Original Title: Drilling Technology Encyclopedia, This Must Be Collected! Drill, as the most common tool in hole processing, is widely used in mechanical manufacturing, especially in the processing of holes in cooling devices, tubesheets of power generation equipment and steam generators. I. Characteristics of drilling Drills usually have two main cutting edges, and when machining, the drill cuts while rotating. The rake angle of the drill is getting larger and larger from the central axis to the outer edge, the cutting speed of the drill is getting higher and higher when it is closer to the outer circle, and the cutting speed decreases to the center, and the cutting speed of the rotary center is zero. The chisel edge of the drill is located near the central axis of rotation. The chisel edge has a large secondary rake angle, no chip space and low cutting speed, so it will produce a large axial resistance. If the chisel edge is ground to Type A or Type 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 shape, material, structure and function of the workpiece, the drill bit can be divided into many types, such as high-speed steel drill bit (twist drill, group drill, flat drill), solid carbide drill bit, indexable shallow hole drill, deep hole drill, trepanning drill and interchangeable head drill bit. 2. 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 edge groove, so the chip shape has a great influence on the cutting performance of the drill. Common chip shapes include flaky chips, tubular chips, acicular chips, conical spiral chips, ribbon chips, fan-shaped chips and powdery chips. Key Chip Control in Drilling When the chip shape is not appropriate, the following problems will arise: ① Fine chips block the cutting edge groove, affecting the drilling accuracy, reducing the life of the drill bit, and even breaking the drill bit (such as powdery chips, fan-shaped chips, etc.); ② Long chips entangle the drill bit, which hinders the operation, causing the drill bit to break or preventing the 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 effects can be improved by increasing the feed rate, intermittent feed, grinding the chisel edge, installing the chip breaker and other methods respectively or in combination, so as to eliminate the problems caused by chips. ② Professional chip-breaking drill can be used for drilling. For example, the addition of a designed chip breaker in the groove of the drill breaks the chips into more easily removed chips. The debris is smoothly discharged along the groove, and the phenomenon of blockage in the groove does not occur. Therefore, the new chip-breaking drill obtains a much smoother cutting effect than the traditional drill. At the same time, the short scrap iron makes the coolant flow to the drill tip more easily, which further improves the heat dissipation effect and cutting performance in the processing process. Moreover, because the new chip-breaking edge penetrates the whole groove of the drill bit, it can still maintain its shape and function after many times of 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. III. Drilling accuracy The accuracy of the hole is mainly composed of the hole size, position accuracy, coaxiality, roundness, surface roughness and hole burr. Factors affecting the accuracy of the hole to be machined during drilling: Expand the full text ① Bit clamping accuracy and cutting conditions, such as tool holder, cutting speed, feed rate, cutting fluid, etc.; ② Drill size and shape, such as drill length, blade shape, drill core shape, etc.; ③ Workpiece shape, such as orifice side shape, orifice shape, thickness, clamping state, etc. Reaming Reaming is caused by the oscillation of the drill during machining. The swing of the tool holder has a great influence on the hole diameter and the positioning accuracy of the hole, so when the tool holder is worn seriously, it should be replaced with a new one in time. When drilling a small hole, DHD Drill bit , it is difficult to measure and adjust the swing, so it is better to use a drill with a thick shank and a small edge diameter with a good coaxiality between the blade and the shank. When the regrinding drill is used for machining, the reason for the decrease of hole accuracy is mostly due to the asymmetry of the back shape. Controlling the height difference of the cutting edge can effectively restrain the amount of hole expansion. Roundness of the hole Because of the vibration of the drill bit, the drilled hole shape is easy to be polygonal, and the lines like rifling 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 rotary centers during drilling, and they vibrate at the frequency of exchanging once every 600. The main reason for the vibration is that the cutting resistance is unbalanced. When the drill bit rotates for one turn, the resistance is unbalanced during the second turn of cutting due to the poor roundness of the processed hole. The last vibration is repeated again, but the vibration phase has a certain deviation. Cause rifling pattern to appear on that walls of the hole. When the drilling depth reaches a certain degree, the friction between the drill edge and the hole wall increases, the vibration decreases, the rifling disappears, and the roundness becomes better. The orifice of this pass is funnel-shaped when viewed in longitudinal section. For the same reason, pentagonal and heptagonal holes may also appear in cutting. In order to eliminate this phenomenon, in addition to controlling the factors such as Chuck vibration, cutting edge height difference, back and edge shape asymmetry, measures such as improving drill rigidity, increasing feed per revolution, reducing clearance angle and grinding chisel edge should be taken. Drill holes on sloped and curved surfaces When the cutting surface or penetration surface of the drill is inclined, curved or stepped, the positioning accuracy is poor, and the tool life is reduced due to the radial single-sided cutting surface of the drill. To improve the positioning accuracy, the following measures can be taken: 1. Drill the center hole first ； 2. Milling hole seat with end mill ； 3. And that selection cut-in property is good Drill bit with good rigidity; 4. Reduce the feed speed 。 