Mass Media Finishing Processes Explained

By David Davidson posted 01-02-2016 22:48

  

MECHANICAL SURFACE PREPARATION

Mass Media Finishing Processes Explained

David A. Davidson

Deburring/Surface Finishing Specialist, dryfinish@gmail.com
SME Machining/Material Removal Technical Community


Photo courtesy M. Riley, BV Products, Melbourne, Australia

 What is mass finishing?

Mass finishing is a term used to describe a group of abrasive industrial processes by which large lots of parts or components made from metal or other materials can be economically processed in bulk to achieve one or several of a variety of surface effects. These include deburring, descaling, surface smoothing, edge-break, radius formation, removal of surface contaminants from heat treat and other processes, preplate and prepaint or coating surface preparation, blending in surface irregularities from machining or fabricating operations, producing reflective surfaces with nonabrasive burnishing media, refining surfaces, and developing superfinish or microfinish equivalent surface profiles.

All mass finishing processes utilize a loose or free abrasive material referred to as media within a container or chamber of some sort. Energy is imparted to the abrasive media mass by a variety of means to impart motion to it and to cause it to rub or wear away at part surfaces. Although by definition, the term mass finishing is used generally to describe processes in which parts move in a random manner throughout the abrasive media mass, equipment and processes that utilize loose abrasive media to process parts that are fixtured come under this heading also.

Why mass finishing?

Nearly all manufactured parts or components require some measure of surface refinement prior to final assembly, or the final finish or coating required to make the parts acceptable to the consumer or end-user. Most manufacturing companies who employ mass finishing techniques do so because of the economic advantages to be obtained, especially when compared with manual deburring and surface finishing techniques. Mass finishing processes often reduce or eliminate many procedures that are labor intensive and require extensive part handling. This is especially important in meeting increasingly stringent quality control standards, as most mass finishing processes generate surface effects with part-to-part and lot-to-lot uniformity that cannot be replicated with processes in which parts are individually handled. It has become a manufacturing engineering axiom that part reject and rework rates will plummet, if a mass finishing approach can be implemented to meet surface finish requirements.

Although each of the mass finishing process types carries with it a unique set of process strengths and weaknesses, all of them are sufficiently versatile to be able to process a wide variety of part types successfully. A plethora of abrasive media types, sizes, and shapes makes it possible, in many cases, to achieve very different results within the same equipment, ranging from heavy grinding and radiusing to final finishing. Components from almost every conceivable type of material have been surface conditioned using mass finishing techniques including ferrous and nonferrous metals, plastics, composition materials, ceramics, and even wood.


Mass finishing processes (barrel, vibratory cemtrifugal etc.) can be utilized on a wide variety of materials.  Although commonly thought of as processing technology for metal parts, variants of the technology have also been also used on various plastic, composite, ceramic and even wooden components

Mass finishing cautions

Despite the immense versatility of these types of processes, some potential process limitations should be noted. It can be difficult to selectively treat certain part areas to the exclusion of other areas, which might have critical dimensional tolerance requirements. Unless masked or fixtured, all exterior areas of the part will be affected by the process to a greater or lesser degree, with effects on corners and edges being more pronounced than those on flat areas, and with interior holes, channels, and recesses being relatively unaffected in the more common processes.

Care must be exercised in media size, shape selection, and maintenance to prevent media lodging in holes and recesses, which might require labor-intensive manual removal. Some parts have shapes, sizes, or weights that may preclude them from being finished in some mass finishing processes because of the risk of impingement from part-on-part contact or of nesting due to certain features of the parts interlocking together when in proximity. Additionally, most processes that use water in conjunction with the abrasive media create an effluent stream, which must be treated prior to discharge into municipal sewage or other disposal.

Mass finishing part of the manufacturing process

Much time and money can be saved both in mass finishing process operations and in process development if finishing considerations are given sufficient weight at the design, production, and quality control stages. Although it is a rule more breached than observed, it should be noted that mass finishing processes are not, and were never intended to be, methods for rectifying errors made in earlier stages of the manufacturing process. It should be equally obvious that processes developed for parts made with tools and dies that are sharp will no longer produce the same results when that tooling becomes dull. Mass finishing processes can produce remarkably uniform results if process parameters are followed carefully, but this assumes some measure of uniformity of surface condition for a given part within a lot, and from lot-to-lot, as received in the finishing area.



