ASM Metals HandBook Vol. 14 - Forming and Forging
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Fig. 3
Principle of straightening by bending. (a) Manual straightening with a grooved block. (b) Straightening in
a press. (c) Simplest form of rotary straightening. (d) Two-roll straightening. (e) Five-roll straightening. (f
1
and
f
2
) Two arrangements of rolls for six-roll straightening. (g) Seven-
roll straightening. (h) Wire straightening. In
all methods shown, the bar is supported at points A and B, and force at C on the convex side causes
straightening. See text and subsequent illustrations for details of the straightening methods shown here.
Shafts for centrifugal irrigation pumps are an example of parts that usually must be manually straightened because of the
accuracy required and the necessity of doing the work at the installation site. Most of these shafts are 3.0 to 6.1 m (10 to
20 ft) long, have diameters of 19 to 50 mm ( to 2 in.), and are of cold-drawn 1045 steel. The steel supplier generally
straightens the cold-drawn stock within 0.13 or 0.25 mm (0.005 or 0.010 in.) TIR in 3.0 m (10 ft), but the shafts are often
bent slightly in transport and in handling. It is common to hand straighten the shafts at installation--within 0.13 mm
(0.005 in.) TIR in 6.1 m (20 ft). The shafts are rotated on supports and are deflected with a lever.
Special cold-drawn sections as long as 3.7 m (12 ft) are commonly straightened manually. Many special sections are
similar enough to standard flats that they can be straightened in standard two-direction roll straighteners, but quantities
are often too small to warrant the cost of special rolls. Other special sections may be too complex in shape for machine
straightening.
Special sections almost always have twist after cold drawing. The twist must be removed before the section can be
straightened. One end of the section is held in a vise or in a special fixture, while the other end is twisted with a wrench or
special handle. When the twist is corrected, the bar can be straightened to remove camber and bow.
Manual Peening. Straightening of a workpiece by peening (material displacement) is accomplished manually by
placing the workpiece on heavy, flat plates and rapidly hammering on the concave side of the distorted portion of the
workpiece surface. Elimination of the distortion allows the workpiece to lie flat. This method is applicable when
straightening round parts with a high degree of hardness, such as drill bit blanks. Unfortunately, the technique is time
consuming and does require highly skilled operators.
High-Production Peening. In peen straightening, also known as pulse straightening, the workpiece is pulsed on the
low side opposite the high point of the bend (Fig. 4). The pulsating tooling compresses the workpiece material and
burnishes its surface. To counteract existing stresses in the workpiece, the material expands to straighten the part. The
metal structure is stabilized, the part retains its straight configuration in storage, and the material is unaffected by the
shock and vibration encountered during subsequent machining operations. This method is gaining wide acceptance in
high-volume production operations in which cast and forged steel camshafts, hardened steel transmission shafts,
crankshafts, and disk plates require straightening.
Stretch Straightening. Many bars and shapes can be
easily straightened by stretching. However, this
technique is usually confined to the straightening of
shapes that are uniform in cross section and length. The
advantages of stretch straightening include low costs for
tooling and for maintenance, simplicity of operation, and
(usually) completion of straightening in one operation.
The disadvantages include waste by trimming 152 to 467
mm (6 to 18 in.) from the ends of bars damaged by
gripping, the need (usually) for two men to do the
straightening, the need for cutoff equipment, and
production of only 30 to 40 bars per hour.
A stretch straightener has two heads with grips that clamp on the ends of the bar. One head can be adjusted to suit the
workpiece length. The other head (tailstock) is powered for stretching and for rotation to correct twist in the workpiece.
Stretching machines are made in sizes to exert stretching forces of 135 to 4450 kN (15 to 500 tonf) to workpieces that
may be from 6 to 30 m (20 to 100 ft) long. Stretching must stress the work beyond its yield strength. For complete
straightening, the bar should be stretched 2%. To accomplish this and to overcome the greater strength caused by work
hardening, the stretching machine needs a capacity of 10 to 15% beyond the yield strength of the bar.
