Mass Finishing

Note: This article was originally presented at Buff, Brush & Polishing Techniques (February 25-26, 2002, Nashville, TN) and is reproduced courtesy of SME.

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Note: This article was originally presented at Buff, Brush & Polishing Techniques (February 25-26, 2002, Nashville, TN) and is reproduced courtesy of SME.

Mass Finishing is a surface improvement process that is accomplished by the careful manipulation of many variables. The surface requirements as well as the beginning condition and the identification of surface residues are essential to the development of the process. The goal of the process may be any of the following: Cleaning, deburring, surface improvement, inhibiting or drying. Each of these may be achievable through several processes where the determining factors become efficiency, production rate and economy.

Once the desired function is identified, the processing variables can be established. These variables include, machine type and associated action; type and size of media; compound type; concentration and the flow rate of the processing fluid; time cycle; and, most importantly, the part. All of these variables are essential for the development of the mass finishing process; however, the successful operation of any process, including mass finishing, is process control. This is best accomplished by 1) identifying all the process variables and understanding the interrelationship of the variables; and 2) controlling all the proceeding processes that feed the mass finishing process.

Cleaning is the primary function in mass finishing, because it is the basis for all finishing operations. In order to efficiently debur, burnish, inhibit, plate or paint a part, it must be clean. Cleaning is simply the removal of unwanted residues from the surface of the part, these residues may be present as dust or particulates, cutting or stamping fluid, die lubes, corrosion inhibitors, oxidation or scale. Each of these soils requires its own type of cleaning process. Please refer to the cleaning document at the end of this presentation for further description of the cleaning process. The cleaning process is primarily a chemical action but is greatly enhanced by mechanical action i.e. - media and machine. The factors that most greatly affect cleaning are type and quantity of soil, compound type, concentration, temperature, solution contamination, time, agitation and rinsing.

Deburring employs the use of abrasive action to grind away machining burrs and sharp stamping edges. This action is optimized by transmittance of the impingement energy of the abrasive on the part to sliding action that allows the media to impact or grind the burr or edge. This action is similar to a grinding wheel or a file. The key to this operation is keeping the media clean so that it maintains maximum cut. Again cleaning is critical. One of the most important criteria is the suspension and removal of metallic fines and media residues. Similar operations include radiusing and grinding, where the abrasive action is targeted at sharp edges or at extreme surface roughness. For instance, sand castings have a typical rough surface that must be ground to accept subsequent finishes. Likewise, hand tools are typically rough ground with abrasive belts then heat-treated. After heat treat the part is finish ground in mass finishing equipment to remove the rough scratch lines.

The next mass finishing operation is surface improvement or, more accurately, surface refinement. All of the mass finishing operations improve the surface condition of the part, however, surface refinement produces a smoother finish. Generally these finishes are measured by a profilometer and expressed in terms of Ra, or the average distance between the peaks and valley of the scratch pattern. The part may be also measured in reflectivity, however, it is important to know that reflectivity does not necessarily result from a low Ra. For example a machined surface may be reflective but have a high Ra. Mirror-like finishes occur when reflectivity and Ra are both optimized.

One of the most common refinement processes is preplate finishing. This process uses lightweight media, typically plastic, with light cutting abrasive. The resulting scratch pattern is indirectional (much like deburring) but very fine with a low Ra value. Probably the most common improvement process is burnishing. This process results in reflective surfaces by peening or rolling the surface of the part with an extremely hard substance. The media impacts the part and with the proper selection of variables, the impingement is minimized, and the burnish roll is maximized.

Another process that usually takes place in surface improvement operations, particularly in burnishing, is brightening. Most cosmetic finishes require brightness or luster in addition to reflectivity. This is primarily accomplished with chemical selection in the improvement process. Typically the use of mild acids or strongly chelated alkaline compounds result in bright finishes. This brightening however is not limited to burnishing. It is also commonly seen in preplate and deburring applications. It is sometimes referred to as bleaching, but essentially is nothing more than tarnish removal or de-oxidation.