Mass Finishing processes can be used to develop highly refined isotropic surfaces and even polished micro-finishes such as that shown on this automotive engine component.  To produce super-finishes on this level may requite a sequence of processes.  While at times these processes are used to develop cosmetic or decorative finishes, but often finishing to this level can be functionally important beyond creating the quality aesthetic.  These plateaued or planarized isotropic surface finishes can improve wear resistance and, prevent premature fatigue failure. they also can promote better lubricant distribution and reduce friction and component operating temperatures.  Many of these processes also have a cold-hardening effect and can develop useful and beneficial compressive stress equilibrium to parts materially lengthening their useful service life.  The methods also offer noted improvement in part-to-part and lot-to-lot uniformity when compared to methods that rely on individual part processing and handling.  [Photo credit:  Matk Riley, BV Products, Australia]

Mass finishing equipment

One of the more obvious factors influencing mass finishing processes is equipment selection. There are five major equipment groups as follows: barrel, vibratory, centrifugal barrel, centrifugal disk, and spin/spindle finishing.

As Table I shows, there are variations within each major grouping, and each equipment group has its own set of advantages. The first four groups are primarily used with parts immersed within a body of abrasive media and are capable of some independent movement within that mass. On occasion, fixturing or some subcompartmentalization may be used to isolate delicate or critical parts from each other. Part-on-part contact may also be minimized by using higher media-to-part ratio combinations. Common media-to-part ratios for noncritical parts run anywhere from 1:1 to 1:4 by volume. Parts with a higher need for cushioning and protection may utilize media/part ratios as high as 10:1 to 15:1. In contrast, all spin/spindle finishing processes utilize fixturing of parts, and in most cases movement of the fixture develops much of the action needed to abrade the parts.

Table I. Mass finishing equipment selection considerations

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Horizontal barrel

 

Oblique barrel

 

Round vibrator

 

Tub vibrator

 

Centrifugal barrel

 

Centrifugal disk

 

Spin/spindle finish

 

 

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Time cycles

 

Long

 

Longer

 

Medium

 

Medium

 

Short

 

Short

 

Very short

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Media wear

 

Slow

 

Very slow

 

Moderate

 

Moderate

 

High

 

Very High

 

Very High

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Media size

 

Large

 

Large

 

Medium

 

Medium

 

Small

 

Small

 

Very small

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Equipment cost

 

Low

 

Low

 

Moderate

 

Moderate

 

High

 

High

 

High

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Typical kinds of processes

 

Heavy radiusing, burnishing, dry polishing

 

Drying, part-on-part finishing

 

Deburring, smoothing, burnishing, preplate

 

Deburring, stock removal, large parts

 

Micro-finishing, polishing, fast stock removal

 

Aggressive stock removal, smoothing, deburring

 

Aggressive deburring, stock


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

removal, no impingement, dry color buff

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Part size limitations

 

Medium

 

Small to medium

 

Restricted length by bowl diameter; flat parts nest

 

Almost unlimited, very large per machine

 

Small to moderate, fixturing or compartments possible

 

Part length severely restricted by size of chamber

 

Some part geometry restriction

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Type of energy

 

Rotational, gravity slide

 

Rotational, gravity slide

 

Kinetic, vibratory

 

Kinetic, vibratory

 

Centrifugal, pressure

 

Centrifugal, toroidal

 

Spin, media resistance

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Continuous or batch

 

Batch

 

Batch

 

Continuous possible

 

Continuous possible

 

Batch

 

Batch

 

Batch Liquid

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Compound usage

 

Low

 

Low

 

High with flow-through systems

 

High with flow-through systems

 

Low

 

High with flow-through systems

 

Medium

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Working capacity

 

50%

 

35%

 

80-90%

 

80-90%

 

60% wet, 80-90% dry

 

30-40%

 

N/A; Fixtured

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Exterior or interior part areas

 

Concentrates on exterior corners, edges

 

Concentrates on exterior corners,edges

 

Interior and exterior

 

Interior and exterior

 

Exterior; some interior

 

Exterior and interior similar

 

Dependent on fixture orientation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Media/parts material handling

 

Awkward with external separation

 

Easier unloading than horizontal barrel

 

Automated internal separation

 

Discharge chute to exterior separation

 

Manual load, machine unload

 

Manual or automatic

 

Manual or robotics

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In-process inspection?