Some straightening can be done by stretching only to the yield strength of the work, but this would not be sufficient to
remove completely some sharp bends and twists. A stretcher with a capacity of 1340 kN (150 tonf) can straighten, to
some extent, a low-carbon steel bar with a cross section of 9700 mm
2
(15 in.
2
), 6450 mm
2
(10 in.
2
), of austenitic stainless
steel, or 4550 mm
2
(7 in.
2
) of ferritic stainless and can do complete straightening on 8400 mm
2
(13 in.
2
) of low-carbon
steel, 5150 mm
2
(8 in.
2
) of austenitic stainless, or 3900 mm
2
(6 in.
2
) of ferritic stainless steel. Figure 5 shows the
relationship between stretcher force and the cross-sectional area of the bar. The deviation that remains after stretch
straightening can sometimes be corrected by manual straightening.
Fig. 4
Schematic of principles involved in straightening
by automatic peening.
Fig. 5 Relationship between stretch
er force and area of bar cross section in the stretch straightening of steel
bars of various yield strengths.
Most hot-rolled bars can be straightened by stretching. Straightening by stretching also works well on rolled and extruded
bars of aluminum and austenitic and ferritic stainless steels, but not on martensitic stainless steels unless they are first
annealed. Low-carbon steels are easy to stretch straighten, but annealing before stretching becomes more necessary as the
carbon content increases. Because it is slow and limited in effectiveness, stretch straightening is not widely used.
Straightening of Bars, Shapes, and Long Parts
Straightening by Heating
Alloy steel bars and shapes with a hardness exceeding 50 HRC, as well as fabricated stainless steel parts, frequently warp
because of the stress set up during fabrication, machining, or heat treatment. These items can usually be straightened by
the application of heat and, in most cases, force. The heat can be localized in the area to be straightened, or the entire
piece can be heated--either to the tempering temperature or to about 30 °C (50 °F) below it. (Heating to a temperature
above that required for tempering will reduce the hardness, as will prolonged heating at the tempering temperature itself.)
Low-carbon steel bars can also be straightened by heating.
Localized Heating. Torches are used to apply heat to the convex side of warped parts. A small area is heated to a dull
red. The localized heating causes the workpiece to expand, but some straightening occurs during cooling. Skillful heating,
cooling, and gaging of the workpiece can result in reasonable straightness.
Torch heating causes soft spots in hardened steel workpieces. Localized heating with a torch can also cause localized
residual tensile stress that can be undesirable even in an unhardened workpiece if it is subjected to cyclic loading.
In press straightening with the use of localized heat, the workpiece is supported at each end with suitable blocks. A stop
block is placed directly under the ram to limit the amount of deflection. With the high points of its curvature up, the
workpiece is pressed down until it rests lightly on the stop block; heat is then applied. For a heat-treated workpiece, the
amount of heat is usually governed by the original tempering temperature, and the distance the workpiece can be
deflected and released without fracture depends on the type and hardness of the steel, the heat treatment, and the shape of
the workpiece. Another method of controlling deflection without breakage is to place the workpiece on shims while it
rests on a flat surface, apply pressure to the surface of the workpiece, then heat and release. If the workpiece is still not
straight, it will be necessary to use more shims and reheat or to allow the workpiece to cool longer before releasing the
pressure.
Where a flame cannot be effectively directed or may damage the metal, a small weld bead can sometimes be used as the
source of heat. Weld beads are applied to the convex area, allowed to cool, and machined off if necessary.
Heating Below Tempering Temperature. Heating and press straightening are generally not applicable to steel at
high hardness levels. The force required to cause permanent set is close to the rupture strength of the steel, and even with
extreme care, failure is probable. At medium and lower hardness levels, heating to a temperature about 30 °C (50 °F)
below the tempering temperature will permit press straightening to be done successfully. Straightening becomes more
difficult as the part cools, and only slight corrective straightening should be attempted at the lower temperature levels.
