Straightening (manufacturing process)

Straightening can be classified in main group two - forming - in addition to the manufacturing processes according to DIN 8580 .

Straightening is comparable to bending . The difference is that a workpiece is reworked in a separate straightening process after a bending process to improve dimensional accuracy. While bending describes a rather rough shaping process, straightening focuses on shaping the workpiece precisely. Often steel parts that have been subjected to heat treatment (e.g. hardening , annealing ) also have to be straightened.

Presses and bending devices that are electromechanically driven, possibly also hydraulically, are often used as aids for straightening. More modern straightening benches run a fully automatic, computer-controlled and monitored program in order to process workpieces down to the tightest tolerances.

Straightening procedure

conditions

During straightening, the geometry of a workpiece is deformed in such a way that the specified tolerances are adhered to. This is done either without measuring the start / end geometry (shape deviation and material properties are always approximately the same before the straightening process, so that a constant result is always achieved with a fixed deformation) or the geometries are recorded before, during and after Straightening is recorded - the deformation processes are adapted to the measured deviations by an (NC) control.

Round parts

When straightening round parts, there are roll straightening and bending straightening processes. Sensors (e.g. digital measuring devices) are used to measure the deviation. With rotary straightening, they only have to record the deviations that occur during a rotation. One setting is reduced to checking that it is always in contact with the workpiece during the entire measurement.

The roll-straightening process is often used for workpieces that are still at the beginning of the creation process (e.g. workpiece blanks after the forging process).

The bending-straightening procedure requires a measurement of the concentricity deviations so that they can be brought within their tolerance by bending.

Roll straightening

With roll straightening, the workpiece is set in rotation by a motor via a force-fit connection to a workpiece holder. During this rotary movement, the workpiece is strongly deformed (beyond the elastic limit) and the deformation is gradually (or continuously) reduced during the rotary movement. This straightening process mostly affects the entire workpiece (usually one point in the middle).

Bending straightening

When bending straightening, the workpiece geometry must have been measured prior to the actual straightening process in order to be able to correctly position the deviation to be straightened. This is done fully automatically on such machines (measurement of the deviation, positioning of the workpiece, performing the bending stroke). Then an NC axis performs a bending stroke (as in roll straightening, the workpiece is pressed against two abutments by a punch). This bending stroke is adapted to the measured deviation by the NC control. The machine is capable of learning and can adapt the deformation to the values measured after a bending stroke.

Molding straightening

When straightening molded parts, a workpiece that is not rotationally symmetrical is straightened within its tolerances (e.g. cast aluminum parts). This requires that the measuring devices are calibrated to their target value (a master workpiece is inserted, the measuring devices are set up and these are compared to the known deviations). The deformation can then be brought about by bending.

Straightening by means of high-frequency hammering

When straightening using a high-frequency hammer, plastic deformations and residual compressive stresses are introduced into defined areas of the workpiece or assembly. Weldments are z. B. often directed in the area of the welds by an aftertreatment. Depending on the construction and the choice of post-treatment location, excellent dimensional accuracy can be achieved during straightening. In addition, in contrast to other straightening processes, the service life of cyclically loaded constructions can in many cases be increased.

Straightening pipes

Straightness deviations in pipes

In the various pipe manufacturing processes, the pipes are generally so imperfect that a straightening process is switched on. While in the past straightened tubes had to be straight by eye, the straightness was therefore left to the subjective perception of the machine personnel, today more and more precise quantitative straightness requirements are being made that can only be achieved with modern straightening machines.

So z. For example, it is required that a pipe over a reference length of 1 meter must not have a straightness deviation greater than 0.2 mm. A ruler 1 m long placed on the pipe jacket must not be more than 0.2 mm away from the pipe jacket. With longer pipe lengths, proportionally larger deviations occur due to a linear link between the straightness deviation and the reference length.

Types of pipe straightening machines

The most important machine group for tubes should be the continuously operating helical roll straightening machines. The long contact length of each roller due to its hyperbolic shape leads to the introduction of the bending forces as a line load (not as a point load), which has a positive effect on the surface. First, the inclined roller straightening machines can be divided into two categories, depending on whether the material to be straightened rotates or not during the straightening process. If the straightness deviation before the straightening is very large, a solution with only axially displaced material to be straightened has great advantages. The rotary straightening machine and the wing straightening machine should be mentioned here. Very "crooked" material to be straightened could quickly become damaged and unusable by rotation when it is introduced into the machine. With all other tube straightening machines, the straightening material rotates with it during the straightening process. The number of straightening rollers used is adjusted depending on the area of application:

Two-roll straightening machines for bare and scaled thick-walled tubes (and bars)
Six-roll straightening machines for tubes made of steel and non-ferrous metal with high surface quality (precision tube)
Seven-roll straightening machines especially for scaled pipes with medium wall thicknesses
Nine-roll straightening machines as high-performance straightening machines (300 m / min) also for scaled pipes with medium wall thicknesses
Ten-roll straightening machines (CNC-controlled)

Straightening principle of the inclined roller straightening machine for pipes

Two-roll straightening machine In this machine with a vertical roll arrangement, the top roll is hyperbolic and the bottom roll is usually cylindrical. The rollers are set at an angle of 14 ° to 21 ° with respect to the material to be straightened axis, so that the object to be straightened is forced to rotate and feed. The tube is bent plastically by the lower roller into the concave contour of the upper roller. A plastic oval deformation of the pipe cross section can be superimposed on this plastic longitudinal bending. This improves the straightness, especially at the pipe ends. The straightening gap is limited laterally by guide rulers.

Straightening rolled sheets

Sheets rolled in rolling mills are straightened after rolling. A distinction is made here between hot straightening and cold straightening , depending on the temperature of the sheet . To do this, the sheet is passed through a group of upper and lower rollers so that it runs through a kind of serpentine line and is bent in both directions. The deflection is set so that a straight sheet metal reaches the yield point in both bending directions, but does not exceed it. Uneven sheet metal sections exceed the yield point and are plastically (permanently) straightened, while straight sections retain their desired shape.

Web links

Lecture Production Technology ( PDF ( Memento from September 27, 2011 in the Internet Archive ))
Metal processing ( PDF )
Manufacturing engineering manual [1]