 

No

 

Yes

 

Yes

 

Yes

 

No

 

Yes, usually

 

Not usually

 

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Shown above is an example of an octagonal horizontal rotary barrel.  This style of equipment has been in use in North America and Europe since the early part of the twentieth century for bulk-processing large numbers of metal parts that required deburring, edge-break, radiusing, smoothing, burnishing and polishing.  This style of equipment was widely used for abrasive finishing metals in various abrasive media along with water and compounds.  Dry processing with finer polishing abrasives and natural materials were used to produce fine finishes on plastics and many non-ferrous metals with surfaces that rivaled that of traditional buffing and polishing operations.  This kind of finishing is still common place in some industries that require high quality decorative finishes in both North America and throughout Europe.

Barrel finishing

Barrel finishing is unquestionably the oldest of the mass finishing methods, with some evidence indicating that crude forms of barrel finishing may have been in use by artisans as far back as the ancient Chinese and Romans as well as the medieval Europeans.

In this method, action is given to the media by the rotation of the barrel. As the barrel rotates, the media and parts within climb to what is referred to as the turnover point. At this point, gravity overcomes the cohesive tendencies of the mass, and a portion of the media mass slides in a retrograde movement to the lower area of the barrel. Most of the abrading or other work being performed on parts within the barrel takes place within this slide zone, which may involve as little as 10-20% of the media mass at any given moment. A variety of process elements may have an effect on this slide zone and its efficiency. Some of these are noted in Table II

Table II. Barrel processing variables

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Barrel processing condition

 

Process effect

 

 

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Media/parts fill level too low (Fill level well below the 50% mark)

 

Reduced length of slide zone, usually insufficient cushioning and protection of parts, slower cycle times with possibility of rougher finish and potential for part damage

 

 

 

 

 

 

 

Optimum media/parts fill level (50-60%)

 

Longest possible slide zone, abrading efficiency considered to be at maximum

 

 

 

 

 

 

 

Media/parts fill level too high

 

Restricted slide zone with increased time cycle. May be desirable for processing delicate parts, which require more cushioning and slower media action.

 

 

 

 

 

 

 

Media size and abrasive content

 

Larger media and coarser abrasive content will perform deburring and develop generous radii rapidly but will generate less refined surfaces. Smaller media and finer abrasive grit will produce more refined finishes.

 

 

 

 

 

 

 

Water level

 

As a general rule, water levels are kept close to the top of mass, higher levels will retard movement and generate more refined finishes, low levels may accelerate movement and create rougher surface finishes.

 

 

 

 

 

 

 

Rotational speed

 

Movement within the barrel is usually measured in surface feet per minute (sfpm) The optimum speed for deburring operations is usually 100-200 sfpm, where burnishing operations are usually performed in 40-90 sfpm range. A 30-in. barrel will achieve 180 sfpm if rotated at 24 rpm, and will produce 60 sfpm at 8 rpm. Rotational speeds faster than these norms may cause parts to cascade outside the media mass. Excessively slow speeds lengthen time cycles.

 

 

 

 

 

 

 

Compound

 

Many barrel finishing compounds have high foaming characteristics. The foam fills the upper area of the barrel and often acts as a buffer or cushion, which curbs the action inside the barrel. In some barrel processes, the barrel contents will be rinsed and a new compound added, which will change the nature of the process, allowing for a two-stage process in the same equipment.

 

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Tumbling barrels are available in a variety of configurations, the most common being a horizontally oriented octagonal chamber, which provides a much more efficient media lift than a purely cylindrical shape. Other configurations include barrel chambers mounted on pedestals, barrels with front or end loading, perforated barrels encased in a water tank or tub, and so called triple-action polygonal barrels. Also used extensively are oblique barrels, similar in some respects to small batch concrete mixers. This equipment is used for light deburring and finishing as well as part drying. It has the advantage of permitting operator inspection while in process, and its open end can be tilted down for ease of unloading, but it is much less efficient than horizontal equipment, and suffers from the tendency of parts and media to segregate in extended time cycles.