Considerable skill is required to perform such operations and to hold tolerances within 0.08 to 0.25 mm (0.003 to 0.010
in.) over a length of 0.45 to 1.22 m (18 to 48 in.).
After the workpiece has been straightened, it is tempered to the required hardness. Tempering relieves stress set up during
straightening and during the hardening cycle. This stress will often deform the workpiece; consequently, workpieces
straightened by heating and pressing should be clamped in restraining fixtures during tempering. Fixturing can correct a
slight distortion and prevent distortion during tempering.
Temper straightening is used to correct the distortion caused by heat treatment. The workpiece is first tempered to a
hardness somewhat higher than required, then clamped in a straightening fixture and tempered to the required hardness.
The greater the hardness difference between the first and the corrective tempering operations, the more accurate the
dimensions will be. Temper straightening is most successful at hardness levels of 55 HRC and lower.
Deep-hardening alloy and tool steels that are being martempered to minimize distortion should be held straight during the
cooling period after austenitizing and until the completion of martempering. If straightness is not maintained throughout
martempering, the workpiece will warp as martensite continues to form. Straightening should be done below 480 °C (900
°F). Cold bars or chills contacting the high side will more rapidly extract the heat from the workpiece and aid in
straightening.
Straightening of Bars, Shapes, and Long Parts
Straightening by Heating
Alloy steel bars and shapes with a hardness exceeding 50 HRC, as well as fabricated stainless steel parts, frequently warp
because of the stress set up during fabrication, machining, or heat treatment. These items can usually be straightened by
the application of heat and, in most cases, force. The heat can be localized in the area to be straightened, or the entire
piece can be heated--either to the tempering temperature or to about 30 °C (50 °F) below it. (Heating to a temperature
above that required for tempering will reduce the hardness, as will prolonged heating at the tempering temperature itself.)
Low-carbon steel bars can also be straightened by heating.
Localized Heating. Torches are used to apply heat to the convex side of warped parts. A small area is heated to a dull
red. The localized heating causes the workpiece to expand, but some straightening occurs during cooling. Skillful heating,
cooling, and gaging of the workpiece can result in reasonable straightness.
Torch heating causes soft spots in hardened steel workpieces. Localized heating with a torch can also cause localized
residual tensile stress that can be undesirable even in an unhardened workpiece if it is subjected to cyclic loading.
In press straightening with the use of localized heat, the workpiece is supported at each end with suitable blocks. A stop
block is placed directly under the ram to limit the amount of deflection. With the high points of its curvature up, the
workpiece is pressed down until it rests lightly on the stop block; heat is then applied. For a heat-treated workpiece, the
amount of heat is usually governed by the original tempering temperature, and the distance the workpiece can be
deflected and released without fracture depends on the type and hardness of the steel, the heat treatment, and the shape of
the workpiece. Another method of controlling deflection without breakage is to place the workpiece on shims while it
rests on a flat surface, apply pressure to the surface of the workpiece, then heat and release. If the workpiece is still not
straight, it will be necessary to use more shims and reheat or to allow the workpiece to cool longer before releasing the
pressure.
Where a flame cannot be effectively directed or may damage the metal, a small weld bead can sometimes be used as the
source of heat. Weld beads are applied to the convex area, allowed to cool, and machined off if necessary.
Heating Below Tempering Temperature. Heating and press straightening are generally not applicable to steel at
high hardness levels. The force required to cause permanent set is close to the rupture strength of the steel, and even with
extreme care, failure is probable. At medium and lower hardness levels, heating to a temperature about 30 °C (50 °F)
below the tempering temperature will permit press straightening to be done successfully. Straightening becomes more
difficult as the part cools, and only slight corrective straightening should be attempted at the lower temperature levels.
Considerable skill is required to perform such operations and to hold tolerances within 0.08 to 0.25 mm (0.003 to 0.010
in.) over a length of 0.45 to 1.22 m (18 to 48 in.).
