by Steve Meyer, Contributing Editor
One company is leveraging CAD, vision systems and robotics to make dramatic improvements in grinding large castings. The next generation of grinding will not look anything like the past.
Traditional machining and grinding processes have been around for a long time. Historians record that one of the earliest known lathes was operating around 1483. Machines that cut metal in complex shapes were the foundation of the industrial revolution, and for that matter, all of modern manufacturing. Computer Numerical Control (CNC) has been in use since the 1950s. A lot of effort goes into the making of metal parts, and a lot of effort is expended to make parts more efficiently. 3D printing of metal parts is the beginning of the next generation of part making, but for now it is only cost effective in complex designs.
In engineering school, undergraduate mechanical engineers are taught graphics in order to document and communicate part design. The process requires that we think about a part as if it were inside a glass cube and imagine the edges, holes and features of the part’s shape as they would appear on the 2D surfaces of the cube. Complex parts often require a great deal of detail in order to capture all of the information needed to produce the desired part. Combustion engine blocks are a great example of this complexity.
Computer Aided Design (CAD) has been around since the 1970s—at first, as very expensive workstations that allowed designers to work in 2D and 3D without pencil and paper. In the 1990s, lower cost programs on desktop systems replaced the workstation. 3D solid modeling software became integrated with machining when editors for CNC part programming became available extensions of the software. Much of the lead time and the cost associated with starting up a new part were reduced, which changed industry forever.
All of these tools and training are based on Cartesian (perpendicular) orientation as the standard means of visualizing and processing parts. Machine tools are organized the same way, using three linear, perpendicular axes to mill, bore, drill or tap a part. 5-axis machining centers are more flexible adding 2 rotary axes to the 3 Cartesian, but this is a lot more expensive and doesn’t solve all problems. As the part size increases, the forces necessary to support the machine tool goes up arithmetically. The result is that machining centers for parts like engine blocks end up costing millions of dollars.
In complex parts, several different machines may be necessary to perform all of the processes required. Conventional manufacturing procedures require time consuming setups from one machine to another, which presents the potential for catastrophic error. The further along a part goes in the process, the more cost the scrap part represents. In addition, multiple machine tools contribute to extremely high parts cost.
So the ideal solution to grinding large, complex castings would be a machine that can process the part in a single work cell using a single part setup. Which is exactly what Chris and Randy Sutton decided to create at their engineering firm Sutton Engineering Technologies (SET) in Alabaster, Ala. The Sutton brothers and their team of engineers have decades of experience between them, and a vision to solve production problems with solutions that work better.
Sutton Engineering has successfully integrated a Kuka Titan series robot with a 40 hp spindle motor as the end effector. This formidable looking combination creates a giant robotic grinding tool. The robot has 6 axes of motion, with a nearly 12-ft reach, enabling a large work envelope with amazing dexterity. In addition to the robot’s size, it has a payload capability of 1,650 pounds giving it the heft to carry the massive 40-hp spindle motor for grinding, and the ability to generate enough force at the end of the spindle to do some serious material removal.
The main event—grinding metal
One of the major limitations in any grinding environment is the speed at which material can be removed. This is a function of material hardness and the cross section, or volume of the material to be removed. High speed spindle motors are used to improve the material removal by simply speeding things up, typically at speeds of 10,000 to 40,000 rpm. However, unwanted variations in the amount of material removed can show up in the grinding process.
Traditional grinding is the result of a grinding tool engaging a workpiece. This is generally done as an open-loop positioning function in a machine. If the grinding is done with a CNC, there are tool wear compensation features to insure accuracy, but it is still strictly a positioning process—and errors can occur. The missing ingredient is measuring the force at the end of the grinding tool.
The SET robotic grinder integrates a strain device between the robot arm and the spindle motor and grinding wheel. The sensor measures the force between the grinding wheel, robot and workpiece in real time. This creates a closed loop around the grinding process, which can be monitored and regulated by the system controller.
When the robot removes material from the work piece and reaches a section that requires multiple grind passes, the strain device detects push back, which is also reflected in the robot’s positioning system. By recording the “out of position” state, the robot can come back and do a second pass in the affected area and eliminate the high spot. The SET process has been tested and is producing precise, repeatable parts on the shop floor.
A Danfoss VLT drive is used to control the induction spindle motor at speeds up to 12,000 rpm. The VFD is able to do speed and torque regulation with sufficient dynamic response to optimize the grinding process. The drive is even able to perform spindle orient cycles to 1/32-in. of rotary position, so that the robot can pick up one of several grinding wheels required for the range of grinding processes.
Single setup with automated homing
One of the most difficult aspects of grinding complex metal castings is orienting the part and creating a home position reference for the machine tool. The SET robotic grinding center handles this problem in a unique way.
Parts ready for grinding are clamped down to a large rotary table that can index like a loader/unloader, giving the robot access to the 2 or 4 parts at a time, depending on size. The parts are fixtured in mirror image—this simplifies programming of the robot’s grinding path. Multiple parts in the work cell also helps ensure high operating effectiveness, so that productivity is maximized.
The work cell includes an operator station where production part numbers can be searched and selected from a list by the operator. After a part number is selected, the vision system scans the part and compares it to a 3D reference image that is on file in the system controller. This step verifies that the correct part is present in the work cell. The vision system also scans for prominent features like casting sprues and vents to determine where the part is and what its orientation is. Using this input, the robot does a multiple point touch-off cycle to provide absolute verification of the part position.
This unique combination of hardware and software features makes the robotic grinding work cell repeatable, reliable and highly efficient at grinding complex parts. Initial demonstrations have shown 300% improvement in cycle times with no scrap.
Currently, there are two, 10-ft diameter tables for parts supported by a third table that can index work in and out of the robot’s work envelope. All three tables are independently controlled by closed-loop hydraulic actuators. The work tables support up to four parts weighing as much as 3,750 lb each. The main table has considerable extra capacity and will support up to 15,000 lb of work. The table system rests on multiple hydraulic cylinders so that it is level and evenly supported.
Robotic grinding goes well beyond 5-axis machining by adding the flexibility of being able to operate inside the part with relative freedom. Cleanup of the interior surfaces of a casting is relatively easy since the robot can reach inside from either side.
The SET Robotic Grinding Center combines CAD, CAM, vision, load cell, spindle drive and robot into an elegant, high productivity work cell that solves many application problems. The economic benefit of the new solution has gained the attention of several large companies that have expressed interest in testing the system for their own unique requirements
Sutton Engineering Technologies
Danfoss North America