Tips and tricks
Drawing, forming and bending dies
DRAWING, FORMING AND BENDING DIES
CONTENTS
- Drawing and Forming Dies
- Examples of Drawing, Bending and Forming Dies
MACHINERY'S REFERENCE SERIES
The Industrial Press, New York, 1914
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Drawing, forming and bending dies
DRAWING AND FORMING DIES
Comparatively little information has been published in book form relating to the subject of drawing dies. This depends partly upon the fact that this line of work in some respects is still in its infancy. While an enormous amount of this kind of work is done daily in hundreds of shops in the country, yet there is a scarcity of definite information as to the fundamental rules that govern this class of work. Some day someone will take up this subject in a manner similar to that in which Mr. F. W. Taylor investigated the art of cutting metals, and then we may be able to lay down exact rules and formulas governing the drawing of metal sheets into shells. At present we must content ourselves with studying a few general principles based upon experience. By giving a great number of examples of work that has been accomplished in practice, the possibilities in this class of work may, however, be indicated.
A large majority of work which, in the past, has been made from castings is now made from sheet steel by drawing up bosses and reinforcing ribs to strengthen the work. The development in the art has been rapid, but has proceeded almost entirely along cut-and-try methods. The definite information that is available, however, relating to the diameters of shell blanks, the depth and diameter reductions of drawn shells, the lubrication of dies, etc., will be given in the following. In addition, a number of practical hints for die-makers in the making of forming dies will be presented, and numerous examples of successful designs of drawing, forming and bending dies. This will give the reader a comprehensive view of the present state of the art.
Depth and Diameter Reductions of Drawn Shells
The depth to which metal can be drawn in one operation depends upon the quality and kind of material, its thickness, the slant or angle of the dies, and the amount that the stock is thinned or "ironed" in drawing. A general rule for determining the depth to which cylindrical shells can be drawn in one operation is as follows: The depth or length of the first draw should never be greater than the diameter of the shell. If the shell is to have a flange at the top, it may not be practicable to draw as deeply as is indicated by this rule, unless the metal is extra good, because the stock is subjected to a higher tensile stress, owing to the larger blank which is necessary for forming the flange. According to another rule, the depth given the shell on the first draw should equal one-third the diameter of the blank. Ordinarily, it is possible to draw sheet steel of any thickness up to 14 inch, so that the diameter of the first shell equals about six-tenths of the blank diameter. When drawing plain shells, the amount that the diameter is reduced for each draw must be governed by the quality of the metal and its susceptibility to drawing.
When a hole is to be pierced at the bottom of a cup and the remaining metal is to be drawn after the hole has been pierced or punched, always pierce from the opposite direction to that in which the stock is to be drawn after piercing. In extreme cases, it is necessary to machine the metal around the pierced hole in order to prevent the starting of cracks or flaws in the subsequent drawing operations.
Prevention of Wrinkles in Drawn Work
The formation of wrinkles in drawing operations is a source of great trouble, and there are many pieces of drawn work which could be performed in a single operation were it not for the wrinkles that would inevitably appear. In drawing operations, the tendency to wrinkle starts with the first contact of the punch upon the metal.
The usual method of preventing wrinkles is to provide the punch with a blank-holder which is operated by springs of sufficient tension to allow the metal to be pulled from beneath it for drawing, but maintaining pressure enough to keep the metal free from wrinkles. At A, in Fig. 1, is shown a section of a simple drawing die in which it will be noticed that the die is provided with a raised ridge around its opening, the blank-holder having a corresponding depression. Consequently, the sheet metal being drawn is pulled over this ridge, and as the space between the blank-holder and the top of the ridge is purposely made slightly less than the thickness of the metal, it will be seen that as the stock passes through this opening any wrinkles are "ironed out" At B, the shell from the dies at A is shown undergoing a second operation.
For strength and protection in hardening, as well as to facilitate the drawing operation, the ridge is provided with a fillet where it joins the flat surface of the die. It is obvious that the addition of this ridge to the drawing die occasions a little extra work in the die-making, but this work is offset by the fact that the blank-holder and upper surface of the drawing die do not have to be ground perfectly smooth and parallel, as is ordinarily required. The size of the ridge around the die should be in proportion to the diameter of the shell.
