Straightening Process For Sheet Metal Parts: The Ultimate Guide

The need for straightening

Due to the influence of factors such as external force or heating, steel will undergo various deformations.

Raw materials such as steel plates and section steels are deformed such as unevenness, bend, twist, wave, etc. before manufacturing.

It is difficult to guarantee the quality of marking, numbering and lofting, as well as parts manufacturing and assembly.

Therefore, before marking, numbering, lofting and forming, the raw materials with excessive deformation must be corrected.

Reasons for deformation of raw materials

Deformation caused by residual stress of steel

During the rolling process of steel, residual stress may be generated and the steel may be deformed.

For example, when rolling a steel plate, due to the failure of the roll adjustment mechanism, the gap between the rolls is inconsistent, and the extension of the steel along the rolling direction is inconsistent.

The part with a small gap has a large extension of the steel; the part with a large gap has a small extension.

Therefore, the larger extension part is hindered by the smaller extension part to generate compressive stress, while the smaller extension part generates tensile stress.

When the steel is cooled faster or due to other reasons, this part of the stress remains in the steel and form the residual stress.

When the steel is subjected to shearing, thermal cutting or other factors, the residual stress is partially released and the steel is deformed.

Deformation caused by steel processing

In the process of steel processing, due to external force or uneven heating, steel may be deformed.

For example, shearing, gas cutting or welding of steel plates will cause deformation of steel due to stress, uneven heating and cooling.

Deformation of steel due to improper transportation and storage

The raw materials used in cold worksheet metal are long and larger steel plates and section steels. If the lifting, transportation and storage are improper, the steel will be bent, twisted and locally deformed due to its own weight.

In summary

There are many reasons for the deformation of steel. If the deformation of the steel exceeds the allowable deviation, it must be straightened.

Straightening principle

Assuming that the steel is composed of multiple layers of fibers in the thickness direction, when the steel is straight, the fiber length of each layer is equal; when the steel is bent, the fiber length of each layer is not equal.

As shown in the figure, when the steel is straight, the lengths of ab and cd are equal; when the steel is bent, the length of c/d/ is shorter, and the length of a/b/ is longer.

Straightening is to use external force or heating to extend the shorter fiber of the steel or shorten the longer fiber. Finally, the fiber length of each part tends to be the same, thereby eliminating the bending, twisting, uneven deformation of the steel or the workpiece.

Straightening principle

Fig.1

Straightening methods

There are many straightening methods, which can be divided into manual straightening, mechanical straightening and flame straightening according to the source and nature of the external force.

Common tools for manual straightening  hand hammer

How to use hand hammer:

Operation requirements:

(1) Master the use of hand hammers through training;

(2) Wipe off the oil stains on the hammer surface and handle before use to avoid slipping off and hurting people;

(3) Check whether the handle is loose to avoid accidents caused by the hammer head coming out.

Hammer

Hammer is the basic tool in cold-worked sheet metal work. It has many shapes and different functions.

(1) Flat-head hammer:

It is mainly used to strike flat surfaces, but can also strike deeper recesses and edge corners, as shown in Figure 2-a.

(2) Stabbing hammer:

It is mainly used to directly stab arc-shaped components, and it can also be used as a prying tool and shim, as shown in Figure 2-b.

(3) Arch hammer:

It is mainly used for the shaping and production of arc-shaped workpieces, such as refurbishing or preparing shaft end caps of small cars, as shown in Figure 2-c.

(4) Intermediate hammer:

Use an intermediate hammer in order to avoid direct hammering of the workpiece, as shown in Figure 2-d.

Hammer

Fig.2 Hammer

(5) Flat head plastic hammer:

It is mainly used to trim box corners and other parts, as shown in Figure 2-e.

(6) Crane hammer:

It is mainly used to eliminate small pits on the surface of the workpiece, as shown in Figure 2-f.

(7) Other forms of hammer

According to the need of hammering, the hammer head can be made into various shapes, such as rubber hammer, wooden hammer, copper hammer, etc., as shown in Figure 2-g.

Hammer

Fig.2 Hammer

The correct use of hand hammer is shown in Figure 3.

Correct use of hand hammer

Fig.3 Correct use of hand hammer

 

1. Manual straightening process

Manual straightening is done with a tool such as a hammer on a flat plate, drill anvil or 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 is convex and the edges are wavy or warped, as shown in Figure 4.

Stretching method to straightening thin metal plates

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 to their original shape with a wrench, as shown in Figure 5.

Twisting straightening strip

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.

