The accuracy of the sheet metal stampings accurately shows the stamping accuracy of the stamping die. The linear dimensional accuracy and shape accuracy of any punching piece mainly depend on the blanking accuracy of the die blanking and the three-dimensional forming stampings. Therefore, the stamping precision of the single-station composite mold and the multi-station continuous mold of the multi-step composite stamping is the most typical and representative among the many types of stamping and the stamping of different structures.
Punching precision analysis of die
The actual error analysis of the punching of the stamping die to the end of the period of failure and scrapping can be seen in the period and trend of the increase, so as to analyze the factors of the increase. The error of the punching of the new die before the first sharpening is the so-called initial error; the punching error of the die after 20 times of sharpening to the point of failure before scrapping is called the conventional error; and before the die is scrapped The maximum allowable error of the last batch of qualified punches punched out is called the limit error. At the scene, the basis for determining the sharpening life of the die is the burr height of the punching and blanking of the punch. This is especially true for composite dies because any forming part has a blanking operation (blank blanking or punching). Therefore, it is very important to check and measure the height of the burr of the punching piece according to the enterprise standard or JB4129-85 "the height of the stamping burr height".
The initial error of the die is usually the smallest of the punch errors over the life of the die. Its size mainly depends on the manufacturing precision and quality of the die, the size of the punch, the thickness of the material, and the size and uniformity of the gap value. The manufacturing precision and quality of the die depend on the molding process. For the medium carbon steel composite blanking die with material thickness t≤1mm, the experimental results and production practice have proved that the burr height of the punching punch made by WEDM is higher than that of the continuous grinding coordinate with the forming mill or NC and CNC. The die-punching parts manufactured by the precision grinding process are 25% to 30% higher. This is because the latter not only has high processing precision, but also the roughness Ra of the processed surface is one order of magnitude smaller than the former, and can reach 0.025 μm. Therefore, the manufacturing precision and quality of the die determine the initial punching accuracy of the die, and also the initial error of the punch.
The conventional error of the punch is the actual error of the punch until the last punch is punched out after the first sharpening to the last sharpening. As the number of sharpening increases, the size increment caused by the natural wear of the cutting edge gradually increases, and the error of the punching piece also increases. When the error exceeds the limit deviation, the punch is not qualified, and the die is invalidated. The upper hole and the inner shape of the punching piece will gradually become smaller due to the wear of the punching die; the shape blanking size of the punching piece will gradually increase due to the wear of the die. Therefore, the upper hole and the inner shape of the punching piece are subjected to a one-way positive deviation tolerance and are molded to be close to or almost equal to the maximum maximum size. Similarly, the shape of the blank is marked with a tolerance of one-way negative deviation and is molded close to or almost equal to the minimum minimum size. This expands the conventional error range of the punch, increases the number of times the die can be sharpened, and increases the life of the die.
The limit error of the punch is the actual maximum size error allowed for the punch with the limit deviation. This type of stamping is usually the last batch of qualified stampings that are punched out before the die is rejected.
A comprehensive analysis of the fluctuations, fluctuations, trends, and regularities of various die blanking errors throughout the life of the die reveals that the dominant part of the punching error is constant; due to the natural wear of the cutting edge or cavity The increase of the error increases with the increase of the number of punching and punching of the die, and the error is gradually increased; and the increment of some errors is unconventional and unpredictable. Therefore, all kinds of die blanking errors are composed of several parts, such as fixed error, increasing error, systematic error and accidental error.
