As the most common tool for hole processing, drill bits are widely used in mechanical manufacturing. In particular, they are widely used and important for processing holes in cooling devices, tube sheets of power generation equipment, steam generators and other parts.
Drill Cutting Characteristics
The drill bit usually has two main cutting edges. During processing, the drill bit cuts while rotating. The rake angle of the drill bit increases from the center axis to the outer edge. The closer to the outer circle, the higher the cutting speed of the drill bit. The cutting speed decreases toward the center. The cutting speed of the drill bit’s rotation center is zero. The chisel edge of the drill bit is located near the center axis of rotation. The secondary rake angle of the chisel edge is large, there is no chip space, and the cutting speed is low. Therefore, a large axial resistance will be generated.
Drills can be divided into many categories according to the shape, material, structure, function, etc. of the workpiece, such as high-speed steel drills (twist drills, group drills, flat drills), solid carbide drills, indexable shallow hole drills, deep hole drills, trepanning drills and replaceable head drills.
Drill Bit Chip Breaking and Chip Removal
The cutting of the drill is carried out in a narrow hole, and the chips must be discharged through the drill groove. Therefore, the chip shape has a great influence on the cutting performance of the drill. Common chip shapes include flake chips, tubular chips, needle chips, conical spiral chips, ribbon chips, fan-shaped chips and powder chips.
The Key to Drilling Processing – Chip Control
When the chip shape is inappropriate, the following problems will occur:
- Fine chips block the cutting edge, affecting drilling accuracy, reducing drill life, and even breaking the drill (such as powdery chips, fan-shaped chips, etc.).
- Long chips wrap around the drill, hindering operation, causing drill damage or blocking cutting fluid from entering the hole (such as spiral chips, ribbon chips, etc.).
How to Solve the Problem of Improper Chip Shape:
- The chip breaking and chip removal effects can be improved by increasing the feed rate, intermittent feed, grinding the chisel edge, installing a chip breaker, etc., respectively or in combination, to eliminate the problems caused by chips.
- Professional chip breaking drills can be used for drilling. For example, adding a designed chip breaker in the groove of the drill bit breaks the chips into easier-to-remove chips. The chips are discharged smoothly along the groove and will not be blocked in the groove. Therefore, the new chip breaker drill achieves a much smoother cutting effect than traditional drills.
At the same time, the short and broken iron chips make it easier for the coolant to flow to the drill tip, further improving the heat dissipation effect and cutting performance during the processing. And because the newly added chip breaker penetrates the entire groove of the drill bit, it can still maintain its shape and function after multiple grinding. In addition to the above functional improvements, it is worth mentioning that the design strengthens the rigidity of the drill body and significantly increases the number of holes drilled before a single grinding.
Drilling Precision
The accuracy of the hole is mainly composed of factors such as hole diameter, position accuracy, coaxiality, roundness, surface roughness and hole burr.
Factors Affecting the Accuracy of the Processed Hole During Drilling:
- The clamping accuracy and cutting conditions of the drill, such as the tool holder, cutting speed, feed rate, cutting fluid, etc.
- The size and shape of the drill, such as the length of the drill, the shape of the blade, the shape of the drill core, etc.
- The shape of the workpiece, such as the side shape of the hole, the shape of the hole, the thickness, the clamping status, etc.
Hole Expansion
Hole expansion is caused by the swing of the drill bit during processing. The swing of the tool holder has a great influence on the hole diameter and the positioning accuracy of the hole. Therefore, when the tool holder is severely worn, it should be replaced with a new one in time. When drilling small holes, it is difficult to measure and adjust the swing. Therefore, it is best to use a coarse shank small blade diameter drill with good coaxiality between the blade and the shank. When using a re-ground drill bit for processing, the reason for the decrease in hole accuracy is mostly due to the asymmetric shape of the back. Controlling the height difference of the blade can effectively suppress the amount of hole cutting and expansion.