After the workpiece has been straightened, it is tempered to the required hardness. Tempering relieves stress set up during
straightening and during the hardening cycle. This stress will often deform the workpiece; consequently, workpieces
straightened by heating and pressing should be clamped in restraining fixtures during tempering. Fixturing can correct a
slight distortion and prevent distortion during tempering.
Temper straightening is used to correct the distortion caused by heat treatment. The workpiece is first tempered to a
hardness somewhat higher than required, then clamped in a straightening fixture and tempered to the required hardness.
The greater the hardness difference between the first and the corrective tempering operations, the more accurate the
dimensions will be. Temper straightening is most successful at hardness levels of 55 HRC and lower.
Deep-hardening alloy and tool steels that are being martempered to minimize distortion should be held straight during the
cooling period after austenitizing and until the completion of martempering. If straightness is not maintained throughout
martempering, the workpiece will warp as martensite continues to form. Straightening should be done below 480 °C (900
°F). Cold bars or chills contacting the high side will more rapidly extract the heat from the workpiece and aid in
straightening.
Straightening of Bars, Shapes, and Long Parts
Straightening in Presses
Round bars up to 50 mm (2 in.) in diameter and from 0.6 to 3.0 m (2 to 10 ft) in length are often straightened in an arbor
press. Larger workpieces are similarly straightened in power presses, which may have power rolls and hoists to move the
work.
The principle of press straightening is illustrated in Fig. 3(b). The bar to be straightened is supported at points A and B
with the convex side of the bow or kink toward point C. Sufficient force is applied at C to cause the bar to become bowed
in the opposite direction. The force must be great enough to exceed the elastic limit of the material, but it must set up just
enough strain in the bar to allow it to return to the straight position (but no farther) when the pressure is released. The
greater the bow in the bar and the higher its elastic limit, the greater the force required to produce the correct amount of
strain. To straighten a bar by press straightening, the metal must be capable of cold deformation, and it must strain
harden.
In press straightening, the operator usually locates kinks or bows in round bars by holding a piece of chalk close to the
surface of the bar and then rotating the bar so that any high spots will be marked by the chalk. The high spot is then
brought under the ram of the straightening press, and sufficient force is applied to remove the kink or bow. In shapes
other than rounds, the out-of-straight condition must be detected visually or with the aid of a straightedge. This type of
straightening requires considerable skill on the part of the operator. A straightening press is sometimes referred to as a
gag press and the straightening operation as gagging.
Hydraulic or mechanical presses are used to straighten bars, shapes, and shaftlike parts before, between, and after heat-
treating operations. Some bars that are roll straightened do not meet straightness requirements and must receive a final
press straightening. Presses are also used to straighten large diameter bars in preparation for turning or grinding. Finish-
ground or turned products with highly finished surfaces are press straightened to avoid the spiral marks produced by
rotary straightening. Press straightening does not change the size of the bar, but rotary straightening--because of the
rolling action or the alteration in residual stress caused by bending or both--may cause a change in bar size.
Some high-strength steels and stainless steels are too hard to be straightened in any way except in a press, and some
metals are too hard to be straightened without heat, unless the bar is first annealed. The cold straightening of bars of these
metals may cause the bars to break.
Press straightening is easier when the bar has a hardness of less than 40 HRC. The bar can be retempered to relieve the
stress introduced during straightening.
In a straightening press, a round bar is usually set on spring-loaded rollers near the ends of the bar. Thus, the bar can be
rotated on its axis while a dial gage shows any deviations from straightness. As the press ram moves down, it presses the
bar into V-blocks that support the straightening pressure. This action is repeated, sometimes with the bar shifted or the
roller supports moved, until the bar is straight enough to meet specifications. The V-blocks and roller supports can be
moved to change the leverage and to adjust the application of the force. A straightening press is better suited to the
correction of short bends and kinks than to the correction of long bends. A straightening press can hold the distortion of
heat-treated bars and shafts to 0.25 mm (0.010 in.) or less, as shown in Examples 1 and 2.
