Have you ever considered the importance of straightening in sheet metal fabrication? This crucial process ensures the accuracy and quality of the final product. In this article, we’ll explore the various methods and techniques used to straighten sheet metal parts, drawing from the expertise of seasoned professionals in the field. By the end, you’ll gain a deeper understanding of how this often-overlooked step can make all the difference in your manufacturing process.
Steel components can experience various deformations due to external forces, thermal stresses, or inherent material inconsistencies. Raw materials such as steel plates and structural sections often exhibit deformations like unevenness, bending, twisting, and warping prior to the manufacturing process. These irregularities pose significant challenges in maintaining the precision and quality of subsequent operations, including marking, numbering, lofting, and the overall manufacturing and assembly of parts.
The presence of these deformations can lead to several critical issues:
To mitigate these challenges and ensure the highest quality in the final product, it is imperative to implement a straightening process before proceeding with marking, numbering, lofting, and forming operations. This crucial step involves carefully applying controlled force or heat treatment to restore the material to its intended geometry, thereby:
By addressing deformations early in the manufacturing process, engineers can significantly improve the efficiency, precision, and reliability of the entire production workflow.
Deformation caused by residual stress in steel
During the steel rolling process, residual stresses can develop, leading to material deformation. For example, if the roll adjustment mechanism malfunctions, the gap between rolls may become inconsistent while rolling a steel plate. This results in non-uniform extension of the steel along the rolling direction.
Areas subjected to smaller roll gaps experience greater extension, while those with larger gaps undergo less extension. Consequently, compressive stresses develop in regions of greater extension, and tensile stresses form in areas of lesser extension.
When the steel cools rapidly or due to other factors, these stresses remain trapped within the material, forming residual stresses. Upon subsequent operations such as shearing, thermal cutting, or other processes, these residual stresses are partially released, causing the steel to deform.
Deformation induced by steel processing
During steel processing, deformation can occur due to external forces or non-uniform heating. For instance, operations like shearing, flame cutting, or welding of steel plates can lead to deformation due to localized stress concentrations and thermal gradients during heating and cooling cycles.
Deformation resulting from improper transportation and storage
Raw materials used in cold working sheet metal, such as large steel plates and structural sections, are susceptible to deformation during handling. Improper lifting techniques, transportation methods, or storage practices can cause bending, twisting, and localized deformation due to the material’s self-weight and stress concentrations.
Mitigation strategies
To minimize deformation in steel components:
The straightening process in metal fabrication is based on the manipulation of internal stresses within the material structure. This principle can be understood by considering steel as a composite of multiple fiber layers arranged along its thickness.
In an ideally straight piece of steel, all fiber layers exhibit equal length. However, when the steel undergoes bending, these layers experience differential elongation. As illustrated in the figure, the initially equal lengths of “ab” and “cd” become unequal upon bending. The outer fibers (represented by “a/b”) elongate, while the inner fibers (represented by “c/d”) compress.
Straightening aims to restore uniformity in fiber lengths through two primary mechanisms:
The goal of both approaches is to equalize the length of all fiber layers, thereby eliminating residual stresses that cause bending, twisting, or uneven deformations in the steel or workpiece. This process not only improves geometric accuracy but also enhances the material’s mechanical properties and fatigue resistance.
Modern straightening techniques often employ a combination of mechanical and thermal methods, utilizing advanced sensing and control systems to achieve precise and uniform results across various steel grades and product geometries.
Fig.1
There are various methods for straightening steel, which can be classified into manual, mechanical, and flame straightening, depending on the source and type of external force applied. Each method has its specific applications, advantages, and limitations in metalworking processes.
Common tools for manual straightening – Hand hammer
How to use a hand hammer for steel straightening:
Operational Requirements:
Hammer
Hammer is an essential tool in cold working sheet metal, and it comes in many shapes and serves various purposes.
(1) Flat-head hammer:
It is mainly used to strike flat surfaces and also used to reach deeper recesses and edge corners, as shown in Figure 2-a.
(2) Stabbing hammer:
It is mainly used for directly striking arc-shaped components, but it can also function as a prying tool and shim, as shown in Figure 2-b.
(3) Arch hammer:
It is mostly used for shaping and producing arc-shaped workpieces, such as refurbishing or preparing end caps for small car shafts, as shown in Figure 2-c.
(4) Intermediate hammer:
The intermediate hammer is used to avoid direct hammering on the workpiece, as shown in Figure 2-d.
Fig.2 Hammer
(5) Flat Head Plastic Hammer:
It is mainly used for trimming box corners and other parts, as shown in Figure 2-e.
(6) Cross Peen Hammer:
It is mainly used to eliminate small pits on the surface of the workpiece, as shown in Figure 2-f.
(7) Other Types of Hammers:
Depending on the specific needs of the hammering process, the hammer head can be shaped in various forms, such as rubber, wooden, or copper hammers, as shown in Figure 2-g.
Fig.2 Hammer
The correct use of hand hammer is shown in Figure 3.
Fig.3 Correct use of hand hammer
Manual straightening is performed using tools such as a hammer, a flat plate, a drill anvil, or a bench vise. Common manual straightening methods include the extension method, twisting method, bending method, and stretching method.
