96
Process used in making hole:
Drilling Punching
Various Forming Processes
Relative thickness of material
x
100 2
3 5
8 10
15 20
30 70
0.75 0.57
0.48 0.41
0.40 0.34
0.32 0.26
0.22 0.70
0.60 0.52
0.50 0.50
0.48 0.46
0.45
7.5
SPINNING
Spinning is the process of forming a metal part from a circular blank of sheet metal or from a length of tubing over a mandrel with tools or rollers. There are three types of spinning processes:
Type I Spinning In type I spinning, also called manual spinning, a circular blank of sheet metal is held against a form
block and rotated while a rigid tool is used to deform and shape the workpiece over the form block. Fig. 7.13 schematically illustrates the manual spinning process. The tools may be operated manually or by a
hydraulic mechanism.
Forming stages
Workpiece
Form block
Fig. 7.13
Manual spinning process.
Fig. 7.14 illustrates representative shapes that can be produced by manual spinning. The advantages of manual spinning, as compared to other drawing processes, are the speed and economy of producing
prototype samples or small lots, normally less than 1,000 pieces. Tooling costs are less and the invest- ment in equipment is relatively small. However, spinning requires more skilled labor. In Fig. 7.15 is
shown a relative cost comparison for manufacturing a round sheet metal shell by deep drawing and by
manual spinning.
Various
Forming Processes
97
I
Fig. 7.14
Characteristic shapes of shells formed
by
spinning.
I
Spinning Deep drawing
1 2
3 4
5 6
Number
of
parts
Fig. 7.15
Cost comparison
for
manufacturing a round sheet shell
by
conventional spinning and
by
deep drawing. Type
spinning Type
spinning is the process of forming complex shapes, such as cones with tapering walls; sym- metrical-axis curvilinear shapes, such as nose cones; and hemispherical and elliptical tank closures with
either uniform or tapering walls. The process is also known as shear spinning, and the shape is generated by keeping the diameter of the workpiece constant, as illustrated in Fig 7.16. Although a single roller can
be used, two rollers are desirable to balance the radial forces acting on the form block. During spinning, normal wall thickness is reduced. In shear spinning over a conical form block, the thickness Tof the spun
part is given by the formula:
=
T
7.18 where:
a
=
half angle of cone T
=
blank thickness. An important factor in shear spinning is the spinnability of the metal. The spinnability is the smallest
thickness to which a work piece can be spun without fracture.
98
Various Forming Processes Roller
Roller
Fig. 7.16
Shear spinning method.
Type spinning
Type spinning is known as tube spinning and it consists of reducing the thickness of a cylindrical
work piece while it is spinning on a cylindrical form block, using rollers Fig. 7.17. There are two meth-
ods: forward and backward. In either example, the reduction in wall thickness results in a longer tube. The ideal tangential force in forward tube spinning may be calculated by the following formula:
where: AT
=
T,
=
average
flow
stress of the material,
=
feed. Because of friction and another influencing factors, the force exerted is about twice that of the ideal force.
Roller
Fig. 7.17
Tube spinning method.