1, fixed error
The new die is put into use on the specified stamping equipment until the entire (total) life of the failed scrap, and the dominant part of the qualified punch error is fixed, the so-called fixed error. The size is the deviation of the qualified punch that is punched before the first blanking of the new die, that is, the initial error of the die, and the punching die at this time has the initial punching precision. After the sharpening of the die, due to the wear of its working parts (convex, concave die), the dimensional error is changed, so that the increment of the punching part increases gradually with the increase of the number of sharpening, so the punching precision after punching is also called " The sharpening accuracy is lower than its initial accuracy. The fixed error of the die blank depends on the following elements:
(1) Material type, structure (shape) size and material thickness of the stamping
The size of the blanking gap and its uniformity have a decisive influence on the dimensional accuracy of the blank. Different blanking processes, different material types and different material thicknesses, the gaps are very different, and the stamping precision varies greatly. The same type of mold m = 0.34 2mm material thickness, the center of the hole H62 brass material sheet gear composite die-punch, when the gap C = 0.5% t (single side), punched with a composite fine die, punching The dimensional accuracy reaches IT7, the punching piece is straight and has no arch bending, the vertical plane of the punching surface can reach 89.5°, and the surface roughness Ra is 0.2μm; while the ordinary composite die is punched, the gap C=5%t (single side) The initial error of the punching piece is the initial punching precision of the die is 1T9, the roughness of the punching surface is 12.5μm, and the height of the burr is 0.10mm; or the punch is punched by continuous die, the gap C=7%t (single Edge), the initial punching accuracy is IT11, the punching surface is rougher, and even the steps visible to the naked eye. Usually, the punch material and its thickness t are the main basis for selecting the punching gap. Once the gap is selected, the body of the fixed error of the plane dimension of the punch is determined; the stiffness and the three-dimensional shape of the punch affect its shape accuracy.
(2) Stamping process and die structure type
Different stamping processes are used, and the accuracy and fixed error of the punching parts are quite different. In addition to the above example of the chip gear, the fine punching process and the ordinary punching punching accuracy are different by an order of magnitude from the fixed error. Even in ordinary punching, different gap blanking is used, and the fixed error is greatly different. For example, H62 brass blanking parts with thickness t=1.5mm, C ≤ 40% t single-side type I small gap blanking ratio C 8% 8% (single side) type III large gap blanking, punching fixed The error will increase by 40% to 60%, and the accuracy will be reduced by at least one level. In addition, with or without laps, the error of the punch is much larger than that of the lap. There is no splicing and punching. The accuracy of the blank-free punching parts is lower than that of the IT12 grade, and most of the stampings with the edge-stitching precision are between IT11 and IT9, and the punching thickness of the thickness of t>4mm will be lower.
Different die structure types, due to the difference in the thickness of the stamping material and the manufacturing precision, the fixed error of the punching piece is different. In the composite mold, the multi-station continuous composite mold has a larger fixed error than the single-station composite blanking die because of the continuous repeated positioning of the punching piece and the large mold error.
(3) Die manufacturing process
The main working parts of the die, that is, the processing procedures of the convex and concave molds, have low technical requirements for operation, and can form a relatively complicated cavity at a time. However, the processed surface is about 0.03~0.05mm thick, which is the residual dendritic austenite structure of high temperature ablation. The hardness can be as high as HRC67-70, there are micro-cracks, and it is easy to chip or peel off during punching. According to a research report by Corrada, Italy, “The wire-cutting process has an adverse effect on the surface metallurgical structure. In fact, the metallographic structure has been changed. We must use diamond powder grinding or CNC continuous-track coordinate grinding (on wire cutting parts). For finishing." In recent years, Switzerland and Japan have conducted in-depth research and major improvements on electrical processing equipment to produce fully functional high-precision NC and CNC wire cutting machines with machining accuracy of ±0.005 to 0.001 mm or even smaller. . The surface roughness Ra of the machined surface can reach 0.4 μm. According to the investigation of 12 domestic production line cutting machine factories in recent years, the domestic wire cutting machine processing precision can be up to ±0.008~±0.005mm for each type of wire cutter, which is generally ±0.01mm or larger. It can also reach ±0.005mm, and the processing surface roughness Ra value is greater than 1.6μm. However, the electrical processing ablates the metal surface to alter and damage the characteristics of the metallurgical structure of the machined surface, unless the hazardous layer is removed by grinding or other processing. Therefore, it is difficult to achieve the die by only the electric machining method, including wire electric discharge and electroporation, especially the high precision and high life die for the dimensional accuracy and the surface roughness Ra of the working parts.
