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Grinding is the process of removing metal by the application
of abrasives which are bonded to form a rotating wheel. When
the moving abrasive particles contact the workpiece, they act
as tiny cutting tools, each particle cutting a tiny chip from the
workpiece. It is a common error to believe that grinding
abrasive wheels remove material by a rubbing action; actually,
the process is as much a cutting action as drilling, milling, and
lathe turning.
The grinding machine supports and rotates the grinding
abrasive wheel and often supports and positions the workpiece
in proper relation to the wheel.
The grinding machine is used for roughing and finishing flat,
cylindrical, and conical surfaces; finishing internal cylinders
or bores; forming and sharpening cutting tools; snagging or
removing rough projections from castings and stampings; and
cleaning, polishing, and buffing surfaces. Once strictly a
finishing machine, modem production grinding machines are
used for complete roughing and finishing of certain classes of
work.
Grinding machines have some special safety precautions
that must be observed. These are in addition to those safety
precautions described in Chapter 1.
SAFETY PRECAUTIONS
GRINDING MACHINE SAFETY
Grinding machines are used daily in a machine shop. To
avoid injuries follow the safety precautions listed below.
Wear goggles for all grinding machine operations.
Check grinding wheels for cracks (Ring Test Figure 5-11)
before mounting.
Never operate grinding wheels at speeds in excess of the
recommended speed.
Never adjust the workpiece or work mounting devices
when the machine is operating
Do not exceed recommended depth of cut for the grinding
wheel or machine.
Remove workpiece from grinding wheel before turning
machine off.
Use proper wheel guards on all grinding machines.
On bench grinders, adjust tool rest 1/16 to 1/8 inch from
the wheel.
TYPES OF GRINDING MACHINES
From the simplest grinding machine to the most complex,
grinding machines can be classified as utility grinding
machines, cylindrical grinding machines. and surface
grinding machines. The average machinist will be concerned
mostly with floor-mounted and bench-mounted utility
grinding machines, bufting machines. and reciprocating
surface grinding machines.
UTILITY GRINDING MACHINES
The utility grinding machine is intended for offhand
grinding where the workpiece is supported in the hand and
brought to bear against the rotating grinding abrasive wheel.
The accuracy of this type of grinding machine depends on the
operator’s dexterity. skill, and knowledge of the machine’s
capabilities and the nature of the work. The utility grinding
machine consists of a horizontally mounted motor with a
grinding abrasive wheel attached to each end of the motor
shaft.
The electric-motor-driven machine is simple and
common. It may be bench-mounted or floor-mounted.
Generally, the condition and design of the shaft bearings as
well as the motor rating determine the wheel size capacity of
the machine. Suitable wheel guards and tool rests are
provided for safety and ease of operation. Grinding machines
come in various sizes and shapes as listed below.
Floor Mounted Utility Grinding Machine
The typical floor-mounted utility grinding machine stands
waist-high and is secured to the floor by bolts. The floormounted
utility grinding machine shown in Figure 5-1
mounts two 12-inch-diameter by 2-inch-wide grinding
abrasive wheels. The two wheel arrangement permits
installing a coarse grain wheel for roughing purposes on one
end of the shaft and a fine grain wheel for finishing purposes
on the other end this saves the time that would be otherwise
consumed in changing wheels.
Each grinding abrasive wheel is covered by a wheel guard
to increase the safety of the machine. Transparent eyeshields.
spark arresters. and adjustable tool rests are provided for each
grinding wheel. A tool tray and a water pan are mounted on
the side of the base or pedestal. The water pan is used for
quenching carbon steel cutting took as they are being ground.
Using the 12-inch wheel, the machine provides a maximum
cutting speed of approximately 5.500 SFPM. The 2-HP
electric motor driving this machine has a maximum speed of
1.750 RPM.
Bench Type Utility Grinding Machine
Like the floor mounted utility grinding machine, one coarse
grinding wheel and one fine grinding wheel are usually
mounted on the machine for convenience of operation. Each
wheel is provided with an adjustable table tool rest and an eye
shield for protection. On this machine, the motor is equipped
with a thermal over-load switch to stop the motor if excessive
wheel pressure is applied thus preventing the burning out of
the motor. The motor revolve at 3.450 RPM maximum to
provide a maximum cutting speed for the 7 inch grinding
wheels of about 6,300 surface feet per minute (SFPM).
Bench-Type Utility Drill Grinding Machine
The bench-type drill grinding machine is intended for drill
sharpening. The accuracy of this type of grinder is not
dependent on the dexterity and skill of the operator because
the drill is placed in a holding device. The holding device
places the drill in the correct position for the clearance and
included angle. For more information on this machine refer to
chapter 4.
Bench-Type Utility Grinding and Buffing
Machine
The bench-type utility grinding and buffing machine is more
suitable for miscellaneous grinding, cleaning, and buffing It is
not recommended for tool grinding since it contains no tool
rests, eyeshields, or wheel guards. This machine normally
mounts a 4 inch-diameter wire wheel on one end. The wire
wheel is used for cleaning and the abrasive wheel is used for
general grinding. One of the two wheels can be removed and a
buffing wheel mounted in its place for buffing and polishing.
The 1/4-HP electric motor revolves at a maximum of 3,450
RPM. The maximum cutting speed of the 4-inch-diameter
wheel is approximately 3,600 SFPM.