Extension method
The extension method is mainly used when the middle of the sheet metal is convex and the edges are wavy or warped, as shown in Figure 4.
Figure 4 Stretching method to straightening thin metal plates
Twisting method
The twisting method is used to correct the distortion of the strips. In operation, the strips are held in a bench vise and twisted back to their original shape using a wrench, as shown in Figure 5.
Fig.5 Twisting straightening strip
Bending method
The bending method is used to straighten various bent bars and strips that bend in the width direction.
Stretching method
The stretching method is used to correct a variety of slender wires, which is shown in Figure 6.
Fig.6 Stretch straightening of straight materials
Straightening of the Bulging Surface:
Straightening of Edge Warp:
Figure 7
Straightening of Diagonal Warp
Step 1: Place the warped sheet on the platform and hold the sheet metal with your left hand and the hammer with your right hand.
Step 2: Begin tapping along the unwarped diagonal line, and then extend to both sides in order to stretch and correct the sheet, as shown in Figure 8.
Step 3: After the sheet has been mostly corrected, use a wooden hammer to make a final adjustment strike to ensure that the entire tissue is stretched evenly.
Patting and Pushing Straightening of Sheet Metal
As depicted in Figure 9, use a clapper (made of iron) to tap the sheet in order to shorten the protruding part and stretch the tensed part under pressure, achieving the goal of straightening.
Fig.8 Straightening of diagonal warpage
Fig.9 Patting and pushing straightening of sheet metal
Straightening of Deformed Convex Curvatures
As depicted in Figure 10, the hammer should be aligned with the center of the top iron to start with, and then the straightening process is carried out using the hammer.
Fig.10 Straightening of deformation of curved convex
The hand holding the hammer should not grip too tightly, instead relying on the wrist for force. The percussion speed should be around 100 strikes per minute.
Straightening of Concave Surface Flaring
As depicted in Figure 11, the jacking iron should be positioned slightly above the hammering point, which is the elevated portion of the uneven surface.
Fig.11 Straightening of concave surface flaring
This allows the plate to be subjected to a force between the head iron and the point of hammering.
Straightening of Large Concavity
As shown in Figure 12, first the middle part of the concavity is heated to a hot pink state using a blowtorch, and then the lower side of the middle part is lifted using a top iron, resetting the original concavity.
Next, the hammer and the top iron are used together to gradually even out the raised part, restoring the original geometric shape.
Figure12 Straightening of large concavity
Straightening of Large Curvature Surfaces
As depicted in Figure 13, when straightening parts with a large surface curvature (such as a high convex surface) like a fender, they can be first heated with a flame, then lifted with a top iron, and finally flattened with hammering to achieve the original shape.
Figure 13 Straightening of a large curvature surfaces
Straightening of Small Dents
① As shown in Figure 14, the tip of a pick hammer is used to flatten the depression from inside to outside.
Fig.14 Straightening of small dents
②As shown in Figure 15, a scabling rod is used to reach into the narrow space and pry the depression flat.
This method is generally used to pry the recesses of the doors, rear fenders and other enclosed body panels.
Fig.15 Use a crowbar to pry out the depression
③As shown in Figure 16, the depression is flattened with a depression puller.
Fig.16 Use a puller to flatten the depression
It is mainly used for enclosed body panels or wrinkles that are inaccessible from behind.
④ The pull rod is used to flatten the depression, as shown in Figure 17, the protrusion is lower by knocking and pulling, and the depression is raised.
Fig.17 Use a pull rod to flatten the depression
Straightening of Twisted Flat Steel
Step 1: Hold the flat steel on the bench vice.
Step 2: Hold the other end of the flat steel with a tenor wrench, force the flat steel twisted in the opposite direction of twisting, as shown in Figure 18.
Fig.18 Use a bench vise to correct distortion
Step 3: After the distortion is basically eliminated, peening is used to correct it.
Step 4∶ When punching, the flat steel inclined, the flat portion rests on the platform, and the twisted warped portion extends beyond the platform, as shown in Figure 19.
Fig.19
Step 5∶Use a hammer to strike the part that is slightly warped upwards outside the platform, the distance between the strike point and the platform is about twice the thickness of the sheet, and move the flat steel to the platform while striking.
Step 6: Turn 180° and repeat the same hammering until corrected.
Deformation and straightening of angle steel (Figure 20)
Fig.20 Deformation of angle steel
Step 1: Put the outer bend angle steel and the inner bend angle steel on the cylindrical iron knot or the platform with holes.
Step 2: Bend the angle to the outside, punch the edges of the two right-angle sides, punch in from the edge, as shown in Figure 21(a).
For the inner bend angle, it needs to beat the roots of the two right-angle sides, as shown in Figure 21(b).
Fig.21 Straightening of angle steel
Step 3: Clamp one end of the twisted angle to a bench vise.
Step 4: Hold the right-angle side of the other end of the angle steel with a dead wrench, and force the angle steel to twist in the opposite direction and slightly exceed the normal state of the angle steel, as shown in Figure 22.
