Honing is a mechanical means of stock removal that uses
spring loaded abrasive stones as the cutting tool. The stone may be composed
of aluminum oxide, silicon oxide, or in some cases diamond grains held
together by a vitrified or organic bond. The number of stones in
the tool head, their length and width, are determined by the size and nature
of the work to be performed. Both horizontal and vertical honing machines
are available. Engine cylinders and other parts with short bores are generated
honed on vertical machines, while long holes in parts, such as cannons
or rifle barrels, are honed in horizontal machines. Honing produces geometrically
accurate forms by correcting various inaccuracies remaining from previous
operations, such as high spots, chatter marks, out-of-roundness, taper,
or deviations in axial straightness. Tolerances within 0.0001 in. are easily
maintained. Honing produces a characteristic cross-hatch matte finish or
lay pattern made up of "hills and valleys." Each minute scratch serves
as an oil reservoir for lubricants, thus diminishing the possibility of
wear on a workpiece in service by minimizing friction and heat. The surface
generated by honing is free of torn, smeared, or "burned" metal.
What Requires Honing?
Parts with tight tolerances.
Parts too difficult to machine economically.
Parts too rough, needing improved microfinish.
Parts undersized in machining.
Parts out of round, out of tolerance.
Parts that shrunk in heat treat.
The process of lapping is performed on workpieces by
manual or by machine methods, principally to increase accuracy. Other important
advantages which are obtained automatically as a part of the process are
the correction of minor surface imperfections, improvement of surface finish,
and achieving a close fit between mating surfaces. Lapping is a gentle,
final operation commonly used to microfinish flat or cylindrical surfaces,
but the process is also adaptable to spherical or specially formed surfaces.
All lapping methods are done at low speed. Lapping is not considered a
stock removal process. Standard practice is to provide only 0.0005 in.
of stock allowance for lapping, and preferably less. Lapping can be performed
either with cast iron laps using loose abrasives or with bonded abrasive
Superfinishing is a proprietary name given to a microfinishing
process that produces a controlled surface condition on parts which is
unobtainable by any other method. It produces the ultimate in the refinement
of metal surfaces. Superfinishing is an abrading process in which the cutting
medium for cylindrical work is a loosely bonded abrasive stick or stone.
An abrasive cup wheel is used for flat or spherical work. The process
consists of removing fragmented or smear metal from the surface of a dimensionally
finished part formed by a previous operation, notably by turning or grinding,
but possibly by honing or lapping. Dimensional changes are principally
limited to the removal of high spots. Superfinished parts are bright and
reflective with an undisturbed crystalline structure.
Mass Surface Finishing Process
Precisely controlled results can be consistently obtained by certain
modifications of the mechanical grinding and honing processes. Each of
these mass-finishing processes use a mixture of abrasive grain media, principally
aluminum oxide or silicon carbide, together with special compounds and
water. Mass surfaces finishing process include:
Orbital or centrifugal finishing
Although not strictly considered mass-finishing processes,
other surfaces finishing process explained in this section are:
Abrasive belt finishing
A typical barrel finishing machine is a horizontally
mounted eight sided unit with a water tight loading door. The barrel is
loaded within 50-60% of its rated capacity with parts, media, and compound.
As the barrel turns, parts and media are carried up the side until gravity
causes them to slide down again. This action is schematically illustrated
in the figure below. All the work is performed during this slide.
Barrel finishing is relatively slow. During the time
that a part of the load is being carried up to the start of the slide,
no work is being performed. The barrel can be only partially filled; if
it were full, there would be no work at all.
The action also dictates the top speed of the barrel (i.e., it is going
too fast when the work begins to cascade, with consequent nicking and marring
of the work). The optimum operating condition is achieved when the barrel
is rotated at a speed just below the cascade point.
The barrel tends to abrade edges and exposed surfaces
much more than recesses and the insides of the bores. Large corner radii
can be formed on parts without difficulty, and large burrs can be removed
The barrel is, of course, a batch type operation; it is
usually not compatible with in-line processing. Barrels come in a range
of sizes from a fraction of a cubic foot to as large as 40 cu. feet. Rotational
speeds of barrels range from 50 to 200 fpm. Low surface speeds are usually
employed for burnishing, while faster surface speeds are used for heavier
stock removal operations.
Vibratory finishing in its simplest denominator is the
finishing of parts in a tub whose vibrating motion creates an abrasive
action in the loose abrasive. The action removes burrs, rounds, corners,
finished bores, and other concealed surfaces. It is faster and more aggressive
than barrel finishing; the cycles are in terms of minutes rather than hours.
Figure below shows one model of a vibratory finishing machine which has
an open top U-shaped tub. Since the tub remains upright at all times, it
may be loaded to about 90% of capacity. As it vibrates, the mass of media,
parts, and compound usually move in an elliptical path, the plane of which
depends upon the form of the tub. The abrasive is in constant motion against
the work, deburring, rounding corners, and working on all the surfaces
to which it has access.
Another type of machine features a round bowl. Parts
travel in a circular path and tend to space themselves at regular intervals.
The vibratory action is rapid and constant. The tub on most vibratory machines
is mounted on sprigs and is vibrated by eccentric or magnetic mechanisms.
In contrast to the barrel, which produces a relatively long scratch pattern,
the vibrator produces a short choppy pattern. The work of course can be
examined at any time simply by picking it up out of the tub.
The vibrating mechanism of the tub, although not a complicated piece
of machinery, is subject to a greater degree of wear than the simple barrel.
