MASS FINISHING EQUIPMENT: Barrel and Vibratory Systems
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. 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.
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.
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.
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 25-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 machineryis also used extensively for batch applications and can be easily subcompartmentalized 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.
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 vibratorymotion of the mass. Too little will permit a soils/sludge buildup on the media, reducing itscutting 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 makingone 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 ultimate 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 the short stroke movement or amplitude characteristic of vibratory finishing.
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- February 21, 2012 / 5:00 pm
- Abrasive Media, Aircraft Industry Finishing, Barrel Finishing, Deburring, Dry Media Process Finishing, Dry Process Finishing, Edge Contour, Mass Finishing, Radius or Edge Contour, Tub Vibratory Finishing, Tumbling -Finishing Media, Vibratory bowl finishing equipment, vibratory finishing machines