Hypertherm HD4070 Rev.8 - Manual (Page 147)

OPERATION
4
HD4070 Instruction Manual 4-37
Straightness of the cut surface
A typical plasma cut surface is slightly concave.
The cut surface may become more concave, or convex. Correct torch height is required to keep the cut
surface acceptably close to straight.
A strongly concave cut surface occurs when the torch-to-work distance is too low. Increase the arc
voltage to increase the torch-to-work distance and straighten the cut surface.
A convex cut surface occurs when the torch-to-work distance is too great or the cutting current is too
high. First, reduce the arc voltage, then reduce the cutting current. If there is overlap between different
cutting currents for that thickness, try the consumables designed for the lower current.
Additional improvements
Some of these improvements involve trade-offs, as described.
Smoothness of cut surface (surface finish)
• (HyDefinition and HT4400 only) On mild steel, a higher concentration of N
2
in the O
2
-N
2
shield mixture may
produce a smoother cut surface.
Trade-off: This may produce more dross.
• (HyDefinition and HT4400 only) On mild steel, a higher concentration of O
2
in the O
2
-N
2
shield mixture may
increase the cutting speed and produce less dross.
Trade-off: This may produce a rougher cut surface.
Piercing
The pierce delay must be sufficiently long that the arc can pierce the material before the torch moves, but not so
long that the arc “wanders” while trying to find the edge of a large hole.
When piercing maximum thicknesses, the ring of dross that forms during the pierce may become high enough to
contact the torch when the torch begins to move after the pierce is complete.
• A “flying pierce,” which makes the pierce while the torch is moving, may eliminate the torch vibration that
follows contact between the torch and the ring of dross.
• In some Hypertherm systems, the shield gas pressure automatically increases during pierce delay.
• If the above steps do not solve the problem, increasing the setting of the shield gas pressure may help blow
the molten metal away during piercing.
Trade-off: This may reduce starting reliability.
How to increase cutting speed
• Decrease the torch-to-work distance.
Trade-off: This will increase the negative cut angle
Note: The torch must not touch the workpiece while piercing or cutting.