The primary shape product from which a seamless tube can be made may
be a can or an extruded heavy wall tube. A tube with a welded seam may
be produced by Roll-Forming from a strip. The primary process entitled
"Tube Extrusion" for the fabrication of heavy wall large diameter
tube is handled separately. Cans, also called cups, as a primary shape
can be made by each of the following three processes, or by combinations
- Impact Extrusion, as described in Chapter (6)
- Deep Drawing, as described in Chapter (10)
- Ironing, as described in Chapter (11)
of the "Handbook
of Metalforming Processes", by Avitzur. Published by John Wiley,
1983. Tube making is described in Chapter (9).
For example, a shallow can of relatively thick wall can be produced
by either deep drawing or impact extrusion. The wall thickness can be reduced
The cans made by the processes described above may on occasions be used
as the raw material for the production of tube by removing the bottom of
the can. This procedure then may compete with other tube making procedures,
such as tube extrusion. The different routes are then defined as The "Pass
Reduction Schedules" or simply "Reduction Schedules." The
reduction schedule contains the number of passes by each process, i.e.,
deep-drawing, redraw, sinking, plug drawing, etc. The pass reduction schedule
contains also the details for each pass, including, die angle, reduction,
friction value, etc. Intermittent treatments such as intermediate anneals
are also part of the pass reduction schedule.
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The alternate Reduction Schedules for the subsequent finishing sequence
of the tube are described in this section. For a schematic diagram of possible
pass reduction schedules see Fig. <1> below. To simplify the diagram,
intermittent anneals are omitted from Fig. <1>. Many alternate routes
may lead to the same final product. Circumstances in different facilities
may lead to different optimal paths.
For any selected Alloy a pass reduction schedule, similar to Fig. <1>
could be constructed, with details of the process, the tooling, the number
of passes and the reduction, die angles, lubrication, intermittent anneals,
and speeds for each pass. These pass reduction schedules should be constructed
by the team from the plant, together with Metalforming personnel. It is
a trial and error process. While goals may be set, the diagram will always
remain an evolving document. Reduction schedules always change, hopefully
for the better.
PASS REDUCTION SCHEDULE FIGURE <1>
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This page focuses on selected problem areas in the secondary processing
of tube, reducing its wall thickness and/or diameter. Mainly we deal with
tubes that began as hot extruded tubes, deep drawn tubes, or roll formed
and welded tubes. These secondary processes of tube sinking, drawing, and
ironing are described in Chapter (9) of the Handbook. It also deals with
surface defects such as scratches, and lubricant bonding on the surface
of the extruded billet. We focus on the pass reduction scheduling, i.e.,
selection of reductions per pass, appropriate die angle, and the sequencing
of sinking, drawing, with mandrel, and floating plugs, and ironing. We
deal with the design of, and with the introduction of more frequent use
of, the floating plug, instead of sinking. The proper use of the ironing
process to achieve thinner wall on finer tube is also treated.
First we classify and describe the processes briefly. For a deeper analysis
see Chapter 9 of the Handbook, and access the software on tube sinking
which is based on the analysis. The software for the analysis of tube sinking
is instrumental in providing graphs for the design of the die and plug
for tube drawing with floating plugs.
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When a tube is pulled through a (conical) converging die to decrease
its outer diameter, without internal tooling (see Fig. <7>) the process
is called free tube drawing or tube sinking. It can be performed on a draw
bench to form straight tubes of finite length, or it can be performed on
bull block, to make a continuous coiled tube.
In Fig. <7> a tube of original outer radius Ro and inner radius
Ri is pulled through a conical die of semi-cone angle a with an exit hole
of radius Rof. The original thickness of the tube, to, is given by the
difference Ro - Ri. The tube, while passing through the die, shrinks in
outer radius from the original radius Ro to a final nominal radius Rof.
In wire- and tube-drawing practice it may sometimes be observed that the
exit diameter of the product is slightly smaller than the exit of the die.