The precision grinding method is used to manufacture the die, especially the high precision and high life die, such as: thin material small gap composite blanking die, multi-station continuous composite die, etc., with high dimensional accuracy and working surface roughness Ra Small value, high mold life and so on. The processing technology has been changed from the ordinary machine tool roughing to the electric wire cutting or electroporation roughing. Finally, the precision grinding is also gradually filtered by the forming mill, the optical curve grinding and the manual coordinate grinding to the continuous track coordinates. Grinding and NC and CNC continuous track coordinate grinding, the processing roughness can reach ± ​​0.001 ~ 0.0005mm, the processing surface roughness Ra value can reach 0.1 ~ 0.025μm. Therefore, the die manufactured by the process can satisfy the requirements of the die, especially the various composite molds, regardless of the dimensional accuracy and the surface roughness of the working parts, and is higher than the die manufactured by the electric machining process.
(4) The size and uniformity of the gap
Deep drawing, bending, flanging and other sheet forming parts are generally first blanked (unloaded) out of the flat plate to unfold the blank, and also formed after the blanking and cutting to obtain a single finished punch. Therefore, punching operations, including common punching, cutting, trimming, etc., are necessary for each sheet metal stamping. Therefore, the punching clearance has a decisive influence on the dimensional accuracy of the outer diameter of the punching member. The punching gap is small and uniform, which makes the punching size more accurate. For forming dies such as drawing and bending, the gap will increase the dimensional error and rebound of the punch. Uneven gaps can increase the burr of the punch and cause uneven wear of the cutting edge.
(5) Elastic deformation of stamping equipment
During the stamping process, the press will produce a certain elastic deformation after being loaded. Although this deformation varies with the magnitude of the pressing pressure and has obvious directionality, the stampings are mainly for embossing, embossing, leveling, embossing, pulsating, squeezing, squeezing with volumetric stamping properties. The punching of the shape, the flanging, the upsetting, the flattening, the thinning and the deep drawing, etc., have a significant influence on the dimensional accuracy of the stamping.
2, increasing error
During the stamping process, there is a strong friction between the die-punched raw material and the die edge and the cavity surface under the action of strong punching force. In the long-term continuous stamping process, the strong friction causes a certain natural wear on the working parts of the die, that is, the surface of the convex and concave molds, thereby changing the size and shape of the die cutting edge and the cavity, so that the punching piece is generated and gradually increases with the life of the die. This large part of the error is called "increasing error." New die blanks have only fixed errors. The incremental error occurs when the die work parts wear out. In the multi-station continuous blanking and continuous composite molds, especially the guide plate type multi-station continuous mold, the punching method and the punching waste are removed from the die by the missing parts method, and the blanking and blanking die are multi-made. The hole is formed to facilitate the pushing and unloading of the punch and the waste. Generally, the edge of the concave die is inclined outward by an angle α, and usually takes a=1/4° to 3° depending on the thickness of the blank, and the edge of the die is sharpened after being worn. After the concave die is sharpened, the planar size is inevitably increased, and the size increment ΔL depends on the a-angle size and the number of sharpenings, and the increment of each sharpening. △L can be calculated according to the formula: △L=2×h×tgα, when α=0.5°, each sharpening amount is 0.15mm, after sharpening 20 times, the total sharpening amount h=3mm, then press The formula calculates ΔL = 2 × 3 × tg 0.5 ° = 0.053 mm, and the unilateral net increase ΔL / 2 = 0.0265 mm. △L=0.00265mm per edge grinding. The single-station composite mold is mostly a die-opening, and the punching die is mostly a straight-walled hole. Since the punch is required to enter the die cavity when punching, about one punch is thick. Therefore, the side of the cutting edge is also very worn. After the sharpening, the size of the cutting edge will also change, which will increase the error of the punching piece, but the increment is small.
