Five key issues in drilling
As the most common tool in hole processing, drill bits are widely used in machinery manufacturing, especially for the processing of parts holes such as cooling devices, tube plates of power generation equipment, and steam generators. The application is particularly extensive and important.
1. Characteristics of drilling
The drill bit usually has two main cutting edges. During machining, the drill bit cuts while rotating. The rake angle of the drill bit increases from the center 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. The cutting speed of the rotation center of the drill bit is zero. The lateral edge of the drill bit is located near the center axis of rotation, the secondary rake angle of the lateral edge is large, there is no chip space, and the cutting speed is low, so it will produce a large axial resistance. If the cutting edge of the cutting edge is sharpened to type A or type C in DIN1414, the cutting edge near the center axis is a positive rake angle, which can reduce the cutting resistance and significantly improve the cutting performance.
According to the workpiece shape, material, structure, function, etc., drills can be divided into many types, such as high-speed steel drills (twist drills, group drills, flat drills), solid carbide drills, indexable shallow hole drills, deep holes Drills, sleeve drills and interchangeable head drills.
2. Chip breaking and chip evacuation
The cutting of the drill bit is carried out in a hole with a narrow space. The chips must be discharged through the cutting edge groove of the drill bit, so the shape of the chip has a great influence on the cutting performance of the drill bit. Common chip shapes include flake chips, tube chips, needle chips, tapered spiral chips, ribbon chips, fan chips, powder chips, etc.
The key to drilling-chip control
When the chip shape is not suitable, the following problems will occur:
① Fine cuttings 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.).
②Long chips are wound around the drill bit, hindering 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 inappropriate chip shape:
①The methods of increasing feed amount, intermittent feed, grinding the cutting edge, installing chip breaker, etc. can be used separately or in combination to improve the chip breaking and chip evacuation effect and eliminate the problems caused by chip.
②It can be drilled with professional chip breaking drill. For example, adding a designed chip breaker to the groove of the drill bit will break the chips into chips that are easier to remove. The debris is smoothly removed along the groove, and blockage in the groove will not occur. Therefore, the new chip breaker has a much smoother cutting effect than the traditional drill bit.
At the same time, the short pieces of iron chips make the coolant flow to the drill tip more easily, which further improves the heat dissipation effect and cutting performance during 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 grindings. 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.
3. Drilling accuracy
The accuracy of the hole is mainly composed of factors such as aperture size, position accuracy, coaxiality, roundness, surface roughness and orifice burr.
Factors that affect the accuracy of the processed hole during drilling:
① The clamping accuracy and cutting conditions of the drill, such as tool holder, cutting speed, feed rate, cutting fluid, etc.
② Drill bit size and shape, such as drill bit length, blade shape, core shape, etc.
③ The shape of the workpiece, such as the shape of the side of the hole, the shape of the hole, the thickness, the clamping state, etc.
Reaming is caused by the wobble of the drill bit during processing. The swing of the tool holder has a great influence on the aperture and the positioning accuracy of the hole, so when the tool holder is seriously worn, the new tool holder should be replaced in time. When drilling small holes, the measurement and adjustment of the swing are more difficult, so it is best to use a coarse shank small blade diameter drill with a good coaxiality between the blade and the shank. When using the re-grinding drill, the reason for the decrease in hole accuracy is mostly due to the asymmetric shape behind. Controlling the height difference of the cutting edge can effectively suppress the cutting expansion of the hole.
Due to the vibration of the drill bit, the hole pattern drilled is easily polygonal, and the lines of the hole appear like lines on the wall. Common polygonal holes are mostly triangles or pentagons. The reason for the triangle hole is that the drill has two centers of rotation when drilling, and they vibrate at a frequency of 600 exchanges every interval. The main reason for the vibration is the unbalanced cutting resistance. Well, the resistance is unbalanced during the second cutting, and the last vibration is repeated again, but the vibration phase is shifted to a certain extent, resulting in the appearance of double-track lines on the hole wall. When the drilling depth reaches a certain level, the friction between the edge face of the drill bit and the wall of the hole increases, the vibration is attenuated, the double-track line disappears, and the roundness becomes better. This type of orifice is funnel-shaped from the longitudinal section. For the same reason, pentagonal and heptagonal holes may appear in cutting. In order to eliminate this phenomenon, in addition to controlling the vibration of the chuck, the height difference of the cutting edge, the asymmetry of the back face and the shape of the blade petals, it is also necessary to increase the rigidity of the drill, increase the feed per revolution, reduce the back angle, and grind Measures such as horizontal blades.
Drill holes on inclined and curved surfaces
When the cutting surface or drilling surface of the drill bit is inclined, curved or stepped, the positioning accuracy is poor. Since the drill bit is a radial single-sided eating knife at this time, the tool life is reduced.
In order to improve the positioning accuracy, the following measures can be taken:
① Drill the center hole first.
②. Mill the hole seat with an end mill.
