Mass finishing as a process automates the mechanical as well as chemical finishing of simple as well as complex workpieces or tools of different shapes. It involves the use of energy generating equipment, media as well as other compounds to transmit energy from the equipment through the media to the components or workpieces being processed at the time. The machine is built in such a way as to enable the media flow around, into and through the workpieces or parts being finished. Depending on certain conditions, the process may be wet or dry and can work on a lot of parts at a time.
In simpler terms, mass finishing describes the continuous flow and vibration of abrasive media around non-fixated parts. This vibration causes the parts to move randomly within the mass of abrasive media.
1. Choosing the Right Mass Finishing Technique
A number of considerations come into play when choosing an equipment for mass finishing. These factors among others may include the availability of labor for assistance in the equipment process, production time/ schedule, capital, the parts being processed etc.
Among other mass finishing techniques, the most common choices manufacture look to when faced with the need to perform mass finishing activities are the tumbling barrel and vibratory tumbler finishing processes. These are versatile processes and the ability to choose one which will suit your needs and goals depend largely on how well you understand each process, costs and of course benefits.
If you’ve come to a point where you need to deburr, clean, surface finish or polish more than one part at a time, this article may just be helpful in getting you on the right track to making the right choice.
2. Mass Finishing Techniques
There are several mass finishing machines exist. However, the most commonly used are the barrel tumbler and the vibratory tumbler.
2.1 Barrel Tumbler
The action of a barrel tumbler can be compared to that of tiny pieces of rocks tumbling or skidding down the slope of a hill. The tumbler is built in such a way that its corners lift the load as it rotates up to a point where it begins to fall or slide down the sides of the barrel. As workpieces become abraded as they rub against the media as well as other parts of the system.
2.2 Vibratory Tumbler
The vibratory tumbler is built in a manner that allows the cutting material to surround the parts. The rotating weigh shakes the tub in a circular path. This circular movement causes the entire load to be lifted at an angle and then dropped. As the load falls, the tub returns quickly to an upward position, applying an upward angular force which sets in motion a shearing action that involves the parts and media rubbing against each other.
2.3 Centrifugal disc finishing machines
Rapidly moving into being the second most preferred mass finishing method for installations, centrifugal disc mass finishing machines have an advantage of speed which offsets some its disadvantage of being limited to batch operations. Although mass finishing equipment manufacturers are making the process increasingly acceptable, the higher operating cost continues to discourage prospective users.
2.4 Centrifugal Barrel Machines
This equipment features a hexagonal or octagonal shaped barrel. As it revolves around its axes, it passes a centrifugal force to the contents, significantly enhancing finishing performance. The major disadvantage of this method is its ability to cause parts to impinge with other parts which may lead to damage of work-pieces if preventive measures are not taken.
2.5 Comparison of barrel tumbling and vibratory tumbling
The barrel tumbler grinds at an applied force, while the vibratory tumbler moves faster than a freefall. The extra force in the barrel tumbler causes it to have a vibratory effect up to five times the vibratory tumblers freefall Force. Furthermore, the barrel tumbler restricts the entire cutting action to only about 20 to 30% of the total load.
However, the vibratory tumbler allows each load to be cut with each pulse of about 1800 beats a minute. This is the reason why the vibratory tumbler has such shorter cycle time as compared to the barrel tumbler.
Barrel finishing systems produce a more uneven surface. It ensures the corners are well rounded off then goes ahead to deburr more materials from surfaces. Therefore, at such times when this brute stock effect is desirable, the barrel tumbler will be your best bet.
The barrel tumbler features a peening action that can be used to work tougher parts. It uses hardened and polished steel shots to produce quick luster on parts. Although this media may also be used in the vibratory media, the barrel tumbler will produce a denser surface and greater luster.
