|Straightening - Course: Technique of working sheet metals, pipes and sections. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 16 p.)|
|Hints on Labour Safety|
|1. Purpose of Straightening|
|2. Tools, Devices and Auxiliary Means|
|3. Processes within the Material|
|4. Techniques of Straightening|
According to the intensity and kind of deformation of metal sheets and pipes and the shape of sections, the most useful and effective techniques are applied leading to the restoration of the original form in cold as well as red-hot condition of the material.
4.1. Techniques of Eliminating Bendings and Bucklings
Long, slightly bent, thin sections and thin pipes can be straightened by hand in a vice. For this purpose, the materials are firmly clamped in the vice and drawn into the desired direction. The deformed spot must be placed directly beside the vice.
Figure 16 Straightening of long sections by hand in the vice
Bendings and bucklings at short sections and narrow strips of metal sheets are evened out by simple fastening of the vice jaws. For this purpose, the respective spot of the material is directly clamped into the jaws of the vice. However, the degree of elastic recovery must be equalized afterwards by hammering on a support
Figure 17 Straightening of short sections directly with the help of the vice
Bent buckled sections can be straightened under section punches of the hand screw press or hydraulic press.
Figure 18 Straightening of bent sections by the press using different sectional punches
Bent or buckled thin flat materials and metal sheets can be straightened by hammer blows in the vice or under the flat punch of the hand screw press.
Figure 19 Straightening of bent flat material by the press using the flat punch
Bent or buckled stronger flat materials are evenly worked by the face of the locksmith's hammer over the entire spot which is deformed. With soft materials, a wooden, rubber or light metal hammer is used.
Suitable backings are the anvil or the straightening plate.
Figure 20 Straightening of thicker flat material by the locksmith's hammer
Bent flat materials and thin angle sections can be straightened by hammer blows from one side by the peen of the locksmith's hammer or by the sweeping hammer. The blows are done evenly from the head end to the end on the compressed side of the material. Suitable backings are the anvil or the straightening plate.
Figure 21 Straightening of thin angle sections by the locksmith's hammer
Bent and buckled thicker angle sections can also be straightened by local heating up by the welding torch. The straight material is quickly heated up by the welding torch till it is red-hot, the neighbouring sports are cooled by pieces of cloth that are constantly kept wet, so that the heat is not abstracted. The heated material is compressed, because it cannot expand to the sides. When cooling down, it shrinks in the compressed spot - the bening disappears.
Figure 22 High-temperature straightening of strong angle sections with the welding torch
Bent and slightly buckled short pipe sections can be straightened by filling the pipe with fine-grained sand and closing it firmly on either side. Then, it is bent back in cold condition in the vice or straightened in warm condition after having been heated up by the welding torch, in the smith's hearth or furnace. The sand filling prevents a new deformation of the pipe section.
Figure 23 Straightening of pipes that had been filled with sand and firmly closed before
How can slightly bent thin sections be
How can smaller bucklings at sections and metal sheet strips be
What must be considered when straightening bent
4.2. Techniques for Eliminating Twistings
Twisted smaller flat materials can be straightened by means of tongs. They can also be placed between the jaws of the vice and straightened by closing the jaws. The hand screw press may be used as well.
Figure 24 Straightening of thin flat material by means of tongs
Larger twisted flat material are clamped in the vice and turned back into their normal form by means of the moving iron. The respective spot must be directly beside the vice, the moving iron is applied closely beside, too.
Figure 25 Straightening of distorted flat material by means of the moving iron
Twisted smaller square sections can be straightened with appropriate moving irons in the same manner. Larger square materials are straightened on the turning machine. Twisted angle and T-sections can be clamped in the vice and straightened by clamps.
Figure 26 Straightening of distorted square sections by means of the moving iron
4.3. Techniques of Eliminating Dents and Corrugations in Metal Sheets
Small dents in metal sheets are straightened by beating with the face of the hammer directly on the dent. Pay attention, that the face of the hammer is larger than the surface of the dent Smaller dents can also be straightened under the flat punch of a hand screw press.
Figure 27 Straightening of a large bulge in sheet metal by means of the locksmith's hammer
Larger dents in metal sheets are straightened by drawing blows struck with the face of the locksmith's hammer, the straightening hammer or with the mallet or light metal hammer from the rim of the dents outwards. Outwards the blows must be placed denser and denser.
Large dents can also be straightened by precise local heating of the metal sheet with the blowpipe.
Figure 28 Straightening of a large bulge in a piece of sheet metal by means of the straightening hammer
Corrugated metal sheets are straightened by hand by striking hammer blows from the rim of the material towards the centre. In doing so, the blows of the hammer must be placed more directly towards the centre in order to stretch the material more in this place.
A straightened metal sheet must lie evenly and must not buckle when slightly bent to and from.
Figure 29 Straightening of a corrugated metal sheet by means of the straightening hammer
Corrugated and warped metal sheets and sections can be straightened by means of the roller straightening machine.
Figure 30 Straightening by means of roller straightening machines
By hammering, the density of the material is increased causing an increased hardness and brittleness. If hammered workpieces shall be bent afterwards, it may come to fissures or fractures.
Do not use too many hammer blows.
Should many hammer blows be required nevertheless, the workpiece should be soft-annealed in between times.
How are large bulges straightened in metal
Why is it not recommendable to strike many hammer blows when