Читать книгу Die Design Fundamentals - Vukota Boljanovic - Страница 8
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THE MATERIAL STRIPS
3.5 Methods for Producing Strips
Most stampings are made of steel. Carbon content varies from AISI-SAE 1010 to AISI-SAE 1030 and, therefore, most blanks are in the machine or cold-rolled steel range. Stampings are also made from these other materials:
1.Aluminum
2.Brass
3.Bronze
4.Copper
5.Stainless steel
6.Silicon steel
7.Fiber
8.Plastic sheet, etc.
Steel is an alloy of iron and carbon. Carbon must be present to the extent of about 0.05 percent by weight in order for the material to be known as “steel” rather than commercial iron. The composition and processing of steels are controlled in a manner that makes them suitable for numerous applications. They are available in various basic product shapes: sheet, strip, sheet, and plate.
3.2.1 Hot-Rolled Steel
Hot-rolled sheets are formed easily. Low-carbon sheets are used for tanks, barrels, pails, farm implements, lockers, cabinets, truck bodies, and other applications where scale and discoloration are not objectionable because surfaces are painted after forming. Hot-rolled sheets are readily available in thicknesses ranging from #30 gage (0.012 in. or 0.3 mm) to #7 gage (0.1875 in. or 4.8 mm).
a) Pickled and Oiled Sheets
Pickling, or the immersing of hot-rolled sheets in acid solution, results in smooth, clean, scale-free surfaces having a uniform gray color. Oiling protects the surfaces against rust.
These sheets are readily stamped or welded. Long-lasting painting or enameling is possible because of the absence of scale. Pickled and oiled sheets are used for household appliances, automotive parts, toys, and the like.
b) Copper-Bearing Sheets
Copper-bearing sheets are hot-rolled sheets having 0.20 percent minimum copper content. They are used for parts designed for outdoor exposure, or for indoor use under corrosive conditions. These sheets have a service life from two to three times longer than can be expected from non-copper-bearing steels. They are used for roofing and siding, farm and industrial buildings, truck bodies, railroad cars, farm implements, signs, tanks, dryers, ventilators, washing machines, and other similar applications.
c) Medium-Carbon Sheets
Hot-rolled sheets having a 0.40 to 0.50 percent carbon content provide hardness, strength, and resistance to abrasion. They can be heat-treated to make the material even harder and stronger and are primarily used for scrapers, blades, hand tools, and the like.
3.2.2 Cold-Rolled Sheets
Cold-rolled sheets have a smooth, deoxidized satin finish, which provides an excellent base for paint, lacquer, and enamel coating. Thicknesses are held to a high degree of accuracy. Cold-rolled steel is produced by the cold rolling of hot-rolled sheets to improve size and finish. Refrigerators, ranges, panels, lockers, and electrical fixtures are among their many uses.
a) Possibility of Deformation
Six tempers of cold-rolled steel sheets and strips are available; it is important to know exactly what operations can be performed on each (Figure 3.1):
1. Hard. Hard sheets and strips will not bend in either direction of the grain without cracks or fracture. These tempers of steel are employed for flat blanks that require resistance to bending and wear. Direction of grain is shown along lines A in the illustration. Hardness is Rockwell B 90 to 100.
2. Three-quarter hard. This temper of steel will bend a total of 60 degrees from flat across the grain. This is shown as dimension B in the illustration. Hardness is Rockwell B 85 to 90.
3. One-half hard. This temper will bend to a sharp 90-degree angle across the grain, shown as dimension C. Hardness is between Rockwell B 70 and 85.
4. One-quarter hard. This commonly used temper of steel will bend over flat on itself across the grain and to a sharp right angle along the grain. Hardness is Rockwell B 60 to 70.
Figure 3.1 Various tempers of cold-rolled steel from hard (1) to dead soft (6) and kinds of deformation possible with each.
5. Soft. This temper will bend over flat upon itself both across the grain and along the grain. It is also used for moderate forming and drawing. Hardness is Rockwell B 50 to 60.
6. Dead soft. This temper of steel is used for deep drawing and for severe bending and forming operations. Hardness is Rockwell B 40 to 50.
b) Finish
Cold-rolled steel is available in three grades of finish:
1. Dull finish. This is a gray lusterless finish to which lacquer and paint bond well.
2. Regular bright finish. This is a moderately bright finish suitable for most work. It is not recommended for plating unless buffed first.