Treatment of burrs During drilling, burrs will appear at the entrance and exit of the hole, especially when machining materials with high toughness and thin plates. The reason is that when the drill bit is about to drill through, the material to be processed is plastically deformed. At this time, the triangular part that should be cut by the cutting edge of the drill bit near the outer edge is deformed and bent outward under the action of axial cutting force, and further curled under the action of the chamfer of the outer edge of the drill bit and the edge surface of the cutting edge, forming curling or burrs. IV. Processing conditions of drilling In the catalogue of general drill product sample book, there is a Reference Table of Basic Cutting Parameters arranged by processing materials. Users can refer to the cutting parameters provided to select the cutting conditions for drilling. Whether the selection of cutting conditions is appropriate or not should be judged comprehensively through trial cutting according to the factors such as machining accuracy, machining efficiency and drill life. 1. Drill life and machining efficiency On the premise of meeting the technical requirements of the machined workpiece, whether the drill bit is used properly or not should be measured comprehensively according to the service life and machining efficiency of the drill bit. Cutting path can be selected as the evaluation index of drill life, and feed speed can be selected as the evaluation index of machining efficiency. For high speed steel drill bit, the service life of the drill bit is greatly affected by the rotary speed, and is less affected by the feed per revolution, so the machining efficiency can be improved by increasing the feed per revolution, and the longer service life of the drill bit can be ensured. However, it should be noted that if the feed per revolution is too large, the chip will be thickened, resulting in difficulty in chip breaking. Therefore, the range of feed per revolution that can successfully break the chip must be determined through trial cutting. For cemented carbide drill, the cutting edge has a large chamfer in the negative rake angle direction, and the optional range of feed per revolution is smaller than that of high-speed steel drill. If the feed per revolution exceeds this range during machining, the service life of the drill will be reduced. Because the heat resistance of cemented carbide drill is higher than that of high-speed steel drill, the rotary speed has little effect on the life of the drill, so the method of increasing the rotary speed can be used to improve the processing efficiency of cemented carbide drill and ensure the life of the drill. 2. Reasonable use of cutting fluid The cutting of drill is carried out in the hole with narrow space, so the type of cutting fluid and the way of injection have a great influence on the life of drill and the machining accuracy of the hole. Cutting fluid can be divided into two categories: water-soluble and water-insoluble. The water-insoluble cutting fluid has good lubricity, wettability and adhesion resistance, and also has anti-rust effect. Water-soluble cutting fluid has good cooling property, no smoke and no flammability. For the sake of environmental protection, the amount of water-soluble cutting fluid is large in recent years. 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, so attention must be paid in use. Whether it is water-soluble or non-water-soluble cutting fluid, the cutting fluid must fully reach the cutting point in use, and the flow, pressure, nozzle number and cooling mode (internal cooling or external cooling) of the cutting fluid must be strictly controlled.
V. Re-sharpening of drill bit Bit regrinding discrimination The criteria for re-sharpening of drill bits are: 1. Cutting edge Wear loss of chisel edge and edge land; 2. Dimensional accuracy and surface roughness of the hole to be machined ； 3. The color of the swarf , shape; 4. Cutting resistance (Indirect values of spindle current, noise, vibration, etc.); 5. Processing quantity, etc 。 In actual use, accurate and convenient criteria should be determined from the above indicators according to the specific situation. When the wear loss is used as the criterion, the best regrinding period with the best economy should be found out. Because the main grinding parts are the back of the head and the chisel edge, if the wear of the drill bit is too large, the grinding time is too long, the grinding amount is too large, and the number of times of regrinding is reduced (the total service life of the tool = the service life of the tool after regrinding × the number of times of regrinding), which will shorten the total service life; When the dimensional accuracy of the hole to be machined is used as the criterion, the column gauge or limit gauge shall be used to check the cutting expansion and non-straightness of the hole. Once the control value is exceeded, it shall be re-sharpened immediately. When the cutting resistance is used as the criterion, it can be stopped automatically immediately when the set limit value (such as spindle current) is exceeded; When the limit management of processing quantity is adopted, the above discrimination contents shall be integrated and the discrimination criteria shall be set. Method for sharpening drill bit When re-sharpening the drill, it is better to use a special machine tool for drill sharpening or a universal tool grinder, which is very important to ensure the service life and machining accuracy of the drill. If the original drill type is in good processing condition, it can be reground according to the original drill type; if the original drill is defective,dth hammer bit, the rear shape can be properly improved and the chisel edge can be grinded according to the use purpose. Attention shall be paid to the following points during sharpening: 1. Prevent overheating ? Avoid reducing the hardness of the drill bit; 2. Damages on the drill bit shall be (Especially the damage on the edge surface of the blade) is completely removed; 3. The drill pattern shall be symmetrical ； 4. Be careful not to damage the cutting edge during sharpening. , and remove the burrs after sharpening; 5. For cemented carbide drill bits ? The sharpening shape has a great influence on the performance of the drill. The drill shape when leaving the factory is the best drill shape obtained through scientific design and repeated tests. Therefore, the original edge shape should be maintained when re-sharpening. This article is from Momo's WeChat public account [UG NC Programming]. You can get the tutorial after paying attention to it. Return to Sohu to see more Responsible Editor:. wt-dthtools.com