3, system error
The error caused by the die feed, the material and the positioning system to the punch during the stamping process is the systematic error. It is based on the feeding mode and the different feeding device, the difference of the position of the material and the distance of the feed, the difference of the structure of the positioning device, etc., so that the error of the punching piece fluctuates within a certain range. If the die block and positioning system are properly designed and matched, it can reduce the excessive feed error caused by poor feeding mode or improper feeding device.
The material fed into the die is generally limited by the stop pin or the side blade stop. The composite blanking die, the integrated composite die and the single punching die often use a cylindrical head or a hook-shaped head to fix the retaining pin, and the single-station composite die also has a movable retaining pin. In the multi-station continuous mode, the initial (temporary) stopper pin is used and the feeding limit is matched with the fixed stopper pin, the side edge and the guiding pin. When the mold material hits the stopper pin, it stops feeding and reverses backward to form a gap with the material surface of the stopper pin. The size of this gap is related to the feeding push (pull) force, and the feed force will be larger, but regardless of the gap size, the feed distance will be negatively affected by ≤0.5mm, using the active stop pin and the original use ( Temporary) block pin, this error will be even larger. If the edge layout is adopted, the width of the lap can compensate the feeding error, so there is no influence on the precision of the single die and the single-station composite die. However, the shape and position accuracy of the multi-station continuous mode and multi-station continuous composite die-punching parts, especially the inner hole (inner shape), has the most obvious influence on the coaxiality of the outer profile. In order to reduce this coaxiality deviation, a positive pin is usually placed on the end face of the blanking punch or a process guiding hole is provided on the edge of the blank. Since the station arrangement of the multi-station continuous mode is first punched and then blanked or formed, the guiding pin is inserted into the punched hole shape and then punched in the subsequent working position of the punching. For the hard steel plate with material thickness t≤1~4mm, the guiding pin can reduce the feeding error to ±0.04~0.20mm, the value of t is large and the error is large.
4, sporadic error
Some accidents or sudden situations in the stamping process cause new errors in the punching, which further reduces the accuracy of the punching and is difficult to prevent. This kind of accidental error is often caused by occasional mistakes in operation, local and invisible defects of the material, sudden looseness or damage of the mold parts, accidental failure of the stamping equipment, interference and impact of accidents at the work site, etc. .
The use of various types of composite die stamping has higher technical requirements for the operator. For the raw material and blank size and surface quality, the pre-die stamping inspection must be carried out. It is not uncommon for the strip thickness difference and the tolerance to be caused by the “same plate differenceâ€. The material size is too poor and the burr is too large, the surface is scratched and bumped, and the unevenness and rust and crusting are eliminated. Once the inspection is missed and not eliminated in time, the precision of the punching piece is inevitably reduced. Lubrication in composite die stamping is particularly important. Excessive, uneven or leaky coating of the material and mold can cause fluctuations in the size error of the punch. As for the thin material, there are “stackingâ€, crushing and bumping of the punching piece, uneven feeding force, punching the edge into the cutting edge or the cavity, the punching part is not punched out or the working face of the die is not removed, and the punching is punched. Crushing... These operational errors can be reduced as long as the operation is careful.
So far, the domestic final assembly and repair die often use a dead wrench, adjustable wrench, socket wrench screwdriver to tighten the screws and bolts, but it is difficult to achieve the reasonable pre-tightening force required for the screw connection. A tight threaded joint is the main component joint of the die. For dies that are subject to high speeds, high pressures, strong vibrations, and continuous violent alternating loads, it is fatal that the threads are loose during work! The torque of tightening the screw is not enough. After the die is used for a long time, the screw will loosen due to vibration; if the tightening torque of the screw is too large, the screw will be deformed or even broken. Due to the difference in gender and physical strength of the workers, the same thread has different degrees of tightening, and the appropriate tightening torque is certain. It is not uncommon to cause mold damage or even equipment accidents due to loose or broken screws on the die. Therefore, the final assembly or repair die should use a torque wrench to tighten the screws on the die to ensure the reliability of the tight thread connection. Due to improper screw tightening, the split edge is loosely misaligned during the stamping process, the insert is loose, the mold shank is loose and rotated, the discharge plate is detached, and the platen of the fixed die is moved or loosened... not only the stamping size is changed, but also the shank can be changed. Into an accident.