It is obvious that the completed shell will have a ridge left at the edge. On work which is to be wired or for work on which the edge is to be turned over, this additional ridge is no detriment as it can be made use of directly. Moreover, if a succeeding operation is to follow, deepening the shell slightly, this ridge will provide the surplus metal required. This point is illustrated at C. In other cases, the - extra metal left at the edge may be removed when the shell is trimmed. It is claimed that this improvement in drawing dies is being employed with success. By its use, wrinkles are absolutely prevented, and, moreover, the drawing operation puts less stress upon the metal.
Practical Hints for Die makers in making Drawing and Forming Dies
In making templets and blanks, always file them straight and square across the edge. In developing the blank always keep a templet or reference blank, so that it will be at hand if alterations are found necessary. Each time a change is made the previous blank which was kept for reference is marked to designate it from others. The marks may be "M" for model or "S" for sample. It should be remembered that metal will not draw around sharp corners, and that corners over which the metal is to be drawn should be rounded to a true radius. In all cases when making blanks for forming punches and dies consider the thickness of the metal.
In forming blanks they should always be bent with the grain of the metal and not across it, particularly on sharp bends. By the "grain" is meant the way in which the metal is drawn when passing through the rolls. If it is required to make bends at right angles to each other or approximately so, the blanks should be punched out diagonally across the grain. It is sometimes found necessary to form blanks from unannealed stock, that is stock which has been rolled to a certain degree of hardness. In bending this metal it springs back more or less after being struck in the die. This makes it necessary to make a more acute angle or a smaller radius on the punch and die, than is required on the finished product. This difference can be ascertained only by the cut-and-try method. When producing a short bend in blanks in such a position and of such a nature that the blank slips away from under the punch when it is descending into the die, a spring pad is fitted into the die with the lower part of the bend shaped into it, and flush with the top surface of the die. This holds the metal securely against the punch in its descent into the die and insures perfect duplicates of the product. Where holes in a blank come near a bend, a strain in the metal is set up during the bending operation which elongates the holes. This makes it necessary sometimes to pierce the holes slightly oval in the opposite direction before forming. In testing the shape of a forming die before it is hardened, always apply a small amount of oil to the surface so that the blank will not bruise or scratch the die, which would be the case if the die were left dry.
Never leave the inside corners of a die sharp when they can just as easily conform to the radius formed by bending the stock around the punch. This will strengthen the die and lessen the possibility of its cracking when hardened. When necessary, one forming die can be made to form bends in several pieces which have the same form tut are of different lengths. This is accomplished by equipping the die with interchangeable gages or guide strips. Never leave any file marks on the working portions of the punch or die, as these will be reproduced on the blank. A screw hole in a die should be tapped a little larger than the screw, as the die shrinks somewhat in hardening.
When a punch or die is heated in a charcoal or a soft coal fire, the dust and ashes should be thoroughly scraped off the working portion before dipping, so that the water will have a free action upon the steel. Bending and forming dies, unless there is danger of cracking or breaking of weak parts, should be as hard as fire and water will make them. After hardening they may be warmed over a slow fire until water "sizzles" on them. Some toolmakers, when hardening a punch or die, apply cyanide of potassium to the working portions of the steel before dipping. They claim for this that the outer surface of the steel is rendered harder by the application of this casehardening substance and thus will be better fitted to withstand the wear to which it is subjected. The practice is strongly condemned for this reason: If, as is often the case, the tool should fail to harden, this fact will be concealed by the casehardened outer coating, and the tool will respond to the file test as being hard whether it is or not. Gage plates should never be secured with two screws and one dowel pin. It is far more practical to use one screw and two dowel pins in most cases. A good method of holding gage plates before their exact position is determined is to clamp them to the die with fillister screws having washers under their heads, and to drill the holes in the gage plate about 1/16 inch larger than the diameter of the screws, so that the gage plate may be shifted around. Always drill the screw holes for the gage plates through the die so that in case a new gage plate is required the holes will be spotted from the die. Whenever the gage plate comes close to the working portion of the die, cut the punch back far enough so that the body of the punch will come within 1/8 or 1/4 inch of the gage plate. In making gage plates for locating large blanks of irregular shape, they should be made to fit the blank only at the point where accuracy is essential, and not to conform exactly to the irregular shape of the blank.
