Drilling Process

1. Characteristics of Drilling

A drill bit usually has two main cutting edges, and when machining, the drill bit is rotating while cutting. The front angle of the drill bit is increasing from the center axis to the outer edge, the closer to the outer part of the drill the higher the cutting speed, the cutting speed decreases toward the center, and the cutting speed of the rotary center of the drill bit is zero. The cross cutting edge of the drill bit is located near the center axis of rotation, and the subfrontal angle of the cross cutting edge is larger, so there is no chip space, and the cutting speed is low, resulting in a larger axial resistance. If the crossed cutting edge is ground to DIN 1414 type A or C, and the cutting edge near the center axis has a positive front angle, the cutting forces can be reduced and the cutting performance can be significantly improved.

Depending on the shape, material, structure and function of the workpiece, there are many types of drills, such as high speed steel drills (twist drill, group drill, flat drill), integral carbide drills, indexable shallow hole drills, deep hole drills, sleeve drills and interchangeable drill bits.

2. Chip Breaking and Chip Removal

The cutting of the drill bit is carried out in the narrow space of the hole, and the chips must be discharged through the cutting groove of the drill bit, so the shape of the chips has a great influence on the cutting performance of the drill bit. The common chip shapes include flake chips, tubular chips, needle chips, conical spiral chips, ribbon chips, fan-shaped chips, powder chips, and so on.

When the chip shape is not appropriate, the following problems arise.

(1) Fine chips blocking the cutting edge groove, affecting drilling accuracy, reducing the life of the drill bit and even causing the drill bit to break (such as powder chips, fan-shaped chips, etc.).

Long swarf wraps around the drill bit, hindering operations, causing the drill bit to break or blocking the cutting fluid from entering the hole (e.g., spiral swarf, ribbon swarf, etc.).

How to solve the problem of inappropriate chip shape.

1) Increase the feed, intermittent feed, resharpen the cross edge, install chip breaker and other methods to improve chip breakage and chip evacuation to eliminate the problems caused by chips.

Professional chip breaker bits can be used for drilling. For example, adding a designed chipbreaker in the groove of the drill interrupts the chips into easier to remove chips. The chips are removed smoothly along the groove without clogging in the groove. As a result, the new chip breaker drill achieves a much smoother cutting action than conventional drills.

Also, the short chips allow coolant to flow more easily to the drill tip, further improving heat dissipation and cutting performance during the machining process. And because the new chip breakers penetrate the entire groove of the drill, they retain their shape and function even after multiple resharpenings. In addition to these improvements, it is worth noting that the design increases the rigidity of the drill body, significantly increasing the number of holes drilled before a single regrind.

3. Drilling Accuracy

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

Factors that affect the accuracy of the hole being machined when drilling and machining.

(1) Clamping accuracy of the drill and cutting conditions, such as toolholder, cutting speed, feed, cutting fluid, and so on.

② Drill bit size and shape, such as bit length, cutting edge shape, and core shape.

(iii) The shape of the workpiece, such as the shape of the side of the orifice, the shape of the orifice, the thickness, and the state of loading.

Reaming

Hole reaming is caused by the oscillation of the drill bit during machining. The oscillation of the drill bit has a great influence on the diameter of the hole and the positioning accuracy of the hole, so when the drill bit is worn out, the drill bit should be replaced with a new one. When drilling a small hole, it is difficult to measure and adjust the swing, so it is better to use a coarse shank with a small cutting edge that has a good coaxiality with the shank. The decrease in hole accuracy when using a resharpening bit is often caused by the asymmetry of the back shape. Controlling the cutting edge height difference can effectively suppress the amount of hole expansion.

