During the drilling process, breakage of the drill bit is a common issue, especially when working with deep holes, small diameters, or hard materials like castings. After careful analysis, we found that the main cause is the constant feed rate throughout the entire drilling operation. As the drill bit approaches the bottom of the workpiece, the material at the exit of the hole begins to bulge, causing the bit to get stuck. At this point, the temperature of the bit rises significantly, increasing the risk of fracture. Therefore, the most effective way to prevent breakage is to automatically reduce the feed rate as the bit nears the end of the hole.
Through extensive testing, we developed the device shown in Figure 1, which effectively achieves this goal. The sleeve 2 has a tapered tail that fits into the drill spindle 1. A through hole is drilled into the cylindrical part of the sleeve, and a ball 4 is placed inside. This ball is in contact with the inner surface of the movable ring 3, which is conical in shape. At the lower end of the mandrel, there is a drill chuck 6 and a drill bit 7, while the upper part of the mandrel slides inside the sleeve. A removable shifting ring 9 limits the downward movement of the mandrel. Additionally, the height of the baffle 10 can be adjusted, and it is equipped with a thrust bearing for smooth operation.
The working principle of the device is as follows: When the drill spindle rotates along with the sleeve 2, the drilling starts with a normal feed amount S0. Initially, the axial force from the contact between the movable ring 3 and the ball 4 remains balanced. As the sleeve 2 moves downward during cutting, the distance L1 between the end of the movable ring 3 and the thrust bearing 11 gradually decreases. When the drill bit reaches the point where the metal starts to bulge (at a distance L from the bottom), the movable ring 3 contacts the thrust bearing 11 and stops moving. However, the sleeve 2 continues to move downward with the original feed rate S0, causing the ball to shift radially. This radial movement reduces the axial feed rate to SCB, which is less than S0, allowing the bit to pass through the workpiece safely.
The instantaneous feed rate SCB depends on the ratio of the cone angle a of the inner surface of the movable ring 3 to the cone angle b of the outer surface of the mandrel. As shown in Figure 2, the feed rate remains consistent only when a equals b. Due to the difference in axial slip speed, the feed rate naturally adjusts. To customize the feed rate according to different needs, the conical surfaces can be designed irregularly—either convex outward or concave inward.
Since the device has been in use for over a year, its effectiveness has been clearly demonstrated, particularly in preventing drill bit breakage when drilling small, deep holes. Moreover, the device helps avoid machine overload by allowing the movable ring 3 to rise as the drilling force increases, compressing the spring in the process. If a limit switch is installed in the bushing, the drive mechanism will automatically stop when the cutting force reaches a set threshold, ensuring safe and controlled operation.
Vacuum Forming Block Moulding Machine,Automatic Vacuum Block Moulding Machine,,Eps Block Foaming Machine
Hangzhou Sutuan Machinery Co.,Ltd.
Leading EPS EPP ETPU machinery manufacturer , https://www.sutuanmachinery.com