③Select a bit with good penetration and good rigidity.
④ Reduce the feed rate.
Treatment of glitches
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 penetrate through, the material to be processed is plastically deformed. At this time, the triangular part that should be cut by the edge of the drill bit near the outer edge is deformed and bent outwards after being subjected to the axial cutting force, and the outer edge of the drill bit Under the action of chamfering and edge banding, it is further curled to form a curl or burr.
4. Processing conditions for drilling
The general catalog of drill bit products has a "Basic Cutting Amount Reference Table" arranged by processing materials. Users can refer to the cutting amount provided by them to choose the cutting conditions for drilling. Whether the selection of cutting conditions is appropriate should be judged comprehensively by trial cutting, based on factors such as machining accuracy, machining efficiency, and bit life.
1 Drill bit life and processing efficiency
Under the premise of meeting the technical requirements of the workpiece to be processed, the proper use of the drill bit should be measured based on the service life of the drill bit and the processing efficiency. The evaluation index of the service life of the drill bit can be the cutting distance; the evaluation index of the processing efficiency can be the feed rate. For high-speed steel drill bits, the service life of the drill bit is greatly affected by the rotation speed, and is less affected by the feed per revolution. Therefore, the machining efficiency can be increased by increasing the feed per revolution, while ensuring a long 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 chip. Therefore, the range of feed per revolution that can successfully break the chip must be determined by trial cutting. For carbide drills, the cutting edge has a large chamfer in the direction of the negative rake angle. The selectable 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 use of the drill will be reduced. life. Because the heat resistance of cemented carbide drills is higher than that of high-speed steel drills, the speed of rotation has little effect on the life of the drill. Therefore, the method of increasing the speed of rotation can be used to improve the processing efficiency of cemented carbide drills while ensuring the life of the drill.
2 Reasonable use of cutting fluid
The cutting of the drill bit is performed in a hole with a narrow space, so the type of cutting fluid and the method of injection have a great influence on the life of the drill bit and the machining accuracy of the hole. Cutting fluid can be divided into two major categories of water-soluble and water-insoluble. The water-insoluble cutting fluid has good lubricity, wetting and anti-adhesion properties, and also has anti-rust effect. The water-soluble cutting fluid has good cooling performance, no smoke and no flammability. In consideration of environmental protection, the usage of water-soluble cutting fluid has been relatively large in recent years. However, if the dilution rate of the water-soluble cutting fluid is improper or the cutting fluid deteriorates, it will greatly shorten the tool life, so care must be taken during use. Whether it is water-soluble or non-water-soluble cutting fluid, the cutting fluid must be fully reached to the cutting point during use, and the flow rate, pressure, number of nozzles, cooling method (internal or external cooling) of the cutting fluid must be strictly controlled.
5. Re-sharpening of the drill bit
Discrimination of bit regrind
The criteria for re-sharpening the drill bit are:
①Abrasion amount of cutting edge, transverse edge and edge face of cutting edge;
②Dimensional accuracy and surface roughness of the processed hole;
③ The color and shape of chips;
④Cutting resistance (indirect values of spindle current, noise, vibration, etc.);
⑤ Processing quantity, etc.
In actual use, according to the specific situation, determine the accurate and convenient judgment criteria from the above indicators. When using the amount of wear as a criterion, the best refining period with the best economy should be found. Since the main sharpening parts are the back of the head and the transverse edge, if the wear of the drill is too large, the blade wears too much time, the grinding amount is large, and the number of regrindable times is reduced (total tool life of the tool = life of the tool after regrind × available) Re-grinding times), on the contrary, it will shorten the total service life of the drill bit; when using the dimensional accuracy of the processed hole as the criterion, apply a column gauge or limit regulation to check the cutting expansion, unevenness, etc. of the hole. Once the control value is exceeded, it should be immediately Re-sharpening; when cutting resistance is used as the criterion, it can be used to automatically stop the machine if it exceeds the set limit value (such as the spindle current); when the limit of processing quantity is used, the above criteria should be integrated to set the criteria.
Sharpening method of drill bit
When re-sharpening the drill bit, it is best to use a special machine tool or universal tool grinder for sharpening the drill bit, which is very important to ensure the service life and machining accuracy of the drill bit. If the original drill type is in good condition, it can be reground according to the original drill type; if the original drill type is defective, the rear shape can be appropriately improved and the cross edge grinding can be performed according to the purpose of use.
Pay attention to the following points when sharpening:
①. Prevent overheating and avoid reducing the hardness of the drill bit.
②All damages on the drill (especially those on the edge of the blade) should be removed.
③The drilling pattern should be symmetrical.
④ Be careful not to damage the edge of the blade during sharpening, and remove the burr after sharpening.
⑤ For cemented carbide drills, the shape of the sharpening has a great influence on the performance of the drill. The drill type at the factory is the best drill type obtained through scientific design and trial and error. Therefore, the original blade type should be maintained when re-sharpening.