Composite Drill Bit: Characteristics, Structure And Selection

For the machining of large diameter holes on parts, high-speed steel fried dough twist drills are generally used to drill holes first, and then expand the drilling (or boring) to gradually reach the required size.

This process has the advantages of low efficiency, short tool life and high cost.

Composite drill bit (also known as modular drill bit) is produced under such circumstances.

It is made by adding high-speed steel center drill bit on the basis of the structure of carbide indexable blade drill bit. It is specially used for the processing of large hole diameter. It does not need to enlarge the hole step by step, and it can reach the required size at one time.

Composite drill bit is shown in Fig. 5-15.

composite drill bit

Fig. 5-15 composite drill bit

(1) Structure composition and characteristics of composite bit

1. Structure composition and characteristics of compound drill (see table 5-7).

Table 5-7 structure composition and characteristics of composite bit

FunctionsStructure composition and characteristics of compound drill 
1. center drillIt is made of high-speed steel or cemented carbide for centring. Use the axial screw to adjust the bit extension length to compensate the wear value
2. cutting headCoated cemented carbide indexable blade shall be used for asymmetric installation with staggered teeth. The blade material can be determined according to the processed material and drilling conditions
3. connecting ringIt transmits torque and plays a safety role. It is respectively connected with drill pipe cutting tool head by double convex cross key
4. cooling ringThe cutting fluid cooling device has the function of cooling and promoting chip removal, and can prolong the service life of the center drill and indexable blade
5. drill pipeOne end is connected with the spindle, the other end is connected with the cutter head, and there is a cooling hole inside. One drill pipe can replace 10~15 cutting heads of different sizes to meet the requirements of different diameters. The drill pipe can also be lengthened to meet the requirements of different depths

2. Scope of application and reasonable selection

Composite drill bits are generally applicable to large-scale CNC boring and milling machines, boring machines and rocker drills to process large apertures of carbon steel, alloy steel, non-ferrous metals and cast iron, requiring sufficient power, high rigidity and precision of the machine tools.

The general drilling diameter range is  φ58~  φ170mm, which can be machined to  φ holes above 300mm.

Generally, the ratio of drilling depth to drilling diameter is less than 5, and it can reach 8 in some cases.

The ratio of the most suitable drilling depth to diameter is shown in Fig. 5-16.

The requirements for workpiece surface shape during machining are shown in Fig. 5-17.

Fig. 5-16 ratio of optimum drilling depth to diameter

Fig. 5-17 requirements of composite drill for workpiece surface shape

When using composite drill bits, it must be noted that drilling can only be done on the plane, and it is not allowed to drill on the arc surface, curved surface and other irregular surfaces.

If there is a center hole on the plane, its diameter shall be smaller than the diameter of the center drill on the composite drill bit.

The compound drill cannot enlarge the hole.

Before using the compound drill, the distance between the center drill and the cutting tips of the cutting head must be reasonably adjusted, otherwise the center drill will not play the role of centering and guiding.

When applying composite drill bit on radial drilling machine, attention must be paid to:

(1) Select cutting parameters correctly and reasonably.

Due to the poor rigidity of the rocker arm drill, the rocker arm is easy to lift, so the cutting amount should be properly reduced, and the cutting speed has the greatest impact.

(2) The chip groove shape of indexable blade shall be reasonably selected to ensure ideal chip breaking and light cutting.

Generally, the convex trigonal point groove shape shall be selected and matched with the cutting parameters to ensure smooth chip removal.

3. Cutting parameters

Recommended cutting parameters for composite drill bits (see table 5-8).

Table 5-8 recommended cutting parameters of composite bits

Processed material

Tensile strength /mpa

Cutting speed Vc (m/min)

Feed rate f/ (mm/r)

Diameter /mm





Low carbon steel (WC ≤ 0.2%)







Medium carbon steel (Wc=0.3%~0.4%)







Alloy steel 34CrNi3Mo







Alloy steel 42CrMo







Alloy steel 32crmo12







(2) Typical product introduction

1.KSEM PLUS ™ Modular drilling tool

(1) Product introduction.

KSEM PLUS ™ The drill (see Fig. 5-18) adopts a simple and efficient design concept.

This product will ksem ™ The advantages of modular drill and indexable drill are combined.

KSEM PLUS ™ The aperture of the product is  φ28~  φ101mm, length diameter ratio of 3 ~ 10D, suitable for processing steel, cast iron, stainless steel and advanced materials.

Fig. 5-18 ksem plus ™ Modular drilling tool

KSEM PLUS ™ The modular drilling tool is a replaceable drill bit with FDS coupling device, which can quickly and easily replace the cutting blade or drill tip without removing the whole tool from the machine tool.

Because only the worn drill tip needs to be replaced, the drill body can be used for a longer time, so the cost can be saved and the tool inventory can be reduced.

In addition, different specifications and types of drills can be applied to a drill body.

The same as other solid carbide drills or modular drills, KSEM PLUS™ The product has two complete cutting edges to work. Compared with the indexable drill of the same size, it has unparalleled excellent performance, and the productivity can reach 100%.

KSEM PLUS ™ Products can be customized according to customer requirements.

The definitions of A, B, C and D (see Fig. 5-19) in the figure are as follows:

A: DFTD/DFR blades.

“V” cutting edge, with chip breaking groove and blade belt design, has certain guiding function, cutting is light and fast, cutting speed is higher than modular drilling, chip control is good, and stability is increased.


High feed speed, strong centering ability, the same feed speed as modular drill, long service life.

KSEMP+DFTD/DFR blade. KSEMP is a centering drill with 4 blade belts to balance the cutting force and ensure the quality of the hole outlet.

C: 4 cooling holes.

Optimize tool life and chip removal effect.

D: 2 effective cutting edges.

Reduce the cutting load, high feed speed, 4 cutting edges, balance the radial force, allow large feed, small cutting is easy to improve the surface quality of the hole, and improve the machining size and cylindricity.

Fig. 5-19 ksem plus product breakdown

(2) Application cases

1) Part name: cylinder.

2) Industry: petroleum.

3) Machining type: hole machining.

4) Customer demand: improve efficiency and increase tool life.

5) Tool: KSEM1875FDS45B1+,WD50FDS45451M.

6) Cutting parameters: vc=90m/min, n=602r/min, vf=0.2mm/min, hole depth L=304mm.

7) Tool life: 2 times longer.

8) The final result: the tool life was increased by 100% from 73min to 9min.

The surface of the through hole is smooth with good chip breaking.

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