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Gas planing of steel workpieces is an exothermic process where the ignition
temperature lies at 1250.degree. C. It is necessary to preheat to this
temperature a certain smallest portion of the workpiece surface where the
planing process is to be initiated. For this local preheating usually a
flame from a combustion burner is used which, therefore, is an essential
part of the planning burner. At manual as well as machine planing it
usually is necessary that the workpiece and the planing burner with
possibly an ignition device during the ignition process are in a state of
relative standstill. This applies both to so-called cold-planing, which
normally takes place at room temperature, and to hot-planing, which is
carried out at about 800.degree. C. This requirement of standstill, of
course, reduces the capacity of planing machines, because at every
ignition occasion the machine must be stopped. The time losses due to said
requirement are still greater at spot planing where the planing is ignited
at each single surface defect. At a high defect frequency the resulting
reduction in the capacity of the spot planing machine can be so great,
that it is more advantageous to plane the entire surface, in spite of the
high material losses.
For solving this problem, which applies primarily to selective spot
planing, improvements have been proposed with the object of shortening the
ignition process. When the ignition is carried out sufficiently rapidly,
the workpiece can move relative to the planing burner and ignition device
during the ignition period, so that this possibly may be called a flying
start.
In order to accelerate the ignition, at manual planing, a small amount of
iron melt is transferred in the form of some droplets to the preheated
ignition point. The droplets are formed by melting iron wire in a
preheating flame. A similar solution is applied at machine planing, but
there iron powder is added instead of iron wire and blown into the
preheating flame. The combustion heat of the iron in combination with a
favourable effect of the slag on the local heat transfer brings about an
acceleration in the preheating of the ignition surface. Only at low
relative speeds a flying start will succeed Unfortunately, this is not the
only disadvantage involved with the use of iron powder.
It, therefore, had been proposed, instead of combusting iron powder, to use
an electric arc for producing initial melt. The electrode used either is
fusible or non-fusible. In both cases material is heated and melted by
action of an arc at the point of the workpiece surface where the ignition
of the planing process is intended to take place. The use of a fusible
electrode has the advantage of rendering it possible to increase the melt
amount at the ignition point.
During the activation period of the arc, the electrode either may be
stationary or move synchronously with the workpiece. In the first case a
zone of melt arises in parallel with the planing direction, and its length
depends on the burning time of the arc. In the second case the melt forms
a spot, the size of which also is dependent on the burning time of the
arc. Immediately after the fusing is completed, the planing burner starts
operation. In the oxygen stream from the planing burner the zone or spots
of preheated and molten metal are ignited. Owing to the geometry of the
melt and the direction of the oxygen stream, the planing process
practically commences in a point or line, from which it spreads in wedge
shape until it assumes full width.
The described concentration of the melt and preheated metal to a point or a
zone extending along the planing direction shortens the ignition procedure
substantially and thereby renders flying start possible even at pretty
high speeds. A serious drawback of the described method is the formation
of relatively deep starting cavities or starting zones with steep edges.
It usually is troublesome and at times even impossible to remove these
steep flanks by rolling. New secondary defects can thereby arise on rolled
sheet metal.
The method of igniting gas planing according to the invention and the
device for carrying out said method satisfy the requirement on flying
start at gas planing at still higher speeds without showing the aforesaid
drawbacks and disadvantages. The invention is based on the utilization of
a heat source, which in principle is point-shaped and by action on the
surface of the workpiece forms a strip of molten and preheated metal which
substantially is oriented perpendicularly to the flow direction of the
planing oxygen. The device for carrying out the method comprises planing
burners and ignition means, which in their turn consist of a point-shaped
heat souce and a means for controlling the same.
The heat source can be controlled relative to the burner both in parallel
with and perpendicularly to the planing direction. Simultaneously with
said relative movement said heat source can be activated. At a suitable
setting of the relative movement of the ignition device to the burner with
respect to the relative speed of the workpiece to the burner, according to
the invention a strip of molten and preheated metal is formed by the
action of the activated heat source and extends across the planing oxygen
stream.
As heat source in the ignition device primarily an electric arc is used
which burns between the workpiece and a fusible or non-fusible electrode.
The utilization of the invention is not restricted to an electric arc as
point heat source. Similar effect can be achieved also with other heat
sources rich in energy and in principle of point shape, as for example
plasma burner, laser, electron-beam or possibly high-efficiency combustion
burner.
The invention is illustrated in greater detail with reference to the
accompanying drawings, in which
FIGS. 1a and 1b show an example of a device for carrying out the method
according to the invention, where an electric arc is used as point-shaped
heat source which burns between a fusible wire electrode and a workpiece.
FIG. 1a is a lateral view of the device, and FIG. 1b is a view from above
of the device.
FIGS. 2a and 2b show a second example of the device of carrying out the
method, where the arc as point-shaped heat source burns between a fusible
web electrode and a workpiece. FIG. 2a is a lateral view, and FIG. 2b a
view from above the device.
