Tweco XTTM-301 - Manual (Page 52)

Manual 0-4768 4-14 OPERATION
B. Travel Speed
Proper travel speed is indicated by the trail of the arc
which is seen below the plate. The arc can be one of
the following:
1. Straight Arc
A straight arc is perpendicular to the workpiece
surface. This arc is generally recommended for the
best cut using air plasma on stainless or alumi-
num.
2. Leading Arc
The leading arc is directed in the same direction as
torch travel. A five degree leading arc is generally
recommended for air plasma on mild steel.
3. Trailing Arc
The trailing arc is directed in the opposite direc-
tion as torch travel.
A-00662
Standoff
Distance
Straight Arc
Trailing Arc
Leading Arc
Direction of Torch Travel
Shown with
Rack and Pinion
Mounting Assembly
Machine Torch Operation
The arc characteristics vary with gases used, cutting
speed, material, and thickness. Air plasma normally
produces a straight arc (on stainless or aluminum),
while nitrogen plasma generally creates a 5° trailing
arc.
For optimum smooth surface quality, the travel speed
should be adjusted so that only the leading edge of the
arc column produces the cut. If the travel speed is too
slow, a rough cut will be produced as the arc moves
from side to side in search of metal for transfer.
Travel speed also affects the bevel angle of a cut. When
cutting in a circle or around a corner, slow down the
travel speed to achieve a squarer cut. The power
source output should be reduced also.
C. Piercing With Torch
To pierce with the torch, start the arc with the torch
positioned as high as possible above the plate while
allowing the arc to transfer and pierce. This standoff
helps avoid having molten metal blow back onto the
front end of the torch.
When operating with a cutting machine, a pierce or
dwell time is required. Torch travel should not be
enabled until the arc penetrates the bottom of the
plate. As motion begins, torch standoff should be re-
duced to the recommended distance for optimum speed
and cut quality.
Piercing the plate is not recommended on plates thicker
than 1/2 inch (12.7 mm). Blowback from the piercing
operation can shorten the life of torch parts. All pierc-
ing should therefore be done as quickly as possible
and at maximum amperage and maximum standoff.
Pierce off the cutting line and then continue the cut as
needed. Clean spatter and scale from the shield cup
and the shield cap as soon as possible. Spraying or
dipping the shield cup and cap in anti-spatter com-
pound will minimize the amount of scale which ad-
heres to it. This can significantly increase parts life.
4.08 Recommended Cutting
Speeds
Cutting speed depends on material, thickness, and input volt-
age. These other factors may affect system performance:
• Torch parts wear
• Air quality
• Operator experience
• Torch standoff height
• Proper work cable connection
• Alloy content of material
NOTE
This information represents realistic expectations using
recommended practices and well-maintained systems.
Actual speeds may vary from those shown in the charts
depending on the alloy content of the selected material.
See the Appendix pages for complete cutting data.