As is the case with most other mass finishing equipment, polyurethane, rubber, or linings made from similar material are used to extend equipment life, provide some measure of cushioning to parts, and furnish some measure of noise abatement. Although considered by some to be an outdated and obsolete finishing method, barrels still have a place in the finishing engineer's repertoire. Although it is true that it is slower and presents some automation and materials handling challenges, it is sufficiently versatile to perform numerous finishing operations for many manufacturers. Furthermore, barrel finishing provides an excellent alternative for flat parts, which may nest in vibratory systems. Although perhaps requiring some measure of operator experience in order to be used effectively, barrel finishing is capable of producing some unique and desirable surface finishes and is highly efficient in compound and media usage.

Vibratory finishing systems

Vibratory finishing was introduced during the 1950s and, through a succession of design refinements, has become the primary workhorse of the industry. Equipment usually consists of a spring-mounted open chamber, lined with polyurethane or similar material, to which a vibratory motion generator is attached. The motion generator is usually mechanical in nature, consisting of a rotating shaft with eccentric weights affixed. (A few machines make use of electromagnetic motion generators.)

The motion of the media within the chamber can be controlled by adjusting the speed of rotation (frequency ranges between 900 and 3,000 rpm, more commonly between 1,200 and 1,800 rpm), the positioning of the eccentric weights, and the amount of the weight attached (amplitude,the amount of "rise and fall" being imparted to the container and media, can range between 1/16 in. [2 mm] to 3/8 in. [10 mm]). The actual chambers are available in a variety of shapes (round bowl, oval, or U-shaped tub being the most common.) The adjustments noted above will not only affect the vibratory motion of the media, but the roll or forward motion within the chamber (spiral motion in the case of many round bowls).

A number of advantages have manifested themselves over traditional barrel finishing methods. Unlike barrel processing, the entire media mass is in motion at any given time, so parts are being constantly treated, making for shorter cycle times. The entire chamber is utilized to its full capacity and, in many cases, the vibratory motion of the machine can be harnessed to assist in unloading. Many round bowl equipment designs are capable of internal separation, where an integral separation deck is used to separate and retrieve media from parts being unloaded at the end of a cycle. The open nature of equipment allows for ease of operator monitoring of the process on a continuous basis.

Tub vibrators

This equipment ranges in size from 1 ft3 capacity up to 200 ft3.

Tub vibrators are considered to have more aggressive media action than round-bowl machines, and they are capable of processing very large, bulky parts (as large as 6 ft by 6 ft) or potentially awkward part shapes (parts 40-ft long and longer). The vibratory motion generators consist of rotating shafts with sets of eccentric weights attached either at the bottom of the U-shaped tub or one of the sidewalls.

This equipment is usually loaded from the top of the chamber, and usually unloaded through a discharge door located on a side panel. Parts and media can be screened on an external separation deck. This arrangement allows for relatively quick load/unload or media changeover cycles when compared with other equipment.

Tub-shaped or tubular-shaped vibrators are commonly utilized for continuous high volume applications where the time cycle required to process the parts is relatively short. Media return conveyors and feed hoppers are used to meter the correct ratio of media and parts to the loading area of the machine, while media and parts are separated on a continuous basis by a screen deck located at the unload or discharge area of the machine. Tub-type machinery is also used extensively for batch applications and can be easily subcompartmentalized for parts that require total segregation from each other. A typical machine is shown in Fig. 1

This tub-style vibratory finishing machine being used to replace manual finishing procedures on titanium aerospace air frame components. This type of equipment is especially useful for processing larger structural parts, when coupled with automated handling equipment it can also process unusually large volumes of parts on a continuous process if the parts can be processed within the cycle time permitted by the dwell time of the parts traveling from one end of the tub or tube to the discharge end where they are separated from the abrasive media.

Round-bowl vibratory systems

Round-bowl equipment normally has a processing chamber that resembles the bottom half of a doughnut. Although up to 20% slower than tub-style machines, and having occasionally more unwieldy media changeover routines, the advantages in automation and material handling for these machines have often given them an edge in any processing cost per part analysis. The vibratory motion generator on these machines is customarily a vertical shaft mounted in the center-post area of the bowl. Adjustments related to the eccentric weights on this shaft will affect the rolling motion of the media, as well as the forward spiral motion of the media in the bowl chamber. This spiral motion is one of the machine's more salient advantages, as it promotes an even distribution and segregation of parts in the mass, thus lessening the chance of part-on-part contact.

