As the most common tool in hole machining, drill bits are widely used in mechanical manufacturing, especially for the machining of holes in parts such as cooling devices, tube sheets of power generation equipment, and steam generators. The application is particularly extensive and important.
1. Characteristics of drilling
Drills usually have two main cutting edges. During machining, the drill cuts while rotating. The rake angle of the drill bit increases from the central axis to the outer edge. The cutting speed of the drill bit increases as it gets closer to the outer circle, and the cutting speed decreases towards 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 axis of the rotation center, the chisel edge has a large auxiliary rake angle, no chip space, and the cutting speed is low, which will generate 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 different workpiece shapes, materials, structures, functions, 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 hole drills, etc. Drills, trepanning drills and replaceable head drills, etc.
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2. Chip breaking and chip removal
The cutting of the drill is carried out in a hole with a narrow space, and the chips must be discharged through the drill groove, so 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 chips, powder chips, etc.
The Key of Drilling Process--Swarf Control
①Small chips 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 entangle the drill bit, hinder the operation, cause the drill bit to break or prevent the cutting fluid from entering the hole (such as spiral chips, ribbon chips, etc.).
How to solve the problem of improper chip shape:
① Increase the feed rate, intermittent feed, grind the chisel edge, install a chip breaker and other methods to improve chip breaking and chip removal effects separately or in combination, and eliminate problems caused by chips.
②It can be drilled with a professional chip breaking drill. For example: Adding a designed chip breaker to the groove of the drill bit breaks the chips into chips that are easier to remove. Debris is smoothly discharged along the groove without clogging in the groove. Therefore, the new chip breaking drill has obtained 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 machining. And because the newly added chip breaker penetrates the entire groove of the drill bit, it can still maintain its shape and function after repeated grinding. In addition to the above functional improvements, it is worth mentioning that this design strengthens the rigidity of the drill body and significantly increases the number of drilled holes 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 burrs.
Factors that affect the accuracy of the processed hole during drilling:
① Clamping accuracy and cutting conditions of the drill bit, such as tool holder, cutting speed, feed rate, cutting fluid, etc.;
②Drill size and shape, such as drill length, blade shape, drill core shape, etc.;
③The shape of the workpiece, such as the shape of the side of the hole, the shape of the hole, the thickness, the state of the clamping, etc.
1. Reaming
Reaming is caused by the swing of the drill bit during machining. The swing of the tool holder has a great influence on the positioning accuracy of the aperture and the hole, so 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 drill with a thick handle and a small blade diameter with a good coaxiality between the blade and the shank. When using a regrinding drill bit, the reason for the decrease in hole accuracy is mostly due to the asymmetrical shape of the back. Controlling the edge height difference can effectively restrain the cutting expansion of the hole.
2. The roundness of the hole
Due to the vibration of the drill bit, the drilled hole pattern is easily polygonal, and lines like 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 centers of rotation when drilling, and they vibrate at a frequency of 600 times. The main reason for the vibration is the unbalanced cutting resistance. Well, the resistance force is unbalanced during the second round of cutting, and the last vibration is repeated again, but the vibration phase has a certain offset, resulting in rifling 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 hole wall increases, the vibration attenuates, the rifling line disappears, and the roundness becomes better. This type of hole 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 the vibration of the chuck, the height difference of the cutting edge, the asymmetry of the back face and the blade shape, etc., measures should also be taken to improve the rigidity of the drill bit, increase the feed per revolution, reduce the back angle, and grind Chisel and other measures.
3. Drill holes on inclined and curved surfaces
When the cutting surface or the drilling surface of the drill bit is an inclined surface, a curved surface or a step, the positioning accuracy is poor, and since the drill bit is radial and single-sided 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;
3. Choose a drill with good penetration and rigidity;
4. Reduce the feed rate.
4. Handling of burrs
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 bit is about to drill through, the processed material undergoes plastic deformation. At this time, the triangular part that should be cut by the cutting edge of the drill bit near the outer edge is deformed to the outside by the axial cutting force, and the outer edge of the drill bit Under the action of chamfering and land facets, it is further curled to form rolled edges or burrs.