The single-station composite blanking die and the integrated composite die must ensure that the punching member is discharged from the working face of the die less than 50 times of the reciprocating stroke time of the punching slider when using compressed air to blow the punching piece or using the special unloading device to unload the piece. %, can be continuously automatic stamping, there are deep drawing, bending and squeezing, thinning and deep drawing and other process work station dies, and it is not appropriate to make the slider stroke times n ≥ 120 times / min. Otherwise, due to the time lag of the punching member from the cavity to the working face of the die, it is often too late to push and unload the punching die from the die, causing crushing, bumping or even damage.
In the multi-station continuous composite die stamping, the tail and joint of the strip and the material width error cause the feed deflection, the height of the stopper pin to be too small, the hook-shaped pin movement, the foreign matter in the cavity, and the rust. Elimination in time... will increase the punching error.
Factors affecting the precision of sheet blanking
1. The influence of the dynamic precision of the open press on the punching precision of the punching die
The structural design of the die should fully consider the structural type, technical state and dynamic accuracy of the press. For the production of small and medium-sized sheet metal stamping parts, domestically-made open-type presses with a nominal pressure of PG ≤ 2500KN are used in China, including:
(1) J11, J12 series and its modified and improved design of the open single column fixed table, movable table press. Its nominal pressure PG ≤ 2500KN;
(2) J21 series and its modified and improved design of the open double column fixed table press. Its nominal pressure PG ≤ 2500KN, recently produced a large tonnage of PG ≤ 4000KN, but not much use;
(3) J23 series and its modified and improved design of the open double column tilting press. The nominal pressure is mostly PG ≤ 1000KN;
According to JB/T6580-93 "Opening Press Accuracy" industry standard, the factory precision of open press with nominal pressure PG ≤ 2500KN must comply with the specified test items and their tolerances.
Many domestically produced open presses for stamping small and medium-sized stamping parts have been used for many years, and overtime service is quite common. The static accuracy is much lower than the actual project measurement error exceeds the table tolerance, the factory precision has been lost, and the dynamic accuracy after the actual operation is lower.
Since the domestic open presses all use C-shaped frames, their accuracy is far less than that of O-frame closed presses. According to the measured C-frame, the elastic deformation after loading is 0.01mm/10KN on average, which is 10 times that of the O-shaped frame, and is uneven; the opening of the C-shaped frame is the front side of the workbench and the rear side is small. When punching medium and thick plates with large punching force, the dynamic accuracy of the open press will be seriously deteriorated. In the normal production situation, the dynamic accuracy of the domestic open-type press is reflected in the displacement deviation of the cutting edge of the blanking die up to about 0.1~0.15mm, such as the single-side blanking clearance C of the die, according to the ordinary blanking of the instrument product. The part is usually taken as C=5%t, then the displacement deviation is equal to the blanking gap of the punching die of the blank thickness of 2~3mm: t=2mm, C=5%t=0.1mm; t-3mm; C=5% t=0.15mm, therefore, the punching piece with t<3mm is punched on the open press with the unguided open punching die, whether it is punching, blanking, slitting, punching hole, die edge It is possible to collide and peel, at least uneven wear and reduce the life of the die. Although t>3mm, although it can barely be punched, the operation is very dangerous.
2. The influence of the precision of the sliding guide guide frame on the punching precision of the punching die
The use of unguided fixed unloading structure and open structure blanking die, stamping on ordinary open press without safety protection device, especially with plate cutting material and single blank, all used manual feeding is the pressing industry in recent years One of the main causes of personal injury accidents, mold and equipment damage accidents and rising trends. The use of a die with a guiding device and a feeding machine is the main technical measure for reducing the above accidents and improving the precision of the punching and the punching efficiency.
The widely used "Cold Die" GB International and the JB line standard slide guide column mold frame, the guide column and the guide sleeve are slightly gap sliding fit.