DOWNLOAD FREE BOOK:
Drawing, forming and bending dies
A large majority of work which, in the past, has been made from castings is now made from sheet steel by drawing up bosses and reinforcing ribs to strengthen the work. The development in the art has been rapid, but has proceeded almost entirely along cut-and-try methods. The definite information that is available, however, relating to the diameters of shell blanks, the depth and diameter reductions of drawn shells, the lubrication of dies, etc., will be given in the following. In addition, a number of practical hints for die-makers in the making of forming dies will be presented, and numerous examples of successful designs of drawing, forming and bending dies. This will give the reader a comprehensive view of the present state of the art.
Depth and Diameter Reductions of Drawn Shells
The depth to which metal can be drawn in one operation depends upon the quality and kind of material, its thickness, the slant or angle of the dies, and the amount that the stock is thinned or "ironed" in drawing. A general rule for determining the depth to which cylindrical shells can be drawn in one operation is as follows: The depth or length of the first draw should never be greater than the diameter of the shell. If the shell is to have a flange at the top, it may not be practicable to draw as deeply as is indicated by this rule, unless the metal is extra good, because the stock is subjected to a higher tensile stress, owing to the larger blank which is necessary for forming the flange. According to another rule, the depth given the shell on the first draw should equal one-third the diameter of the blank. Ordinarily, it is possible to draw sheet steel of any thickness up to 14 inch, so that the diameter of the first shell equals about six-tenths of the blank diameter. When drawing plain shells, the amount that the diameter is reduced for each draw must be governed by the quality of the metal and its susceptibility to drawing.
When a hole is to be pierced at the bottom of a cup and the remaining metal is to be drawn after the hole has been pierced or punched, always pierce from the opposite direction to that in which the stock is to be drawn after piercing. In extreme cases, it is necessary to machine the metal around the pierced hole in order to prevent the starting of cracks or flaws in the subsequent drawing operations.
Prevention of Wrinkles in Drawn Work
The formation of wrinkles in drawing operations is a source of great trouble, and there are many pieces of drawn work which could be performed in a single operation were it not for the wrinkles that would inevitably appear. In drawing operations, the tendency to wrinkle starts with the first contact of the punch upon the metal.
The usual method of preventing wrinkles is to provide the punch with a blank-holder which is operated by springs of sufficient tension to allow the metal to be pulled from beneath it for drawing, but maintaining pressure enough to keep the metal free from wrinkles. At A, in Fig. 1, is shown a section of a simple drawing die in which it will be noticed that the die is provided with a raised ridge around its opening, the blank-holder having a corresponding depression. Consequently, the sheet metal being drawn is pulled over this ridge, and as the space between the blank-holder and the top of the ridge is purposely made slightly less than the thickness of the metal, it will be seen that as the stock passes through this opening any wrinkles are "ironed out" At B, the shell from the dies at A is shown undergoing a second operation.
For strength and protection in hardening, as well as to facilitate the drawing operation, the ridge is provided with a fillet where it joins the flat surface of the die. It is obvious that the addition of this ridge to the drawing die occasions a little extra work in the die-making, but this work is offset by the fact that the blank-holder and upper surface of the drawing die do not have to be ground perfectly smooth and parallel, as is ordinarily required. The size of the ridge around the die should be in proportion to the diameter of the shell.
It is obvious that the completed shell will have a ridge left at the edge. On work which is to be wired or for work on which the edge is to be turned over, this additional ridge is no detriment as it can be made use of directly. Moreover, if a succeeding operation is to follow, deepening the shell slightly, this ridge will provide the surplus metal required. This point is illustrated at C. In other cases, the - extra metal left at the edge may be removed when the shell is trimmed. It is claimed that this improvement in drawing dies is being employed with success. By its use, wrinkles are absolutely prevented, and, moreover, the drawing operation puts less stress upon the metal.
