Appeal No. 96-2649
Application 08/418,875
is often difficult. It has already been proposed to
form tubes of this type by drawing an expander plug
through a tubular workpiece so as to force the metal of
the tube wall into grooves formed in an external
tubular die. Unfortunately, the rubbing friction of
the expander plug against the internal surface of the
tubular workpiece draws the metal and results in
deformation of the fibers which is detrimental to the
resistance of the fins when these latter are subjected
to the action of a flow of gas at high temperature
[column 1, lines 45 through 57].
To solve the foregoing problems, Portal proposes
a method of fabrication of tubes having a smooth
internal surface and an external surface which is
provided with fins inclined to the axis of the tube,
said method consisting in placing a tubular workpiece
within a die which is made up of a series of finned
elements each designed to reproduce an angular sector
of said fins, in then exerting a substantially radial
stress on said workpiece in a zone of application which
is displaced axially from one end to the other of said
workpiece so as to cause a predetermined thickness of
said workpiece to creep into the die while at the same
time limiting the pressure of air which is enclosed
between said die and the fins progressively as said
fins penetrate within said die [column 2, lines 1
through 14].
The step of exerting a substantially radial stress on the
workpiece to force metal into the fin-forming cavities of the die
is performed by a frusto-conical expander head 24 having recesses
in which are fitted balls 26. A drive mechanism imparts to the
expander head a swash-plate motion having an axial translational
component and a rotational component (see column 3, lines 37
-4-
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Last modified: November 3, 2007