Quality Finishing: Part II Mechanical Finishing: A Review and New Technologies
Part II of this series discusses equipment for mechancial finishing...
The mechanical finishing equipment focused on in this article is the type that relies on some type of motion and some form of force to apply an abrasive material to a part. Motion is acceleration, velocity and deceleration. Of these, velocity is the most significant factor in processing, although rates of acceleration and deceleration can affect a finish. Force for finishing is gravitational, mechanical or centrifugal. Force is mass multiplied by acceleration, so size and speed matter. Kinetic energy also affects the process as media and parts sizes vary, as does the density of different media and part material.
For material removal you need to move the part and abrasive under pressure in a uniform contact fashion against one another at a certain velocity. In simple terms, each machine uses the energy it creates to push, shake or slide the abrasive material against the surface of the part at some rate of speed.
Mass finishing equipment is defined as any machine that can run parts and media together in mass to achieve a certain finishing result. Mass finishing is the equipment and application that will be discussed in more detail. Some of these technologies have been adapted to run a single part by fixture or some other means, but we will focus on equipment capable of running in mass.
The oldest machine for mass finishing is the tumbling barrel. This comes in either the horizontal, closed barrel or the open-end barrel. There are still some applications that only the barrel effectively processes, but for the most part, these units are for small shops with no critical finishing applications. Tumbling is a slow process at best and does not match up well to material handling. Open-end barrels can be automated with loaders and separation, but process times are still lengthy. There is also the possibility of an explosion in closed barrels from gases generated by the process.
In tumbling barrels, the friction is created by gravity as the media slides across the diameter of the barrel. The motion is created by rotating the barrel by motor at an RPM that is fast enough to finish effectively but does not throw the media. The only work being done in a barrel is as parts and media slide together across the top of the mass as the barrel rotates slowly (relative to other technologies). Although pressure is greater at the bottom of the barrel, lack of motion equals lack of work. To optimize the amount of work being done, the barrel should be filled about 60% so the part of the mass doing the work crosses the barrel at it widest point.
By far the most popular form of finishing in the U.S., vibratory finishing can be a reliable, cost-effective solution. This technology has taken many forms, but spinning a shaft with out-of-balance weights produces the motion, with few exceptions. This is true for both vibratory bowls and tubs with the position of the shaft relative to the work chamber varying. Some machines use direct-mounted motors with the weights fastened to their shaft. The alternative is to drive the shaft with the weights via pulley and belt.
In the vibratory bowl machines, the shaft spinning with the out-of-balance weights creates a back-and-forth motion that “scrubs” the media over the parts. The mass itself moves from the outside of the bowl to the inside at top and inside to outside at the bottom of the mass. It also rotates around the chamber. The path followed would be similar to following the wire of a slinky wrapped in a circle with the two ends held together.
The amount of weight set off-center of the shaft affects the degree of amplitude, with the greater weight creating the greater amplitude. An amplitude range of 2-8 mm is common in vibratory finishing. (Lower amplitudes are used for polishing and more delicate operations; larger amplitude for quicker metal removal.) The number of times that the machine shakes is relative to the motor RPM. Most vibratory equipment runs between 1,200 – 1,800 RPM. A machine set with amplitude of 5mm and running at 1,800 RPM would exact approximately 9,000mm of work across a part in a minute.
Out-of-balance weights on a round bowl are found at the top and bottom with the bottom leading the top weight in the direction the mass is moving. Changing the angle of the top and bottom weights to each other can change the tightness of the spiral the mass travels in the bowl. The bottom leading the top weight by 90 degrees is considered common.
In vibratory tubs, the amplitude and rolling motion are created the same way. The only difference would be the lateral rotation that lends itself to unloading in the round bowls is not a factor. Motion in the tub from end to end, for unloading or continuous tub operation, can be created by gravity and raising the tub at one end to create a slope in the direction travel is needed.
Ratios depend on the process needed. General deburring may be at a 3:1 ratio (media to parts), while pre-plate finishing may be at a 6:1 ratio. Pushing more parts into a defined media-to-parts ratio is a common problem in mass finishing as production tries to get more from the process. Media levels and load size (parts) should be clearly defined and checked routinely.
Another reason for the popularity of vibratory machines is their versatility. They can accommodate a range of part sizes, making them easy to adapt from part to part. Vibratory machines allow for easy and fast changeover of media, compound and screens.
Vibratory bowls and tubs allow for peripheral equipment to be added easily. This includes loaders in batch or piece, automatic compounding, separation and media classification. Multiple machines for multiple processes can be set up to load one into another for different applications. For example, a vibratory bowl could be set to debur and then unload into another bowl equipped with heaters and cob for drying. Another common example would be a cleaning and surface improvement process going straight to a polishing or steel ball burnishing process.
Vibratory equipment, despite continually trying to shake apart, is relatively low maintenance equipment if designed and built correctly. Besides lubrication of the main drive and dam blade (if applicable), there are not many items to concern you on these workhorses. If you are going to be using an aggressive media, your unit should have a good quality hot-poured lining of urethane.
Batch bowls and tubs require the entire load to be emptied from the machine for separation, unless magnetism can be used and is effective, and then reloaded. Round bowls with internal separation are very good at separating and unloading the parts with the media never leaving the machine. To further control separation, make sure the length of the screen is adequate. Variable speed controls will also improve your ability to thoroughly separate your parts from the media. Curved wall chambers with spiral bottoms can achieve 100% separation of parts and media in relatively short time.
Although the round bowl and tub are the most common examples of vibratory machines, some others include oval or racetrack; multiple channel (multipass); long radius; continuous (flow through); divided tubs and bowls.
