As the most common tool in hole processing, drill bits are widely used in mechanical manufacturing, especially for the processing of holes in parts such as cooling devices, tube sheets of power generation equipment and steam generators, etc., and the application area is particularly wide and important.
1. Characteristics of drilling Drill bits usually have 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, and the cutting speed of the rotating center of the drill bit 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, which will produce a large axial resistance. If the chisel edge is ground to type A or C in DIN1414, and the cutting edge near the center axis is a positive rake angle, the cutting resistance can be reduced and the cutting performance can be significantly improved. According to the shape, material, structure, function, etc. of the workpiece, drill bits can be divided into many categories, such as high-speed steel drill bits (twist drill bits, group drill bits, flat drill bits), solid carbide drill bits, indexable shallow hole drill bits, deep hole drill bits, sleeve drill bits and replaceable head drill bits. 2. Chip breaking and chip removal The cutting of the drill bit is carried out in a hole with a narrow space, and the chips must be discharged through the drill bit groove. Therefore, the chip shape has a great influence on the cutting performance of the drill bit. Common chip shapes include flaky chips, tubular chips, needle chips, conical spiral chips, ribbon chips, fan-shaped chips, powdery chips, etc. The key to drilling processing-chip control When the chip shape is inappropriate, the following problems will occur: ① Fine chips block the 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 the operation, causing the drill bit to break or hindering the cutting fluid from entering the hole (such as spiral chips, ribbon chips, etc.). How to solve the problem of inappropriate chip shape: ① Increase the feed rate, intermittent feed, grind the chisel edge, install a chip breaker, etc. to improve the chip breaking and chip removal effect and eliminate the problems caused by chips. ② Use a professional chip breaking drill to drill holes. For example: Add a designed chip breaker in the groove of the drill bit to break 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 has a much smoother cutting effect than the traditional drill. 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 passes through the entire groove of the drill bit, it can still maintain its shape and function after multiple grinding. In addition to the above-mentioned 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. 3. Drilling accuracy The accuracy of the hole is mainly composed of factors such as hole size, position accuracy, coaxiality, roundness, surface roughness, and hole mouth burrs. Factors that affect the accuracy of the processed hole during drilling: ① The clamping accuracy and cutting conditions of the drill bit, such as tool holder, cutting speed, feed rate, cutting fluid, etc. ② The size and shape of the drill bit, such as the length of the drill bit, the shape of the blade, the shape of the drill core, etc. ③ The shape of the workpiece, such as the side shape of the hole, the shape of the hole, the thickness, the clamping state, 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, 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 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 asymmetry of the back shape. Controlling the height difference of the blade can effectively suppress the amount of cutting and expansion of the hole. The roundness of the hole Due to the vibration of the drill bit, the hole type drilled is easy to be polygonal, and rifling-like lines appear on the hole wall. Common polygonal holes are mostly triangles or pentagons. The reason for the triangular hole is that the drill bit has two rotation centers when drilling, and they vibrate at a frequency of 600 exchange intervals. The main reason for the vibration is the unbalanced cutting resistance. When the drill bit rotates one turn, the roundness of the processed hole is not good, which causes the unbalanced resistance during the second turn of cutting, and repeats the last vibration again, but the vibration phase has a certain offset, resulting in the appearance of rifling lines on the hole wall. When the drilling depth reaches a certain level, the friction between the edge of the drill bit 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. In order to eliminate this phenomenon, in addition to controlling factors such as chuck vibration, cutting edge height difference, back and blade shape asymmetry, measures such as improving drill bit rigidity, increasing feed per revolution, reducing back angle, and grinding chisel edges should also be taken. When drilling on inclined and curved surfaces, the engagement surface or penetration surface of the drill bit is an inclined surface, curved surface or step, the positioning accuracy is poor. Since the drill bit is radially engaged on one side at this time, the tool life is reduced. To improve the positioning accuracy, the following measures can be taken: ① Drill the center hole first. ② Use an end mill to mill the hole seat. ③ Select a drill bit with good penetration and good rigidity. ④ Reduce the feed speed. Burr treatment During drilling, burrs will appear at the entrance and exit of the hole, especially when processing materials with high toughness and thin plates. The reason is that when the drill bit is about to drill through, the