Example 1: Straightening of a Heat-Treated Shaft in a Press.
A D2 tool steel shaft with a hardness of 63 to 65 HRC and a straightness specification of 0.25 m (0.010 in.) TIR is shown
in Fig. 6. The shaft was hung in a vertical furnace and preheated to 205 °C (400 °F), 540 °C (1000 °F), and 815 °C (1500
°F) before heating at 540 °C (1850 °F). The part was air cooled to 260 °C (500 °F). Because inspection at that
temperature showed distortion of 0.38 to 0.64 mm (0.015 to 0.025 in.) TIR, the bar was straightened in a manual
hydraulic press to 0.18 mm (0.007 in.) TIR before it cooled to 205 °C (400 °F). At 65 °C (150 °F), the shaft was again
straightened to within 0.18 mm (0.007 in.) TIR.
The shaft was clamped in a V-block for tempering at 150
°C (300 °F) for 6 h, then given a subzero treatment and
straightened to within 0.20 mm (0.008 in.) TIR. The
shaft was again clamped in the V-block, using shims for
straightening, while being retempered at 150 °C (300 °F)
for 6 h. Final straightening in the press was to 0.18 mm
(0.007 in.) TIR, which was better than was required.
Example 2: Straightening of a Heat-
Treated Bar in a Press.
A flat bar of O1 tool steel having a 19 mm ( in.) deep
V-groove along one edge is shown in Fig. 6. The
specified hardness was 61 to 63 HRC. The top and sides
had to be flat within 0.13 mm (0.005 in.).
The bar was preheated to 595 °C (1100 °F), heated at
805 °C (1480 °F), and marquenched at 190 °C (375 °F).
The bar was then clamped in a fixture and cooled to 38
°C (100 °F). Inspection showed 0.15 mm (0.006 in.)
maximum variation for the top and side. The bar was
reclamped in the fixture and tempered at 150 °C (300
°F) to a hardness of 63 to 64 HRC. The bar was
reclamped with shims to straighten it and was
retempered at 165 °C (325 °F) to a hardness of 61 to 62
HRC. The bar was then straightened in a press within 0.13 mm (0.005 in.) at the top and side.
Clamping of Workpiece. If the workpieces in Examples 1 and 2 had been heat treated without being clamped, they
would have been free to distort and would have needed more press straightening. The straightening of bars and shafts
during transformation and during tempering is more efficient and costs less, and it is sometimes the only way in which
straightness specifications can be met. Structural parts for aircraft are commonly straightened by a combination of
methods, as shown in Examples 3, 4, and 5. Example 6 describes a procedure for heat treating and straightening a long,
thin rectangular bar in a press. Round shafts are often straightened before they are ground, as in Example 7 and 8.
Straightening presses are used as accessories to other equipment, such as blooming mills that roll blooms or billets 127 to
178 mm (5 to 7 in.) thick. A 1.8 MN (200 tonf) press with a bed 0.6 × 1.2 m (2 × 4 ft) can straighten such blooms or
Fig. 6
Shaft and grooved bar that were press
straightened after heat treatment. Shaft hardness: 63 to
65 HRC. Bar hardness: 61 to 63 HRC. Dimensions given
in inches.
billets within 6.4 mm per 1.8 m ( in. per 6 ft) in lengths as great as 4.9 m (16 ft) without the use of spacers or shims.
Many blooms or billets do not need straightening--for example, if they are to be cut into pieces for forging stock.
Example 6 describes a procedure for heat treating and straightening a long, thin rectangular bar in a press. Round shafts
are often straightened before they are ground, as in Example 7 and 8.
Example 3:
A structural aircraft part called a cap was made from modified 4330 steel bar 4.7 mm ( in.) thick × 1.7 m (66 in.) long
(Fig. 7). The part was channel shaped with one flange removed for a portion of its length to produce an angle section.
