Traditionally, it was believed that drilling operations must be performed at lower feed rates and cutting speeds. This viewpoint was correct under the processing conditions of standard drill bits. However, with the advent of carbide-tipped drill bits, the concept of drilling operations has evolved.
In fact, by selecting the appropriate hard alloy drill bits, drilling productivity can be significantly increased, and the cost per hole processed can be reduced.
Four basic types of carbide drill bits are available:
Overall, carbide drill bits, carbide indexable drill bits, brazed carbide drill bits, and replaceable carbide crown drill bits. What should one pay attention to when selecting carbide drill bits?
Process Precision
When selecting carbide tipped drill bits, the first consideration should be the dimensional accuracy requirements for the drilling operation. Generally, the smaller the diameter of the hole being machined, the tighter the tolerance. Therefore, drill bit manufacturers typically categorize bits based on the nominal diameter of the hole to be processed. Among the four types of carbide tipped drill bits mentioned, the solid carbide drill bits offer high machining accuracy (the tolerance range for φ10mm solid carbide drill bits is 0 to 0.03mm), making them an excellent choice for drilling high-precision holes; the brazed carbide tipped drill bits or the replaceable carbide inserts drill bits have a tolerance range of 0 to 0.07mm, which is suitable for general precision hole machining; drill bits equipped with carbide indexable inserts are more suitable for heavy-duty rough machining, although their processing costs are usually lower than the others, their accuracy is also lower, with a tolerance range of 0 to 0.3mm (depending on the length-to-diameter ratio of the bit), thus they are generally used for holes with low precision requirements or for finishing holes by changing the boring inserts.
2. Processing Stability
In addition to considering the drilling precision requirements, when selecting drill bits, the stability of the machining machine tool must also be taken into account. The stability of the machine tool is crucial for the safe service life and drilling precision of the drill bit, hence a careful inspection of the working condition of the machine tool's spindle, fixtures, and accessories is necessary.
Additionally, the inherent stability of the drill bit itself should be considered. For instance, solid carbide drill bits are known for their high rigidity, allowing for exceptional machining precision. In contrast, the structural stability of carbide-tipped indexable insert drills is poorer, making them prone to skewing. These drills are equipped with two indexable inserts: the inner insert for machining the central part of the hole, and the outer insert for the peripheral portion from the inner insert to the outer diameter. Since only the inner insert engages in cutting during the initial stage, the drill bit is in an unstable state, which can easily cause the drill body to skew. Moreover, the longer the drill bit, the greater the skewing. Therefore, when using carbide-tipped indexable insert drills with a length exceeding 4D for drilling operations, it is advisable to reduce the feed rate initially until stable cutting is achieved, and then increase the feed rate to normal levels.
Welded carbide drill bits and replaceable carbide crown drill bits are composed of two symmetrical cutting edges that form a self-centering geometric blade shape. This high-stability blade design allows for no reduction in feed rate during cutting into workpieces, except when the drill bit is tilted and cuts at a certain angle to the workpiece surface. In such cases, it is recommended to reduce the feed rate by 30% to 50% during entry and exit. Due to the slight deformation capability of the steel body in these drill bits, they are particularly suitable for lathe processing; whereas solid carbide drill bits, being more brittle, are prone to breakage during lathe processing, especially when the bit's centering is not optimal.
3. Chip Removal and Cooling Fluid
Chipping removal is a crucial issue in drilling and cutting processes. In fact, poor chipping removal is a common problem encountered during drilling and cutting, especially when machining low-carbon steel workpieces, and this issue cannot be avoided regardless of the type of drill used. Machine shops often use external coolant injection to assist with chipping removal, but this method is only effective when the depth of the hole being machined is less than the hole diameter and when cutting parameters are reduced. Additionally, it is necessary to select the appropriate coolant type, flow rate, and pressure that match the drill diameter. For machines without a spindle internal coolant system, a coolant conduit should be used. The deeper the hole being machined, the more difficult it is to remove chips, and the higher the required coolant pressure becomes, so it is essential to maintain the low coolant flow rate recommended by the drill manufacturer. If the coolant flow is insufficient, the feed rate of the machining process should be reduced.
Cost per hole machining
Production efficiency or processing cost per hole is a critical factor affecting drilling operations. To enhance production efficiency, drill bit manufacturers are focusing on researching processing methods that can integrate multiple operational procedures, and developing cutting tools capable of achieving high feed rates and high rotational speeds.
The newly developed replaceable carbide-tipped crown drill bits offer exceptional processing economy. After the bit wears out, users need not replace the entire drill body; they simply replace the carbide-tipped crown, which incurs a cost equivalent to only one resharpening of a welded or solid carbide drill bit. The ease of replacing the carbide-tipped crown and the high repeatability of precision allow the workshop to use a single drill body with multiple crowns for drilling holes of different diameters. This modular drilling system can reduce cataloging costs for drill bits with diameters of 12 to 20mm and also save on the costs of backup tools required for resharpening welded or solid carbide drill bits.
When considering the cost per hole, the total lifespan of the drill bit should also be factored in. Generally, a solid carbide drill bit can be resharpened 7 to 10 times, a brazed carbide drill bit can be resharpened 3 to 4 times, and a replaceable carbide tip drill bit can have its steel body resharpened with a new tip 20 to 30 times when machining steel materials.
