Whether you’re working with a Cartesian, SCARA, or a 6-axis robots, there are a number of details deserving of attention when it comes to automatic screw-driving applications. Namely, you want to make sure that the robot selected is sized correctly. While the robot’s payload capacity is an important factor, there are a number of other details worthy of consideration, for instance:
• While we often consider the weight of the tool when determining payload requirements, it is also important to consider the weight of the interface plate, cables, as well as the air lines & feed tube that are going to be connected to the tool during its operation. In additional, it is important that this weight can be supported by the robot throughout its entire working envelop.
• Moment of Inertia – or the resistance to acceleration or deceleration. The greater the offset is from the centre of gravity of the robot’s interface plate the greater this resistance is going to be. Hence, you’re going to need to consider this should you elect to add additional end of arm tooling. You’re also going to need to consider the length of the screwdriver itself, as well as the screwdriver’s centre of mass, which unfortunately is often not located at its centre. Best practices are to locate the driver’s centre of gravity and have that point married to the centre of the interface plate between the robot & the tool itself. This becomes more critical when the robot is employed at the outer reaches of its working envelop.
• Reverse Thrust – now that we’ve considered the aggregate weight of all the devices affixed to the end of our robot we now need to consider the Reverse Thrust that the robot will be subjected to during the screw-driving process. When the screw becomes engaged with the screw hole we will see a rise in this force that will remain during the entire screw insertion process. If the robot is not able to counter this force the bit may become disengaged with the screw, or angular moment on the 5th or 6th axis of your 6-axis robot may cause side thrust on the driver bit, changing the torque signature due to an increase in thread friction, and in the end insufficient clamp load and improperly assembled parts.
• If you’re working with a 6-axis robot there are optimal positions for the 4th and 5th axis that will help minimize the effects of Reverse Thrust. For SCARA robots support buttresses can be applied to the z-axis, and for Cartesian robots support buttresses can be applied the main mounting axis to help alleviate concerns associated with Reverse Thrust.
Naturally there are a number of other factors that go into the proper integration of an automatic screwdriver onto a robot. These, however, constitute some of the more common concerns that my customers confront. Should you require more elaborate information please let us know. We’re here to help!