Holes 3.2 mm ( in.) in diameter were made in both sides of the angle section and in the channel section. Heat treatment
consisted of suspending the workpiece by the angle end in a salt bath at 845 °C (1550 °F), quenching in salt at 245 °C
(470 °F), and air cooling. After being heat treated, the parts were cleaned and checked for hardness (aim was 46 to 49
HRC). With hardness less than 50 HRC, a subzero treatment was given prior to tempering; with hardness of 50 HRC or
harder, the workpiece was clamped in a fixture, tempered at 315 °C (600 °F) for 5 h, and finally air cooled. The
workpiece warped about 25 mm in 1.7 m (1 in. in 66 in.) after quenching, and a warp of about 6.4 mm ( in.) remained
after fixture tempering. A camber of 6.4 mm ( in.) was easily removed manually, but a workpiece with a camber of
more than 6.4 mm ( in.) was retempered in the fixture before straightening. Shape and straightness were checked in a
fixture using a 0.51 mm (0.020 in.) feeler gage. After being tempered at 315 °C (600 °F), the workpiece had a hardness of
46 to 49 HRC.
Fig. 7 Aircraft part that was straightened by a combination of methods.
Hardness: 46 to 49 HRC. Dimensions
given in inches.
Rotation of about 60° was required to correct for 0.76 mm (0.030 in.) twist. An 89 kN (10 tonf) hydraulic press was used
to correct short bends. The workpiece was supported on slotted blocks, placed about 360 mm (14 in.) apart, which gave
support to the flanges while pressure was applied. A force of about 13 kN (3000 lbf) deflected the channel section 9.5 to
13 mm ( to in.) for a camber correction of 3.3 mm/m (0.020 in. per 6 in.). A force of 4.4 to 6.7 kN (1000 to 1500 lbf)
on the angle section produced about the same deflection and correction.
Stretching was used to maintain spacing and alignment for the 3.2 mm ( in.) diam holes. Because of the thin sections,
the part shrank 0.76 to 1.0 mm (0.030 to 0.040 in.) after quenching in salt at 245 °C (470 °F). To correct this shrinkage,
the part was preheated at 290 °C (550 °F) for 30 min, then clamped in the tempering fixture and heated at 315 °C (600 °F)
for 5 h. As the fixture expanded from the heat, the part was stretched. After slowly cooling in the fixture, the part had a
permanent stretch of 0.51 to 0.76 mm (0.020 to 0.030 in.), which corrected hole alignment and spacing. About 8 min was
required for clamping the part in the tempering fixture, and 22 min for hand straightening.
Example 4:
A welded double-channel structural member made of 4340 steel 1.8 m (72 in.) long and weighing 5.2 kg (11.5 lb) (Fig. 8)
was austenitized at 830 °C (1525 °F) for 40 min, martempered at 245 °C (470 °F), stress relieved at 205 °C (400 °F), and
cleaned. The unusual feature of this operation was the simultaneous straightening, bending, and tempering.
The workpiece was machined on a straight plane and
then fixture bent 7° on one end during tempering. The
fixture was constructed with various gibs and filler
blocks milled to fit the contours of the workpiece,
including allowance for the 7° bend, which extended for
approximately 305 mm (12 in.) (Fig. 8). The workpiece
was then placed on the fixture, and all holding clamps
were placed in position. The fixture-and-workpiece
assembly was then heated to 315 °C (600 °F). At this
time, all clamps were tightened, thus making the 7° bend
at one end. The assembly was then heated (tempered) to
540 °C (1000 °F for 4 h.
After tempering the channel was straightened in a
hydraulic press to achieve final alignment within 0.76
mm (0.030 in.). A special gage was used to inspect alignment and the bend. Time for straightening and gaging was 30
min.
Example 5:
A structural bar with a tapered-width channel 4.8 mm ( in.) thick and 1.7 m (66 in.) long was made of 17-4 PH
stainless steel (Fig. 9). The part was solution treated to 39 to 42 HRC, finish machined, then aged for 1 h at 480 °C (900
°F). It was straightened to within 0.38 mm (0.015 in.) camber in a hydraulic press upon removal from the aging treatment.
