As the most common tool in hole machining, bit is widely used in mechanical manufacturing, especially for the processing of holes of parts such as cooling device, tube plate of power generation equipment and steam generator.
Characteristics of drilling
The bit usually has two main cutting edges. During machining, the bit rotates and cuts at the same time. The front angle of the bit is increasing from the central axis to the outer edge, and the cutting speed of the bit closer to the outer circle is higher, and the cutting speed decreases towards the center, and the cutting speed of the rotary center of the bit is zero. The transverse edge of the bit is located near the axis of the rotary center. The front angle of the secondary edge is large, there is no chip space, and the cutting speed is low, so it will produce a large axial resistance. If the transverse cutting edge is grinded into a or C type in din1414, and the cutting edge near the central axis is 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 kinds, such as high-speed steel drills (twist drills, group drills, flat drills), solid carbide drills, indexable shallow drills, deep drills, nesting drills and replaceable bits.
Chip breaking and chip removal
The cutting of the bit is carried out in the hole with narrow space, and the cuttings must be discharged through the cutting groove of the bit, so the shape of the cuttings has a great influence on the cutting performance of the bit. The common chip shapes are flake chip, tubular chip, needle chip, conical spiral chip, ribbon chip, fan-shaped chip, powder chip, etc.
The key of drilling: chip control
When the chip shape is not suitable, the following problems will occur:
- ① Fine chips block the cutting edge groove, affect the drilling accuracy, reduce the life of the bit, and even break the bit (such as powder chips, fan-shaped chips, etc.);
- ② The long cuttings wrap around the bit, hindering the operation, causing the 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 improper chip shape:
- ① The methods of increasing feed rate, intermittent feed, grinding cross edge and installing chip breaker can be used respectively or jointly to improve chip breaking and chip removal effect and eliminate the problems caused by chip.
- ② Professional chip breaking drill can be used for drilling. For example, adding a designed chip breaking edge in the groove of the drill bit will break the chips into chips that are easier to remove. Debris can be removed smoothly along the trench without blocking in the trench. Therefore, the cutting effect of the new chip breaking drill is much smoother than that of the traditional drill.
At the same time, the short scrap iron makes it easier for the coolant to flow to the drill tip, further improving the heat dissipation effect and cutting performance in the processing. Moreover, because the new chip breaking edge penetrates the whole groove of the bit, it can still maintain its shape and function after several times of grinding. In addition to the above functional improvement, it is worth mentioning that the design strengthens the rigidity of the drilling body and significantly increases the number of drilling holes before single grinding.
The accuracy of hole is mainly composed of aperture size, position accuracy, coaxiality, roundness, surface roughness, burr and other factors.
Factors affecting the accuracy of the hole to be machined during drilling:
- ① Clamping accuracy and cutting conditions of the bit, 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 The shape of the workpiece, such as the shape of the side of the orifice, the shape of the orifice, the thickness, the clamping state, etc.
Reaming is caused by the swing of the drill bit 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 seriously worn, it 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 small bit with a thick shank and a small diameter with a good coaxiality between the edge and the shank. When using the regrinding bit to process, the reasons for the decrease of the hole accuracy are mostly due to the asymmetry of the back shape. Controlling the height difference of the cutting edge can effectively restrain the expansion of the hole.
Roundness of holes
Due to the vibration of the drill, the hole pattern is easy to be polygonal, and there is a pattern like repeated lines on the hole wall. The common polygon holes are mostly triangles or pentagons. The reason for triangular hole is that the drill bit has two rotation centers during drilling, and they exchange frequency vibration once every 600 intervals. The vibration is mainly caused by unbalanced cutting resistance. When the drill bit rotates for one revolution, the resistance is unbalanced during the second revolution due to poor hole roundness, and the last vibration is repeated again, but the vibration phase has a certain deviation, resulting in the hole wall There are double lines. When the drilling depth reaches a certain degree, the friction between the edge of the bit and the hole wall increases, the vibration decreases, the repetition line disappears, and the roundness becomes better. According to the longitudinal section, the orifice is funnel-shaped. For the same reason, pentagonal and heptagonal holes may also appear in cutting. In order to eliminate this phenomenon, in addition to controlling the chuck vibration, the height difference of cutting edge, the asymmetry of back and blade shape and other factors, measures should be taken to improve the bit rigidity, increase the feed per revolution, reduce the back angle, and repair the cross edge.
