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[Factors to consider when using carbide drills]
Release date:[2019/10/31] Is reading[60]次

It has always been believed that drilling operations must be carried out at lower feed rates and cutting speeds, a view that was once correct under the processing conditions of a conventional drill bit. Today, with the advent of carbide drills, the concept of drilling has changed.

In fact, by properly selecting the right carbide drill, the drilling productivity can be greatly improved and the processing cost per hole can be reduced.

Carbide drill bits are divided into four basic types:

Solid carbide drills, carbide indexable insert drills, welded carbide drills and replaceable carbide crown drills. So what should you pay attention to when choosing a carbide drill?

Processing accuracy

When selecting a carbide drill, first consider the dimensional accuracy requirements of the drilling process. In general, the smaller the machined hole diameter, the smaller the tolerance. Therefore, drill bit manufacturers typically classify the drill bit based on the nominal diameter size of the hole being machined. Among the above four types of cemented carbide drills, the solid carbide drill has the highest machining accuracy (the tolerance range of φ10mm solid carbide drill is 0-0.03mm), so it is the best choice for machining high-precision holes; Welded carbide drills or replaceable carbide crowns have a tolerance range of 0 to 0.07 mm, which is suitable for hole machining with general accuracy requirements. Bits with carbide indexable inserts are more suitable for heavy-duty roughing. Although its processing cost is usually lower than that of other drills, its machining accuracy is relatively low, and the tolerance range is 0-0.3mm (depending on the length-to-diameter ratio of the drill), so it is generally used for hole machining with low precision. Or finish the hole by changing the boring blade.

2. Processing stability

In addition to considering the drilling accuracy requirements, the stability of the machine tool must also be considered when selecting the drill bit. Machine stability is critical to the safe life of the drill bit and the accuracy of the drill. Therefore, it is necessary to carefully check the working state of the machine spindle, clamp and accessories.

In addition, the stability of the drill itself should also be considered. For example, solid carbide drills have the best rigidity and therefore high machining accuracy. The cemented carbide indexable insert drill has poor structural stability and is prone to deflection. Two drillable inserts are mounted on the drill bit, wherein the inner insert is used to machine the central portion of the bore and the outer insert is machined from the inner insert to the outer edge portion at the outer diameter. Since only the inner blade enters the cutting in the initial stage of machining, the drill bit is in an unstable state, which easily causes the drill body to deflect, and the longer the drill bit, the larger the amount of deflection. Therefore, when using a carbide indexable insert drill bit longer than 4D for drilling, the feed rate should be appropriately reduced when starting the drilling stage, and the feed rate should be increased to the normal level after entering the stable cutting stage. .

Welded carbide drill bits and replaceable carbide crown drills are self-centering geometric edges consisting of two symmetrical cutting edges. This highly stable cutting edge design eliminates the need for cutting into the workpiece. The feed rate is reduced, except when the drill bit is tilted and installed at a certain angle to the workpiece surface. It is recommended to reduce the feed rate by 30% to 50% during drilling and drilling. Due to the small deformation of the steel drill body of such a drill bit, it is very suitable for lathe machining; while the solid carbide drill bit is more fragile, which is easier to break when used for lathe machining, especially when the drill bit is in poor condition. This is especially true.

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 the poor chip removal (especially when machining low-carbon steel workpieces), and no matter which drill is used, this problem cannot be avoided. The shop often uses external injection of coolant to assist chip evacuation, but this method is only effective if the hole being machined is deeper than the hole diameter and the cutting parameters are reduced. In addition, the appropriate coolant type, flow rate and pressure must be selected to match the drill diameter. For machines that do not have an in-spindle cooling system, a coolant conduit should be used. The deeper the hole being machined, the more difficult it is to remove the chips and the greater the required coolant pressure. Therefore, the minimum coolant flow rate recommended by the drill manufacturer should be guaranteed. If the coolant flow is insufficient, the machining feed rate needs to be reduced.

4. Processing cost per hole

Productivity or processing cost per hole is the most important factor affecting drilling operations. To increase productivity, drill manufacturers are working on machining methods that integrate multiple operating procedures and develop drilling tools that enable high feed and high speed machining.

The newly developed replaceable carbide crown bit has excellent processing economy. After the drill bit is worn, the user does not have to replace the entire drill body, only the cemented carbide crown can be replaced, and the purchase cost is only equivalent to the cost of re-grinding the welded or solid carbide drill bit. Carbide crown replacement is easy and repeatability is extremely high. The machine shop can be equipped with multiple crowns for one hole to process holes of different aperture sizes. This modular drilling system reduces the cataloging cost of drill bits from 12 to 20 mm in diameter while also saving on the cost of backup tools required for regrinding welded or solid carbide drills.

The total life of the drill bit should also be taken into account when considering the cost per hole. In general, a solid carbide drill can only be reground 7 to 10 times, a welded carbide drill can only be reground 3 to 4 times, and a replaceable carbide crown can be used to process steel. At the time, the steel drill body can replace the crown at least 20 to 30 times.