The part could be straightened only until it cooled to 370 °C (700 °F), which took 10 min. Because 20 min was required
to straighten the part, reheating to 480 °C (900 °F) was necessary.
Fig. 9 Setups for straightening and removing twist from a stepped channel. Dimensions given in inches.
The channel was supported on two blocks 406 mm (16 in.) apart while force was applied by a pressure ram through a
block fitted to the inside contour (upper view, Fig. 9). A force of 13.3 kN (3000 lbf) deflected the part about 25 mm (1
in.) for a correction of 0.51 mm (0.020 in.). Force was applied at 50 mm (2 in.) increments along the bar.
Two 44 kN (5 tonf) hydraulic presses mounted on a large steel table were used to remove the twist. One ram held one end
of the part against a block on the table while the second ram untwisted the part (lower view, Fig. 9). A slotted bar served
Fig. 8 Welded structural mem
ber that was straightened
during and after heat treatment.
Dimensions given in
inches.
as a lever to twist the channel 30° for a permanent correction of 0.83 mm per 1 m (0.020 in. per 24 in.) or 1.3 mm (0.050
in.) for the entire length.
Example 6: Straightening a Long, Thin Rectangular Bar in a Hydraulic Press.
A rectangular bar of 17-4 PH stainless steel was 75 mm (3 in.) wide, 2.1 m (84 in.) long, and 6.4 mm ( in.) thick (except
for 75 mm, or 3 in., at each end, where it was 25 mm, or 1 in., thick). The bar was solution treated and finish machined,
which caused a bowing of 6.4 mm ( in.). The bar was clamped in a fixture and aged at 480 °C (900 °F), which reduced
the bowing to 3.2 mm ( in.). After aging, the bar was removed from the fixture, reheated to 425 °C (800 °F), and
straightened in an 89 kN (10 tonf) hydraulic press, using 13.3 kN (3000 lbf) of force between support blocks 406 mm (16
in.) apart. This caused deflection of 19 mm ( in.) for a correction of 0.76 mm (0.030 in.). It took 20 min and two to
three ram strokes per 406 mm (16 in.) setup, as well as five setups per bar, to straighten the bar within 1.5 mm (0.060 in.).
Example 7:
A shaft of medium-carbon steel, 102 mm (4 in.) in diameter × 6.1 m (20 ft) long, was heat treated to 269 to 321 HB,
straightened for turning, turned in a lathe, and then straightened for centerless grinding. The shaft lay on rollers beneath
the ram of the press, which permitted it to be rotated and to be moved along its axis. Spring-loaded blocks supported the
rollers so that straightening pressure would first deflect the springs, letting the shaft down on movable V-block anvils for
straightening. The springs pushed the rollers up, lifting the shaft off the anvils when the ram moved up. The shaft was
rotated under a dial indicator to find the high spots. The spots were marked with chalk so that they could be moved
beneath the ram. The shaft was straightened within tolerance by pressing, moving the shaft, and pressing again. In an 8-h
day, 5 to 15 shafts were straightened.
Example 8:
After being heated to 1010 °C (1850 °F) and air cooled, a shaft of D2 tool steel, 50 mm (2 in.) in diameter × 1.7 m (66 in.)
long, was straightened within 0.51 mm (0.020 in.). The straightening began when the shaft had cooled to 480 °C (900 °F)
from 1010 °C (1850 °F). An 89 kN (10 tonf) hydraulic press applied 8.9 kN (2000 lbf) of force to the shaft, which was
supported on anvil blocks 457 mm (18 in.) apart. The shaft was continuously gaged with dial indicators as it was rotated
on its axis in order to check the location of the high and low points.
Between 480 and 260 °C (900 and 500 °F), the shaft deflected easily under a load of 910 kg (2000 lb), causing a
deflection of 3.2 mm per 508 mm ( in. per 20 in.). When the shaft cooled further, straightening was more difficult and
required increased force and a longer holding time.