Round Round owl vibratory finishing systems, This is the most common style of equipment.  Note the internal separation deck on the first machine with can be used for automated separation of parts from the abrasive processing materials at the end of the finishing cycle. Photo courtesy of Mark Riley, BV Products, Australia

Like tub machines, equipment size varies from small bench models, whose capacity are measured in quarts or gallons, to very large equipment in excess of 100 ft3 capacities. Successful processing requires appropriate media and compound selection, correct amplitude and frequency adjustments of the motion generator, and precisely determined water flow rate and compound metering rates. Unlike barrel systems, whose water levels are determined once at the beginning of the cycle, vibratory systems have a constant input and throughput of water into the system (both flow-through and recirculation systems are employed, although flow-through is generally much preferred).

Water levels are critical to process success. Too much water will impede the vibratory motion of the mass. Too little will permit a soils/sludge buildup on the media, reducing its cutting efficiency. Flow-through functions can be automated with appropriate controls and metering devices. For parts requiring relatively short cycle times, round-bowl machines can be configured to perform in a continuous mode, the parts being metered in and then making one pass around the bowl, and exiting via the internal separation deck. Some designs include a spiral bottom to enhance loading from the machine onto the separation deck, lessening the likelihood of part-on-part contact at the entrance to the separation deck.

Ease of use and economy are the hallmarks of vibratory finishing methods, and have contributed to making this perhaps the most accepted deburring and surface conditioning method for finishing parts in bulk. The equipment performs well in either batch or continuous applications. Standard applications usually can be run most economically in round-bowl-type equipment. Larger parts may require more specialized tub-type equipment, large volumes of parts, which can be processed in relatively short cycles, can make use of continuous tub or bowl equipment, or even multipath equipment. The latter can offer parts transfer from one operation to a secondary-type operation within the confines of the same machine, but different chambers. Vibratory action itself often will preclude the ability to develop superfinishes or microfinishes. These types of finishes are often best attempted in equipment in which the media action has a more rolling, glancing, or linear action than short stroke movement characteristic of vibratory finishing.

 

Centrifugal barrel finishing

Centrifugal barrel finishing (CBF) is a high-energy finishing method, which has come into widespread acceptance in the last 25-30 years. Although not nearly as universal in application as vibratory finishing, a long list of important CBF applications have been developed in the last few decades.

Similar in some respects to barrel finishing, in that a drum-type container is partially filled with media and set in motion to create a sliding action of the contents, CBF is different from other finishing methods in some significant ways. Among these are the high pressures developed in terms of media contact with parts, the unique sliding action induced by rotational and centrifugal forces, and accelerated abrading or finishing action. As is true with other high energy processes, because time cycles are much abbreviated, surface finishes can be developed in minutes, which might tie up conventional equipment for many hours.

The principle behind CBF is relatively straightforward. Opposing barrels or drums are positioned circumferentially on a turret. (Most systems have either two or four barrels mounted on the turret; some manufacturers favor a vertical and others a horizontal orientation for the turret.) As the turret rotates at high speed, the barrels are counterrotated, creating very high G-forces or pressures, as well as considerable media sliding action within the drums. Pressures as high as 50 Gs have been claimed for some equipment. The more standard equipment types range in size from 1 ft3 (30 L) to 10 ft3, although much larger equipment has been built for some applications.

Media used in these types of processes tend to be a great deal smaller than the common sizes chosen for barrel and vibratory processes. The smaller media, in such a high-pressure environment, are capable of performing much more work than would be the case in lower energy equipment. They also enhance access to all areas of the part and contribute to the ability of the equipment to develop very fine finishes. In addition to the ability to produce meaningful surface finish effects rapidly, and to produce fine finishes, CBF has the ability to impart compressive stress into critical parts that require extended metal fatigue resistance. Small and more delicate parts can also be processed with confidence, as the unique sliding action of the process seems to hold parts in position relative to each other, and there is generally little difficulty experienced with part impingement. Dry process media can be used in certain types of equipment and is useful for light deburring, polishing, and producing very refined superfinishes.