The parameters Ro, Ri, Rof, a, the length of the land of the die L (also
called the bearing), and the back tension sxb (if applied) are all independent
parameters of the process.
The inner radius of the emerging tube, Rif, is not directly controlled
by the tooling of the process. The final thickness of the tube (tf = Rof
- Rif) is a dependent parameter, dependent on Ro, Ri, Rof, a, sxb, and
the friction between the tube and the die. The drawing stress sxf required
to perform tube sinking is another dependent process parameter. In the
treatment of tube sinking as performed in the Handbook the dependence of
the dependent process parameters - the drawing stress sxf and the exit
thickness tf - on the independent process parameters Ro, Ri, Rof, a, L,
sxb, and the friction between the tube and the die - is determined. The
optimal die angle that minimizes the drawing force and at the same time
maximizes the maximum possible reduction are also studied. Software, based
on the analysis is available for the determination of the following dependent
- drawing stress
- emerging wall thickness
- optimal die angle
- maximum possible Reduction
on the independent parameters. The understanding of the process of tube
sinking, through the analysis and through experimental work, is essential
for the development of criteria for design and tooling for mandrel and
plug drawing, as discussed next. User friendly software for the analysis
is available through Metalforming Inc. The software is presented and provided
for an on hand experience in the appropriate Seminar on tube making.
TUBE SINKING FIGURE <7> [Cannot perform fsize(): Win32 Error Code = 2
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Mandrel and Plug Drawing
A better control on the inner surface of the tube is provided by placing
a cylindrical mandrel inside the tube at the throat of the die. The mandrel
exert a size determination of the wall thickness and inner diameter, as
well as affecting a smooth mirror like surface finish inside the tube.
In order to be functional the diameter of the mandrel must be larger than
the diameter expected from identical processing by tube sinking. If the
mandrel is smaller it will have no control of the process at all. The mandrel
have only to be slightly larger because otherwise it will cause a large
increase in the drawing force and may lead to tube tearing. The recommended
die angle for drawing with a mandrel is nominally the same as that found
to be the optimal angle for tube sinking.
The two popular designs of mandrels are the stationary and the moving
mandrel (Figs. 9.51 and 9.52a of the Handbook). A stationary mandrel is
clamped to the frame of the draw bench, while a moving mandrel adheres
to the drawn tube through friction and pressure and moves with it. In either
case the shaping of the inner surface of the tube commences only at the
exit of the tube from the die. While converging through the die the inner
surface of the tube deforms freely.
The conical plug (Fig. <8>) mounted at the tip of a stationary
mandrel was first introduced to affect the inner surface earlier than possible
with the cylindrical mandrel. The diameter of the tip of the plug is that
recommended for the mandrel, i.e., slightly larger than that of the emerging
tube when tube sinking is practiced. The cone on the plug is of a slightly
smaller angle than that observed under identical tube sinking conditions.
It is noticeable that the tension on the mandrel depends on friction
between the plug and the tube, and on the cone angle of the plug. The larger
the plug angle the smaller is the tension on the mandrel. Under the proper
conditions the tension on the plug may be eliminated. The plug then places
itself at the throat of the die even when there is no mandrel to hold it
there. Such a plug is called floating plug. The design of the floating
plug, i.e., diameter of the tip, and plug cone angle are determined through
experimental tube sinking, or through the use of the software for the analysis
of tube sinking. When a floating plug is used the tube can be drawn on
a bull block, from coil to coil.
MANDREL AND TUBE DRAWING FIGURE <8>
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The process of ironing of cans is already described earlier. When a
tube is ironed we can't rely on ram force acting on the bottom of the tube.
Ironing of tubes is done with a draw bench, pulling at the exit side of
the die. Usually the clamp or tong may grab a hold of the tube forcing
it onto the mandrel and pulling the two together. However one can pull
on the mandrel itself and if the die angle is small enough the mandrel
will drag the tube with it through friction. (see chapter 11 of the Handbook).
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