Drilling on slopes and surfaces
When the cutting surface or penetration surface of the bit is inclined, curved or stepped, the positioning accuracy is poor. Because the bit is single-sided radial cutting at this time, the tool life is reduced.
In order to improve the positioning accuracy, the following measures can be taken:
- 1. Drill the central hole first;
- 2. Use the end milling cutter to mill the hole seat;
- 3. Select a bit with good cutting and rigidity;
- 4. Reduce the feed speed.
Treatment of burr
In drilling, there will be burr at the entrance and exit of the hole, especially when processing the material and sheet with large toughness. The reason is that when the bit is about to drill through, the material to be processed will have plastic deformation. At this time, the triangular part that should be cut by the edge near the outer edge of the bit will bend outwards under the action of axial cutting force, and further curl under the action of the chamfer of the outer edge of the bit and the edge with edge to form curl or burr.
Processing conditions of drilling
In the general sample catalog of bit products, there is the reference table of basic cutting parameters arranged according to the processing materials. Users can select the cutting conditions for drilling and machining by referring to the cutting parameters provided by them. Whether the cutting conditions are appropriate or not should be judged comprehensively by trial cutting, machining accuracy, machining efficiency, bit life and other factors.
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. Cutting distance can be selected as the evaluation index of bit service life, and feed speed can be selected as the evaluation index of machining efficiency. For the high-speed steel bit, the service life of the bit is greatly affected by the rotation speed and the feed per revolution, so the machining efficiency can be improved by increasing the feed per revolution, while ensuring a longer bit life. However, it should be noted that if the feed per revolution is too large, the chip will thicken, which will cause chip breaking difficulty. Therefore, it is necessary to determine the feed per revolution range of chip breaking smoothly through trial cutting. For cemented carbide bit, the negative rake angle of cutting edge has a larger chamfer, and the optional range of feed per revolution is smaller than that of high-speed steel bit. If the feed per revolution exceeds this range, the service life of the bit will be reduced. As the heat resistance of cemented carbide bit is higher than that of high-speed steel bit, the rotary speed has little effect on the life of bit, so the method of increasing the rotary speed can be used to improve the machining efficiency of cemented carbide bit and ensure the life of bit.
Reasonable use of cutting fluid
The cutting of bit is carried out in the hole with narrow space, so the type and injection mode of cutting fluid have a great influence on the life of bit and the machining accuracy of hole. Cutting fluid can be divided into water-soluble and non-water-soluble. The water-insoluble cutting fluid has good lubricity, wettability and adhesion resistance, as well as antirust effect. The water-soluble cutting fluid has good cooling performance, no smoke and no flammability. Due to the 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 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 be fully reached the cutting point in use. At the same time, the flow, pressure, nozzle number, cooling method (internal or external cooling) of cutting fluid must be strictly controlled.
Regrinding of drill bit
Bit regrinding discrimination
The criteria for regrinding the bit are:
- 1. Wear of cutting edge, cross edge and edge with edge;
- 2. Dimensional accuracy and surface roughness of the hole to be machined;
- 3. Chip color and shape;
- 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 the specific conditions. When the wear amount is used as the criterion, the best economic regrinding period should be found out. Because the main grinding parts are the back of the head and the transverse edge, such as the excessive wear of the drill bit, the excessive wear of the cutting edge, the large grinding amount, and the reduction of the number of regrinding times (the total service life of the tool = the service life of the tool after regrinding × the number of regrinding times), the total service life of the drill bit will be shortened instead; when the dimensional accuracy of the hole to be processed is used as the criteria, the cylinder gauge or limit gauge shall be used to check the cutting expansion of the hole If the straightness exceeds the control value, it shall be reground immediately; if the cutting resistance is used as the judgment standard, it can be automatically shut down immediately if it exceeds the set limit value (such as spindle current); if the processing quantity limit management is used, the above judgment content shall be integrated to set the judgment standard.
