Rational use of the hottest round broach

Rational use of circular broach

circular broach is a common tool for efficient machining of inner holes. Based on the author's long-term experience and verification in the factory, this paper puts forward improvement suggestions for the problems existing in the structure of the circular broach. At the same time, it also analyzes in detail the reasons for the poor surface quality of the broach, the low accuracy of the broach and the low service life of the broach, and lists the improvement measures one by one

Figure 1 combined circular broach teeth and cutting patterns

circular broach is a commonly used tool for efficient machining of inner holes. The round broach is a kind of complex cutting tool with high manufacturing accuracy, strict technical requirements and high price, which should be used reasonably. At present, the circular broach (hereinafter referred to as broach) widely used in production mostly adopts the combined cutter tooth structure shown in Figure 1, that is, the rough cutting teeth and transition teeth adopt the wheel cutting mode without tooth groups, on which a wide arc chip separation groove is opened; The precision cutting gear adopts the same profile cutting method, on which a narrow chip separation groove is grinded. After the author's long-term use and verification in the factory, it is found that due to the structure itself or improper use, this broach often causes problems such as poor broaching surface quality and short tool service life

broach structure problems and improvement

in order to prolong the service life of broach, the structure of ordinary broach is that there are 3 ~ 7 calibration teeth on its calibration part, so that after the cutting teeth are worn, they can be supplemented one by one for fine cutting teeth. Moreover, it is generally believed that the calibration teeth do not participate in the cutting work, so the calibration teeth generally do not wear the chip separation groove (see Figure 1). However, in the actual production, it is observed that the first calibration tooth on the broach is involved in the cutting work (because there is a blunt circle on the edge, it will have a greater squeezing effect on the hole wall and make the workpiece produce elastic recovery and shrink), and it has the function of fine cutting teeth. However, because there is no chip separation groove on it, the cut chips form a ring, which is difficult to roll up and easy to block in the groove, reducing the quality of the machined surface. Therefore, the author suggests that when designing and manufacturing broach, in addition to the last one to two calibration teeth, as shown in Figure 2, arc-shaped chip separation grooves or angular (V-shaped) narrow chip separation grooves should also be grinded alternately back and forth on the other calibration teeth. As shown in Figure 3, the rear corners of the two edges on the chip separation groove can be calculated according to the following formula:

tan α k=tan α f·sin( θ/2)

α F is the rear angle of the groove bottom of the chip separation groove, which is generally taken as 5 °, as shown in section A-A in Figure 3. At this time, the experimental force will not increase; θ Groove angle of chip separation groove

Figure 2 grinding chip separation groove on the calibration tooth

it can be seen from the above formula that when θ= At 0 °, regardless of α How big is f ground, α F is always equal to 0 ° (that is, the case of the U-shaped chip chute shown in Figure 3C). And when θ When the angle increases, α The f angle will also increase. If take α f=5°， θ= 60 ° ~90 °, the calculation from the formula shows that at this time, the back angle on the side edge will increase to 2.5 ° ~3.5 °, which can greatly improve the cutting conditions there

in actual production, due to the narrow width of the angular chip separation groove, which is usually only about 1mm, the round corner of the grinding wheel should be trimmed to an angle of 60 ° ~90 °, and it is required to grind it out on the broach α For the back angle of the groove bottom with f=5 °, the diameter of the grinding wheel cannot be selected to be large, otherwise it will encounter the adjacent cutter teeth. In addition, the grinding wheel will wear quickly at the sharp corner, and it needs to be trimmed frequently, which is difficult in the process. Therefore, when making broach, the operator often uses a thin disc grinding wheel to grind, resulting in a narrow chip separation groove into a U-shaped chip separation groove. Turn off the controller and computer

