03-10-2012, 01:52 PM
Fundamentals of Modern Manufacturing
Fundamentals of Modern.pptx (Size: 1.49 MB / Downloads: 144)
Introduction to Rolling
Rolling is a bulk deformation process in which the thickness of the
work is reduced by compressive forces exerted by two opposing
rolls. The rolls rotate to pull and simultaneously squeeze the work
between them.
The basic process shown in the previous figure is “Flat Rolling”,
used to reduce the thickness of a rectangular cross section. A
closely related process is “shape rolling”, in which a square cross
section is formed into a shape such as an I-beam. (in this lab, you
will only do flat rolling)
Flat rolling and its analysis
Rolling increases work width. This is called “spreading”.
Spreading is expected because of the volume constancy in plastic deformation. Since the material is compressed in the thickness direction, both the length and width will increase provided that the material is not constrained in the width direction.
Spreading is more pronounced with low width-to-thickness ratios and low coefficients of friction, since there is small resistance to flow in the width direction.
The work enters the gap between the rolls at a velocity vo and exits at a velocity vf. Because the volume flow rate is constant and the thickness is decreasing, vf should be larger than vo.
The roll surface velocity vr is larger than vo and smaller than vf. This means that slipping occurs between the work and the rolls.
Only at one point along the contact length, there is no slipping (relative motion) between the work and the roll. This point is called the “Neutral Point” or the “No Slip Point”.
Flat rolling defects
Defects in rolling may be either surface or structural defects:
Surface defects include scale and roll marks.
Structural defects (see next figure) include:
Wavy edges:
bending of the rolls causes the sheet to be thinner at the edges, which
tend to elongate more. Since the edges are restricted by the material at the center, they tend to wrinkle and form wavy edges.
2. Center and edge cracks: caused by low material ductility and barreling of the edges.
3. Alligatoring: results from inhomogeneous deformation or defects in the original cast ingots.
Other defects may includes residual stresses (in some cases residual stresses are desirable).