Surface finishing is an essential part of industrial and mechanical parts manufacturing. The shiny and super smooth metal objects you see are products of surface finishing. This process improves resilience, corrosion resistance, tensile strength, and also removes burrs, defects, and other surface flaws.
Barrel tumbling, also known as barrel finishing, is one of the most popular finishing processes. The process is suitable for mass finishing relatively small parts. Barrel finishing involves a six or eight-sided barrel, which keeps rotating at a fixed speed. A mixture of raw metal parts, media, and compounds is placed in this barrel. The rotating barrel creates friction, rubbing the parts and media against each other.
1. Important Elements of Barrel Tumbling
There are three important elements contribute to a successful barrel tumbling: barrel tumbling equipment, tumbling media, and compound
1.1 Barrel Tumbling Equipment
Barrel tumbling machines mostly come in different affordable designs. The equipment is further sub-divided according to the difference of the vibratory object. This guide is meant to introduce to you the Horizontal Octagonal barrel system as well as the Triple Action Polygonal barrel machines used in mass finishing processes today.
1.2 Tumbling Media
Mass finishing processes perform better when parts do not strike against each other. To achieve this, a material is therefore added to the load to lubricate the process. Experience has shown that a ratio of three parts media to one part workpiece by volume is generally best for achieving better results. Although the size and type of material determine the media be chosen.
They are usually selected such that they are small enough to pass through breaks, holes, and points, yet large enough not to lodge. When a large media is used. Using large media is used, a faster action is achieved. If the media is smaller, however, a finer finish is obtained.
Below are Some Examples of Selected tumble Media
- Porcelain tumbling media: porcelain tumbling media (also called aluminum oxide media) is generally applied in mass finishing processes where an emphasis is laid on deburring and honing. It is more economical compared to the others and tends to last longer.
- Steel tumbling media: Steel media is always being used where there is the need to burnish or brighten workpieces. They are mild in action and when used, they do not cause excess wearing of parts.
- Plastic tumbling media: They are excellent for finishing delicate parts in a process that do not involve the excess wearing of work parts.
- Ceramic tumbling media: Regularity in size and shape of the workpiece needed to be finished will determine if ceramics will work well. Generally, this is used when larger pieces of the workpiece are involved.
Addition of compounds in mass finishing processes is geared towards assisting the various actions to achieve the best results. When compounds are added to mass finishing processes, they produce finer workpieces. They come in different forms including liquids and dry powder.
2. Comparison between the Horizontal, Octagonal barrel and the Triple Action Polygonal barrel
2.1 Horizontal, Octagonal Barrel Tumbling Machine
The Horizontal Octagonal barrel system and the Tilting and open-ended barrel designs have several similarities in common. These two machines may properly fit into the category of the most commonly used systems. With the Tilting and open-ended unit, users can properly monitor their work to make necessary additions if the need arises to varying results.
In contrast, Horizontal barrel systems do not have these features. Rather they are built with doors which are tightly shot during operation. Since there is no means for breathing within this system, pressure may develop in them. These effects mostly occur in operations where the use of acidic media is employed. To make up for this, the machine is incorporated therefore with pressure release valves. These valves must be released before loading or unloading may be done.
2.2 Triple Action Barrel
The Triple Action barrel differs from the Horizontal barrel finishing unit. In this unit, the machine is built to rotate both horizontally and axially. This double rotation results in a strong cutting action faster than that as is obtainable with the Horizontal barrel finishing unit. Therefore, it is relatively faster with a shorter cycle time.
3. Wet barrel tumbling and dry barrel tumbling
There are two main types of barrel tumbling processes namely wet tumbling and dry tumbling. Both methods are cost-effective, require little handling, and finish many parts at the same time.
3.1 Wet barrel tumbling
As the process involves the use of water as a lubricant, it is called wet tumbling. It usually consists of horizontal and oblique barrels made of steel and lined with wood, rubber, neoprene, urethane, PVC or vinyl for better cushioning. Cushioning increases the life of barrels by reducing the contact between metal parts and barrel surface.
The water often washes out the cutting residue to improve efficiency. The level of water in the barrel depends on the desired finishing output. For example, burnishing requires filling the barrel with water up to the level of the load.
As the water level increases, you get a more refined finish and a slower cutting rate. Sometimes, however, using water can undermine the efficiency of the process as the surface tension makes the parts cling together, especially small and flat metal parts. Wet tumbling is often used for removal of excess stock.
3.2 Dry Barrel Tumbling
Dry tumbling entails using a dry material as a lubricating agent or cleaning compound instead of water. This method is used for all phases, including polishing. You can use sand, ceramic parts, wood pegs, and corn cob or walnut shell grit.