Stretch straightening of straight materials

Fig.6 Stretch straightening of straight materials

(1) Straightening of the bulging surface

Step 1: The plate convex face of sheet metal should be placed on the platform, the sheet metal can be held with the left hand and the hammer can be held with the right hand.

Step 2∶Strike should start from around the edge of the sheet and gradually close to the center of the convex drum surface, as shown in Figure 4.

Step 3∶After the sheet is basically corrected, then a wooden hammer should be used for an adjusting strike to make the whole organization spread evenly.

(2) Straightening of edge warpage

Step 1: A wavy sheet should be placed on the platform, the sheet metal can be held with the left hand and the hammer can be held with the right hand.

Step 2: The percussion starts from the middle of the sheet and gradually spreads around, as shown in Figure 7.

Step 3: After the sheet is basically corrected, a wooden hammer is used to perform an adjustment strike to make the entire group stretch evenly.

Straightening of edge warpage

Figure 7

(3) Straightening of diagonal warpage

Step 1: The warped sheet should be placed on the platform, the sheet metal can be held with the left hand and the hammer can be held with the right hand.

Step 2: Tapping starts along the diagonal line without warpage, and then it stretches to both sides in order to make it stretch and correct, as shown in Figure 8.

Step 3: After the sheet is basically corrected, a wooden hammer is used to perform an adjustment strike to make the entire tissue stretch evenly.

(4) Patting and pushing straightening of sheet metal

As shown in Figure 9, using a clapper (iron) to slap on the sheet can shorten the protruding part of the sheet, while the tension part is stretched under pressure to achieve the purpose of straightening.

Straightening of diagonal warpage

Fig.8 Straightening of diagonal warpage

Patting and pushing straightening of sheet metal

Fig.9 Patting and pushing straightening of sheet metal

(5) Straightening of deformation of curved convex

As shown in Figure 10, the hammer should align with the center of the top iron at first, then the hammer dressing is performed.

Straightening of deformation of curved convex

Fig.10 Straightening of deformation of curved convex

The hand holding the hammer should not be too tight, then it needs to strike with the force of wrist.

The percussion speed is about 100 times/min.

(6) Straightening of concave surface flaring

As shown in Figure 11, the jacking iron should be placed slightly above the point of hammering, which is the higher part of the uneven surface.

Straightening of concave surface flaring

Fig.11 Straightening of concave surface flaring

This allows the plate to be subjected to a force between the head iron and the hammer point.

(7) Straightening of large concavity

As shown in Figure 12.

Firstly, the middle part of the concavity can be heated by a blowtorch to a hot pink state, and then the lower side of the middle part can be jacked up by a top iron, so that the original concavity can be initially reset.

Then the hammer and the top iron are used together to gradually flatten the part that becomes taller all around to restore the original geometric shape.

Straightening of large concavity

Figure12 Straightening of large concavity

(8) Straightening of a large curvature surfaces

As shown in Figure 13, when trimming parts with large surface curvature (high convex surface) such as fender etc., it can be heated by flame at first, then jacked up by top iron, and finally hammered and flattened to reach the original shape.

Straightening of a large curvature surfaces

Figure 13 Straightening of a large curvature surfaces

(9) 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.

Straightening of small dents

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.

Use a crowbar to pry out the depression

Fig.15 Use a crowbar to pry out the depression

③As shown in Figure 16, the depression is flattened with a depression puller.

Use a puller to flatten the depression

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.

Use a pull rod to flatten the depression

Fig.17 Use a pull rod to flatten the depression

(10) 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.

Use a bench vise to correct distortion

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.

the twisted warped portion extends beyond the platform

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.

(11) Deformation and straightening of angle steel (Figure 20)

Deformation of angle steel

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).

Straightening of angle steel

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.

Straightening of angle steel distortion

Fig.22 Straightening of angle steel distortion

Step 5: Repeat several times to basically eliminate the distortion of the angle steel.

(12) 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.

Straightening of round steel deformation

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.

(13) Straightening of the rectangular frame

straightening method: A rectangular welded part is shown in Figure 24,

Straightening of rectangular frame

Fig.24 Straightening of rectangular frame

When the frame AD and BC side of the bilateral bending phenomenon, the frame can be erected on the platform.

The outer flange AD is facing upwards, the two ends of the BC side are cushioned and the raised point E is beaten.

If all four sides are slightly curved, it can punch the bump outward or inward respectively.

When the size error is not too large, it needs to erect the frame and beat the end of the longer side to shorten the total length.

If ∠B and ∠D are less than 90°, the method shown in Figure 25 can be used to hammer point B to expand it.