3. Best bright finish. This finish has a high lustre well suited for electroplating. It is the brightest finish obtainable.
c) Stretcher-Leveled Sheets
These are cold-rolled steel sheets that have been further processed by stretcher leveling and resquaring. They are used in the manufacture of metal furniture, table tops, truck body panels, partitions, and other equipment requiring perfectly flat material.
d) Deep-Drawing Sheets
Deep-drawing steel is prime quality cold-rolled steel having a low carbon content. Sheets are thoroughly annealed, highly finished to a deoxidized silver finish, and oiled. Deep-drawing sheets are used for difficult drawing, spinning, and stamping operations such as those which produce automobile bodies, fenders, electrical fixtures, and laboratory equipment.
e) Silicon Steel
Also called “electrical steel,” silicon steel is extensively used for motors and generators. Lighter gages are suitable for transformers, reactors, relays, and other magnetic circuits.
The shearing process involves the cutting of flat material forms, such as sheets and plates. The cutting may be done by different types of blades or cutters in special machines driven by mechanical, hydraulic, or pneumatic power.
Figure 3.2 shows the mechanics of shear in 8 steps:
1.This illustration shows the cutting edges of a die with clearance C applied. The amount of this clearance is important, as will be shown.
2.A material strip is introduced between the cutting edges and is represented by phantom lines. Cutting a material strip occurs when it is sheared between cutting edges until the material between the edges has been compressed beyond its ultimate strength and fracture takes place.
3.The upper die begins its downward travel and the cutting edge of the punch penetrates the material by the amount A. The following stresses occur: The material in the radii at B is in tension; that is, it is stretched. The material between cutting edges C is compressed, or squeezed together. Stretching continues beyond the elastic limit of the material, then plastic deformation occurs. Observe that the same penetration and stretching is applied to both sides of the strip.
4.Continued descent of the upper cutting edge causes cracks to form in the material. These cleavage planes occur adjacent to the corner of each cutting edge.
5.Continued descent of the upper die causes the cracks to elongate until they meet. Here then is the reason for the importance of correct clearance. If the cracks fail to meet, a bad edge will be produced in the blank.
6.Further descent of the upper die causes the blank to separate from the strip. Separation occurs when the punch has penetrated approximately 1/3 of the strip.
7.Continued descent of the upper die causes the blank to be pushed into the die hole where it clings tightly because of the compressive stresses introduced prior to separation of the blank from the strip. In other words, the material at C in step 3 was compressed and it acts like a compressed spring. The blank, confined in the die hole, tends to swell, but it is prevented from doing so by the confining walls of the die block. Conversely, the material around the punch tends to close in and, therefore, the strip clings tightly around the punch.
Figure 3.2 Mechanics of shear-enlarged views of clearance between cutting edges of a shearing die (step 1) and material undergoing shear (steps 2 to 8).
8.The punch has now penetrated entirely through the strip and the blank has been pushed entirely within the die hole. Observe that the edge of the blank and the edge of the strip have identical contours except that they are reversed. The strip will cling around the upper punch with approximately the same pressure as the blank clings within the die hole and a stripper will be required to remove it.
3.3.1 Sheared Edges
It now becomes necessary to understand exactly what occurs when sheet material is cut between the cutting edges of a punch and die. Figure 3.3 shows the cut edge of a blank with correct clearance c applied, enlarged many times to reveal its contour. Observe the following:
The top corner is defined by a small radius R. The size of this radius depends upon the thickness and hardness of the strip and on the sharpness of the punch and die members.
A smooth, straight, burnished band goes around the periphery of the blank. The extent of this band, distance D, is approximately 1/3 the thickness T of the blank when the die is properly sharpened and when the correct clearance has been applied.
The remaining 2/3 of the edge is called the breakoff. The surface is somewhat rough and tapers back slightly. The extent of the taper, distance B, is the amount of clearance between cutting edges. If burrs are produced in cutting the blanks, they occur on this breakoff side of the blank. Burrs are produced when improper clearance has been applied and also when cutting edges become dull. The other side of the blank, which has the radius and smooth, shiny band, is called the burnished side of the blank.
Figure 3.3 Enlarged view of sheared blank edge.