Figure 3 shape of commonly used chip dividing groove

due to the reasonable structure of the circular arc chip dividing groove, its groove angle is larger than that of the V-shaped narrow chip dividing groove, and the knife tip angle at the intersection of the groove and the cutting edge is also large, which is convenient for grinding. The contact surface between the grinding wheel and the workpiece (broach) is large, and the workpiece is not easy to burn during grinding. Therefore, in order to improve the service life of the broach, the broach is not only on the rough cutting teeth and transition teeth, Moreover, the circular arc shaped chip separation groove should also be used as far as possible for the chip separation groove ground on the fine cutting gear

in order to make the chips curl smoothly, the number of slots in the chip separation slot should be increased appropriately. Experience shows that the number of slots should make the width of the cut chip not more than 4mm ~5mm. At present, the number of V-shaped chip separation grooves recommended in some broach design materials is generally small, for example, the diameter is Φ For a 50mm round broach, the number of chip separation grooves is 22, while the number of chip separation grooves on broaches of the same size abroad is 40. Similarly, the number of circular arc chip separation grooves on wheel cutting and combined circular broaches should also be appropriately increased

broaching defects in Figure 4

causes and improvement of poor surface quality of broaching holes

defects such as scale thorns, annular corrugations and scratches shown in Figure 4 will directly affect the surface quality of broaching holes

scale stab

the main reason for scale stab is that the plastic deformation is serious during broaching. Therefore, properly increase the rake angle, reduce the tooth lift, select cutting fluid with good lubrication performance (such as extreme pressure additive cutting fluid containing chlorine), and properly improve the hardness of workpiece material with pre heat treatment (when the workpiece hardness is 240hbs); Or reduce the front angle appropriately to increase the strength of the cutting edge and prevent the edge from collapsing, which can effectively reduce the extrusion bright spots and scratches on the workpiece surface

causes and improvement of low hole drawing accuracy

hole diameter expansion or reduction after drawing

due to the influence of chip buildup and burrs generated during grinding, the deviation of cutting edge position and axis straightness generated during broach manufacturing, and the non concentricity of broach and workpiece preformed holes, the hole diameter after drawing will usually expand. However, when broaching thin-walled workpiece or workpiece materials with high bremsstrahlung, the aperture will be reduced due to the elastic recovery of the workpiece after broaching. When determining the size of broach calibration teeth, the expansion or contraction of the aperture after broaching must be considered, and its value should be determined through tests, generally within the range of 0.01mm ~0.02mm

when using broach, it can also prevent the formation of chip buildup by appropriately reducing the cutting speed and using cutting fluid with good cooling performance, so as to avoid the expansion of aperture. To prevent the hole diameter from shrinking, we can increase the rake angle, keep the cutting edge sharp, and select the appropriate cutting fluid. For example, when broaching 40Cr and 45 steel workpieces with broach, when the aperture size of emulsion becomes smaller, vulcanized oil broaching can be used. This is because the emulsion is a water-based cutting fluid, and the thermal conductivity of water is good, so the thermal expansion of the tool is small; The lubricity of emulsion is worse than that of oil cutting fluid, so the extrusion effect of the tool on the workpiece is also large, and the rebound amount of the workpiece after machining will increase, so the aperture size will be reduced. Using the above rules, the actual machining size can be controlled by changing the type and composition of cutting fluid in actual production, so as to meet the requirements of broaching accuracy and quality

deviation of hole shape after drawing

when the thickness of the workpiece hole wall along the axis direction is uneven in the whole length, the hole shape after drawing will change. If both ends of the workpiece are thin-walled, the workpiece hole is "waist drum shaped" after drawing; The middle part of the workpiece is thin-walled, and the hole of the workpiece after drawing is in the shape of "bell mouth". Therefore, broaching should not be used when the wall thickness of the workpiece is uneven and the thickness difference is too large

Figure 5 effective area of chip holding groove and cross-sectional area of cutting layer

causes and improvement of low service life of broach

insufficient chip holding space of broach

broach is a closed cutting tool. If the chip holding space is insufficient, the chip will be blocked in the chip holding groove, resulting in a sharp increase in broaching force, leading to damage of cutter teeth or breakage of broach. For this reason, when broaching with a purchased broach, the chip holding coefficient K of the broach should be calculated first. As shown in Figure 5, the effective area of the chip holding groove must be greater than the cross-sectional area of the cutting layer, that is,

