by tiny blocks of brilliant white light. Silicon suppresses
the carbon burst even more than nickel. When silicon is
present, the carrier line usually ends abruptly in a white
flash of light.
Spark testing may be done with either a portable or
stationary grinder. In either case, the speed on the outer
rim of the wheel should not be less than 4,500 feet per
minute. The abrasive wheel should be rather coarse,
very hard, and kept clean to produce a true spark
To conduct a spark test on an abrasive wheel, hold
the piece of metal on the wheel in a position that allows
the spark stream to cross your line of vision. By trial and
error, you soon discover what pressure is needed to get
a stream of the proper length without reducing the speed
of the grinder. Excessive pressure increases the tem-
perature of the spark stream. This, in turn, increases the
temperature of the burst and gives the appearance of a
higher carbon content than actually is present. When
making the test, watch a point about one third of the
distance from the tail end of the spark stream. Watch
only those sparks that cross your line of vision and try
to forma mental image of the individual spark. Fix this
spark image in your mind and then examine the whole
While on the subject of abrasive wheels, it is a good
idea to discuss some of the safety precautions associated
with this tool.
l Never use an abrasive wheel that is cracked or
out of balance because the vibration causes the wheel to
shatter. When an abrasive wheel shatters, it can be
disastrous for personnel standing in line with the wheel.
l Always check the wheel for secure mounting and
cracks before putting it to use. When you install a new
wheel on a grinder, be sure that it is the correct size.
Remember, as you increase the wheel radius, the periph-
eral speed at the rim also increases, even though the
driving motor rpm remains the same. Thus, if you should
use an oversized wheel, there is a distinct danger the
peripheral speed (and consequent centrifugal force) can
become so great that the wheel may fly apart. Use
wheels that are designed for a specific rpm. Guards are
placed on grinders as protection in case a wheel should
. Never use a grinder when the guards have been
removed. When turning the grinder on, you should stand
to one side. This places you out of line with the wheel
in case the wheel should burst.
l Never overload a grinder or put sideways pres-
sure against the wheel, unless it is expressly built to
withstand such use.
l Always wear appropriate safety goggles or a face
shield while using the grinder. Ensure that the tool rest
(the device that helps the operator hold the work) is
adjusted to the minimum clearance for the wheel. Move
the work across the entire face of the wheel to eliminate
grooving and to minimize wheel dressing. Doing this
prolongs the life of the wheel.
l Keep your fingers clear of the abrasive surface,
and do not allow rags or clothing to become entangled
in the wheel.
l Do not wear gloves while using an abrasive
l Never hold metal with tongs while grinding.
. Never grind nonferrous metals on a wheel in-
tended for ferrous metals because such misuse clogs the
pores of the abrasive material. This buildup of metal
may cause it to become unbalanced and fly apart.
c Grinding wheels require frequent recondition-
ing. Dressing is the term used to describe the process of
cleaning the periphery. This cleaning breaks away dull
abrasive grains and smooths the surface, removing all
the grooves. The wheel dresser shown in figure 1-3 is
used for dressing grinding wheels on bench and pedestal
grinders. For more information on grinding wheels, you
should consult chapter 5 of NAVEDTRA 10085-B2
(Tools and Their Uses).
Referring now to figure 1-4, notice that in low-
carbon steel (view A), the spark stream is about 70
inches long and the volume is moderately large. In
high-carbon steel (view B), the stream is shorter (about
55 inches) and the volume larger. The few sparklers that
may occur at any place in low-carbon steel are forked,
Figure 1-3.Using a grinding wheel dresser.