Practical Hints for Die makers in making Drawing and Forming Dies
In making templets and blanks, always file them straight and square across the edge. In developing the blank always keep a templet or reference blank, so that it will be at hand if alterations are found necessary. Each time a change is made the previous blank which was kept for reference is marked to designate it from others. The marks may be "M" for model or "S" for sample. It should be remembered that metal will not draw around sharp corners, and that corners over which the metal is to be drawn should be rounded to a true radius. In all cases when making blanks for forming punches and dies consider the thickness of the metal.
In forming blanks they should always be bent with the grain of the metal and not across it, particularly on sharp bends. By the "grain" is meant the way in which the metal is drawn when passing through the rolls. If it is required to make bends at right angles to each other or approximately so, the blanks should be punched out diagonally across the grain. It is sometimes found necessary to form blanks from unannealed stock, that is stock which has been rolled to a certain degree of hardness. In bending this metal it springs back more or less after being struck in the die. This makes it necessary to make a more acute angle or a smaller radius on the punch and die, than is required on the finished product. This difference can be ascertained only by the cut-and-try method. When producing a short bend in blanks in such a position and of such a nature that the blank slips away from under the punch when it is descending into the die, a spring pad is fitted into the die with the lower part of the bend shaped into it, and flush with the top surface of the die. This holds the metal securely against the punch in its descent into the die and insures perfect duplicates of the product. Where holes in a blank come near a bend, a strain in the metal is set up during the bending operation which elongates the holes. This makes it necessary sometimes to pierce the holes slightly oval in the opposite direction before forming. In testing the shape of a forming die before it is hardened, always apply a small amount of oil to the surface so that the blank will not bruise or scratch the die, which would be the case if the die were left dry.
Never leave the inside corners of a die sharp when they can just as easily conform to the radius formed by bending the stock around the punch. This will strengthen the die and lessen the possibility of its cracking when hardened. When necessary, one forming die can be made to form bends in several pieces which have the same form tut are of different lengths. This is accomplished by equipping the die with interchangeable gages or guide strips. Never leave any file marks on the working portions of the punch or die, as these will be reproduced on the blank. A screw hole in a die should be tapped a little larger than the screw, as the die shrinks somewhat in hardening.
When a punch or die is heated in a charcoal or a soft coal fire, the dust and ashes should be thoroughly scraped off the working portion before dipping, so that the water will have a free action upon the steel. Bending and forming dies, unless there is danger of cracking or breaking of weak parts, should be as hard as fire and water will make them. After hardening they may be warmed over a slow fire until water "sizzles" on them. Some toolmakers, when hardening a punch or die, apply cyanide of potassium to the working portions of the steel before dipping. They claim for this that the outer surface of the steel is rendered harder by the application of this casehardening substance and thus will be better fitted to withstand the wear to which it is subjected. The practice is strongly condemned for this reason: If, as is often the case, the tool should fail to harden, this fact will be concealed by the casehardened outer coating, and the tool will respond to the file test as being hard whether it is or not. Gage plates should never be secured with two screws and one dowel pin. It is far more practical to use one screw and two dowel pins in most cases. A good method of holding gage plates before their exact position is determined is to clamp them to the die with fillister screws having washers under their heads, and to drill the holes in the gage plate about 1/16 inch larger than the diameter of the screws, so that the gage plate may be shifted around. Always drill the screw holes for the gage plates through the die so that in case a new gage plate is required the holes will be spotted from the die. Whenever the gage plate comes close to the working portion of the die, cut the punch back far enough so that the body of the punch will come within 1/8 or 1/4 inch of the gage plate. In making gage plates for locating large blanks of irregular shape, they should be made to fit the blank only at the point where accuracy is essential, and not to conform exactly to the irregular shape of the blank.
DOWNLOAD FREE BOOK:
Drawing, forming and bending dies
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