ABRASIVES
Most grinding wheels are made of silicon carbide or
aluminum oxide, both of which are artificial (manufactured)
abrasives. Silicon carbide is extremely hard but brittle.
Aluminum oxide is slightly softer but is tougher than silicon
carbide. It dulls more quickly, but it does not fracture easily
therefore it is better suited for grinding materials of relatively
high tensile strength.
ABRASIVE GRAIN SIZE
Abrasive grains are selected according to the mesh of a sieve
through which they are sorted. For example, grain number 40
indicates that the abrasive grain passes through a sieve having
approximately 40 meshes to the linear inch. A grinding wheel
is designated coarse, medium, or fine according to the size of
the individual abrasive grains making up the wheel.
BONDING MATERIAL
Bond
The abrasive particles in a grinding wheel are held in place
by the bonding agent. The percentage of bond in the wheel
determines, to a great extent, the “hardness” or “grade” of the
wheel. The greater the percentage and strength of the bond,
the harder the grinding wheel will be. “Hard” wheels retain the
cutting grains longer, while “soft” wheels release the grains
quickly. If a grinding wheel is “too hard” for the job, it will
glaze because the bond prevents dulled abrasive particles from
being released so new grains can be exposed for cutting.
Besides controlling hardness and holding the abrasive, the
bond also provides the proper safety factor at running speed. It
holds the wheel together while centrifugal force is trying to
tear it apart. The most common bonds used in grinding wheels
are vitrified, silicate, shellac, resinoid, and rubber.
Vitrified
A vast majority of grinding wheels have a vitrified bond.
Vitrified bonded wheels are unaffected by heat or cold and are
made in a greater range of hardness than any other bond. They
adapt to practically all types of grinding with one notable
exception: if the wheel is not thick enough, it does not
withstand side pressure as in the case of thin cutoff wheels
Silicate
Silicate bond releases the abrasive grains more readily than
vitrified bond. Silicate bonded wheels are well suited for
grinding where heat must be kept to a minimum, such as
grinding edged cutting tools. It is not suited for heavy-duty
grinding. Thin cutoff wheels are sometimes made with a
shellac bond because it provides fast cool cutting.
Resinoid
Resinoid bond is strong and flexible. It is widely used in
snagging wheels (for grinding irregularities from rough
castings), which operate at 9,500 SFPM. It is also used in
cutoff wheels.
Rubber
In rubber-bonded wheels, pure rubber is mixed with sulfur.
It is extremely flexible at operating speeds and permits the
manufacture of grinding wheels as thin as 0.006 inch for
slitting nibs. Most abrasive cutoff machine wheels have a
rubber bond.
GRADES OF HARDNESS
The grade of a grinding wheel designates the hardness of the
bonded material. Listed below are examples of those grades:
A soft wheel is one on which the cutting particles break
away rapidly while a hard wheel is one on which the bond
successfully opposes this breaking away of the abrasive
grain.
Most wheels are graded according to hardness by a letter
system. Most manufacturers of grinding abrasive wheels
use a letter code ranging from A (very soft) to Z (very
hard). Vitrified and silicate bonds usually range from very
soft to very hard, shellac and resinoid bonds usually range
from very soft to hard, and rubber bonds are limited to the
medium to hard range.
BUFFING AND POLISHING WHEELS
Buffing and polishing wheels are formed of layers of cloth
felt or leather glued or sewed together to form a flexible soft
wheel.
Buffing wheels are generally softer than polishing wheels
and are often made of bleached muslin (sheeting), flannel, or
other soft cloth materials. The material is cut in various
diameters and sewed together in sections which are put
together to make up the buffing wheel. The buffing wheel is
often slotted or perforated to provide ventilation.
Polishing wheels are made of canvas, felt, or leather sewed
or glued together to provide various wheel grades from soft to
hard. The harder or firmer wheels are generally used for
heavier work while the softer and more flexible wheels are
used for delicate contour polishing and finishing of parts on
which corners and edges must be kept within rather strict
specifications.
Cylindrical Grinding
If cylindrical grinding is to be performed, such as grinding
of workplaces mounted in the grinding may be done with the
workpiece set up between centers, held in a chuck and
supported by a center rest, or clamped to a faceplate as in lathe
setups.
MOUNTING WORKPIECES
General
Offhand grinding requires no mounting of the workpiece.
Mounting for cylindrical, surface, and tool and cutter grinding
is described below.
Mounting Workpiece for Cylindrical Grinding
Cylindrical grinding may be done with the workpiece setup
between centers, held in the chuck and supported by a center
rest, or clamped to the faceplate as in lathe setups.
Use the following methods when mounting the workpiece
between centers:
Use a dead center in the tailstock spindle. This method is
preferred because it eliminates any error caused by wear
in the machine’s spindle bearings. Before grinding check
the accuracy and alignment of centers and correct if
necessary.
To grind the centers, follow the procedures for grinding
lathe centers in Chapter 7.
After the centers are accurate, align the centers by one of
the methods prescribed for aligning lathe centers.
Position the workpiece between the centers, and use a
lathe dog to revolve the workpiece.
Use the following methods and procedures when mounting
the workpiece for concial grinding.
Workpieces for conical grinding can be set up in a chuck
or between centers.
The table is swiveled to the required taper by means of
the graduations on the end of the table