The location of the burnished side and of the burr side of the blank is very important for performing secondary operations such as shaving, burnishing, and the like. In addition, the burr-side position can influence the functioning or the appearance of the finished stamping.
In blanking, the burnished band goes completely around the blank and the breakoff taper extends completely around the blank on the opposite side. This is not the case for blanks produced in cut-off or progressive dies. In such dies, the burnished side may alternate from side to side in a number of positions. Careful study is needed to ensure that no burr will interfere with the function or appearance of the stamping.
Shearing of material occurs in a continuous action. However, to understand the process, it will be necessary to “stop” the action in its various stages and to examine what occurs.
3.3.2 Clearance
Clearance generally is expressed as a free space between two mating parts. In closed contours, clearance is measured on one side.
a) Insufficient Clearance
The inset at A in Figure 3.4a shows the four effects of insufficient clearance:
•Radius R is smaller than when correct clearance is applied.
•A double burnished band D is formed on the blank edge.
•The breakoff angle B is smaller than when correct clearance is applied
•Greater pressure is required for producing the blank.
Referring to Figure 3.4a:
1.This figure shows cutting edges of a punch and die in partial penetration. It is obvious that cracks have appeared at the punch. Die sides will not meet when extended because the clearance is insufficient.
Figure 3.4 Enlarged views of blank edge sheared: a) with insufficient clearance A and material undergoing shear with insufficient clearance (1 to 3); and b) with excessive clearance A and material undergoing shear with excessive clearance (1 to 3).
2.Continued downward descent of the punch causes elongation of the cracks. The uncut area between them will be broken in a secondary fracture.
3.At the bottom of the stroke, the secondary fracture has occurred. A second burnished band has been produced on the blank edge and on the strip edge. The characteristic contour shown in inset A has been formed.
b) Excessive Clearance
The inset at A in Figure 3.4b shows the four effects of excessive clearance:
•Radius R is considerably larger than when correct clearance has been applied.
•Burnished band D is narrower.
•The break-off angle B is excessive.
•A burr C is left on the blank.
Referring to Figure 3.4b:
1.This shows the cutting edges of a punch and die in partial penetration. Cracks have begun to form at opposite sides
2.Continued downward descent of the punch causes elongation and widening of the cracks. Their alignment is fairly good
3.At the bottom of the stroke, separation has occurred, leaving the characteristic blank edges shown in the inset at A.
When a die is provided with excessive clearance, less pressure is required to effect cutting of the material. For this reason, more clearance is often specified for blanking the heavy gages of stock to reduce pressure on the press.
The first step in the actual production of stampings is to order standard-size sheets of the proper size and thickness from the mill. These are then sheared into strips, as described above. The widths of the strips into which the sheets are to be cut is specified by the die design department. Therefore, let us go over the steps taken in determining strip width so that your understanding will be complete.
Figure 3.5 Typical part drawing.
Figure 3.5 shows a part drawing of a typical representative stamping to be produced in a pierce and blank die.
3.4.1 Blank Layout
Figure 3.6 shows the two possible ways of running the strip of a typical representative stamping (Figure 3.5) through the die. The blanks may be positioned the wide way, necessitating a wide strip, or they may be run the narrow way, permitting the use of a narrower strip. These are called “blank layouts” and it is important that you understand exactly what is meant by the term. A blank layout shows the way in which the designer proposes to produce the blank. For both the wide-run and narrow-run layouts, two holes are to be pierced at the first station and the part is to be blanked out at the second station. It is customary to show small piercing punches in solid black. Section lines are applied through larger piercing punches and through blanking punches, as shown. The strip width and the feed are given directly on the blank layout.
Figure 3.6 Blank layouts necessitating either wide or narrow strips.
Now let us go through the first steps taken in the production of blanks in cut-off dies. Two of the sides of such blanks are originally sides of the material strip, and no scrap bridge is produced as in blanking dies. Figure 3.7 shows a representative stamping having the parallel sides typical of blanks suitable for production in cut-off dies.
Figure 3.8 shows two blank layouts for producing the stamping (Figure 3.7) in a cut-off die. At view A, the part is positioned the wide way in the strip. The edges of the strip are notched at the first station and a rectangular hole is punched. The blank is cut off from the strip at the second station. At view B, the part is positioned the narrow way in the strip. Observe how notching punches are sectioned. The heels C, which prevent deflection of the punches, are shown, but not sectioned. At D, short 45 degree lines and a long vertical line represent the “cut off” line.