Gaging and straightening continued until the shaft had cooled to 65 °C (150 °F); the shaft was then tempered at 480 °C
(900 °F) for 2 h, resulting in a hardness of 59 to 60 HRC. A difficult shaft was sometimes clamped to a 76 × 76 × 1830
mm (3 × 3 × 72 in.) bar with shims and retempered. If the straightness error was 2.0 mm (0.080 in.), a 1.5 mm (0.060 in.)
shim was used on each end for a deflection of 3.5 mm (2.0 + 1.5 mm), or 0.140 in. (0.060 + 0.080 in.), and the shaft was
retempered at 495 °C (925 °F). The higher tempering temperature made the shaft slightly softer, but generally
straightened it within the specified 0.51 mm (0.020 in.).
Straightening of Bars, Shapes, and Long Parts
Parallel-Roll Straightening
Roll straightening is a cold-finishing mill process by which bars and structural shapes are provided with straightness
adequate for most applications. For bars and shapes on which close tolerances must be maintained, roll straightening can
be followed by press straightening.
In one type of roll straightening, square, flat, hexagonal, and other flat-sided bars are continuously passed between sets of
parallel-axis rolls (Fig. 10 and 11). Uniform bends are introduced such that the bar is straight when it leaves the rolls. By
varying the distance between roll centers and the amount of offset, the degree of bend can be adjusted according to the
section size and yield strength of the metal being straightened.
Fig. 10 Arrangement of vertical-shaft and horizontal-
shaft rolls in a roll straightener for straightening a
rectangular-section bar.
Round bars can be straightened on parallel-axis roll
straighteners, but there is no way to prevent a round
workpiece from turning on its axis as it passes through
the machine. In rotary straighteners, round bars rotate
and advance through the rolls so that the bars are bent
uniformly in all planes; the rolls are adjustable so that
the bars emerge straightened. Rotary straighteners for
round bars usually consist of rolls that can be set at
variable angles to each other (see the section "Rotary
Straighteners" in this article.
Parallel-Roll Straighteners. Square, flat,
hexagonal, and other flat-sided bars can be straightened
in both directions in one pass through a straightener
having two sets of parallel rolls in planes 90° to each other (Fig. 10), or in two passes through a straightener having a set
of parallel rolls in only one plane, by turning the bar 90° on its axis between passes.
In machines with a single straightening plane, the rolls are mounted on horizontal shafts as shown in unit 2 of Fig. 10. If
the horizontal rolls are grooved (like the vertical rolls of unit 1, Fig. 10), the straightening in one plane also produces
some straightening in a plane 90° to the first plane.
For high accuracy of straightness and high production rate, a second unit is added in a plane perpendicular to the first unit
(Fig. 10). Unit 1 has vertical shafts for straightening curvature in the horizontal plane; unit 2 has horizontal shafts for
straightening curvature in the vertical plane. The two driven rolls rotate, but are otherwise stationary; the three idler rolls
are adjustable away from and toward the workpiece.
The first driven roll contacted by the bar is set with enough space between the first two idler rolls to curve the bar
uniformly with the concavity toward the first driven roll. As the bar passes over the second idler roll and is held in
position by the second driven roll, the concave side of the bar is reversed. The amount of reversal can be controlled by the
position of the second idler roll, and with that roll properly positioned, the bar will emerge straight from the third idler
roll. With a greater number of rolls, the most severe curvatures are reduced at the entry end of the machine. This results in
less work for the remaining rolls and provides for better straightening of small curvatures. The number of rolls in a set of
straightening rolls ranges from 4 to as many as 13; the most common number is eight or nine.
The amount of adjustment in roll spacing is determined by:
• The resistance of the work metal to deflection beyo
nd its elastic limit. The greater the resistance, the
Fig. 11 Straightening of a hexagonal bar in a two-
plane
roll straightener.