Drill bit, as the most common tool in hole processing, is widely used in mechanical manufacturing, especially for the processing of holes in cooling devices, tube sheets of power generation equipment, steam generators and other parts.
1、 Characteristics of drilling
The drill bit usually has two main cutting edges. During machining, the drill bit rotates and cuts at the same time. The front angle of the drill bit becomes larger and larger from the central 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, and the cutting speed of the drill bit’s rotating center is zero. The horizontal edge of the drill is located near the axis of the rotary center. The lateral edge has a large auxiliary rake angle, no chip space, and low cutting speed, so it will produce large axial resistance. If the transverse edge is grinded to type A or 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 different shapes, materials, structures and functions of workpieces, drills can be divided into many types, such as high-speed steel drills (twist drills, group drills, flat drills), integral carbide drills, indexable shallow hole drills, deep hole drills, sleeve drills, and interchangeable head drills.
2、 Chip breaking and chip removal
The cutting of the drill bit is carried out in a narrow hole, and the chips must be discharged through the cutting groove of the drill bit, so the chip shape has a great impact on the cutting performance of the drill bit. Common chip shapes include flake chips, tubular chips, needle chips, tapered spiral chips, ribbon chips, fan-shaped chips, powdery chips, etc.
When the chip shape is not suitable, the following problems will occur:
① Fine 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.);
② Long chips wrap around the drill bit, hindering operation, causing damage to the drill bit or preventing 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 effect can be improved by increasing the feed rate, intermittent feed, grinding the cross edge, installing chip breaker, etc., respectively or jointly, to eliminate the problems caused by chips.
② A professional chip breaking drill can be used for drilling. For example, the designed chip breaking edge is added in the groove of the drill bit to break the chips into more easily cleared chips. Debris shall be discharged smoothly along the trench without blockage in the trench. Therefore, the new chip breaking drill achieves much smoother cutting effect than the traditional drill.
At the same time, the short scrap iron makes it easier for the coolant to flow to the drill point, further improving the heat dissipation effect and cutting performance during the processing. Moreover, because the newly added chip breaking edge has penetrated the entire groove of the drill bit, its shape and function can still be maintained after several times of grinding. In addition to the above function improvement, it is worth mentioning that the design strengthens the rigidity of the drill body and significantly increases the number of holes drilled before single grinding.
3、 Drilling accuracy
The accuracy of a hole is mainly composed of such factors as hole size, position accuracy, coaxiality, roundness, surface roughness and hole burr.
Factors affecting the accuracy of the hole to be machined during drilling:
① The clamping accuracy and cutting conditions of the drill, such as tool holder, cutting speed, feed rate, cutting fluid, etc;
② Bit size and shape, such as bit length, edge shape, core shape, etc;
③ Workpiece shape, such as orifice side shape, orifice shape, thickness, clamping state, etc.
Counterbore
Reaming is caused by the swing of the drill bit during processing. The swing of the tool holder has a great impact on the hole diameter and the positioning accuracy of the hole. Therefore, when the tool holder is seriously 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 better to use a coarse shank small diameter drill with good coaxiality between the blade and the shank. When using a regrind drill to process, the reason for the decline in hole accuracy is mostly due to the asymmetry of the back shape. The control of the edge height difference can effectively restrain the reaming of the hole.
Roundness of hole
Due to the vibration of the drill bit, the drilled hole is easy to be polygonal, and the hole wall appears like a double line pattern. Common polygonal holes are mostly triangular or pentagonal. The reason for the triangular hole is that the drill bit has two rotation centers when drilling, and they vibrate at a frequency of exchange every 600. The main reason for the vibration is that the cutting resistance is unbalanced. When the drill bit rotates once, due to the poor roundness of the processed hole, the resistance is unbalanced during the second rotation of cutting. The last vibration is repeated again, but the vibration phase has a certain deviation, causing the double lines on the hole wall. When the drilling depth reaches a certain degree, the friction between the drill edge edge and the hole wall increases, the vibration attenuates, the involute disappears, and the roundness becomes better. This type of hole is funnel shaped from the longitudinal section. For the same reason, pentagon and heptagon holes may also appear in cutting. In order to eliminate this phenomenon, in addition to controlling such factors as collet vibration, cutting edge height difference, and asymmetric shape of the back and blade, measures should also be taken to improve the rigidity of the drill bit, increase the feed rate per revolution, reduce the back angle, and grind the cross edge.
Drill holes on slopes and surfaces
When the cutting surface or drilling through surface of the drill bit is inclined, curved or stepped, the positioning accuracy is poor. At this time, the drill bit is cut on a radial single side, which reduces the tool life.