On the other hand, the vibrator lends itself quite well to in-line processing,
with parts moving into the vibrator from another process, and then on to
Vibratory finishing has an advantage over barrel finishing
for parts that are likely to get tangled up; in the vibrator, parts and
media tend to retain their distances. This is a continuous and unbroken
path; hence the parts do not get entangled with each other.
The machines are available in a range of sizes from 1/8
cu. feet to 70 cu. feet capacity. Depending on the machine, they may be
equipped with amplitude settings adjustable from 1/64 in. to 1/4 in. and
with variable frequency controls ranging up to 600 vpm (vibrations per
Spindle finishing is a loose grain process that has been
termed the "form-fitting grinding wheel." The work is chucked on one or
more spindles and lowered into a tub containing an abrasive grain. The
spindles with the attached parts are slowly rotated to expose all surfaces
to a high velocity abrasive stream. During processing, the tub spins up
to 1200 fpm in a direction opposite to the rotation of the spindle. Control
of the finishing process can be achieved through variation of the following
major elements of the cycle:
Speed of rotation of the tub
Speed of rotation of the part on the spindle
Depth of submersion of the part
Angle of the spindle
Length of the cycle
Type and size of abrasive
These elements are all interrelated. If the speed is
too fast, for example, the grain will have a peening action rather than
a cutting action and will roll over the burrs rather than cutting them
and rounding the edges. The part must be covered by abrasive at all times.
The spindle angle determines the degree to which the interior of the
part is worked on. When vertical, it confines the action to the outside
of the part. The more acute the angle, the more action there is on the
inside of the part.
The abrasive used in this operation is aluminum oxide
grain wetted down with water, and with charges of detergent periodically
This process is practical for use on fragile parts ordinarily
fixed to prevent impingement, and where close control of deburring and
edge breaking is imperative. Part geometry is not a problem, as long as
the part can be chucked.
Abrasive Belt Finishing
Abrasive belt finishing is a low cost, relatively fast
finishing process. With the development of better resin bonded belts having
increased flexibility and improved joints coupled with improvements in
machinery, the versatility of the process has reached even greater acceptance
on the production line. Abrasive belt finishing is now an important size
and surfacing process for the precision finishing of flat, concave, and
Stock removal is accomplished by the abrasive grains on a moving belt
as they continuously pass over the work area. In this way, burrs, high
spots, the coarse texture on cast parts, parting lines, or machining marks
are refined or totally removed from a workpiece.
Most production belt grinding and polishing operations
use a lubricant that is applied at frequent intervals. Aluminum alloys
are finished dry. Belt life is extended by using a series of coated belts
of varying degrees of abrasive fineness, usually roughing, polishing, and
fine polishing. The final surface finish produced is superior to that obtained
by milling or turning. Inspite of the term "fine polishing," however, the
resulting surface on the workpiece is always characterizes by fine scratches
Most belt finishing machines consist of a motor driven
contact wheel and idler arrangement over which an endless coated-abrasive-tensioned
belt rides. Flat finishing requires a sturdy support pattern. Contoured
finishing, can be performed by rotating the part against a flat contact
wheel, against a formed contact wheel, or offhand, entirely against the
moving belt without support. Tubing and rolls may be finished in centerless
abrasive belt machines. Fixtures are often used to simplify parts handling
in repetitive operations on large production runs. Wide sheet and coil
stock are finished by abrasive belt machines in a smooth and continuous
ypical parts commonly processed by belt finishing include
Golf club heads
Jet engine turbine blades
Hand tools including screwdrivers, hammerheads, wrenches, and so on.
Belt finishing is often the process selected to remove
welding beads and various other unwanted projections on machine parts.
Polishing, or flexible grinding, is an intermediate,
dimensionless step in the formation of a finished surface. It is generally
preceded by grinding with a solid abrasive wheel and followed by buffing.
A polished surface is accomplished by the cutting action of millions of
small abrasive grains adhering to an endless coated belt or flexible wheel
as they wear away the metal. The complete polishing sequence usually
involves several steps, first to remove the initial scratches and defects
and then to gradually impart the final surface condition.
In addition to the many product applications already
listed under belt finishing, manufacturers of cutlery and small hand tools
are particularly dependent upon polishing for finishing. This process is
also used for such work as "stain finishing," deburring, and for cleaning
up irregularly shaped parts prior to plating or buffing.
Buffing generally follows polishing and is usually the
final operation that is performed on a workpiece. In buffing, the rubbing
action is more gentle than the vigorous and aggressive cutting action employed
in polishing. Buffing removes negligible amounts of material. A buffed
surface is formed in two distinct steps: 1.) Cutting down and 2.) Coloring.
During the initial finishing stage of cutting down, minute scratches left
by polishing and other surface irregularities are reduced or entirely eliminated.
It is during the final stage of buffing (coloring) that the ultimate reflective,
highly lustrous surface is produced.
Buffing wheels, called "buffs," are often fabricated into a number
of piles from a series of individual fabric disks of various kinds. Buffing
wheel speeds are in the range 6500 to 8000 fpm.
The principle abrasive used for buffing compounds on
aluminum, copper, brass, and for zinc alloy die castings is Tripoli. The
buffing compounds may be manually applied to metal products in the same
manner as is outlined for polishing (i.e., solid bar compound). The compound
may be also be supplied to the wheel face in liquid form by using a patented
airless -spraying system. Liquid compounds are highly suitable for machine
or automatic buffing.
The decorative mirror like finish obtained by buffing
is applied to a wide selection of metal products, including objects used
on mobile homes, automobiles, motorcycles, boats, bicycles, as well as
sporting items, tools, store fixtures, commercial and residential hardware,
and household utensils and appliances. Buffing may be specified both prior
to and following plating.