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Mass finishing is the processing of parts or components to be finished in a container, usually with abrasive or nonabrasive media, water and compound. Mass ﬁnishing process automates the mechanical and chemical ﬁnishing of non ﬁxtured complex shaped parts. Action or movement of the container is created to cause media to press and / or scrub against component surfaces, edges and corners, or components to rub against each other, to remove burrs, radius edges and corners, improve surfaces, alter and / or enhance characteristics. Mass finishing is also 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. It typically replaces hand deburring and manual polishing of pieces one at a time.
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. Mass finishing can economically enhance the utility, attractiveness, and value of the metal, plastic, ceramic or other parts manufactured for industrial or consumer use. 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. A mass finishing machine often reduces or eliminates 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 fall down, if a mass finishing approach can be implemented to meet surface finish requirements. While a surface finishing process does not guarantee that the final finish on a product will be a high-quality finish, it does improve surface properties, upgrade product quality, and enhance the image of the metal finishing process.
Mass Finishing Equipments and its Types
There are five major equipment groups as follows: rotary barrels, vibratory tubs and round bowl machines, centrifugal barrel with turret mounted drums, centrifugal disk, and spin/spindle finishing. 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. At times, fixturing 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.
Working Principle of Mass Finishing Equipment
The operating principle of the above mentioned mass finishing equipments are:
1. Rotary barrel finishing (tumbling) is carried out in a rotating container. Tumbling is a slide action process whereas the mass (parts and media) slides down a slope created via the container’s rotation. The mass slide occurs when gravity overcomes a portion of the mass (15-20% more or less) sending it gently sliding down the incline. The optimal loading is 60% of the container volume, which provides the longest slide resulting in smoother finishes and faster cycle times.
2. Vibratory finishing is the most popular method where energy is induced via rotating eccentric weights vibrating the container which are transferred from the walls of the machine to the media and hence to the parts in the tub or bowl. Such a process requires vibratory finishing machines.
3. The centrifugal barrel system uses centrifugal force to increase the pressure of the media on the parts being finished.
4. The centrifugal disc is a stationary round outer wall with a high speed rotation bottom independent of the outer containment wall creating a tornadic motion to the mass around the outer wall thereby imparting an increased pressure of the media on the parts and constant change in speed of the mass.
5. Spindle finishing accomplishes surface improvement by fixturing parts on rotating spindles and plowing them through a mass of loose abrasive, utilizing both pressure and abrasive action.
Applications of Mass Finishing Machines
The major applications of mass finishing machines are:
Cleaning: Cleaning is the primary function of mass finishing, and is the basis for all surface finishing operations. It is a pre-requisite that in order to efficiently debur, surface refine, inhibit, and dry, the part must be clean. Cleaning is simply the removal of unwanted residue from the part, these residues may be present as dust or particulates, cutting or stamping fluid, die lubes, corrosion inhibitors, oxidation or scale.
Deburring: Deburring employs the use of abrasive media to grind away machine lines, belting marks, burrs, sharp edges, slag, and establish radius.
Surface refinement: Surface refinement lowers rough surface finishes to acceptable standards for plating, polishing, anodizing, and painting operations. Generally these finishes are measured by a profilometer and expressed in terms of RA, or the average distance between the peaks and valleys of the parts surface.
Inhibition: Once the part has been cleaned, deburred, and surface refined it must be protected from oxidation and corrosion.
Drying: When the above operations are carried out, the resultant product will be wet. The parts are required to be dried inorder to insure optimum corrosion inhibition.
Some of the other auxiliary functions of mass finishing equipments are: Radiusing, Degreasing, Descaling, Pickling and Burnishing.