Forming By Stretch Press
Stretch-Press Application
Stretch forming provides in extensive but flexible tooling by offering substantial reduction in tool cost, compared to the usage of draw presses. The stretch press is well suited for production requirement that are intermediate between those that can be met by the drop hammer and those requiring the high production process of the automotive industry. This is because of their adaptive nature, when it comes to rapid fabrication of multiple varieties of double-curvature parts. A properly set upped stretch press can process a part, (loading, stretching, and unloading) in an average of two minutes.
Stretch pressing can be used in both a soft or hard stock. But use of a hard stock, such as 24ST, is recommended, since heat treatment is eliminated. With the use of stretch-press, much larger sheets may be formed into double contours, rather than with hydro-pressing.
The number of separated parts going to sub-assembly, and the assembly time is reduced with the used of large sized parts. The main limit of stretch-press method is that only contours relatively symmetrical, and with only slight reverse bends can be formed.
Another disadvantage of this is that a certain percent of scrap, unavoidable since the ends of the sheet must be long enough to be held in clamps. Adding 16 inches to the length and inches to the width for the gripping and trimming allowances is commonly practiced.
Reduction of scrap may be reduced in some extent by forming two or more parts from a single sheet in one part, and then cutting them apart. This results in only a single clamping allowance for the forming of two parts. Unsymmetrical, right-handed, and left-handed parts, on some cases can be formed in this method, but it is a common practice to form both left and right-hand parts in the same die, then snapping half of them through, producing parts opposite hand to that of the die.
Principles of Stretch-Press Forming
Stretch-press forming is different from drawing on a double-action press in both principle, and operation. The aluminum alloy sheet is clamped at opposite ends and stretched over a form, or die to the elastic limit of the material when processed in a stretch-press. (This is contrary to being forced through a die by the action of a punch).
This stretching of the material into the plastic range, eliminates excessive springback, causing the material to set to the shape of the die. When a material is stressed below its elastic limit, there will be a complete elastic recovery, or springback. When a material is stretched into the plastic range, elastic a permanent set will take place.
Direction of Springback
When a part is produced through bending, the material outside the neutral plane is stretched, while the material inside the neutral plane is compressed. The material at this point is under neither tension nor compression, since there is no lengthwise movement.
When a part is formed through stretching, all materials is stretched since the neutral plane is below the plane of the material. When the stretching force is removed, the recovery is in the plane of the material and parallel with the curvature, thus it does not distort the part beyond the useable limits.
Elongation of Material
A stretch-press elongates a sheet and reduces its thickness as much as 5 to 7 percent, however, this is compensated by the increase of tensile strength. Since the stretching operation is conducted in the plastic range, the strength required to stretch the whole part must not exceed the ultimate strength of the metal, to prevent tearing up the material.
How much a material is stretched before breaking is determined by the maximum elongation characteristics of the metal. A 25ST Alclad has a maximum elongation generally specified at about 18 or 19 percent. This value was determined on a 2-inch gage length test specimen, and is not applicable to stretch-forming which might involve working with materials several feet in length. Maximum elongation for test sheets has shown that stretch forming of either 24SO or 24ST Alclad is from about 8 to 10 percent. Eight percent is generally used as the maximum elongation, and 10 percent elongation can only be obtained by extra-careful polishing and lubrication of the die, plus observing every possible precaution in the stretching operation.
Stress-Press Operation
Stretch presses are made in three sizes, 75, 150, and 300 tons capacities. The sheet metal is clamped at both ends in a row of individual pneumatic or hydraulic clamps mounted on supports which are controlled from a push button panel. They can be parallel or at any angle up to 15 degrees with the table. The wedging action between the beveled shoes forces the sheet upwards into the tapered slots, gripping the sheet in place tighter as the pull on the sheet becomes greater. Some of the clamps may be loosened slightly by means of their separate valves to allow the sheet to slip a little, thus preventing tearing at highly stressed points. The material is stretched to shape when the die contacts the sheet and builds up pressure. The speed can be reduced to suit the peculiarities of the die.
The result from stretch-pressing depends largely on the ability of the operator. The characteristics of the material must be known to the operator, allowing him to judge when it can reach its maximum deformation. Generally, wrinkles can be generally be avoided through careful design of the die and studying the material as it is being stretched.
It should be remembered that pressure should be applied in such a way that pressure is equalized, and stress distributed equally throughout the surface of the sheet.