Roundness of the hole

Due to the vibration of the drill bit, the drilled hole is easily polygonal, with a rifling-like pattern on the hole wall. The most common polygonal holes are triangular or pentagonal. The reason for triangular hole is that the drill bit has two rotation centers when drilling, and they vibrate at a frequency of 600 times per interval, mainly because of the unbalanced cutting resistance. When the drilling depth reaches a certain level, the friction between the edge of the drill bit and the hole wall increases, the vibration attenuates, the rifling lines disappear, and the roundness becomes better. This hole pattern shows a funnel shape from the longitudinal profile. For the same reason, a pentagonal or heptagonal hole may appear during cutting. To eliminate this phenomenon, in addition to controlling the chuck vibration, height difference of cutting edge, asymmetric shape of the back and flap, etc., measures such as improving the rigidity of the drill bit, increasing the feed per revolution, reducing the back angle, and resharpening the cross edge should be taken.

Drilling on inclined and curved surfaces

The positioning accuracy is poor when the tool eating surface or drilling surface of the drill is inclined, curved or stepped, and the tool life is reduced due to the radial single-sided eating surface of the drill.

The following measures can be taken to improve the positioning accuracy.

1. Drill the center hole first.

2. Milling the hole holder with an end mill.

3. Use drill bits with good penetration and rigidity.

4. Reduce the feed rate.

Treatment of burrs

In drilling, burrs appear at the entrance and exit of the hole, especially in the processing of tough materials and thin plates. The reason is that when the drill bit is about to drill through, the material to be processed undergoes plastic deformation, at which time the triangular part of the cutting edge which should be near the outer edge of the drill bit is deformed by the axial cutting force and bent outward, and then further curled under the action of the chamfer of the outer edge of the drill bit and the rib of the cutting belt, forming a roll or burr.

4. Machining Conditions for Drilling

The general drill product catalog has a reference table of basic cutting capacity arranged by processing materials, and users can refer to the amount of cutting capacity provided to select the cutting conditions for drilling. Whether the selection of cutting conditions is appropriate or not should be determined by test cutting, according to the factors of machining accuracy, machining efficiency, and drill life.

1、Drill life and processing efficiency

On the premise of meeting the technical requirements of the workpiece to be machined, whether the use of the drill bit is appropriate or not should be comprehensively evaluated based on the life of the drill bit and the machining efficiency. The life of the drill can be evaluated by the cutting distance, while the processing efficiency can be evaluated by the feed speed. As for HSS drill, the service life of drill bit is more affected by the rotation speed and less affected by the feed per revolution, so it is possible to increase the feed per revolution to improve the processing efficiency and ensure the long service life of drill bit. However, it should be noted that if the feed per revolution is too large, the chips will be thickened, resulting in chip breaking difficulties, so it is necessary to determine the range of feed per revolution that can break chips smoothly through test cutting. For carbide drills, the cutting edge has a large chamfer in the direction of the negative front corner of the cutting edge, and the range of feed per revolution is smaller than that of HSS drills, and if the feed per revolution exceeds the range, it will reduce the service life of the drill. As the heat resistance of the carbide drill is higher than the HSS drill, the rotation speed has little effect on the life of the drill, so the method of improving the rotation speed can be used to improve the processing efficiency of the carbide drill, while ensuring the life of the drill.

2、The reasonable use of cutting fluid

The cutting of the drill bit is carried out in a narrow hole, so the type of cutting fluid and the way it is injected has a great influence on the life of the drill bit and the machining accuracy of the hole. Cutting fluids can be divided into two categories: water-soluble and non-water-soluble. Non-water-soluble cutting fluid has better lubrication, infiltration and anti-bonding properties, as well as rust prevention. Water-soluble cutting fluids have better cooling properties, no fumes and no flammability. For the consideration of environmental protection, the use of water-soluble cutting fluids has been large in recent years. However, if the dilution rate of water-soluble cutting fluid is not appropriate or if the cutting fluid deteriorates, it will greatly shorten the life of the tool, so care must be taken in its use. Whether it is water-soluble or non-water-soluble cutting fluid, the cutting fluid must reach the cutting point, and the flow rate, pressure, number of nozzles, and cooling method (internal or external cooling) of the cutting fluid must be strictly controlled.