Drilling technology method, characteristics of drilling technology

As the most common tool in hole processing, drill bits are widely used in mechanical manufacturing, especially for the processing of holes in cooling devices, tube plates of power generation equipment, steam generators and other parts. It is particularly important to master the drilling technology and its characteristics.

1、 Characteristics of drilling

The drill bit usually has two main cutting edges. During machining, the drill bit rotates and cuts at the same time. The front angle of the drill bit becomes larger and larger from the central axis to the outer edge, the cutting speed of the drill bit closer to the outer circle is higher, and the cutting speed decreases toward the center, and the cutting speed of the drill bit’s rotating center is zero. The horizontal edge of the drill is located near the axis of the rotary center. The lateral edge has a large auxiliary rake angle, no chip space, and low cutting speed, so it will produce large axial resistance. If the transverse edge is grinded to type A or C in DIN1414, and the cutting edge near the central axis has a positive rake angle, the cutting resistance can be reduced and the cutting performance can be significantly improved.

According to the different shapes, materials, structures and functions of workpieces, drills can be divided into many types, such as high-speed steel drills (fried dough twist drills, group drills, flat drills), integral carbide drills, indexable shallow hole drills, deep hole drills, nesting drills, and interchangeable head drills.

2、 Chip breaking and chip removal

The cutting of the drill bit is carried out in a narrow hole, and the chips must be discharged through the cutting groove of the drill bit, so the chip shape has a great impact on the cutting performance of the drill bit. Common chip shapes include flake chips, tubular chips, needle chips, tapered spiral chips, ribbon chips, fan-shaped chips, powdery chips, etc.

The key of drilling — chip control

When the chip shape is not suitable, the following problems will occur:

① Fine chips block the edge groove, affect the drilling accuracy, reduce the life of the drill bit, and even break the drill bit (such as powdery chips, fan-shaped chips, etc.);

② Long chips wrap around the drill bit, hindering operation, causing damage to the drill bit or preventing cutting fluid from entering the hole (such as spiral chips, ribbon chips, etc.).

How to solve the problem of improper chip shape:

① The chip breaking and chip removal effect can be improved by increasing the feed rate, intermittent feed, grinding the cross edge, installing chip breaker, etc., respectively or jointly, to eliminate the problems caused by chips.

② A professional chip breaking drill can be used for drilling. For example, the designed chip breaking edge is added in the groove of the drill bit to break the chips into more easily cleared chips. Debris shall be discharged smoothly along the trench without blockage in the trench. Therefore, the new chip breaking drill achieves much smoother cutting effect than the traditional drill.

At the same time, the short scrap iron makes it easier for the coolant to flow to the drill point, further improving the heat dissipation effect and cutting performance during the processing. Moreover, because the newly added chip breaking edge has penetrated the entire groove of the drill bit, its shape and function can still be maintained after several times of grinding. In addition to the above function improvement, it is worth mentioning that the design strengthens the rigidity of the drill body and significantly increases the number of holes drilled before single grinding.

3、 Drilling accuracy

The accuracy of a hole is mainly composed of such factors as hole size, position accuracy, coaxiality, roundness, surface roughness and hole burr.

Factors affecting the accuracy of the hole to be machined during drilling:

① The clamping accuracy and cutting conditions of the drill, such as tool holder, cutting speed, feed rate, cutting fluid, etc;

② Bit size and shape, such as bit length, edge shape, core shape, etc;

③ Workpiece shape, such as orifice side shape, orifice shape, thickness, clamping state, etc.

Counterbore

Reaming is caused by the swing of the drill bit during processing. The swing of the tool holder has a great impact on the hole diameter and the positioning accuracy of the hole. Therefore, when the tool holder is seriously worn, a new tool holder should be replaced in time. When drilling small holes, it is difficult to measure and adjust the swing, so it is better to use a coarse shank small diameter drill with good coaxiality between the blade and the shank. When using a regrind drill to process, the reason for the decline in hole accuracy is mostly due to the asymmetry of the back shape. The control of the edge height difference can effectively restrain the reaming of the hole.

Roundness of hole

Due to the vibration of the drill bit, the drilled hole is easy to be polygonal, and the hole wall appears like a double line pattern. Common polygonal holes are mostly triangular or pentagonal.

Drill holes on slopes and surfaces

When the cutting surface or drilling through surface of the drill bit is inclined, curved or stepped, the positioning accuracy is poor. At this time, the drill bit is cut on a radial single side, which reduces the tool life.

To improve the positioning accuracy, the following measures can be taken:

1. Drill the center hole first;

2. Mill the hole seat with the end mill;

3. Select drill bits with good penetration and rigidity;

4. Reduce the feed speed.

Final burr treatment

During drilling, burrs will appear at the entrance and exit of the hole, especially when processing materials and thin plates with high toughness. The reason is that when the drill bit is about to drill through, the material to be processed will have plastic deformation. At this time, the triangular part which should be cut by the edge of the drill bit near the outer edge will be deformed and bent outward under the action of the axial cutting force, and further curled under the action of the chamfer of the outer edge of the drill bit and the edge of the edge band, forming curls or burrs.