting screws, since they have a decisive influence on the cutting forces.

Among other things, the depth of cut is a key criterion; the application engineers limited this to 2 mm to reduce vibrations. At the same time, they optimized the cutting speeds and feeds so that chatter marks are avoided. The cutting edges are cooled during machining by means of minimum quantity lubrication. Programmable spindle units mounted on the robotic arms are responsible for analyzing the cutting data. They form a seventh axis. This solution achieves a surface finish of Rz = 10 µm / Ra = 2 µm. So, the surface finish required by the user is thus undercut by a factor of two.

At first glance, a workcell with three robots is a complex system. However, its operation is simpler than expected. Kadia designs workcells with usability in mind; the operator does not need to be a CNC programmer or a robot specialist.

"Any skilled metal worker who understands a technical drawing is able to operate our robotic cells,” Weiss explained. “Only one master point is defined for each machining detail. This is easy to correct. The approach paths and transition movements to the next feature are predefined. Every process-relevant dimension on the workpiece can be read in plain text from the drawing."

It follows that if a workpiece is out of tolerance, the operator can quickly and easily correct the corresponding workpiece and tool coordinates on his own.