General all-steel punching die’s punching accuracy
Accuracy of panel punching part is display the press accuracy of the die exactly. But the accuracy of any punching parts’ linear dimension and positional accuracy almost depend on the blanking and blanking accuracy, So that the compound mould of compound punching accuracy is typical ness and representation in the majority.
For the analyses of pracyicable inaccuracy during production of dies to inactivation, we could get the tendency when it is augmentation in most time. From this we could analyses the elements. When the new punch dies pt into production to the first cutter grinding, the inaccuracy produced called initial error; if the die grinding more than twenty times, until it’s discard, the inaccuracy called conventional error; and before the dies discard, the largest error of the last batch permit, called limiting error. At job site, the evidence to confirm life of sharpening is the higher of the blanking, punched hole or punched parts. Because all finished parts had been blanked, so it is especially for the compound dies. Therefore, the analyses of burr and measurement are especially important when do them as enterprise standardization or checked with <<the height of punching part>>.
The initial error usually is the minimal through the whole life of die. Its magnitude depends on the accuracy of manufacture, quality, measure of the punching part, thickness of panel, magnitude of gap and degree of homogeneity. The accuracy of manufacture depends on the manufacture process. For the 1mm thicket compound punching part made in medium steel, the experimental result and productive practice all prove that the burr of dies which produced by spark cutting are higher 25%~~30% than produced by grinder, NC or CNC. The reason is that not only the latter have more exact machining accuracy but also the value of roughness Ra is less one order than the former, it can be reached 0.025um. Therefore, the die’s initial blanked accuracy depends on the accuracy of manufacture, quality and so on.
The normal error of the punch die is the practicable error when the fist cutter grinding and the last cutter grinding before the die produce the last qualified product. As the increase of cutter grinding, caused the measure the nature wear of the dies are gradual increasing, the error of punching part increase also, so the parts are blew proof. And the die will be unused. The hole on the part and inner because the measure of wear will be small and small gradually, and it’s outside form will be lager in the same reason. Therefore, the hole and inner form in the part will be made mould according to one-way positive deviation or nearly equal to the limit max measure. In like manner, the punching part’s appearance will be made mould according to one-way negative deviation or nearly equal to limit mini measure. For this will be broaden the normal error, and the cutter grinding times will be increased, the life will be long.
The limit errors in punching parts are the max dimension error which practicable allowed in the parts with limit error. This kind of parts usually is the last qualified products before the die discard.
For the all classes of dies, if we analyses the fluctuate, tendency of increase and decrease and law which appeared in the die’s whole life, we will find that the master of the error are changeless; the error that because the abrade of the cutter and impression will be as the cutter grinding times increased at the same time. And that will cause the error oversize gradually; and also have another part error are unconventional, unforeseen. Therefore, every die’s error is composed of fixed error, system error, and accident error and so on.
1. Fixed error
At the whole process when the New punching die between just input production to discard, the changeless master error that in qualified part are called fixed error. It’s magnitude is the deviation when the die production qualified products before the first cutter grinding. Also is the initial error, but the die having initial punching accuracy at this time. Because of the abrade of parts, the die after grinding will be change the dimension error. And the increment of deviation will oversize as the times of cutter grinding. So the punching accuracy after cutter grinding also called “grinding accuracy” and lower tan initial accuracy. The fixed errors depend on the elements factor as followed:
(1) The material, sorts, structure, (form) dimension, and thick of panel
The magnitude of punching gap and degree of homogeneity are having an important effect for the dimension accuracy. Different punching process, material, thick of panel, have completely different gap and punching accuracy. A gear H62 which made in yellow brass with the same mode number m=0.34, 2mm thick and had a center hole, when the gap get C=0.5%t (single edge), and punched with compound punching die, and the dimension accuracy reached IT7, the part have a flat surface, the verticality of tangent plane reached 89.5°, its roughness Ra magnitude are 12.5um, height of burr are 0.10mm; and the punching part are punched with progressive die, the gap C=7%t (single edge), initial accuracy are IT11, and have an more rough surface, even can see the gap with eyes. In the usual situation, flushes a material and its thickness t is these lection punching gap main basis. Once the designation gap had determined flushes the plane sizes the fixed error main body; Flushes the structure rigidity and the three-dimensional shape affects its shape position precision.