Earlier, sand was used with smooth stones for metal parts. Recently, however, the trend has shifted towards using an organic material such as corn cob or walnut shell grit for better retention of dirt and even oils. For finishing plastic parts, you need to use softer materials such as wood peg.
4. Parameters Contributing to Successful Mass Finishing
4.1 Barrel Load
If better results are required, barrel load height should not be less than 45% or more than 60% of the entire barrel capacity.
At a load height of 50%, optimum finishing results are achieved. While more action and poorer finish are obtained between 40 and 45%.
After the right media and complementing compounds have been selected, parts are placed in the barrel. These barrels are then rotated at a speed of between 20 to 40 RPM. This will go on for a period of time depending on the desired result.
Depending on the barrel size and capacity as well as items to be placed in it, while also making considerations on the types of workpieces as well as schedule, the RPM is selected to suit whatever purpose as is required. When a barrel systems need to run on just the right amount of speed, it creates the necessary friction between parts. Faster action may be achieved by increasing the speed but may lead to poor finishing. Slower speed, however, may produce finer finishing, however with extended cycle time.
A standard for rotational speed has been in use in Japan for many years. It is calculated from the following equation:
where N = speed of barrel (rpm), D = diameter of the barrel (m or ft), and K = a proportionality constant. The value of K differs depending on the finish. When D is given in meters:
- K = 8 for brilliant finishes
- K = 15–20 for lustrous finishes
- K = 20–25 for medium finishing
- K = 25–30 for rough finishing
4.3 Slide and Slope
Barrel systems are built such that parts go off at tangents. As the barrel rotates, workpieces consequently collide with themselves, the media and compound. Depending on the desired results, the speed of the machine is set. If the speed is too fast, the system may be damaged or become ineffective. When the speed is slower, on the other hand, parts may not be damaged but may, however, lead to longer process cycle.
4.4 Use the Right Cleaning Compound
As is the case with any finishing process, barrel tumbling also requires careful use of cleaning compound. Usually, the media to parts ratio is three to one by volume. If you are finishing fragile or large items, use more media for better protection and higher productivity.
You can use Aluminum oxide for deburring and honing if the parts have irregular shape and size. On the other hand, you can use steel burnishing balls and shapes only for burnishing and brightening. Steel burnishing balls will not remove light burrs. But, you can add a finishing compound to get high-quality finishing.
Preformed plastic media is probably the best for finishing aluminum, die-cast, and delicate parts. You can use ceramic media if retaining the size and shape of the parts is necessary. However, it wears faster than aluminum oxide.
5. Key Benefits of Barrel Tumbling
Most manufacturers tend to use barrel finishing owing to their unique benefits. The good news is barrel tumbling methods are becoming increasingly sophisticated. So, the benefits will continue to grow.
5.1 Better Consistency
When it comes to mass finishing, maintaining consistency is key to a better output. All mass finished parts should exhibit the same level of brightness, smoothness, appearance, and other mechanical properties. This is why manufacturers prefer using barrel tumbling. Compared to hand polishing, it offers a higher level of uniform finishing results.
5.2 Less Time-Consuming
Barrel finishing is one of the fastest finishing processes out there. Usually, a typical barrel tumbling requires just six to 24 hours depending on desired finish, parts, and media. You don’t need to monitor the process continuously. You have to set the parameters such as load volume and time of the cycle and relax just before the process begins. There’s little handling required during the process.
5.3 High Adaptability
The third biggest benefit of barrel tumbling is its adaptability. You can calibrate a tumbling machine as per your desired specifications. Whether you want to remove oils or finish intricate parts with hard-to-reach areas, barrel tumbling is the right choice. You can also use a variety of finishing media and additives to get the desired level of finish. This is why you can process anything, from large metal parts to small and fragile items as well.
5.4 Finishing at Scale
Barrel tumbling is perhaps the best way for finishing a vast quantity of parts in a short time. With little handling required, you can process multiple batches at a time having thousands of components. For longer runs, you can run the system overnight. Plus, you don’t have to worry about the consistency and quality of the finish.
5.5 Other Benefits
It is cost-effective compared to hand finishing techniques. Barrel tumbling increases the toughness and strength of the parts. Tumbling provides a certain amount of stress relief. You can also blend forgings and castings using this process.
Outlined above is the most basic description of different barrel mass finishing process. Since its inception, barrel tumbling has incorporated many other important practices and techniques.
Manufacturers are in search of a cost-effective, efficient, and consistent finishing processes. This is where barrel tumbling comes in. Hopefully, understanding these various benefits and the basics involved in barrel finishing will help you use this process more efficiently. Feel free to tell us about your customized barrel tumbling requirements in your comments.