Die Lubricants
The die must be covered with a lubricant, such as Vaseline, or light grease, or drawing compounds made up of half vegetable and half mineral oil, before the sheet metal is placed in the press. The lubricant used must not break down below 150 degrees Fahrenheit, and should provide viscous lubrication up to a normal pressure of about 5,000 pounds per square inch. It is necessary to remove the compound from the parts within four hours after they are formed, or it will oxidize the surface of the aluminum-alloy. Sheets must be washed with hot water, which also warms the sheet and prevents water spots, since the ordinary degreasing process will not remove the compound.
Die Materials
Kirksite, concrete faced with plaster, and hardwoods are commonly used materials for stretch-press dies. Lead, iron, and fiber have also been successfully used. Kirksite is the most widely used material, since it is easily cast to shape, and it can be reclaimed by melting the worn-out dies, and it can be worked to a smooth surface, permitting the sheet metal to slide over it with very little friction.
Plaster dies are made of reinforced concrete that is faced with a mixture of plaster and special hard plaster known as Hydrocal. A sheet of low-carbon steel, such as S.A.E 1010 can be used to give it a long-wearing surface. A plaster die may be used without this for a fairly large runs but the steel both increases the die's life, and reduces friction.
Wood dies are not as widely used as before. They shrink, especially when in storage for a long period of time. Wood dies may be built in sections, or made of a solid piece which is preferred.
Types of dies
There are two types of stress-press dies:
Has the actual shape of the finished part, which when removed will retain the shape of the die. They are usually made of Kirksite
As its name implies, has a distorted shape, (meaning they do not have the true shape of the finished part. They are usually made of concrete and plaster, and are used usually in forming parts that would be too long to be stretched to their finished shape in the press.
Forming Reverse Contours
Reverse contours may be created on a stretch-press if the die is designed so that the formation of wrinkles is prevented along the central part of the sheet. The reverse curve on the die will tend to cause the material to slip along the curve and wrinkle in the middle as soon as the material along the sides is stretched. Locking beads at both ends of the die may be used to prevent this wrinkling, then at the beginning of the stretch the material will be drawn into these beads, and held crosswise so that it can not slip along the reverse curve of the die and form wrinkles.
Stretch-Forming in Double-Action Press
Application of Double-Action Press
Double-action hydraulic presses can be adapted to stretch-forming when no specially designed stretch presses are available. Skins can be formed in the ST condition at the rate of ninety per hour. The stock must be draped between the clamps and made long enough so that the die, while descending, will stretch the metal sufficiently, producing a permanent set while not producing too much stress to rupture it. Required length may be determined by trial and error, or calculation, and should be kept the same for all sheets for obtaining uniform results. The die should be well lubricated and lowered to stretch the metal in much the same manner as a regular stretch-press.
Details of Clamps
Rods are used to exert clamping pressure on sheet stock. This passes from the hold-down plates up through the main ram to the hold-down ram, or blank-holder. Because of the heavy pressure, both the rails and plates are made with smooth surfaces and need not be serrated to provide sufficient frictional restraint to hold the sheet metal. The smooth surface is an advantage since any serrations or other irregularities of the holding surfaces tend to tear the sheet when it attempts to yield locally within the clamps at is being stretched. The actual range of forming can be extended by allowing the sheet to slip between the clamping plates over certain critical areas.
Stretch-Forming Shallow Contours
IT is often more satisfactory to form the parts in pairs, when parts of very shallow curvature are to be stretch-formed. Some parts can not be satisfactorily stretched individually, since their contours are too flat. When such exceptionally shallow-contoured surfaces are stretched, they have a tendency to overform the center of the part.
Another method that can be successfully applied to stretch-form parts of shallow contours when using a double-action press is to use a die that is deeper than is required by the contour.
Two-Die Set-Up
Production speed may be increased, and waste reduced when parts are formed two at a time. It is similar to an ordinary set-up with only one die, where the sheet metal is gripped between the hold-down plates and the rails, but when two dies are used, additional rail is placed in the center to support the metal sheet at this point. Parts that are conical slightly, the dies should be reversed, end to end; the larger sections of the dies should be on opposite ends, to allow the material to flow over the center rail more uniformly. Satisfactory operation can be achieved by positioning the die carefully on the ram so that they contact the sheet simultaneously at both ends. When very severe stretch-forming is necessary to form the parts, the material may first be stretched partially, annealed, and finally, finished-stretched.
|