Parts Finishing Comparison Chart
|System Type||Action||Characteristic Results||Special comments|
|Barrel Tumbling Equipment||Causes load to tumble down in a free fall||Large spans. Relatively poor in recesses. Suitable for largely exposed workpieces||Mostly requiring good handling equipment. Lasts longer|
|Vibratory Tumbling Equipment||Parts may scrub against media due to vibrating action||Small finishing radius ranging between 0.010 to 0.020. Can produce a smooth and finely finished surface. Works with a speed much faster than of a barrel tumbler||Suitable for large work-pieces. Can be automated. Appropriate for delicate parts and close tolerance work|
|Centrifugal Disc Finishing Machines||Similar in action to a barrel tumbler, however, enhanced by centrifugal force.||Results comparable to a barrel tumbler, but much faster. High pressure which may roll over burrs.||Requires higher care regarding handling.|
Optimum finishing cycle, appropriate for small parts with small media.
|Centrifugal Barrel Finishing Machines||High energy force with gravity force||Create an isotropic finish in a short period of time||Well suited for small parts, high valued parts, and parts that sensitive for the impingement|
3. Important Factors for Equipment Selection
3.1 Parts Surface Requirement
This is the first thing you need to consider. Because it determines what choice to make regarding choosing the right mass finishing processes. Understanding what you want to achieve with your surface finishing machine is key to knowing what option to go for.
- Deburring: This works best with ceramics and plastic media having edges and holes. Softer action for light deburring is mostly achieved using rounded media.
- Surface finish texture: Any media works well for this depending on what you want. If you’re looking to do some work on surface finishing action, then using a ceramic media would generally give a duller finish. Plastic media, however, is known to give a brighter finish.
- Polishing: If your intention is to polish your workpieces, then organic media such as walnut shells as well as porcelain pins will be just perfect.
- Cleaning a laser or water jet cut finish: Ceramic media tend to produce a more aggressive process. This is will work on all edges to produce smoother parts that feed seamlessly into a machine.
- Removing Rust: Ceramic media works well for this process since it is more aggressive compared to the others.
3.2 Parts Materials
Parts Materials is also an important factor for consideration is the question of what materials you’re working with. Below is a list of common materials and what generally works well with them.
- Stainless steel: This generally requires ceramic media
- Aluminum: This requires plastic media
- Brass: This also requires plastic media
- Cast iron: This works well in ceramic media
- Plastic: Depending on the nature of the plastic material, a choice is made. This, however, works more with a plastic media
3.3 Parts Size and Shape
Yet another critical factor that affects the choice of mass finishing is the size and shape of the material or workpiece to be finished.
The table below shows the relationship between shape/size and what method is chosen.
|Parts conditions||Most suitable media|
|Very small||Ceramic media is generally used|
|Intercalate||Plastic/ porcelain media produce sharper action on delicate parts|
|Medium||Larger media would lead to a shorter process time|
|Highly machined||Could be plastic or ceramic|
|External Thread||A light deburring process generally requires a non-aggressive plastic media|
Cost is an important factor that can affect the choice of a system. Due to their massive sizes, vibratory systems are more expensive than barrel tumbling machine. Furthermore, barrel tumbling systems wear out tumbling media at a rate which is half the pace of vibratory systems. Although, they take a longer time to do the same job.
3.5 Tumbling Media Selection
Ceramic tumbling media is made with abrasive filler in similitude of manner as a grinding wheel is made. However, plastic tumbling media is mixed with abrasive and cast to shape. Ceramic media uses aluminum oxide as filler while plastic media uses quartz or silica for cleaner results.
To achieve a precise work, randomly shaped media, either man-made or natural,re rarely used. This is because they tend to jam in the holes and do not deburr into corners or recessed areas.
Much of the experience of using finishing equipment effectively can only be gained by actually working with the equipment. It is also imperative to keep careful records of each run in a bid to learn its effect and also to allow the results to be duplicated later should the need arise. Thus, there is no easy way to know which system is best.
Through many years of experience, we have found that to arrive at the proper choice, the particular requirement for the job must be studied, while having a prior knowledge of the characteristics of the performance of each system. This would be an added advantage.