It has always been believed that drilling must be performed at a lower feed rate and cutting speed. This view was once correct under the processing conditions of ordinary drills. Nowadays, with the advent of carbide drills, the concept of drilling has also changed.
In fact, by correctly selecting the right carbide drill bit, drilling productivity can be greatly improved, and the cost of processing per hole can be reduced.
Cemented carbide drill bits are divided into four basic types:
Solid cemented carbide drill bits, cemented carbide indexable insert drill bits, welded cemented carbide drill bits and replaceable cemented carbide crown drill bits. So, what should you pay attention to when choosing a cemented carbide drill?
1. Machining accuracy. When choosing cemented carbide drills, the dimensional accuracy requirements of drilling must be considered first. Generally speaking, the smaller the aperture to be processed, the smaller the tolerance. Therefore, drill manufacturers usually classify drills according to the nominal diameter of the hole being machined. Among the above four types of cemented carbide drills, solid cemented carbide drills have the highest machining accuracy (the tolerance range of φ10mm solid cemented carbide drills is 0～0.03mm), so it is the best choice for machining high-precision holes; The tolerance range of welded cemented carbide drills or replaceable cemented carbide crown drills is 0～0.07mm, which is more suitable for hole machining with general accuracy requirements; drills with cemented carbide indexable inserts are more suitable for heavy-duty rough machining. Although its processing cost is usually lower than other types of drills, its processing accuracy is also relatively low, with a tolerance range of 0～0.3mm (depending on the length-to-diameter ratio of the drill), so it is generally used for hole processing with low precision. , Or finish the hole finishing by replacing the boring blade.
2. Processing stability. In addition to considering the drilling accuracy requirements, the stability of the processing machine tool should also be considered when selecting a drill. The stability of the machine tool is very important to the safe service life and drilling accuracy of the drill bit. Therefore, it is necessary to carefully check the working status of the machine tool spindle, fixtures and accessories.
In addition, the stability of the drill bit itself should also be considered. For example, solid carbide drills have the best rigidity, so they can achieve high machining accuracy. The cemented carbide indexable insert drill bit has poor structural stability and is prone to deflection. Two indexable inserts are installed on this drill bit. The inner insert is used to machine the center part of the hole, and the outer insert is used to machine the outer edge from the inner insert to the outer diameter. Since only the inner blade enters the cutting at the initial stage of processing, the drill bit is in an unstable state, which can easily cause the drill body to deviate, and the longer the drill bit, the greater the amount of deflection. Therefore, when using a cemented carbide indexable insert drill with a length of more than 4D for drilling, the feed should be appropriately reduced at the beginning of the drilling phase, and the feed rate should be increased to the normal level after entering the stable cutting phase .
The welded cemented carbide drill bit and the replaceable cemented carbide crown drill bit are composed of two symmetrical cutting edges with a self-centering geometric edge type. This high-stability cutting edge design makes it unnecessary when cutting into the workpiece Reduce the feed rate, except when the drill is installed obliquely and cut in at a certain angle to the surface of the workpiece. At this time, it is recommended to reduce the feed rate by 30% to 50% when drilling in and out. Because the steel drill body of this kind of drill bit can produce slight deformation, it is very suitable for lathe processing; while the solid carbide drill bit is more brittle, it is easier to break when used for lathe processing, especially when the drill bit is not well centered. This is especially true at times.
3. Chip removal and coolant. Chip removal is a problem that cannot be ignored in drilling. In fact, the most common problem encountered in drilling is poor chip removal (especially when machining low-carbon steel workpieces), and this problem cannot be avoided no matter what kind of drill is used. Processing workshops often use external coolant injection to assist chip removal, but this method is only effective when the depth of the processed hole is smaller than the diameter of the hole and the cutting parameters are reduced. In addition, a suitable coolant type, flow rate and pressure must be selected to match the diameter of the drill bit. For machine tools without a cooling system in the spindle, coolant pipes should be used. The deeper the hole to be processed, the more difficult it is to remove chips and the greater the coolant pressure required. Therefore, the minimum coolant flow recommended by the drill manufacturer should be ensured. If the coolant flow is insufficient, the machining feed must be reduced.
4. Processing cost per hole. Productivity or processing cost per hole is the most important factor affecting drilling processing. In order to improve productivity, drill manufacturers are working on researching machining methods that can integrate multiple operating procedures, and developing drilling tools that can achieve high-feed and high-speed machining.
When considering the processing cost per hole, the total life of the drill should also be included. Generally speaking, a solid carbide drill bit can only be regrinded 7-10 times, and a welded carbide drill bit can only be regrinded 3 to 4 times, while the replaceable carbide crown drill bit is processing steel materials. When the steel drill body can replace the tooth crown at least 20 to 30 times.