Practicality and questions of cost effectiveness often determine whether high-energy methods are selected over conventional barrel or vibratory processes. If acceptable surface finishes can be developed in a few hours, conventional equipment is usually the most economic alternative. CBF equipment's strong suit is the ability to develop surface finishes that may require over-lengthy time cycles in conventional equipment and the ability to develop a wide range of special surface finishes required for demanding and critical applications.

Centrifugal disk

Another high-energy finishing method that has become popular in recent years is the centrifugal disk. Most equipment is in the form of a cylinder or bowl with a spinning disk at the bottom. This disk propels the media upward against the interior sidewalls of the cylinder, which act as a brake, causing the mass to turn over and return to the center of the disk, where it is set in motion again. This unique media action is said to perform abrading operations at five to 10 times the speed of conventional vibratory action. As the machine is basically an open end chamber, in-process inspection and monitoring are possible. Faster time cycles can also reduce work in progress and make the equipment a good choice for manufacturing cells. In general, larger or lengthy parts are not good candidates for disk finishing and, at times, higher than usual media-to-part ratios must be maintained to avert part-on-part contact. Equipment size ranges from 1/2 ft3 (15 L) bench-top models to 20 ft3 (600 L) floor machines.

One critical area of attention on this equipment is the gap between the spinner disk and the ring located around the exterior of the disk. Particles or fines of media that are capable of lodging in this area may cause significant damage to certain types of equipment. Correct media maintenance and attention to water flow-through rates can be an important factor in extending the useful service life of main components. Some equipment has the ability to run either wet or dry process media. Many equipment models, however, are designed for dedicated use in either wet or dry finishing and should not be used in the other mode without extensive consultation with the manufacturer. A centrifugal barrel finishing system is shown in Fig. 2

.Centrifugal Barrel finishing machine for high speed wet abrasive and dry processing. Larger machines such as 220 or 330 liter capacity machines shown to the left can process larger parts (for example, superfinishing 40 inch crankshafts) or large volumes of parts. Portable machines such as the one shown to the right can be used in dental, jewelry, medical, electronic and other small part applications with very fine media to produce fine finishes.  Photo courtesy MFI, Howard Lake, MN

Spin/spindle finish equipment

Spindle finishing is performed by fixturing parts at the end of a (stationary, rotating, planetary, or oscillating) spindle, and arranging for the part to be immersed in a mass of fine media, which may be vibrating, stationary, or directed at the workpiece by a spinner arrangement or rotation of the entire media chamber. As all parts must be fixtured, impingement from part-on-part contact is nonexistent. Time cycles can be very short, ranging from a few seconds to a few minutes. Equipment from various manufacturers may feature single or multifixture capabilities. Types of operations vary from heavy abrasive operations for deburring and stock removal, to the use of very fine dry polishing media in some equipment to develop color-buff-type finishes. One recent development in spindle finishing is the turbofinish method, which involves the high-speed rotation of components in a fluidized bed of fine abrasive or polishing material.

Turbo-Finish Model TF-522 Turbo-Abrasive Machining Center for high speed dry horizontal spindle finishing.  Photo courtesy Dr. Michael Massarsky, Turbo-Finish Corp., Barre, MA  USA

Mass finishing media

Media can be generally defined as the loose material contained in the work area of a mass finishing machine, which, when in motion, performs the work desired on part surfaces. Media may be natural or synthetic, abrasive or nonabrasive, random or preformed in shape. Much of the versatility inherent to mass finishing processes can be traced to the wide array of media types, sizes, and shapes available to industry. What follows is a rundown of the more commonly used media types.

Natural/mineral media

Crushed and graded stone was once the predominant source for tumbling abrasives in the early days on barrel finishing. Raw source material included both limestone and granite. Some naturally sourced materials still find some barrel finishing applications today, such as corundum and novaculite. As a general rule, problems with fracturing, rapid wear and attrition rates, lodging, and disposal of the high amount of solid or sludge waste material created mitigates against crushed and graded mineral materials being an effective media for most applications.

Agricultural media

A variety of granular media such as ground corn cob, walnut shell, pecan shell, sawdust, and wooden pegs are used in all of the equipment discussed. These dry process media are used in conjunction with various fine abrasive compounds similar to compounds that might be used in buffing applications. These media are often used in secondary cycles, after initial cutting and smoothing, to produce very fine reflective finishes. Attractive decorative finishes can be produced for jewelry and other consumer articles and, by extension, very low Ra finishes can be produced for precision industrial components.