Figure 3.7 Typical part for production in cut-off die.
Figure 3.8 Blank layout for part shown in Figure 3.7 run, either the wide (A) or narrow (B) way.
Blank layouts are drawn to explain the proposed operation of a die to others. When die designers are given a part print of a stamping for which a die is to be designed, they proceed to lay out a suitable scrap strip. Then they section significant punches and add cut-off lines to make the proposal layout clearer. This is the blank layout, and it must be approved by the group leader or chief engineer before design of the die is begun. When an outside engineering office is doing the work for a manufacturing company, the blank layout is submitted to the customer for approval.
3.4.2 Stripper Sheet
Sizes of sheets as they are manufactured by the mill are given in steel catalogs. Here is a representative list for #18 Gage (0.0478 in. or 1.2 mm) cold-rolled steel:
30 in. × 96 in. (762 mm × 2438 mm)
30 in. × 120 in. (762 mm × 3048 mm)
36 in. × 96 in. (914 mm × 2438 mm)
36 in. × 120 in. (914 mm × 3048 mm)
48 in. × 96 in. (1219 mm × 2438 mm)
48 in. × 120 in. (1219 mm × 3048 mm).
Figure 3.9 Number of strips obtainable with wide strip blank layout.
Figure 3.10 Strips per sheet with wide strip blank layout for production in a cut-off die.
The next step is to select the sheet that will be most economical, that is, the sheet from which a maximum number of strips can be cut, leaving a minimum amount of waste.
a) Wide Run
Strip width is taken from the blank layout. Divide the value given into the values for “width of sheet” in the steel catalog, and compare to determine which sheet leaves the smallest remainder. Figure 3.9 shows a sheet 48 by 120 inches (1219 mm × 3048 mm) divided into strips when the typical representative blank is run the wide way.
Figure 3.10 shows the sheet divided into strips for producing parts in a cut-off die when the blank is run the wide way.
Figure 3.11 Number of strips obtainable with narrow strip blank layout.
Figure 3.12 Strips per sheet with narrow strip blank layout for production in a cut-off die.
b) Narrow Run
Next, we must know how many blanks are produced per sheet with the blanks positioned the narrow way in the strip. With blanks arranged the narrow way, more strips are cut from the sheet, but fewer blanks are contained in each strip.
Figure 3.11 shows the same sheet divided into strips when the typical representative blanks are to be run the narrow way. More strips are produced from the same size of sheet.
Figure 3.12 shows the sheet divided into strips for producing a part in a cut-off die when the blank is run the narrow way.
3.4.3 Strip Layout
After it has been decided how the blanks are to be run (wide or narrow way), a stock layout is prepared complete with the following dimensions:
•Strip width. This dimension is used in selecting the proper width of sheet from which strips are to be cut.
•Feed. This is the amount of travel of the strip between stations. This dimension is used in selecting the proper length of sheet.
Figure 3.13 Complete strip layouts for blanks run either the wide (A) or narrow (B) way.
Figure 3.13 shows complete strip layouts for the typical representative blanks run either the wide (view A) or narrow (view B) way. Two views are applied, ordinarily. These are exactly the views of the strip that will be drawn on the die drawing except that an end view of the strip is added to the die drawing. The die is then actually designed around these views.
View A illustrates a strip layout for a blanking die in which the blank is run the wide way. View B shows a layout in which the blank is run the narrow way. For this particular job, more blanks per sheet are produced when the blanks are positioned the wide way and there is less waste. Therefore, all else being equal, this method of positioning the blanks would be selected.
Figure 3.14 Strip layouts for blanks run either the wide (A) or narrow (B) way.
Figure 3.14 shows strip layout for production of parts in cut-off die. The strip layout is prepared and copies are sent to the purchasing department and to the shear department. From the layout, sheets are ordered and, upon delivery, they are sheared to the strip width given on the layout. View A shows a representative strip layout for a blank for a cut-off die positioned the wide way, and view B shows a layout for a blank positioned the narrow way.
