Author: Tronserve admin
Friday 30th July 2021 06:44 PM
Mass Finishing Processes Explained
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. This processing is performed to achieve one or several of a variety of edge and surface effects. These include deburring, descaling, surface smoothing, edge-break, radius formation, removal of surface contaminants from heat treat and other processes, pre-plate, pre-paint or coating surface preparation, blending in surface irregularities from machining or fabricating operations, producing reflective surfaces with nonabrasive burnishing media, refining surfaces, and developing super-finish or micro-finish 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 rejects and rework rates will plummet if a mass finishing approach can be implemented to meet surface and edge-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 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 a municipal sewage or another 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 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 processing segmentation 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.
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, the 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 Table I. Mass Finishing Equipment Selection Considerations
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.
30 x 36 in. octagonal horizontal barrel for dry process finishing plastics and light metals
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.
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 unloading or discharge area of the machine. Tub-type machinery is also used extensively for batch applications and can be easily sub compartmentalized for parts that require total segregation from each other.
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.
Media action in a round-bowl vibratory finishing system being used here as a deburring tool. Photo courtesy of Mark Riley, BV Products, Melbourne, 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 super-finishes or microfinishes. (Exception: some micro-finished or super-finished surfaces on ferrous and other alloys are being developed in vibratory equipment with chemically accelerated surface finishing compounds. These acidic based compounds can remove surface peaks and asperities to develop very low micro-inch finishes.) These types of finishes can also be developed without acidic chemical acceleration in centrifugal iso-finishing equipment in which the media action has a more rolling, glancing, or linear action than short stroke movement characteristic of vibratory finishing.