k= (π h2/4)/(l0hd)>1

H is the depth of the chip holding groove; L0 is the broaching length; HD is the thickness of the cutting layer, and hd=2fz on the combined broach, as shown in Figure 1, where F2 is the tooth lift; K is the chip holding coefficient, and its size is related to the processed material and tooth lift. Generally, k=2~3.5 can be taken. When machining castings and tooth lift are large, K takes the smaller value

improper geometric parameters of the tool

the rake angle of the broach is improperly selected, the back angle is too small or the blade belt is too wide, the chip separation groove worn on the fine cutting gear is a U-shaped groove, the cutting conditions are poor, and the cutter teeth wear fast. In serious cases, the broach will be stuck in the workpiece hole and the broach will be broken. The rake angle of broach can usually be taken as 5 ° ~18 °. When pulling plastic materials, it can be taken as the large value, and when pulling brittle materials, it can be taken as the small value. The back angle of broach can be taken as 3 ° ± 30 ° on the cutting tooth and 1 ° ± 30 ° on the calibration tooth. The blade width is usually 280hbs on rough cutting teeth and transition teeth), and there is oxide skin on the surface, which will reduce the tool life. Therefore, the material of the workpiece must be checked before drawing. If the hardness of the material is too high, heat treatment can be used to reduce it appropriately; Or use high-performance high-speed tool steels with high hardness and wear resistance, such as w2mo9cr4vco8 (M42), W6Mo5Cr4V2Al, to make broaches, and foreign countries also use broaches made of powder metallurgy high-speed steel (PM HSS); A thin layer of wear-resistant tin can also be coated on the surface of broach teeth; Or install a replaceable carbide extrusion ring at the end of the broach, as shown in Figure 6

excessive wear of broach

in order to prevent excessive wear of broach and avoid damage to the cutter teeth due to excessive cutting force, it is necessary to control the wear amount on the back surface of the broach cutter teeth, which should not exceed 0.2mm~0.3mm. In factories, it is often controlled by the number of workpieces pulled out when the above wear amount is reached

insufficient power and poor rigidity of broaching machine

insufficient power and poor rigidity of broaching machine cause vibration during broaching and cause tool teeth to collapse or break. Broaching machine with good rigidity and sufficient power can be used for broaching

improper broaching speed

if the broaching speed is too high and the cutting temperature is high, the broach life will be reduced. Therefore, for broaches made of general-purpose high-speed steel such as W6Mo5Cr4V2, the rough drawing cutting speed is generally 3~7m/min, and the fine drawing cutting speed is generally 1~3m/min; When the strength and hardness of the workpiece material are high, the broaching speed should be taken as a small value

the type of cutting fluid or pouring method is improper

the type of cutting fluid and pouring method not only affect the service life of broach, but also affect the machining accuracy and surface roughness. When processing holes in steel parts, emulsions or extreme pressure emulsions with a concentration of 10% - 20% can be used, as well as vulcanized oil or composite oil. When pulling the casting hole, it can be cut dry, but it is best to use ordinary emulsion or kerosene. If vulcanized oil is used when processing steel parts, the life of broach life and urban greening consumables can be improved. The supply of cutting fluid should be sufficient. The cutting fluid should be supplied before the broach enters the workpiece hole and just pulled out of the workpiece hole. The supply amount should generally not be less than 5~15l/min

in addition, the correct use and storage of broach also have an impact on the life of broach. After each workpiece is broached, the chips adhered to the cutting edge shall be cleaned with a copper wire brush. If it cannot be removed with a copper wire brush, it can be gently wiped with an oilstone. But never use a wire brush or cotton yarn. At the same time, during the manufacturing, transportation, storage and use of broach, the performance analysis and typical application of vehicle polypropylene materials shall not damage the teeth and edges of broach

repair of broach

the price of broach is very expensive. In order to prolong its service life, the broach has been reground for many times, and its diameter becomes smaller and will be scrapped. It will be repaired and used often. At this time, the carbide tool with negative rake angle can be used to extrude the rake face of the cutter teeth one by one on the lathe. After extrusion, the straight warp can be increased by 0.01mm~0.02mm, and then ground to reach the specified size. (end)