Straightening of rectangular frame

Fig.25

2. Mechanical straightening process

Manual straightening is inefficient and labor-intensive, so it is only suitable for small parts.

For larger size workpieces, special machinery is used for straightening.

Mechanical straightening is through the straightening machine for many times repeated bending of the steel plate, so that the length of the steel plate of unequal fibers tend to be equal, thereby achieving the purpose of straightening.

(1) Mechanical straightening of sheet metal parts

Operation requirements

(1) Use the roller leveler correctly;

(2) Level the deformed sheet metal parts as required;

(3) Do not put your hands around the roller during operation.

Operation steps

(1) Mechanical leveling of metal sheets:

Leveling method: As shown in Figure 26, the gap between the rollers is adjusted according to the plate thickness.

The quality of the straightening depends on the accuracy of the roller.

Mechanical leveling of sheet metal

Figure 26 Mechanical leveling of sheet metal

(2) Rolling of preformed workpieces

Rolling method: As shown in Fig. 27, the rollers below the workpiece are first replaced by rollers with a slightly smaller curvature than the rollers above the workpiece.

The bottom roller is then lifted by a quick-release device, while the workpiece is placed between the rollers, and the pressure of the bottom roller is adjusted so that the workpiece can slide between the rollers under moderate pressure.

Rolling pre-formed parts

Figure 27 Rolling pre-formed parts

Note: Fully rolled to avoid local extension.

Use the template to check the curvature of the workpiece at any time.

After rolling the sheet metal parts in one direction in turn,  the workpiece needs to be turned to 90°.

After repeating the above operation, the rolling line will cross the original direction, as shown in Figure 28.

rolling line will cross the original direction

Fig.28

(3) Wave wrinkles of rolled flat sheet metal

Rolling method: As shown in Figure 29, the moving direction of the metal plate during rolling is diagonal to the original moving direction, the pressure is kept uniform, and it moves steadily to avoid ripples again.

Wave wrinkles of rolled flat sheet metal

Fig.29

(4) Forming method for large sheet metal

Forming method: As shown in Fig. 30, two people are required to hold the workpiece during the rolling of large sheet metal parts according to the requirements of the workpiece, and then it is moved forward and backward on the rolling machine as described above.

Forming method of large sheet metal parts

Figure 30 Forming method of large sheet metal parts

3. Flame straightening process

Flame straightening is the straightening of deformed steel using the method of local flame heating.

Metal materials have the property of thermal expansion and contraction.

The flame straightening uses this new deformation to correct the original deformation.

1. Heating position, flame energy rate and straightening

The effect of flame straightening depends mainly on the heating position and the energy rate of the flame.

Different heating positions can correct the deformation in different directions.

If the location is chosen incorrectly, not only will it not be able to correct the deformation, but it will make the deformation more complicated and serious.

2. Heating method

(1) Spot heating: The heated area is a circle-shaped point with a certain diameter range, so it is called spot heating, as shown in Figure 31a.

(2) Linear heating: The heated area is linear in a certain range, so it is called linear heating, as shown in Figure 31b.

(3) Triangular heating: The heating method in which the heating area is triangular is called triangular heating, as shown in Figure 3-30c.

Heating method

Fig.31 Heating method

3. Flame straightening operation

  1. Flame straightening of the central convex workpiece

Step 1: The sheet metal is placed on the platform and pressed around the board with clips.

Step 2: The convex can be heated by point heating, as shown in Figure 32(a).

Explanation: Linear heating can also be used, as shown in Fig. 32 (b).

Step 3 : After leveling, the clips are tapped with a hammer in a horizontal direction to release the clips and take out the sheet metal.

Flame straightening of the central convex workpiece

Fig.32 Flame straightening of the central convex workpiece

  1. Flame straightening of edge wavy workpieces

Step 1: Use the clips to press the sheet on the platform on three sides, and do not clamp the side where the wavy deformation is concentrated, as shown in Figure 33.

Step 2∶Heat the board in a linear manner from the flat place on both sides of the convexity, and then round up to the convexity, the order of heating as shown by the arrows in Figure 33.

Explanation:

The length of the heating line is usually 1/3 to 1/2 of the board width, and the distance of the heating line depends on the height of the bulge; the higher the bulge is, the closer the distance should be, which is usually 20 to 50 mm.

If there is unevenness after the first heating, the second heating straightening can be repeated, but the position of the heating line should be staggered with the first one.

Flame straightening of edge wavy workpieces

Figure 33 Flame straightening of edge wavy workpieces

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