In FIG. 1a the numeral 1 designates a control means for an ignition wire
11, which is shown in starting position for preheating and fusing a
strip-shaped ignition area. 2 Designates a drive means, for example a
compressed-air cylinder, which during the fusing period drives the control
means 1 in a pendulum-like movement 10 about a fixed axle 3. The pendulum
plane of the control means forms a certain angle with the direction of the
relative movement 12 of the workpiece 7 to a planing burner 6 (see FIG.
1a). A wire electrode 11 is advanced from a magazine 5 by a feed means 4.
During ignition and planing, the planing burner 6 directs planing oxygen
stream in the direction 8 and possibly a preheating flame to the place to
be planed. The dashed line in FIG. 1a indicates the position 9 for the
control means after completed preheating and fusing of a strip-shaped
ignition area. In FIG. 1a further is shown a crack as an example of a
surface defect to be planed. 13 indicates the crack in the position when
the ignition device is activated and the fusion of the electrode
commences, and 14 with dashed line indicates the position of the crack
after completed fusion when the planing can be ignited.
In FIGS. 1 and 2 the planing device is stationary, and the workpiece moves
in relation to the planing device in a direction opposed to the planing
direction. It does not affect the method of igniting the planing according
to the invention if, if instead, the relative movement would be effected
by a stationary workpiece and a moving planing device. The speed and angle
of the pendulum movement 10 is chosen according to the invention always
with respect to the speed of the relative movement 12. The pendulum
movement speed component in parallel with the movement 12 shall be equal
to the speed of this movement. In this way, during the activation period
of the ignition device, said component of the relative movement of the
electrode 11 to the workpiece 7 is substantially equal to zero. The
transverse component, however, of the pendulum movement is independent of
the workpiece movement. At the beginning of the pendulum movement 10 with
the ignition device in starting limit position 1, the electric arc is
ignited. The arc burning between the electrode and workpiece preheats and
melts a small spot of the workpiece surface closest to the electrode
point. Simultaneously also the electrode 11 fuses which is advanced by the
wire feed unit 4. Fused electrode material is transferred to the
workpiece. Energy to the arc is fed from a current source connected
between the workpiece and a contact means on the electrode control. The
current source with associated electric circuit is not shown in the
Figure.
During the burning of the arc the electrode control means 1 moves from the
starting position to the limit position 9. As a result thereof and of all
total movements (the pendulum plane forms an angle with the relative
movement direction 12) a strip of preheated and molten metal 15 is formed
on the workpiece. At a suitable position of the axle 3 and the choice of a
suitable time for commencing the movement 10, the melt 15 after completed
movement 10 shall be directly in front of the extension 8 of the nozzle
passageway, in which the planing oxygen flows out from the burner 6. When
the oxygen flow is started at this moment, the metal in the strip 15 is
ignited, and thereby also the planing process is ignited.
As a consequence of the spread of preheated and molten metal according to
the invention, the planing commences across the entire width corresponding
to the length of the strip 15 which in FIG. 1 is equal to the width of the
burner 6. This also implies that the planing groove starts with full
width, so that no wedge-shaped starting cavity with sharp edges can arise.
The planing groove starting with full width also renders it possible for
the planing to commence immediately close to the surface defect 13, 14. A
shortening of the starting distance of the planing groove, thus, saves
material and simultaneously time by shortening the planing time.
The geometry of the ignition zone which according to the invention extends
in the transverse direction in front of the burner, besides, renders it
possible to start the planing process more reliably, more rapidly and more
easily. A further important advantage of the ignition method according to
the invention is that also surface defects can be planed which are located
near the front edge of the workpiece. The conventional ignition methods
using point heat sources do not render this possible with maintained
planing direction on the workpiece.
It is not necessary that the ignition strip 15 has a width equal to the
burner 6. The igniting according to the invention, of course, can take
place with the strip 15 being wider than the burner. Tests under service
conditions have shown that flying start can be obtained also with the
strip being substantially shorter than the burner.
As wire electrode in FIG. 1 advantageously an iron wire can be used, the
fusion of which increases the amount of the ignition melt. For carrying
out the ignition method according to the invention, of course, also
another metal or alloy can be used as electrode material which burns
exothermally or in another way favourably acts upon the igniting of the
planing process. The invention is not restricted, either, to the using of
only one wire electrode. Two or more such electrodes in parallel can be
used advantageously. Arcs can be produced between each electrode and the
workpiece or only between the electrodes proper. The arcs are fed from one
common or several separate current sources. Control and contact means for
two or more electrodes can be arranged so that their arcs either follow
each other or operate in parallel with each other.
The same effect can possibly also be achieved by means of a simple ignition
device with a multiple ignition wire by repeating the fusion period, if
the return from end position 9 to starting position 1 in FIG. 1 is carried
out sufficiently rapidly.
By using two or more electrodes, or by repeating the fusion period it is
possible to additionally increase the ignition speed for flying start.
