The primary difference between DIN 915 and DIN 916 set screws lies in their design and application. The DIN 915 set screw features a dog point, which is intended to fit into a prepared hole or slot on a shaft. In contrast, the DIN 916 set screw has a cup point that creates a circular dent to secure the screw to the shaft. This distinction affects their usage in various mechanical applications. Understanding these differences is crucial for selecting the appropriate set screw for specific tasks.