4. Drilling processing conditions
The catalog of the general drill bit product catalog has a "Basic Cutting Amount Reference Table" arranged by processing materials. Users can refer to the cutting amount provided by it to select the cutting conditions for drilling. Whether the selection of cutting conditions is appropriate should be comprehensively judged through trial cutting and based on factors such as machining accuracy, machining efficiency, and drill life.
1. Drill life and processing efficiency
Under the premise of meeting the technical requirements of the workpiece to be processed, whether the drill bit is used properly should be comprehensively 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 selected from the cutting distance; the evaluation index of the processing efficiency can be selected from the feed speed. For high-speed steel drill bits, the service life of the drill bit is greatly affected by the rotation speed and less affected by the feed rate per revolution. Therefore, the processing efficiency can be improved by increasing the feed rate per revolution, while ensuring a longer drill life. But it should be noted that if the feed rate per revolution is too large, the chips will thicken and cause chip breaking difficulties. Therefore, it is necessary to determine the range of feed rate per revolution that can break chips smoothly through trial cutting. For cemented carbide drills, 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 drills. If the feed per revolution exceeds this range during processing, the use of the drill will be reduced. life. Since the heat resistance of cemented carbide drills is higher than that of high-speed steel drills, the rotation speed has little effect on the life of drills. Therefore, the method of increasing the rotation speed can be used to improve the processing efficiency of cemented carbide drills while ensuring the life of drills.
2. Rational use of cutting fluid
The cutting of the drill bit is carried out in a hole with a narrow space, so the type of cutting fluid and the injection method have 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 water-insoluble. Non-water-soluble cutting fluid has good lubricity, wettability and anti-adhesion, and also has anti-rust effect. Water-soluble cutting fluid has better cooling performance, no smoke and no flammability. In consideration of environmental protection, water-soluble cutting fluids have been used in large quantities 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 to it during use. Regardless of whether it is water-soluble or non-water-soluble cutting fluid, the cutting fluid must fully reach the cutting point during use, and the flow, pressure, number of nozzles, and cooling method (internal cooling or external cooling) of the cutting fluid must be strictly controlled.
5. Re-sharpening of drill bits
Discrimination of drill regrinding
The criteria for judging that the drill bit needs to be re-sharpened are:
1. The wear amount of cutting edge edge, chisel edge and land face;
2. Dimensional accuracy and surface roughness of the processed hole;
3. The color and shape of chips;
4. Cutting resistance (indirect values such as 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 specific conditions. When the amount of wear is used as the criterion, the best regrinding period with the best economy should be found. Since the main sharpening parts are the back of the head and the chisel edge, if the wear amount of the drill bit is too large, the sharpening time will be long, the grinding amount will be large, and the number of regrinding will be reduced (the total service life of the tool = the life of the tool after regrinding× Regrinding times), on the contrary, will shorten the total service life of the drill bit; when using the dimensional accuracy of the processed hole as the criterion, use the column gauge or limit gauge to check the cutting expansion and straightness of the hole. Once the control value is exceeded, it should be Immediately re-sharpen; when cutting resistance is used as the judgment standard, methods such as immediate automatic shutdown beyond the set limit value (such as spindle current) can be adopted; when processing quantity limit management is used, the above judgment content should be integrated to set the judgment standard.
Drill sharpening method
When re-sharpening the drill bit, it is best to use a special machine tool for drill sharpening 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 shape is in good processing condition, it can be reground according to the original drill shape; if the original drill shape is defective, the rear shape can be appropriately improved and the chisel edge can be ground 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;
2. The damage on the drill bit (especially the damage on the edge of the margin) should be completely removed;
3. The drill type should 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 bit. The drill shape at the factory is the best drill shape obtained through scientific design and repeated tests. Therefore, the original edge shape should generally be maintained when re-sharpening.