To improve the positioning accuracy, the following measures can be taken:
1. Drill the center hole first;
2. Mill the hole seat with the end mill;
3. Select drill bits with good penetration and 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 and thin plates with high toughness. The reason is that when the drill bit is about to drill through, the material to be processed will have plastic deformation. At this time, the triangular part which should be cut by the edge of the drill bit near the outer edge will be 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 edge band, forming curls or burrs.
4、 Processing conditions for drilling
The general catalog of drill products contains the Reference Table of Basic Cutting Parameters arranged according to the processing materials. Users can select the cutting conditions for drilling by referring to the cutting parameters provided. Whether the selection of cutting conditions is appropriate should be judged comprehensively by trial cutting according to factors such as machining accuracy, machining efficiency, drill life, etc.
1. Bit life and machining efficiency
On the premise of meeting the technical requirements of the workpiece to be processed, the proper use of the drill should be comprehensively measured according to the service life and processing efficiency of the drill. The cutting distance can be selected as the evaluation index of the bit service life; Feed speed can be selected as the evaluation index of machining efficiency. For high-speed steel drill bits, the service life of the drill bit is greatly affected by the rotary speed, and is less affected by the feed rate per revolution. Therefore, the machining efficiency can be improved by increasing the feed rate per revolution, while ensuring a longer life of the drill bit. However, it should be noted that if the feed rate per revolution is too large, the chip will thicken, causing difficulties in chip breaking. Therefore, it is necessary to determine the range of feed rate per revolution for smooth chip breaking through trial cutting. For cemented carbide drill bits, there is a large chamfer in the negative rake angle direction of the cutting edge, and the optional range of feed rate per revolution is smaller than that of high-speed steel drill bits. If the feed rate per revolution exceeds this range during processing, the service life of the drill bit will be reduced. As the heat resistance of cemented carbide bit is higher than that of high-speed steel bit, and the rotary speed has little influence on the life of the bit, the method of increasing the rotary speed can be adopted to improve the processing efficiency of cemented carbide bit and ensure the life of the bit.
2. Rational use of cutting fluid
The drill bit is cut in a narrow hole, so the type of cutting fluid and injection method have a great impact on the life of the drill bit and the machining accuracy of the hole. Cutting fluid can be divided into water-soluble and water-insoluble. The water-insoluble cutting fluid has good lubricity, wettability and adhesion resistance, and also has the function of rust prevention. Water soluble cutting fluid has good cooling property, no smoke and no flammability. In consideration of environmental protection, water-soluble cutting fluid has been widely used in recent years. However, if the dilution ratio of water-soluble cutting fluid is improper or the cutting fluid deteriorates, the tool life will be greatly shortened, so attention must be paid in use. Whether it is water-soluble or water-insoluble cutting fluid, the cutting fluid must reach the cutting point fully in use, and the flow, pressure, number of nozzles, cooling mode (internal or external cooling), etc. of the cutting fluid must be strictly controlled.
5、 Re sharpening of drill bit
Judgment of drill regrinding
The criteria for regrinding the drill bit are:
1. Wear amount of cutting edge, cross edge and edge with edge;
2. Dimensional accuracy and surface roughness of the machined hole;
3. Color and shape of chips;
4. Cutting resistance (spindle current, noise, vibration and other indirect values);
5. Processing quantity, etc.
In actual use, accurate and convenient criteria shall be determined from the above indicators according to specific conditions. When the wear amount is used as the criterion, the best economic regrinding period should be found. Since the main grinding parts are the back of the head and the horizontal edge, such as the excessive wear of the drill bit, the excessive wear of the edge, the large amount of grinding, and the reduced number of regrinding times (the total service life of the tool=the service life of the tool after regrinding × Regrinding times), on the contrary, it will shorten the total service life of the drill bit; When the dimensional accuracy of the hole to be machined is used as the judgment standard, 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, re grinding shall be carried out immediately; When the cutting resistance is used as the criterion, the machine can be automatically shut down immediately if it exceeds the set limit value (such as the spindle current); When the processing quantity limit management is adopted, the above judgment contents shall be integrated and the judgment standards shall be set.
Grinding method of drill bit
When re sharpening the drill, it is better to use a special machine tool or a universal tool grinder, which is very important to ensure the service life and machining accuracy of the drill. If the original drilling type is in good processing condition, it can be reground according to the original drilling type; If the original drill type has defects, the rear shape can be properly improved and the cross edge can be grinded according to the purpose of use.
Pay attention to the following points when grinding:
1. Prevent overheating and reduce bit hardness;
2. The damage on the drill bit (especially the damage on the edge of the blade) shall be completely removed;
3. The drill type shall be symmetrical;
4. Take care not to damage the cutting edge during grinding, and remove the burrs after grinding;
5. For cemented carbide drill bits, the grinding shape has a great impact on the performance of the drill bit. The drill type when leaving the factory is the best one obtained through scientific design and repeated tests. Therefore, the original cutting edge should be kept when re grinding.
Post time: Oct-10-2022