24. Rolling and Bending

• The basic process is,

1. We start with raw material in some form, typically there is one long dimension. The material may be hot or cold.

2. The material is then fed in between rollers.

3. The rollers apply a force to the material to thin or reshape the original cross section.

4. The material emerges from the other side of the rollers in a new shape.

5. The material may then be taken off, passed through another set of rollers, coated with oils, drawn, etc.

• There are two types of rolling,

flat rolling: reduces the thickness of a sheet of material.

shape rolling: produces new parts with a complex cross section.

• Materials that have been rolled typically have a wrought structure with the grains broken down.

• Rollers play a large part in continuous casting after the molten metal is poured off into a bloom, or some equivalent form.

24.1 Basic Theory

• While the rollers are in contact with the work there is friction and force applied. There is typically slip between rollers and the work, but this slip is not constant over the surface of contact. The figure below illustrates the forces acting on a roller.

 

24.2 Sheet Rolling

• While rolling a sheet the rollers will be under significant forces. This will lead to deflections at the centers of the rollers. To overcome this rollers are made with a slight barrel shape. therefore when rolling the deformed rollers will take the desired shape.

• When rolling sheets have a tendency to spread. This means that the width of the sheet increases when it is rolled.

• Input and output materials in rolling are,

Sheets: up to 1/4” thick

Plates: between 1/4” to 12” thick

Billet: a square cross section of 6” or less per side

Bloom: a square cross section of 6” or more per side

Rods

Bars

Beams

• When rolling the material may be processed the following ways,

hot rolling is done above the recrystallization temperature (850°F for Al, and 1250°F for steel) and results in a fine grained wrought structure. The surface quality (500-1000 micro in.) and final dimensions are less accurate.

cold rolling is done near room temperature and produces better surface finishes (32-125 micro in.) and dimensional accuracy (0.004”-0.014”), but with strain hardening.

pack rolling involves rolling multiple sheets of material at once, such as aluminum foil.

• Defects in flat rolling include,

waviness

tearing on the sides (edge), or in the middle (zipper), or between the top and bottom faces (alligatoring)

spalling is cracking or flaking of surface layers results when improper material used in hot rolling

heat checking is cracking caused by thermal cycling this results when improper material used in hot rolling

• Residual stresses are also built up in rolled materials. The two possible variations are,

tension outside, compression inside: the result of large rollers, or high reductions

compression outside, tension inside: the result of small rollers or small reductions per pass

• In commercial rolling mills some techniques are used,

two-high, three high, four-high, cluster mills: multiple rollers can be used to increase the stiffness of the contact rollers.

tandem rolling: a number of rollers are used in series. Each point reduces the material thickness a step.

lubricants: used with cold rolling

coolants: used with hot rolling to cool the rolls and break up scale

24.3 Shape Rolling

• In sheet rolling we are only attempting to reduce the cross section thickness of a material. If instead we selectively reduced the thickness we could form complex section easily. This technique is called shape rolling.

• In practice we can make complex cross sections by rolling materials in multiple passes. We can’t do this in one pass because we would overwork the material, and it would crack.

• Some of the types of shape rolling are listed below,

ring rolling: a ring shaped part is rolled between two rollers.

thread rolling: a round shaft is placed between two flat surfaces having flattened screw thread projections. The surfaces are compressed and moved tangentially to produce threads on the shaft.

cross section: a billet or bloom is passed through a set of rollers that slowly transform it to the final shape.

• We may also use rolling to make seamless tube with the Mannesmann process,

1. A bar (cylinder) is rolled radially between two rollers.

2. The force applied by the rollers creates a stress concentration at the center of the bar which may or may not lead to a central crack in the bar.

3. A mandrel is forced into the center where it pierces the hole, and ensures a desired size.

4. The rollers are oriented so that they slowly pass the bar through and onto the mandrel.

5. The finished tube is removed from the mandrel.

24.4 Bending

• After basic shearing operation, we can bend a part to give it some shape.

• Bending parts depends upon material properties at the location of the bend.

• Some of the things that may/do occur in bends,

material at the outer bend radius undergoes tensile plastic deformation

material at the inner bend radius undergoes compressive plastic deformation

the width (along the bend axis) will reduce in length based on poissons ratio

if the bend radius is too small the plastic deformation at the outside of the bend will result in fracture.

• The basic calculations for a bend radius are shown below,

 

• The strain on the outermost fibers of the bend is,

 

• After sheet metal is bent it will tend to spring back to a lesser angle. The following formula relates bend radius before and after release.

 

• There are a variety of methods for spring-back compensating,

trail and error to over bend

calculated over bend

special punch/die designs

• The maximum bending forces may be calculated using the relationship below,

 

• Press Brakes a small presses that will bend a sheet metal piece over several feet.

• Some other operations done to sheet metal parts,

beading: the edges of the sheet metal are bent back (in a rounded shape) to stiffen the edges and eliminate the sharp edge.

flanges: a hole can be formed by punching through, and a lip (the flange) remains about the edge of the hole

Hemming: like a bead, but the edge is bent back as a flat.

Roll Forming: Bends can be made along long axes by using rollers.

• When planning to bend a piece, car must be take not to punch holes too close to the bend.

 

• Quality problems that occur in rolling are,

flatness

twist

straightness

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