Fig.22 Straightening of angle steel distortion
Step 5: Repeat several times to basically eliminate the distortion of the angle steel.
Straightening of Round Steel Deformation
As shown in Figure 23, the round steel is mostly bent and deformed, and the straightening only needs to place the round steel on the platform so that the protrusions are upward.
Fig.23 Straightening of round steel deformation
Use a suitable intermediate hammer to place the round steel protrusions, and then hit the top of the intermediate hammer to correct.
Straightening of the Rectangular Frame
straightening method: A rectangular welded part is shown in Figure 24,
Fig.24 Straightening of rectangular frame
When the frame AD and BC sides both exhibit bending, the frame can be placed on the platform with the outer flange AD facing upward. The two ends of the BC side are cushioned, and the raised point E is beaten. If all four sides are slightly curved, the frame can be punched outward or inward, respectively.
For small size errors, the frame can be placed and the end of the longer side beaten to shorten the total length.
If angles B and D are less than 90 degrees, the method shown in Figure 25 can be used to hammer at point B to expand it.
Fig.25
Manual straightening is time-consuming and labor-intensive, making it suitable only for small components. For larger workpieces, specialized machinery is utilized for straightening.
Mechanical straightening is performed using a straightening machine that repeatedly bends the steel plate multiple times, causing the unequal fibers of the steel plate to tend towards equality and ultimately achieving the goal of straightening.
(1) Mechanical straightening of sheet metal parts
Operation Requirements:
Operation Steps:
(1) Mechanical leveling of metal sheets:
Leveling Method: As shown in Figure 26, adjust the gap between the rollers to match the plate thickness.
Note that the quality of the straightening depends on the accuracy of the rollers.
Figure 26 Mechanical leveling of sheet metal
(2) Rolling of Pre-Formed Workpieces
Rolling Method: As shown in Figure 27, first replace the rollers below the workpiece with rollers that have a slightly smaller curvature than the rollers above the workpiece.
Next, lift the bottom roller using a quick-release device and place the workpiece between the rollers. Adjust the pressure of the bottom roller so that the workpiece can slide between the rollers under moderate pressure.
Figure 27 Rolling pre-formed parts
Notes: Ensure the workpiece is rolled fully to prevent localized elongation. Utilize a template to continuously monitor the curvature of the workpiece. After rolling the sheet metal parts in one direction, the workpiece must be rotated 90 degrees. Upon repeating this process, the rolling lines will intersect the original direction, as depicted in Figure 28.
Fig.28
Rolling Method for Wave Wrinkles of Rolled Flat Sheet Metal:
As illustrated in Figure 29, the movement direction of the metal plate during rolling should be diagonal to its original movement direction. Maintain a consistent pressure and move steadily to avoid the formation of new ripples.
Fig.29
(4) Forming Method for Large Sheet Metal
Forming Method: As shown in Figure 30, two people are required to hold the workpiece while rolling large sheet metal parts, according to the requirements of the workpiece. Then, the workpiece should be moved forward and backward on the rolling machine, as described previously.
Figure 30 Forming method of large sheet metal parts
Flame straightening is a method of correcting deformation in steel through local flame heating. The process takes advantage of the property of metal materials to expand and contract with changes in temperature. By heating a specific area with a flame, the new deformation can be used to correct the original deformation.
1. Heating position, flame energy rate and straightening
The effectiveness of flame straightening mainly depends on the location of the heating and the energy rate of the flame. Different heating positions can correct the deformations in different directions. However, if the location is chosen incorrectly, it may not only fail to correct the deformation, but also make it more complex and severe.
2. Heating method
(1) Spot heating: The heated area is circular in shape with a certain diameter range, hence it is referred to as spot heating, as shown in Figure 31a.
(2) Linear heating: The heated area is linear within a specific range, and is therefore called linear heating, as illustrated in Figure 31b.
(3) Triangular heating: The heating method where the heated area is triangular is called triangular heating, as depicted in Figure 31c.
Fig.31 Heating method
3. Flame straightening operation
Flame Straightening of Central Convex Workpiece:
Step 1: Place the sheet metal on the platform and secure it with clips around the perimeter.
Step 2: Heat the convex area using spot heating, as shown in Figure 32(a). Alternatively, linear heating can also be used, as illustrated in Figure 32(b).
Step 3: Once straightened, use a hammer to tap the clips horizontally to release them and remove the sheet metal.
Fig.32 Flame straightening of the central convex workpiece
Flame Straightening of Edge Wavy Workpieces:
Step 1: Secure the sheet metal to the platform on three sides using clips, leaving the side with the concentrated wavy deformation unclamped, as shown in Figure 33.
Step 2: Heat the board in a linear manner, starting from the flat area on both sides of the convexity, and gradually working your way up to the convexity, as indicated by the arrows in Figure 33.
Explanation:
The length of the heating line should be 1/3 to 1/2 the width of the board, and the distance between the heating lines should be adjusted based on the height of the bulge. Higher bulges should have a closer distance, typically 20 to 50 mm.
If the first heating results in unevenness, repeat the straightening process with a second heating, staggering the position of the heating line from the first one.
Figure 33 Flame straightening of edge wavy workpieces
As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.
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