Inhibition can also be addressed as a mass finishing operation. Once the part has been mechanically processed by cleaning, deburring or burnishing, the process investment needs to be protected, kept from oxidation or corrosion. Three items are required for oxidation to take place: the part or material to be oxidized, oxygen and moisture. If any of the three is removed, then oxidation will not occur. Obviously, the part must remain; therefore, the corrosion inhibitor must either block oxygen or moisture from coming in contact with the part. In general, inhibitors deposit a chemical layer on the surface of the part, which either absorbs or repels the oxygen or moisture.

The last mass finishing function is drying. The result of the previous processes generally results in a wet part. To insure optimum corrosion inhibition the parts need to be dried. This can be accomplished in mass finishing equipment with specialized media and is usually augmented by heat. The material of choice for this application is ground corncob, which is highly absorbent yet releases the moisture upon heating. Other materials such as walnut shells, glass beads and non-abrasive media can also be used, but are much less effective.

It is important to understand that the mass finishing processes are not independent of each other. It is common to have several or all of the functions in the overall process of finishing the part.

Example. Axe Head: Beginning condition abrasive, belt ground and heat-treated. The overall process was to descale (clean) and ground for 18-24 hours, steel media burnish for 2 hours, finally the part was inhibited and dried.

There are four basic elements of mass finishing that encompass all the processing variables. The first and most important is the part. Mass finishing lends itself to a wide range of parts including: toys, jewelry, medical tools and implants, orthodontics, magnets, rubber O-rings, valve bodies, hand grenades, shell casings, ammunition, gun components, turbine blades, fasteners, gears, hinges, plumbing hardware and fittings and aircraft components. The only real limit is the imagination. Some parts may be processed in a similar fashion, but each part requires a specific process and corresponding controls. Among the variables to control relative to the part are surface condition, residues and the quantity or extent of burrs. Obviously, a cast part will process differently from a machined part. Consistency of the part is also critical. For instance, if the process is designed around a burr of 0.125 x 0.006 inch and the burr changes to 0.200 x 0.015 inch there will be a profound impact on the process.

The second element of mass finishing is media, which provides several qualities to the process. It separates the parts from contacting each other as well as performing work on the part. In some applications known as part on part, the part is the media. It also carries the process fluid to the part and accesses nooks and crannies of various parts. Media is chosen on the basis of application, type, composition, size and shape. It is classified as abrasive and non-abrasive. The abrasive media is generally used for deburring, grinding, radiusing and preplate finishing. Ceramic media is used for heavy to light cutting applications. Its composition is varied to optimize wear and cut rates. Plastic media is generally used for less aggressive deburring and preplate finishing. The plastic binder provides a reduced impingement compared to the hard ceramic and is generally formulated with less aggressive abrasives. Natural stone has some limited application in mass finishing but lends itself to unpredictable break down and inconsistent finishing. The non-abrasive media is generally used for burnishing, high polishing or drying. The most common of these types is steel media and is made from hardened carbon steel or stainless steel. Steel media has some unique shapes such as ball cones, pins and oval balls, which give a specific finish. These generally require slightly acid process fluids. Porcelain and non-abrasive ceramic are available in the standard shapes and are generally run with alkaline fluids, which aid in the glazing process. Walnut shell and treated cob meal are generally used to wipe or lap the part to provide an extremely high luster. A new introduction in mass finishing is the use of a special formulated dry media. It is formulated to eliminate the use of water, compound, effluent and any treatment for effluent discharge. This media, unlike conventional media, is formulated to stay active throughout its life. It will not glaze and lose cut and/or finish capability as would be the case with conventional media. It is presently offered in grades from fine cut to extra cut depending on the application required. The cost savings with dry media are no water, no compound and no treatment cost.

The third element of mass finishing is the process fluid, or compound and water solution. It is responsible for cleaning the part and the media, carrying away broken down media and abraded pieces of the part, brightening the part, modifying the action of the process and providing inhibition. The variables to be controlled are cleaning-suspension ability, pH, foam level, inhibition and water quality (hardness). Most process fluids are at a concentration of less than 2%; therefore, the water quality is a major variable to be controlled. Typically hardness and pH of the feed water are most closely watched.
The fourth element of mass finishing is the equipment. There are various types of equipment that present the part with different types of action. As a natural out growth of the equipment, process time is determined and must be weighed against production rate, efficiency and labor requirements. There are three types of finishing equipment: rotary, vibratory and Centrifugal. Each type has a characteristic action associated with its operation and a given set of variables that modify the action.