material being processed undergoes plastic deformation. At this time, the triangular part that should have been cut by the cutting edge of the drill bit near the outer edge is 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 land, forming a curling edge or burr. 4. Drilling processing conditions In the general drill product catalog, there is a "Basic Cutting Quantity Reference Table" arranged according to the processing material. Users can refer to the cutting quantities provided to select the cutting conditions for drilling processing. Whether the cutting conditions are appropriate should be judged comprehensively through trial cutting based on factors such as processing accuracy, processing efficiency, and drill life. 1 Drill life and processing efficiency Under the premise of meeting the technical requirements of the workpiece being processed, whether the drill is used properly should be comprehensively measured based on the drill life and processing efficiency. The evaluation index of the drill life can be selected from the cutting distance; the evaluation index of the processing efficiency can be selected from the feed speed. For high-speed steel drills, the drill life is greatly affected by the rotation speed and less affected by the feed per revolution, so 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, making it difficult to break the chips, so the feed per revolution range 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 drill life will be reduced. Since the heat resistance of carbide drills is higher than that of high-speed steel drills, the rotation speed has little effect on the life of the drills. Therefore, the method of increasing the rotation speed can be used to improve the processing efficiency of carbide drills and ensure the life of the drills. 2 Reasonable use of cutting fluids The cutting of drills is carried out in holes with narrow spaces, so the type of cutting fluid and the injection method have a great influence on the life of the drills and the processing accuracy of the holes. 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, and also have anti-rust effects. Water-soluble cutting fluids have good cooling properties, no smoke and no flammability. For environmental protection considerations, the use of water-soluble cutting fluids has been relatively large in recent years. However, if the dilution ratio of the water-soluble cutting fluid is improper or the cutting fluid deteriorates, the tool life will be greatly shortened, so care must be taken during use. Whether it is water-soluble or non-water-soluble cutting fluid, the cutting fluid must be fully applied 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.
5. Drill bit resharpening Drill bit resharpening judgment The judgment criteria for drill bit resharpening are: ① The wear of the cutting edge, chisel edge, and edge surface of the cutting edge; ② The dimensional accuracy and surface roughness of the processed hole; ③ The color and shape of the chips; ④ Cutting resistance (indirect values such as spindle current, noise, vibration, etc.); ⑤ The number of processes, etc. In actual use, accurate and convenient judgment criteria should be determined from the above indicators according to specific circumstances. When the wear amount is used as the judgment criterion, the best resharpening period with the best economic efficiency should be found. Since the main grinding parts are the back of the head and the transverse edge, if the drill wear is too large, the grinding time is long, the grinding amount is large, and the number of times that can be re-grinded is reduced (the total service life of the tool = the tool life after re-grinding × the number of times that can be re-grinded), it will shorten the total service life of the drill; when the dimensional accuracy of the processed hole is used as the judgment standard, the column gauge or limit gauge should be used to check the cutting expansion, non-straightness, etc. of the hole. Once the control value is exceeded, it should be re-sharpened immediately; when the cutting resistance is used as the judgment standard, the method of automatic shutdown immediately when exceeding the set limit value (such as spindle current) can be adopted; when the processing quantity limit management is adopted, the above judgment contents should be combined to set the judgment standard. Drill bit grinding method When re-sharpening the drill bit, it is best to use a special drill bit grinding machine or a universal tool grinder, which is very important to ensure the service life and processing accuracy of the drill bit. If the original drill type is in good processing condition, it can be re-grinded according to the original drill type; if the original drill type has defects, the back shape can be appropriately improved according to the purpose of use and the transverse edge can be repaired. The following points should be noted when sharpening: ①. Prevent overheating to avoid reducing the hardness of the drill bit. ② All damage on the drill bit (especially damage on the edge of the blade) should be removed. ③ The drill shape should be symmetrical. ④ Be careful not to damage the cutting edge during sharpening, and remove the burrs after sharpening. ⑤ For carbide drill bits, the sharpening shape has a great influence on the performance of the drill bit. The drill shape at the factory is the best drill shape obtained through scientific design and repeated tests. Therefore, the original blade shape should generally be maintained when re-sharpening.