For this job we find that exactly the same number of blanks are produced with blanks positioned the narrow way as for wide-run positioning; there is no waste in either method. When blanks can be run either way, select the wide run method for three reasons:
•Fewer cuts will be necessary for producing the strips.
•The feed is shorter when running strips through the die, thus reducing the time required.
•More blanks are produced per strip and fewer strips have to be handled.
Figure 3.15 Strips ready for feeding either the wide (A) or narrow (B) way.
Figure 3.16 Strips ready for feeding either the wide (A) or narrow (B) way in a cut-off die.
3.4.4 Strips
Figure 3.15 shows a strip ready for feeding either the wide or narrow run. View A shows one of the strips ready to be fed through the die with blanks to be removed from it positioned the wide way. At view B, the blanks are positioned the narrow way in the strip. Five of the parts have been blanked out of each strip. After the strip is run completely through the die, only a narrow scrap bridge is left.
Figure 3.16 shows a strip ready for feeding either the wide or narrow run in a cut-off die. View A shows one of the strips ready to be fed through a die, with blanks to be removed from it positioned the wide way. At view B, the blanks are positioned the narrow way. Five blanks are shown in each strip. Because they are run in cutoff dies, no scrap bridge is produced.
3.5 METHODS FOR PRODUCING STRIPS
3.5.1 Shearing
The oldest and simplest method of producing metal strips is by shearing. In the steel mill, metal is formed into large sheets by rolling and trimming. A sheet that is to be cut into strips is introduced under the blade of a shear. Gages register the edges of the sheet for cutting correct widths of strips. Descent of the shear blade causes each strip to be parted from the sheet. Advancing the sheet against the gages brings it into position for cutting the next strip and this process is repeated until the sheet has been cut entirely into strips. Figure 3.17 at A shows a sheet in position under the shear blade C ready to be cut. At B, the blade has descended and the strip has been cut from the sheet.
Figure 3.17 Producing metal strips from sheet by shearing.
The power shear can cut material in any direction—lengthwise of the sheet, across the sheet, or at any angle.
3.5.2 Slitting
Slitting machines (Figure 3.18) are also used for producing material strips. In slitting operations, the sheet is fed though rotating cutting rolls, and all strips are cut simultaneously. In the illustration, cutting rolls A are mounted the proper distances apart on arbors B. The cutting edges of the rolls are separated by the required amount of clearance to effect cutting of the material as shown in inset C. Turning the rolls under power causes the sheet to advance, and it is cut into strips. As many as 20 or more strands can be cut at one time. In other types of slitters, the sheets are pulled through the rolls instead, and the rolls are free to turn.
Figure 3.18 Cutting rolls for slitting strips from sheet.
Figure 3.19 The various edge contours shown are the result of different production processes.
Slit strips are very accurate in width, flatness, and parallelism of sides because accuracy is built into the machine instead of depending upon the operator. Unlike the shear, which can cut strips only as long as the blade, the slitter will cut continuously to any length, without limit.
3.5.3 Edge Contour (Contour of the Edge of a Strip)
The contour of the edge of a strip (Figure 3.19) depends upon the process by which the strip is produced. Five contours may be recognized:
1.Strips produced in a shear have the burnished bands along the edges on opposite sides of the strip. If burrs are produced because of dull cutting edges, they will also occur on opposite sides of the strip. In addition, sheared strips often become spiraled or curved because the upper blade of the shear is at an angle to the lower blade. This makes the strips difficult to feed through the die unless they are first straightened.
2.Strips produced in the slitter have the burnished bands on the same side of the strip. Blanks produced from these strips in cut-off dies have a better appearance and they are fed more easily because they are straighter. Sheared strips or slit strips may be produced in the shear department of the plant, or they may be ordered directly from the mill.
3.Mill-edge strips have a radius at each corner. They are produced by rolling sheared or slit strips at the mill. Mill-edge strips are used for long stampings, such as for handles, shelf brackets, and other parts where sharp edges would be objectionable.
4.Rolled-edge strips have a full radius at each side, rolled at the mill. They are used for parts where appearance is a deciding factor, such as in ornamental grills, gratings, and the like.
5.Square-edge strips are ordered from the mill when the sides of the strips must be square and smooth. The widths of these strips are held very accurately. Square-edge strips are also specified when blanks are to be bent or formed edgewise. The square edges prevent cracking or splitting in the bending or forming operation.
