Tweco 300 MST Arcmaster - Manual (Page 41)

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7.0 BASIC MIG WELDING GUIDE
7.01 Setting of the Power Source
The setting of the ARC MASTER 300 MST requires some practice by the operator, the welding Power
Source/Wirefeeder having two control settings that have to balance. These are the Wirespeed control
and the Voltage Control. The welding current is determined by the Wirespeed control, the current will
increase with increased Wirespeed, resulting in a shorter arc. Less wire speed will reduce the current
and lengthen the arc. Increasing the welding voltage hardly alters the welding current level, but
lengthens the arc. By decreasing the voltage, a shorter arc is obtained with little change in
welding current.
When changing to a different electrode wire diameter, different control settings are required. A thinner
electrode wire needs more Wirespeed to achieve the same current level.
A satisfactory weld cannot be obtained if the wirespeed and voltage switch settings are not adjusted to
suit the electrode wire diameter and dimensions of the workpiece.
If the Wirespeed is too high for the welding voltage, “stubbing” will occur as the wire dips into the
molten pool and does not melt. Welding in these conditions normally produces a poor weld due to lack
of fusion. If, however, the welding voltage is too high, large drops will form on the end of the electrode
wire, causing spatter. The correct setting of voltage and Wirespeed can be seen in the shape of the weld
deposit and heard by a smooth regular arc sound.
7.02 Position of MIG Torch
The angle of MIG torch to the weld has an effect on the width of the weld run.
Figure 8. MIG Torch Angle
7.03 Distance from the MIG Torch Nozzle to the Work Piece
The electrode stick out from the MIG Torch nozzle should be between ¼” to 3/8”. This distance may
vary depending on the type of joint that is being welded.