I first built a machine for grinding
and figuring mirrors according to the directions in the There are directions for
a machine available on the web.(2) The machine worked, but it was gutless and
took a very long time to polish out a 6" blank. I was in a hurry and decided
to build a more powerful version of a Wig machine. The turntable tended to skip
in the initial version, so I decided to drive the table directly. Pictures of
this second grinding machine are shown below. It used a 200000 RPM motor from
Herbach and Rademan.(3) The motor is geared down from 325 RPM, to a turntable
speed of 24 RPM. The overarm stroke is about 9 Ronnie. These rates can be adjusted
by changing pulleys in the drive chain. I have made a number of mirrors with it
this new machine.
Cylinder wheels, type number 2, may be arranged
for grinding on either the periphery or side of the wheel.
Tapered
Tapered
wheels, type number 4, take tapered safety flanges to keep pieces from flying
if the wheel is broken while snagging.
Straight Cup
The straight
cup wheel, type number 6, is used primarily for cylindrical surface grinding,
but can also be used for offhand
I gradually became aware of three
limitations of this machine: the table rocked, and the stroke length was not continuously
adjustable. The table rocked because I was resting it on three wheels for support
which is one of the trademarks of a machine, but I was driving it with a central
shaft, and the two support systems were not perfectly aligned. When I made the
connection to the central shaft so that it would flex, the support by the three
wheels worked very well. The turntable would hold lots of weight, but when sideways
force was applied to Buffing Abrasives
Buffing abrasives are comparatively
fine and are often made up in the form of paste, sticks, or cakes; the abrasive
being bonded together by means of grease or a similar vehicle. The abrasive sizes
for buffing are 280, 320, 400, 500, and 600. Some manufacturers use letters and
numbers to designate grain size such as F, 2F, 3F, 4F, and XF (from fine to very
fine). Pumice, rottenstone, and rouge are often used as buffing abrasives.
Lapping
Abrasives
Only the finest abrasives are used for lapping. These may be
either natural or artificial. Abrasives for lapping range from No. 220 to No.
600 or No. 800 which are very fine flours. Lapping compounds are generally mixed
with water or oil so that they can be readily applied to the lap.
the
table (i.e. when polishing) the table tended to rock and table base lifted up
off one of the wheels. This made me very nervous about putting much force on the
machine when polishing, and I suspect it also introduced zones on the mirror surfaces.
The machine polished out blanks nicely, but the surfaces needed lots of hand work
to clean them up.
I rebuilt the machine when I became aware of this machine
had two features that attracted my attention: a continuously adjustable clutch,
and a turntable supported by a central shaft. I rebuilt my machine to include
these two features. The rebuilt Corbett clutch follows the design except I made
mine out of aluminum rather than plywood. I used cork gasket material as the clutch
contact surface. The clutch works great, and was not hard to make.
The turntable
rests on a 12" pulley, and I set mine up with a thrust bearing to support
the weight. The 3/4" steel shaft extends about 1" above the pulley,
and the turntable has a 3/4 bronze sleeve epoxide into a hole in the center. Between
support from the pulley and the shaft-sleeve contact, the turntable does not rock.
I have polished out one 13" mirror on the newest version of the machine,
and it worked very nicely. Pictures of the rebuilt grinding machine are shown
below.
grinding of flat surfaces. Plain or beveled faces
are available.
Flaring Cup
The flaring cup wheel, type number 11,
is commonly used for tool grinding. With a resinoid bond, it is useful for snagging.
Its face may be plain or beveled.
Dish
Chapter 15
Cylindrical
Grinders and Cylindrical Grinding Machines
The grinding machine is used for
roughing and finishing flat, cylindrical, and conical surfaces; finishing internal
cylinders or bores; 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, . 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. These are in addition to those safety precautions described in Chapter
1.
The grinding machine supports and rotates the grinding abrasive wheel
and often supports and positions the workpiece in proper relation to the wheel.
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. (Ring Test Figure
5-11) before mounting.
Grinding machines have some special safety precautions
that must be observed.
SAFETY PRECAUTIONS
GRINDING MACHINE SAFETY
Grinding machines are used daily in a machine shop. To avoid injuries follow
the safety precautions listed below. each particle cutting a tiny chip from the
workpiece
Wear goggles for all grinding machine operations.
Check
grinding wheels for cracks
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,
buffing 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
Grinding wheel travel - 7 l/2-inch
vertical.
Grinding wheel travel - 5 1/2-inch horizontal.
Table travel
- 6 inches.
Slitting saws with attachment - 12-inch diameter.
Distance
between centers - 14 inches.
Swing on centers (diameter) - 4 l/2-inch diameter.
Swing in work head (diameter) - 4 l/2-inch diameter.
The
typical floor-mounted utility grinding machine stands waist-high and is secured
to the floor by bolts. The floor-mounted 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 Cylindrical 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.
Bench-Type Tool and Cutter Grinder
The bench-type tool and cutter grinder, see Figure 5-4, was designed primarily
to grind end mills. It can also grind a large variety of small wood and steel
cutters as well as slitting saw cutters up to 12 inches in diameter using the
saw grinding attachment. Capacity of the typical bench-type tool and cutter grinder
is as follows:
Nonspecialized cylindrical grinding machines
in the Army maintenance system include the tool post grinding machine and the
versa mil attachment.
Tool Post Grinding Machine
The tool post grinding
machine, see Figure 5-5, is a machine tool attachment designed to mount to the
tool post of engine lathes. It is used for internal and external grinding of cylindrical
workpieces. Refer to Chapter 7 for a description of this machine.
Milling
and Grinding Lathe Attachment
Also called a Versa-Mil this attachment is
a versatile machine tool attachment that mounts to the carriage of a lathe. It
performs internal and external cylindrical grinding among its other functions.
Refer to Chapter 9 for a description of this machine.
SURFACE GRINDING
MACHINE
The surface grinding machine is used for grinding flat surfaces.
The workpiece is supported on a rectangular table which moves back and forth and
reciprocates beneath the grinding wheel. Reciprocating surface grinding machines
generally have horizontal wheel spindles and mount straight or cylinder-type grinding
abrasive wheels.
Grinding Plain Milling Cutters
Plain milling cutters
with saw-tooth type teeth are sharpened by grinding the lands on the periphery
of the teeth. The lands may be ground using a straight grinding wheel or a cup-shaped
grinding wheel.
The important consideration when grinding this type of
cutter is the primary clearance angle or relief angle of the land (Figure 5-19).
If the primary clearance angle is too large, the cutting edge will be too sharp
and the cutter will dull quickly. If the primary clearance angle is too small,
the cutter will rub rather than cut and excessive heat will be generated.
The
primary clearance angle (Figure 5-19) should be between 3° and 5° for
hard materials and about 10° for soft materials like aluminum. For cutters
under 3 inches in diameter, a larger clearance angle should be used: 7° for
hard materials and 12° for soft materials.
The clearance angle for
end and side teeth should be about 2° and the face of these cutters should
be ground 0.001- or 0.002-inch concave toward the center to avoid any drag.
To
grind the lands of milling cutter teeth to primary clearance angle, the teeth
are positioned against the grinding wheel below the wheel's axis (Figure 5-20).