(2) Punching craft and molder structure type
Uses the different ramming craft, flushes a precision and the fixed error difference is really big. Except that the above piece gear example showed, the essence flushes the craft and ordinary punching flushes a precision and the fixed error differs outside a magnitude, even if in ordinary punching center, uses the different gap punching, the fixed error difference very is also big. For example material thicket=1.5mm H62 brass punching, selects C ≤ the 40%t unilateral I kind of small gap punching compared to select C ≤ 8%t (unilaterally) III kind of big gap punching, will flush a fixed error to enlarge 40% ~ 60%, the precision at least will fall a level. Side in addition, whether there is picks builds a row of type side, flushes an error to have far to be bigger than has builds a row of type to flush. Side not builds a row of type to flush. Side not builds a row of type to flush a precision to be lower than the IT12 level side, but most has builds a row of type to flush a precision in IT11 between ~ IT9 levels, material thicket > 4mm flushes, the size precision can lower some. Different die’s structure type, because is suitable the ramming material to be thick and the manufacture precision difference, causes to flush a fixed error to have leaves. Compound die center, multi-locations continuous type compound die because flushes continuously to duplicate the localization to add on the pattern making error to be bigger, therefore it flushes a fixed error compound punching die to want compared to the single location Big 1 ~ 2 levels
(3) The craft of punching die’s manufacture
The main work of punching die namely is raised, the concave mold processing procedure, to operates on the specification not to be high, can time form a more complex cavity. But its processing surface approximately is thick > 0.03 ~ 0.05mm is the high temperature ablation remaining furcated austenite organization, degree of hardness may reach as high as HRC67 ~ 70, has the micro crack, easily when punching appears broke the cutter or flaking. The Italian Corrada Corporation's related memoir called "the line cut the processing construction to have the disadvantageous influence to the superficial gold, in fact already changed the gold construction. We must use the Jin'gang stone powder to grind or the numerical control continual path coordinates rub truncate (cut to line) to make the precision work ". In recent years country and so on Switzerland and Japan, has conducted the thorough research to the electrical finishing equipment and a bigger improvement, makes function complete high accuracy NC and the CNC line cutter, the processing precision may reach ±0.005 ~ 0.001mm,even is smaller. The processing surface roughness Ra values can achieve0.4 um. According to the recent years to the domestic 12 production lines cutter factory investigation and study, the domestically produced line cutter processing precision different factory different model line cutter might reach ±0.008 ~ ±0.005mm, generally all in ±0.01mm or bigger somewhat, was individual also can achieve ±0.005mm, the processing surface roughness Ra value was bigger than1.6um. However, the electrical finishing ablation metal surface thus the change and the damage machined surface mental structure character can not change, only if with rubs truncates or other ways removes this harmful level. Therefore, merely uses electricity machining, including the spark cutting and the electricity perforation, achieves with difficulty punching, especially high accuracy, high life punching die to size precision and work components surface roughness Ra value request.
With precisely rubs truncates the law manufacture punching die, specially makes the high accuracy, the high life punching die, such as: Thin material small gap compound punching die, multi- locations continuous type compound die and so on, has the size precision high, the work component machined surface roughness Ra value is small, the mold life higher characteristic. Its processing craft at present changed the electrical fire by the past ordinary engine bed rough machining spark cutting or the electricity puncher rough machining, finally precisely rubs truncates, also from takes shape rubs, optics curve rubs, the manual grid reference rubs gradually filters the continual path grid reference to rub and NC and the CNC continual path grid reference rubs, Processing coarseness may reach ±0.001 ~ 0.0005mm, the processing surface roughness Ra value may reach 0.1 ~ 0.025um. Therefore, with this craft manufacture the die, regardless of the size precision, the work components surface roughness, all can satisfy die, each kind of compound request, the die is especially higher than the electrical.
(4) Gap size and degree of homogeneity
The flange and other sheet forming agene rally all must first punching (fall material) the plate to launch the semi finished materials, after also has the forming to fall the material, the incision obtains the single end product to flush. Therefore punching the work, including is commonly used punching hole, the margin, cut side and so on, regarding each kind of sheet pressing portal is necessary. Therefore punching the gap to flushes an out form in chprecision to have the decisive influence. Punching the gap small and is even, may cause punching the size gain high accuracy. Regarding draw ability is curving and so on mould, the gap greatly will decide an increase flushes the oral area size error and the snapping back. The gapnon-uniformity can cause to flush a burr enlarges and incurs cutting edge the non-uniform attrition.
(5) Ramming equipment elastic deformation in the ramming process
After the punch press load bearing can have the certain elastic deformation. Although this kind of distortion quantity according to flushes the pressure the size to change also to have the obvious directivity, but on the pressing part, mainly is to has the volume ramming archery target stamping, embosses, the equalization, the pressure is raised, the wave, flushes crowds, the shape, the flange, hits flatly, thinly changes draw ability and so on the craft work punching forming flushes, has the significant influence to its ramming aspect size precision.
Material Behavior
Auto Forge allows the material to be represented as either an elastic-plastic material or as a rigid-plastic material. The material is assumed to be isotropic, hence, for the elastic-plastic model, a minimum of three material data points are required; the Young’s modulus (E), the Poisson’s ratio (μ), and the initial yield stress (y). For a rigid-plastic material, only the yield stress is required. These data must be obtained from experiments or a material handbook. These values may vary with temperature in a coupled analysis. This is prescribed using the TABLES option. The flow stress of the material changes with deformation, so called strain hardening or work hardening behavior and may be influenced by the rate of deformation. These behaviors are also entered via the TABLES option.