High Energy Centrifugal Disk
Centrifugal disk finishing is considered 4-15 times faster than vibratory finishing processes. It is also very fast from floor to floor, including loading, processing, unloading, 100% separation and can incorporate rinsing, drying and multi-batch processing. If you are doing a high volume of parts within a certain size range (6 inches or less) the centrifugal disk is an excellent option.
The energy in the disk is created by a spinning bottom disk, which pushes the media outward, where it climbs the stationary wall of the chamber. Against the chamber wall a braking action occurs and gravity brings the media back down to the center bottom of the chamber. This torroidal action constantly repeats itself, creating a finishing environment where the parts are finished quickly. A sliding motion, where the energy created in the bowl is enhanced by centrifugal forces working against the physical barrier of the process chamber, produces a uniformed and aggressive action. High-energy equipment will allow the smaller media needed for certain applications to work the same as more aggressive large media. Despite the aggression of the process in regards to metal removal or polishing, certain parts are less likely to impinge than in vibratory machines.
Machines ranging in size from ¼-12 cu ft run at different RPM. The RPM is tied to the diameter of the spinner and calculated to produce a certain peripheral velocity that works best within the process chamber
Centrifugal disk finishing is the best for assuring complete, automated unloading and separation. Not only does it 100% unload and separate, it can be done in as few as seven minutes. The disk machines also have a relatively small footprint.
The critical area of this technology is the gap between the moving bottom disk and the stationary bowl. Different approaches have been taken to reduce wear and damage to this area. The technology has been around for more than 30 years, is extremely popular in countries like Japan and the gap is manageable. It would benefit you to check the history of your vendors design and talk to people who use this technology as it has been clouded with many different designs from different vendors offering different solutions.
High-Energy Centrifugal Barrel
Not a new technology but one that has seen resurgence in past years is centrifugal barrel finishing. Although there are different configurations for the machines, the most common have become the units with four equal-size barrels set up opposite each other on a turret. These machines range from a few liters up to 800 liters or larger. Most common applications fit in the 30-240 liter range. The motion of these units can create forces up to 30 times gravity to be used in the finishing process. This allows for process times that are 8-30 times that of a vibratory process.
The machine creates its energy by turning the main turret at up to 250 RPM. Each barrel is set to counter rotate so that they basically stay in the same position (like you are when seated on a ferries wheel), or rotate backwards at some ratio. This ratio could be from 1:1 to 4:1 depending on the make of machine. Some machines also support the barrels on both sides while others just on a single side.
In regards to versatility, centrifugal barrels offer the widest range of processing options. The machines can perform the most aggressive cutting or produce the highest polishes by simply changing the media and compound. Processes include both wet cutting and dry polishing with organic (cob, shell) media. Process times rarely exceed 60 minutes, usually for polishes. Deburring can happen in minutes with the right process parameters.
These units create both heat and pressure that allow the exceptional finishing times. Thirty-minute cycles can generate enough heat to cause burns. In the wet processes this can produce hot slurry, so caution must be taken in opening the barrels after processing. Longer time cycles often require a delay time and external cooling before the chambers can be unloaded. It is this heat and pressure that allows the dry organic media to work like a buffing wheel and produce high polishes.
The down side to this technology has always been floor-to-floor time, especially loading and unloading. With several closed barrels filled with a mix of parts, media, water and compound, each must be loaded and unloaded individually. Recent designs incorporate loaders and separators into the machines but operators must still control the function and assure the units are loaded correctly. Certain critical applications often require layering of the parts as they are loaded, making up a large percentage of the floor-to-floor time. The finishes produced, however, often justify the process, since handwork is the sole alternative.
Early on, these machines were known to be a maintenance problem. Many design upgrades and a better understanding of the variables that affect the machine have all but eliminated this concern. With balanced loads and good maintenance, these units will run trouble free.
Two other types of high-energy equipment are drag finishers and spindle finishers. Both require the fixturing of parts on multiple arms that are immersed and rotated in the media mass. In Spindle finishing, the tub spins the mass at a high speed and the parts are rotated around as the media pass over it. Drag finishers spin the fixtured parts through the mass at high speed while rotating them. Both are very effective processes for parts that cannot be subject to any impingement. Both are also capable of dry organic processing to high polishes.
All of the above machines are capable of running parts in mass by providing an energy that allows the abrasive media to do its job. Each has its advantages and disadvantages. By knowing what technologies are available for consideration, and studying the parameters discussed above that affect your overall process, you should be able to determine what technology is best for your application.
To get an idea of the hourly cost it takes to run each machine the following can be used:
- Tumbling Barrel: $35.00/hr.
- Vibratory Tub: $30.50/hr.
- Vibratory Bowl: $24.75/hr.
- Centrifugal Disk: $25.25/hr.
- Centrifugal Barrel: $29.75/hr.
The variables that help calculate this cost change from plant to plant and area-to-area, so you should spend the time to see what your own cost are. The key to accurately defining your cost is to accurately define the parameters of your process.
The secret to the processing ability of any technology is how it creates and balances velocity and pressure. This, combined with an understanding of abrasives (media) and compounds, is how an effective process is determined. Understanding these factors will help you see your mass finishing as a process that can be defined and managed and, therefore, improved as needed.
Keep in mind, however, an effective mechanical finishing operation must also make sure your parts move smoothly through the process, minimizing handling time and cost. This floor-to-floor movement will have a great influence on the type of equipment that will best suit your production environment. Defining all the variables that apply to your particular process will allow the efficiency and economy you need from it.