T.O. 33B-1-1
3-63
on the gun controls the discharge of the powder-air mixture and blow-off air. More elaborate production systems have
been built using this same principle of operation. In these cases, the discharge nozzles are mechanically controlled, as
is the movement of parts through the machine. Spent powder is automatically retrieved and reused.
Figure 3-32. Squeeze Bottle Applicator
3.5.5.7
Effects Of Part Surface Condition / Orientation.
Clean, smooth surfaces are best for successful dry powder testing when the surface is horizontal. If the surface is
rough, powder tends to gather and be held mechanically in depressions on the rough surface. A stronger stream of air
than normal may be required to blow off this loose powder. Care must be taken in the inspection of such rough areas
(for example, a rough weld bead) so that weakly held indications are not also blown away. By watching the area very
carefully while applying powder and blowing off the excess, weak indications can often be seen as the powder shifts.
For very critical inspections, the weld bead is sometimes machined away. Indications of discontinuities, which are
below the surface, are more readily formed on the smooth machined surface of the weld. If the surface being tested is
vertical or even at an angle to the horizontal, an extremely smooth surface becomes a disadvantage, since the dry
powder tends to slide off easily, and weak leakage fields may not be able to hold it in place. Under these circumstances,
a slightly roughened surface give better results.
3.5.5.8
Inspection Technique Variables.
The two basic inspection variables to be considered are the types of current to be used, and the current/particle
application technique. The type of current is dictated by the location of the defects, whether they are on the surface of
the part, or located wholly below the surface. The choice of current is between AC and some form of DC. If the defect
is on the surface, either AC or DC may be used, and the choice is determined by other considerations. If the defect lies
below the surface, AC SHALL NOT be used.
3.5.5.8.1
Current.
AC versus DC is the first basic choice to be made, since the skin effect of AC at 50 or 60 hertz limits its use to the
detection of defects that are on the surface, or only a few thousandths of an inch below it. However, the skin effect of
AC is less at lower frequencies, resulting in deeper penetration of the lines of force. At 25 hertz the penetration is
deeper, and at frequencies of 10 hertz and less, the skin effect is almost nonexistent. If the defects sought are on the
surface, AC has several advantages. The rapid reversal of the field imparts mobility to the particles. The dancing of
the powder helps it to move to the area of leakage fields and to form stronger indications. Alternating current has