Preformed media

Finishing Media, shown are examples of plastic or resin-bonded media, non-abrasive porcelain and ceramic abrasive cutting media. Photo courtesy of M. Riley, BV Products, Melbourne, Australia

These media have largely replaced the crushed and graded mineral materials mentioned above. Media preforms are made from either extruded ceramic/abrasive shapes, which are fired, or resin-bonded, or which have been molded. The preform concept was an important one for the finishing industry. Unlike the more random shaped mineral media, size and shape preform selection could prevent media lodging and promote access to complex part shapes. The uniformity and predictable wear rates of the media also made it possible to prevent both lodging and separation problems caused by undersized, worn media. A wide variety of shapes have been developed by various manufacturers over the years to accommodate these requirements, including cones, triangles, angle-cut cylinders, wedges, diamonds, tristars, pyramids, arrowheads, and others.

Ceramic media are generally harder and more abrasive and are customarily used for more aggressive applications. Plastic media, as a rule, are somewhat softer and capable of producing finer finishes.


A ceramic preformed abrasive media used for barrel, vibratory and finishing operations
[3/8 inch angle cut triangle shaped]  The shapes are extruded, cut and fired to produce hardened
shapes that are made up of abrasive and ceramic filler and bonding materials. Widely used in
tumbling, vibratory and centrifugal applications for deburring, edge contour, smoothing and
cleaning  of parts in bulk.

Burnishing media

Media made from case hardened steel, stainless steel, and other formulations are used widely in barrel and vibratory equipment to produce burnished surfaces. These media are very heavy (300 lb/ft3 versus 100 lb/ft3 for ceramic media) when compared with other media types and are nonabrasive in nature. It should be noted that not all vibratory equipment can turn or roll steel media. Because of the weight, enhanced or heavier duty equipment may be necessary. The media performs by peening or compressive action; surface material is not removed, as is the case with abrasive media. Burnishing processes with steel media can be used either to develop reflective decorative finishes or provide functional finishes. One attribute of burnishing processes is that part surfaces are often work-hardened, which can extend the service life of components in moving assemblies. Steel media can be extremely long lasting, if care is taken to prevent corrosion of surfaces while in use and/or storage. Nonabrasive porcelain media are also used for some burnishing procedures and are prevalent in some centrifugal applications.


Non-abrasive media can be used in burnishing operations such as this one showing processing of automotive racing wheels.  These kinds of non-abrasive media shapes
are widely utilized in barrel, vibratory and centrifugal equipment to burnish and brighten a variety pf metal parts. Unlike polishing processes which tend to level surfaces by fine abrasive action on surface peaks, these materials flatten or level surface profiles by rolling or flattening the peaks by compression. Photo courtesy of M. Riley, BV Products, Melbourne, Australia

Compounds

Many abrasive and burnishing applications use water with specially formulated compound additives. The proper selection and dosage of these additives (in either liquid or dry powder form) can have a critical effect on the viability of the process. These compounds perform an assortment of functions including water conditioning or softening, pH control, oil/soil and metallic and abrasive fine suspension to prevent redeposition on part surfaces, rust inhibition, cleaning, foam development or control, as well as media lubricity control. Some special compounds are used to chemically accelerate finishing cycle times; some of these may be intensely caustic or corrosive and may require some special handling.

--
Dave Davidson - Deburring/Finishing Specialist
SME Manufacturing C248 - Spokane, Washington
Advisor: Machining/Material Removal Technical Community

(e) dryfinish@gmail.com | (t) 509-230-6821
(i) http://dryfinish.wordpress.com

SEE Also the Video:
https://youtu.be/dUdKjaysTYM


REFERENCES:

See Also:  http://www.slideshare.net/dryfinish/november-2012-f4-deburring-1-final

http://www.slideshare.net/dryfinish/its-the-finish-that-counts

https://youtu.be/dUdKjaysTYM


 


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01-18-2016 07:18

This is an excellent presentation that illustrates mass media processing with specific detail to clearly quantify, in numerical terms, the types of applications available. From simple external finishing through internal intersection deburring, the advantages and disadvantages of each type of mass finishing are suggested including impact on surface finish and hardness to the relative cost of final product...