Hole Roundness
Due to the vibration of the drill bit, the drilled hole is easy to be polygonal, and rifling lines appear on the hole wall. Common polygonal holes are mostly triangular or pentagonal. The reason for the triangular hole is that the drill bit has two rotation centers when drilling. They vibrate at a frequency of 600 rotations, and the main reason for the vibration is unbalanced cutting resistance. When the drill bit rotates one turn, the resistance is unbalanced during the second turn of cutting due to the poor roundness of the processed hole, and the last vibration is repeated again. However, there is a certain offset in the vibration phase, resulting in rifling lines on the hole wall.
When the drilling depth reaches a certain level, the friction between the edge of the drill bit blade and the hole wall increases. The vibration is attenuated, the rifling disappears, and the roundness becomes better. This hole type is funnel-shaped from the longitudinal section. For the same reason, pentagonal and heptagonal holes may also appear during cutting. To eliminate this phenomenon, in addition to the vibration of the chuck and the height difference of the cutting edge. In addition to controlling factors such as the asymmetry of the back face and blade shape, measures should also be taken to increase the rigidity of the drill bit, increase the feed per revolution, reduce the back angle, and sharpen the chisel edge.
Drilling Holes on Inclined and Curved Surfaces
When the cutting surface or penetration surface of the drill bit is an inclined surface, a curved surface or a step, the positioning accuracy is poor. Since the drill bit cuts radially on one side at this time, the tool life is reduced.
To improve positioning accuracy, the following measures can be taken:
- Drill the center hole first.
- Use an end mill to mill the hole seat.
- Use a drill with good penetration and rigidity.
- Reduce the feed rate.
Burr Treatment
During drilling, burrs will appear at the entrance and exit of the hole, especially when processing tough materials and thin plates. The reason is that when the drill is about to penetrate, the material being processed undergoes plastic deformation. At this time, the triangular part that should be cut by the edge of the drill near the outer edge is deformed and bent outwards under the action of the axial cutting force. It is further curled under the action of the chamfer of the outer edge of the drill and the edge of the land, forming a curled edge or burr.
Drilling Processing Conditions
Whether the cutting conditions are appropriate should be determined through trial cutting and comprehensively judged based on factors such as machining accuracy, machining efficiency, and drill life.
Drill Bit Life and Processing Efficiency
Whether the drill is used properly under the premise of meeting the technical requirements of the workpiece being processed. It should be comprehensively measured mainly based on the service life of the drill and the processing efficiency. The evaluation index of the service life of the drill can be the cutting distance. The evaluation index of the processing efficiency can be the feed speed.
For high-speed steel drills, the service life of the drill is greatly affected by the rotation speed and less affected by the feed per revolution. Therefore, the processing efficiency can be improved by increasing the feed per revolution, while ensuring a longer drill life. However, it should be noted that if the feed per revolution is too large, the chips will thicken and make chip breaking difficult. Therefore, the range of feed per revolution that can successfully break the chips must be determined through trial cutting.
For carbide drills, the cutting edge has a larger chamfer in the negative rake direction, and the optional range of feed per revolution is smaller than that of high-speed steel drills. If the feed per revolution exceeds this range during processing, the service life of the drill will be reduced. Since the heat resistance of carbide drill bits is higher than that of high-speed steel drill bits, the rotation speed has little effect on the life of the drill bit. Therefore, the method of increasing the rotation speed can be used to improve the processing efficiency of carbide drill bits while ensuring the life of the drill bit.
Reasonable Use of Cutting Fluid
The cutting of the drill is carried out in a hole with a narrow space. Therefore, the type of cutting fluid and the injection method have a great influence on the life of the drill and the processing accuracy of the hole. Cutting fluids can be divided into two categories: water-soluble and non-water-soluble. Non-water-soluble cutting fluids have good lubricity, wettability and anti-adhesion properties, and also have anti-rust effects. Water-soluble cutting fluids have good cooling properties, do not smoke and are non-flammable.
However, if the dilution ratio of the water-soluble cutting fluid is improper or the cutting fluid deteriorates, the service life of the tool will be greatly shortened. Therefore, it must be used with care. Whether it is a water-soluble or non-water-soluble cutting fluid, the cutting fluid must be fully delivered to the cutting point during use. At the same time, the flow rate, pressure, number of nozzles, cooling method (internal cooling or external cooling) of the cutting fluid must be strictly controlled.
