Factors to be considered when choosing carbide drills

It has always been thought that drilling must be performed at lower feed rates and cutting speeds, and this view used to be correct under the machining conditions of ordinary drills. Today, with the advent of carbide drills, the concept of drilling has also changed.

In fact, through the correct selection of suitable carbide drills, drilling productivity can be greatly improved and the cost per hole can be reduced.

Carbide drills 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?

1. Machining accuracy

When choosing a carbide drill, the dimensional accuracy requirements of drilling need to be considered first. Generally speaking, the smaller the hole diameter to be machined, the smaller the tolerance. For this reason, drill bit manufacturers typically classify drill bits according to the nominal diameter size of the hole being machined. Among the above four types of carbide drills, solid carbide drills have the highest machining accuracy (the tolerance range of φ10mm solid carbide drills is 0-0.03mm), so it is the best choice for machining high-precision holes; The tolerance range of welded carbide drills or replaceable carbide crown drills is 0-0.07mm, which is more suitable for hole processing with general accuracy requirements; drills with carbide indexable inserts are more suitable for heavy-duty roughing. Although its processing cost is usually lower than that of other drills, its processing accuracy is also relatively low, with a tolerance range of 0 to 0.3mm (depending on the length-diameter ratio of the drill), so it is generally used for hole processing that does not require high precision. , or finish the hole by changing the boring insert.

2. Processing stability

In addition to considering the drilling accuracy requirements, the stability of the machining machine should also be considered when selecting a drill bit. The stability of the machine tool is critical to the safe life of the drill bit and the drilling accuracy, so the working condition of the machine tool spindle, fixture and accessories needs to be carefully inspected.

In addition, the stability of the drill bit itself should also be considered. For example, solid carbide drills are the most rigid and can therefore achieve high machining accuracy. The cemented carbide indexable insert drill has poor structural stability and is prone to deflection. This drill is equipped with two indexable inserts, 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 deflection of the drill body, and the longer the drill bit, the greater the deflection. Therefore, when drilling with a cemented carbide indexable insert drill with a length exceeding 4D, the feed rate should be appropriately reduced at the beginning of the drilling stage, and the feed rate should be increased to the normal level after entering the stable cutting stage. .

Welded carbide drills and replaceable carbide crown drills are self-centering geometry with two symmetrical cutting edges. This highly stable cutting edge design makes it unnecessary to cut into the workpiece Decrease the feed rate, except when the drill is installed obliquely and cuts in at a certain inclination angle to the workpiece surface. At this time, it is recommended to reduce the feed rate by 30% to 50% when drilling in and out. Since the steel drill body of this type of drill bit can produce slight deformation, it is very suitable for lathe processing; while the solid carbide drill bit is more brittle when used in lathe processing, especially when the drill bit is in poor centering condition. even more so.

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 evacuation (especially when machining low carbon steel workpieces), and no matter what kind of drill is used, this problem cannot be avoided. Machine shops often use external coolant injection to assist chip evacuation, but this method is only effective when the depth of the hole being machined is smaller than the diameter of the hole and the cutting parameters are reduced. In addition, the type of coolant, flow rate and pressure must be selected to match the diameter of the drill bit. For machines that do not have an in-spindle cooling system installed, a coolant conduit should be used. The deeper the hole being machined, 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 guaranteed. If the coolant flow is insufficient, the machining feed should be reduced.

4. Processing cost per hole

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

The newly developed replaceable carbide crown drills have excellent machining economy. After the drill bit is worn out, the user does not need to replace the entire drill body, but only needs to replace the carbide tooth crown. The purchase cost is only equivalent to the cost of regrinding the welded or solid carbide drill bit. Carbide crowns are easy to replace and have high repeatability. The workshop can use one drill body with multiple crowns to machine holes of different diameters. This modular drilling system reduces the cost of cataloguing drills 12 to 20mm in diameter, while also saving the cost of backup tools when regrinding welded or solid carbide drills.

When considering the cost per hole, the total life of the drill should also be taken into account. Generally speaking, 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 drill can be used to process steel. The steel drill body can replace the tooth crown at least 20 to 30 times.