The first type is the earliest form of mass finishing, which results from rotating a process vessel, usually hexagonal or octagonal in shape on a single axis. In this equipment, the parts roll in the mass as a result of the motion of the drum. The motion is referred to as the "climb and slide." The part climbs with the mass then slides to the bottom. The work is done in the slide motion. The drums may be run horizontally with a sealed chamber or on an incline with an open end. The variables to control are speed of rotation, size of the drum (which affects the speed of the slide) and configuration of the drum.

The second type of equipment is vibratory, which works on the principle of attaching a drive shaft to the process vessel. The shaft is then fitted with adjustable weights at eccentric positions, as the shaft rotates its imparts a wobble or vibration to the assembly. The vibration requires a dynamic operation, which is accomplished by mounting the process vessel on springs. There are two basic designs of vibratory equipment, the tub and bowl. The tub is a U-shaped vessel of varying length. It can be designed to run batch or continuous processing. The bowl is essentially the same design, which has been wrapped around a center column. To extend the process time in either machine for a continuous process, the channel length must be extended. This results in a long tub, however the bowl concept can wrap the channel in a spiral, resulting in a reduction of floor space for the overall process. This type of equipment is controlled by feed rate, roll rate and amplitudes of the vibration. These parameters are adjusted by the quantity and relative position of the weights on the drive shaft.

The third type of equipment is centrifugal in which a spinning action imparts an increased force on the media and part. These are generally much more aggressive operations with stock removal and media wear rates in the range of 6-20 times that of rotary or vibratory equipment. Process time is correspondingly reduced by a factor of 6 to 20 times. The process is typically run as a batch operation and can be greatly automated to enhance productivity; however, truly continuous processing is not possible.

The first type of centrifugal equipment is the centrifugal barrel, which grew out of the rotary barrel design. In this design, a series of barrels are mounted on a turret. The barrel and the turret both rotate. Most designs set the rotation in opposite directions, which provides a uniform consistent action in the processing mass. Some designs have the flexibility to alter the rotation of the barrel or turret so that the forces exerted are maximized when the rotation directs the mass to the outside of the barrel. The result is a very aggressive process that limits its use to certain parts.

The second type of centrifugal equipment employs spinning disc in the bottom of the process vessel. This results in a torroidal action that minimizes impingement of the part with media but enhances sliding abrasion. The limiting factor in this application is part size. The configuration of the unit will allow only certain size parts for processing. Centrifugal finishing equipment is governed by several variables, including speed and orientation of rotation and the size of the process chamber.

A new introduction to mass finishing is the SpiraBlast (patent pending) finishing machine. This is a marriage of two age-old technologies, blasting and vibratory finishing. In this equipment, parts are fed either batch style or continuously into a vibratory finishing machine to which blasting equipment is added. The blast equipment can be either suction or pressure varieties. The vibratory action presents the parts to the blast stream, with or without the use of media, allowing complete and uniform coverage of difficult to reach areas without the use of fixtures. Advantages of SpiraBlast:

  1. Smaller footprint than conventional blasting equipment
  2. Can be designed as part on part batch process
  3. Can be designed as one-piece flow
  4. Can use metallic abrasive
  5. Can use non-metallic abrasive
  6. Uses 1/20th of the abrasive
  7. Longer nozzle life
  8. Can be either suction or pressure
  9. Secondary abrasive action.

The SpiraBlast is also capable of running odd shapes of parts that may need to be fixtured in conventional equipment. It can also be called the fixtureless fixture machine. This is certainly the newest innovation in the field of mass finishing.

Each of these elements of mass finishing imparts its own variables to the process; however, the interrelationship of these variables is most critical to the operation. For example in centrifugal finishing, we take advantage of highly aggressive cutting action. We would not normally use a low abrasive media and a very lubricious, high-foaming compound. In process development the key is to optimize all variables and exploit the synergism that exist in process.