Another method of increasing the amount of metal with a temperature lying
above the ignition temperature at the application of the method according
to the invention is the utilization of chemical combustion heat prior to
the commencing of the planing proper. The fusing of the electrode, for
example, can be carried out in oxidizing atmosphere. The oxygen required
can be supplied by an auxiliary oxygen nozzle suitably disposed and, for
example, coupled mechanically to the control means of the electrode. The
device proper is so simple that it is not shown in FIG. 1. The same effect
can be achieved, alternatively, by using a tube electrode, through which
oxygen is fed to the fusion point or which is filled with a suitable
oxidation agent.
In the foregoing the application of a fusible electrode and arc for forming
the ignition strip 15 on the surface of the workpiece has been described.
It is obvious, that the principle of the invention is not changed by
using, instead, a non-fusible electrode. In such a case, only the arc
heats and melts the workpiece. The device for carrying out this method
does not in principle differ from that shown in FIG. 1. The only
difference is that no wire magazine 5 and feed means 4 are provided. The
control and contact means 1, 9 are replaced by a holder for non-consumable
electrode which performs the same pendulum movement.
In the same manner as at the use of consumable electrode also at the use of
a non-consumable electrode an auxiliary nozzle can be used, which blows
oxygen on the melt already during the preheating, in order to increase the
temperature and the amount of the molten metal. It is further obvious
that, instead of a simple non-consumable electrode, several more advanced
heat sources of point-type can be used, based on the use of an arc, for
example a plasma burner. It also is obvious that for the igniting of the
planing according to the invention also suitable non-electric point heat
sources can be used, for example lasers rich in energy. In such a case the
electrode control means 1 in FIG. 1 is replaced by an optical deflection
system, which during the ignition procedure projects the laser beam on the
workpiece, so that the resulting strip of metal heated to ignition
temperature extends across the presupposed planing direction.
It also is apparent from the description of the invention, that the desired
effect according to the invention can be achieved also when the pendulum
movement 10 shown in FIG. 1 between the starting position and the position
9 is replaced by another movement, which per se is known, for example a
longitudinal movement. The invention only requires that the result of the
total relative movement of the workpiece and ignition device during the
action of the ignition device on the workpiece is a strip of preheated
metal. Said strip in principle shall extend across the flow direction of
the planing oxygen. In practice certain tolerances must be permitted,
depending on the accuracy with which relative movements can be controlled.
Another example of an alternative solution of planing ignition is shown in
FIGS. 2a and 2b. At this alternative the transverse component of the
movement of the electrode control is replaced by a controlled or free
movement of an electric arc along the width of a fusible strip electrode.
In FIG. 2a the numeral 16 designates the control and contact means for the
strip electrode 26 advanced from a container 20 by the feed means 19.
Between the electrode 26 and the workpiece 22 an electric arc 29 fed from
a conventional weld current source burns during the activation period of
the ignition device. The current source is connected between the workpiece
22 and the contact means 16, but for reason for simplicity it is not shown
in the Figure. During the activation of the ignition device the control
means 16 of the electrode performs a pendulum movement 25 about the axle
18 by means of the drive mechanism 17. The pendulum movement speed between
starting position 16 and end position 24 indicated by dashed line is
adjusted to the speed of the workpiece movement 30. Hereby, the position
of the end of the strip electrode 26 (and the arc 29 burning to the
workpiece 22) is stationary relative to the workpiece 22. The arc 29
burning during the activation period of the ignition device is controlled
by means of a magnetic field produced by electromagnets 27, 28. During the
pendulum movement 25, the arc shall at least once move from one edge of
the strip electrode over to the other edge thereof.
In FIGS. 2a and 2b, furthermore, a surface defect is shown in position 31
at the beginning of the activation period and by a dashed line in position
32 after completed fusion of the strip electrode. The result of the
pendulum movement 25 and of the effect of the arc 29 is a strip of molten
and preheated metal 33. It is located in front of the planing burner 21
and approaches the same at the relative speed 30. The arrow 23 indicates
the flow direction of planing oxygen from the burner 21. The oxygen is
opened when the melt 33 approaches the extension of the direction 23. In
the oxygen flow the melt 33 is ignited and ignites the planing process
immediately and simultaneously in the entire planing groove width. The
ignition procedure is thereby completed, and the ignition device can
return to non-operative position. In this position the ignition device
waits for a control signal for a new activation between position 16 and
24.
It is apparent from the description of the invention, that a replacement of
the pendulum movement 25 in FIG. 2 by another equivalent longitudinal
movement between the starting and end position of the control means should
not affect the principle of the ignition method according to the
invention. It also is apparent, that the width of the strip electrode
which in FIG. 2 is equal to the width of the burner can be both narrower
and wider than the burner without affecting the principle of the
invention. It is also unessential for carrying out the method, if for
control of the arc movement along the strip other means than magnetic
fields are used, for example the free movement of the arc due to a
successive fusion of the strip.
The method according to the invention is suitable for use at igniting with
flying start at high as well as low speeds. It is, therefore, obvious to
the expert that the ignition method also can be used in such extreme cases
when the relative speed of the workpiece to the burner is equal to zero.
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