Grinding method of bit
When regrinding the bit, 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 bit. If the original drill pattern is in good processing condition, it can be reground according to the original drill pattern; if the original drill pattern is defective, the back shape can be properly improved and the cross edge can be reground according to the use purpose.
Attention shall be paid to the following points during grinding:
- 1. Prevent overheating and reduce bit hardness;
- 2. The damage on the bit (especially the damage on the edge with edge) shall be removed completely;
- 3. The drilling pattern shall be symmetrical;
- 4. Pay attention not to damage the edge during grinding, and remove the burr after grinding;
- 5. For cemented carbide bit, the grinding shape has a great influence on the bit performance. The drill type at the factory is the best drill type obtained through scientific design and repeated tests. Therefore, the original edge type should be kept when grinding again.
Drilling technology of stainless steel flange
Because of the poor thermal conductivity and small elastic modulus of the stainless steel flange, it is not easy to make holes. In order to solve the drilling problem of stainless steel flange, the best way is to select the appropriate tool material, determine the reasonable geometric parameters of the tool and the cutting parameters of the tool.
Tool geometric parameters
When using ordinary high speed steel bit to drill holes, the cutting force and cutting temperature are concentrated on the drill tip. In order to improve the durability of the cutting part of the bit, the angle of the top angle can be appropriately increased to facilitate chip removal. However, when the top angle is increased, the transverse edge of the bit becomes wider, which results in the increase of cutting resistance. Therefore, it is necessary to grind the transverse edge of the bit to round the corner between the cutting edge and the cylindrical surface, so as to increase the strength of the transverse edge.
Selection of cutting parameters
The choice of cutting parameters in drilling should be based on the basic point of reducing cutting temperature, because high-speed cutting will increase cutting temperature, and high cutting temperature will aggravate tool wear, so the most important cutting parameters is to choose cutting speed.
Some problems in drilling process of stainless steel flange
- 1. The choice of cutting tool material, because of the large cutting force and high cutting temperature when making holes for stainless steel parts, the cutting tool material should be hard alloy with high strength and good thermal conductivity as far as possible. Because the main characteristics of this kind of material are high toughness and serious hardening, the chip produced by cutting these materials in the form of unit chip will make the tool vibrate, which is easy to cause micro collapse of the blade. Therefore, the first thing to consider when choosing ceramic tool to cut this kind of material parts is micro toughness.
- 2. In order to avoid chip sticking to the cutter, the front and back surfaces of the cutter should be carefully sharpened to ensure a small roughness value, so as to reduce chip outflow resistance and avoid chip sticking to the cutter.
- 3. The cutting edge of the tool should be sharp to reduce hardening. The feed and back draft should not be too small to prevent cutting in the hardened layer and affect the service life of the tool.
- 4. Attention shall be paid to the grinding of chip breaking groove. Due to the strong and tough features of stainless steel chips, the grinding of chip breaking groove on the front cutter face of the tool shall be appropriate, so as to facilitate the cutting process to interrupt, contain and discharge chips.
- 5. The selection of cutting oil for hole making and the performance of cutting oil directly determine the accuracy of hole making. Therefore, when selecting cutting oil, the appropriate cutting oil is selected according to the different material of the workpiece. For materials that are easy to cut, low viscosity will be selected on the premise of preventing burr for the easy cleaning of finished products. The cutting oil containing sulfur and chlorine compound additives should be used for the hard workpieces, but it should be degreased as early as possible to avoid rusting.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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