To ensure the quality, precision and die life of the blank, it is important to ensure that the gap is consistent and uniform on a reasonable basis during blanking. Under normal circumstances, the guide frame guide gap is smaller than the punching clearance to implement the guide of the punch to the concave die. Only when the guide gap is less than half of the punching clearance, can the guide be completely accurately guided, and the punch can be uniformly and uniformly punched. Cut the gap to effectively improve the die life. According to the above situation, regardless of the comprehensive influence of the limited error on the guide frame guiding precision and the punching precision, which is inevitably generated by the mold core loading mold frame, according to the provisions of the international "punching clearance", under the premise of ensuring accurate guidance It is allowed to punch the 08F, 10F, 20 and other grades of low carbon steel plate to the minimum material thickness tmin.
The precision that ordinary all-steel stamping parts can achieve.
Empirical calculation method for error of ordinary all-steel die blank
1. Ordinary blanking die blanking error
For the punching error of the single-slot blanking die and the composite punching die, that is, the punching-type composite die for ordinary punching, when using the edge-cutting punching, only the fixed error WG and the incremental error WZ are used. The algebraic sum of the sporadic error WO is its conventional planar linear dimensional error W∑.
The WG value is related to the die manufacturing precision, the thickness of the blank material t, and the ratio of the blanking gap to the material thickness. In the instrument industry, the size of the punching piece is small, the material is thin, and the dimensional accuracy is high. The (4~5)%t (single side) small punching gap is used, and the punching precision is also high. The fixed error WG of the punching piece is The initial punching accuracy (deviation value) of the die is also relatively high.
The tolerance value obtained by the initial accuracy grade of the punching piece is the WG value, and the WZ value depends on the number of sharpening and the sharpening increment. The composite blanking die with the straight wall die edge is mostly used, and the WZ value is small and dependent. There are many influencing factors, such as changes in the wear of the die and the die material, and it needs to be measured after repeated sharpening. The WO value is an unknown number, and in most cases, WO=0. Therefore, the composite blanking die normally has a punching error W∑=WG+WZ.
3. Comprehensive composite die blanking error
The single-station integrated composite mold with forming and volume stamping operations, such as drawing, bending, flanging, squeezing, countersinking, etc., the size error calculation of the punching and blanking parts of the punching is the same as that of the punching type composite die. The error of the forming and volume stamping parts depends on the manufacturing precision of the cavity, the dimensional change caused by the cavity wear, and the dimensional error of the punching direction caused by the elastic deformation of the stamping equipment. Therefore, for the dimensional error of such composite die-punching parts, in addition to the plane (lateral) error compared with the punching-type composite mode calculation, it is also necessary to calculate the dimensional error of the punching direction according to the following formula:
Open press using C-frame
Closed press using O-frame
JC, JO—the deviation of the punching piece along the punching direction, ie the thickness direction, mm:
P—The maximum punching force during stamping, KN.
The calculation conditions of the above formula are based on the initial manufacturing accuracy of the new die after it is put into production. If the die is sharpened, it should be calculated according to its sharpening accuracy.
4, multi-station continuous composite die blank error
The calculation of the dimensional error value of the punching direction can be calculated according to the formula (1) or the formula (2) depending on the use of the punching device.
The horizontal or horizontal dimension error should be calculated as follows:
In the formula, WG-fixed error can determine the initial accuracy level of the die according to Table 6 and obtain the fixed error value of the punch, mm;
WC—system error can be determined according to the feeding mode and type of feeding device used in die stamping or by actual measurement, mm;
WZ—increasing error, according to the given formula, according to the taper of the cavity of the die, the number of sharpening, the amount of sharpening, mm;
WO—incidental errors, which do not occur under normal conditions, are not a batch of punches, and are usually not considered.
For multi-station continuous blanking and continuous composite die with station number ≤5, it is recommended to calculate the blank value of the blank by the following empirical formula:
Where, W∑—multi-station continuous composite die-punching piece plane size deviation, mm;
Mδ—feed feed distance error, mm. The value can be measured and the relevant manual can be obtained;
N—The number of stations.
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