In uneasily tension tests of most metals (and many other materials), the following phenomena can be observed. If the stress in the specimen is below the yield stress of the material, the material will behave elastically and the stress in the specimen will be proportional to the strain. If the stress in the specimen is greater than the yield stress, the material will no longer exhibit elastic behavior, and the stress-strain relationship will become nonlinear.
Within the elastic region, the stress-strain relationship is unique. Therefore, if the stress in the specimen is increased from zero to s1, and then decreased to zero, the strain in the specimen is also increased from zero to e1, and then returned to zero. The elastic strain is completely recovered upon the release of stress in the specimen.
The loading-unloading situation in the inelastic region is different from the elastic behavior. If the specimen is loaded beyond yield, where the stress in the specimen is s2 and the total strain is e2, upon release of the stress in the specimen the elastic strain, is completely recovered. However, the inelastic (plastic) strain remains in the specimen. Similarly, if the specimen is loaded to point 3 and then unloaded to zero stress state, the plastic strain remains in the specimen. It is obvious that is not equal to. We can conclude that in the inelastic region
• Plastic strain permanently remains in the specimen upon removal of stress.
• The amount of plastic strain remaining in the specimen is dependent upon the stress level at which the unloading starts (path-dependent behavior).
In addition to elastic material constants (Young’s modulus and Poisson’s ratio), it is essential to be concerned with yield stress and work hardening slopes in dealing with inelastic (plastic) material behavior. These quantities vary with parameters such as temperature and strain rate, further complicating the analysis. Since the yield stress is generally measured from uneasily tests, and the stresses in real structures are usually multiracial, the yield condition of a multiracial stress state must be considered. The conditions of subsequent yield (work hardening rules) must also be studied.
Yield Conditions
The yield stress of a material is a measured stress level that separates the elastic and inelastic behavior of the material. The magnitude of the yield stress is generally obtained from an uneasily test. However, the stresses in a structure are usually multiracial. A measurement of yielding for the multiracial state of stress is called the yield condition. Depending on how the multiracial state of stress is represented, there can be many forms of yield conditions. For example, the yield condition can be dependent on all stress components, on shear components only, or on hydrostatic stress. MSC.Marc Auto Forge uses the von Misses yield criteria.
Von Misses Yield Condition
Although many forms of yield conditions are available, the von Misses criterion is the most widely used. The von Misses criterion states that yield occurs when the effective.
Effects on Yield Stress
This section describes MSC.Marc Auto Forge capabilities with respect to the effect of temperature and strain rate. MSC.Marc Auto Forge allows you to input a temperature-dependent yield stress. To enter the yield stress at a reference temperature, use the model definition options ISOTROPIC. To enter variations of yield stress with temperatures, use the model definition options TEMPERATURE EFFECTS. Repeat the model definition options TEMPERATURE EFFECTS for each material, as necessary. The effect of temperatures on yielding is discussed further in “Constitutive Relations”. MSC.Marc Auto Forge allows you to enter a strain rate dependent yield stress, for use in dynamic and flow problems. To use the strain rate dependent yield stress in static analysis, enter a fictitious time using the TIME STEP option. The zero-strain-rate yield stress is given on the ISOTROPIC option. Repeat the model definition option STRAIN RATE for each different material where strain rate data are necessary. Refer to “Constitutive Relations” for more information on the strain-rate effect on yielding.
Work hardening Rules
In an uneasily test, the work hardening slope is defined as the slope of the stress-plastic strain curve. The work hardening slope relates the incremental stress to incremental plastic strain in the inelastic region and dictates the conditions of subsequent yielding. The isotropic work hardening model is used in MSC.Marc Auto Forge. The uneasily stress-plastic strain curve may be represented by a piecewise linear function through the WORK HARD option. As an alternative, you can specify work hardening through the user subroutine WKSLP.
There are two methods to enter this information, using the WORK HARD option. In the first method, you must enter work hardening slopes for uneasily stress data as a change in Cauchy or true stress per unit of logarithmic plastic strain and the logarithmic plastic strain at which these slopes become effective (breakpoint).
In the second method, you enter a table of yield stress, plastic strain points. This option is flagged by adding the word DATA to the WORK HARD statement.
Isotropic Hardening
The isotropic work hardening rule assumes that the center of the yield surface remains stationary in the stress space, but that the size (radius) of the yield surface expands, due to work hardening.
A review of the load path of an uneasily test that involves both the loading and unloading of a specimen will assist in describing the isotropic work hardening rule. During unloading, the stress state can remain elastic or it can reach to a subsequent (reversed) yield point. The isotropic work-hardening rule states that the reverse yield occurs at current stress level in the reversed direction.
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