The power source with high open circuit voltage is required for cutting thick work-piece. The open circuit voltage is
related with air type, for example, by using argon the open circuit voltage may be lower, while it is higher by using air,
nitrogen, hydrogen.
8)
Cutting speed
The cutting speed is related with many parameters. The main parameters determining cutting speed include
work-piece thickness, cutting current, air flux and nozzle aperture. A proper drag is allowed during cutting. The cutting
speed should be increased as much as possible, but the incision quality must be guaranteed.
Eliminate the cutting burr
1)
The characteristics of the incision burr
The ordinary cutting surface is smooth and clean, but if the parameter selection is not suitable, and
electrode centering is not good, then burr may be formed on the cutting surface.
Slag is formed by molten metal and its oxide which is adhesive to the bottom edge of incision and solidified.
The reason for forming this slag is that the molten metal adhesive strength is bigger than the gravity and blow
strength of metal oxide.
When cutting the alloy steel, the molten metal is difficult to be blown away because of its bad fluidity, in
addition, the alloy steel have bad thermal conductivity, the incision bottom is over-heated easily, the left molten
metal and incision bottom melts into one, thereby the irremovable and tough burr is formed.
On the contrary, the incision bottom is difficult to be melted together with molten metal, and the burr
formed under incision is come off easily.
2)
The factors affecting the forming of burr
The fluidity of the molten metal is not good, when the power is too small or the plasma arc compression
effect is not good, the temperature of molten metal during the cutting process is low, fluidity is weak, even if
the air current blow force is strong, it is still difficult to blow away the metal completely, so the burr is formed.
When cutting the thick plate, the burr is caused by the drag of overlarge cutting seam. During the cutting
process, the heat received by different parts of metal is different, the heat on the upper incision is larger than
that of the lower incision, so the upper part melting speed is faster than that of lower part, thereby a distance
between them is formed, it is called drag L of cutting seam. The drag size is related to plasma arc shape and
cutting speed. When the flame is short while the cutting speed is too fast, drag L increases, so the vertical and
horizontal blow force of the arc is formed, the vertical one helps to blow away the molten metal, while the
horizontal one makes the molten metal flow backward along with incision bottom, this over-heated metal will
melt parts of the bottom metal again, then the burr is formed when they cool down and melt together.
The burr is caused by overheat bottom. When the cutting speed is too slow, but the incision bottom is so
over-heated that it melts, the liquid metal flows to bottom metal and combines into one which makes the
difficulty to blow away molten metal by air current, and then the burr is formed.
The air current blow force is not enough. When cutting with plasma, the arc blow force consists of the air
current blow force and the arc electromagnetic force, the air current blow force acts the main function. If the
air current blow force is not strong enough, it cannot ensure all the burr is flown away, then the burr is formed.
3)
The measurement to eliminate burr
Ensure the centering between electrode and nozzle precisely, so that the compression of the plasma arc is
not damaged, the concentration of flame and cutting capacity can be guaranteed.
Enough power to ensure the fluidity of molten metal, as well as increase the stability of the cutting speed
and operation. This makes it possible to adopt large air flux to enlarge the air blow force, and beneficial for
eliminating burr.
Adjust suitable air flux and cutting speed. If the air flux is too small, the blow force is not enough, while if too
big, the plasma arc will be shorten, the incision will be “V” shape, the drag enlarge. The burr can be formed
under both conditions. When cutting speed is too slow, the incision is large and rough, the bottom is easy to be