Process

 

Honing

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?

 

Lapping

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 wheel laps.

Superfinishing

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:
Although not strictly considered mass-finishing processes, other surfaces finishing process explained in this section are:

Barrel 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 without rollover.

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

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 another.
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 minute).

 

Spindle Finishing

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:
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 pumped in.

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

Process

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 convex surfaces.
 
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 or gritlines.

 

Machines

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 motion.

Product Applications

ypical parts commonly processed by belt finishing include :
Belt finishing is often the process selected to remove welding beads and various other unwanted projections on machine parts.

Polishing

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.

Product Applications

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

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.

 

Product Applications

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.