Step-by-Step Through the Tumble Finishing Process
Barrel finishing is a cost- and time-effective surface finishing process. The method is completed by tumbling parts in a rotating barrel, creating a rolling acting between the materials. Let’s take a look at how wet and dry barrel tumbling works.
The selection and usage of tumblers, abrasives, lubricating agents, carrying agents, and polishing agents is critical to producing good surface finishes utilizing barrel finishing.
Barrel finishing, also known as barrel tumbling, is a surface-improvement operation in which a mixture of parts, media, and compounds is placed in a six- or eight-sided barrel and rotated at a predetermined speed to round corners, deburr, grind, descaling, deflashing, improving surface finish, burnishing, polishing, and radiusing parts in bulk. It operates by tumbling pieces in a spinning barrel, causing friction between the parts and other elements such as media and compounds.
Highlights of the Tumbling
- Parts can be finished more cheaply than by hand.
- A large number of pieces can be processed at the same time.
- Very minimal handling is required.
- Tumbling makes parts tougher and stronger.
- Tumbling can help with stress relief.
- Castings and forgings can be combined.
- Deburring and burnishing can be used to provide a high polish on machine parts and stampings.
- The systems can run overnight on extended runs.
- Tumbling time can be saved by carefully machining your pieces.
Wet tumbling and dry tumbling are the two methods of barrel finishing. Wet tumbling is almost primarily utilized to remove surplus stock. Dry tumbling is utilized throughout the process, including polishing.
Tumbling in the Rain
Wet tumbling frequently employs horizontal and oblique barrels. The horizontal barrel’s form is often octagonal or hexagonal. Although the oblique barrel is easier to load and unload, the horizontal barrel is favored because to its bigger capacity and improved tumbling movement.
Barrels are often built of steel and lined with wood, rubber, neoprene, urethane, PVC, or vinyl. The lining absorbs the impact of the falling components against the barrel, extending the barrel’s life. The most common barrel dimensions range from 18′′ to 36′′ in diameter and from 18′′ to 42′′ in length. The diameter is usually less than the length. Depending on the barrel size and the articles to be tumbled, horizontal barrel speeds range from 20 to 38 RPM. Slower speeds, for example, are required for fragile parts to avoid damage.
Water is poured to within three to five inches of the load after a barrel has been loaded to about half of its capacity with components and media. This can be changed in either direction. The faster the cut, the lower the water level. The greater the amount of water utilized, the finer the finish and the slower the cutting rate. The water should be about level with the load when burnishing.
The compound is added last to the barrel. Tumbling might take anywhere from 6 to 24 hours, depending on the amount of material to be removed from the pieces. To raise the load weight of very light plastic parts, smooth ceramic or hardwood media must be added in a dry process.
Following a run, the parts and barrel should be thoroughly washed with fresh water. After that, the parts are removed and dried using one of various ways, including tumbling with corn cob grit, lying under heat lamps, or spinning dry. Rinsing media after use can help prevent contamination or unwanted effects if it is reused later.
Water is used in most mass finishing procedures to lubricate and remove cutting residue. Without water, the imbedded abrasives would scrape and blacken the parts. When mixed with our compounds, the water not only transports the abrasives and cleans the media, but it also functions as a cushion to protect the parts.
However, water can be an impediment at times, such as when processing thin, flat components (where the water surface tension causes the parts to cling together) or when polishing or smoothing a plastic part (which can absorb the water and become too soft to resist scratching or deformation). When the medium is excessively heavy and hard, polishing operations on jewelry are restricted to the smoothness of the shine created. So, how can we run parts without using water safely?
Tumbling on a Dry Surface
The horizontal octagonal barrel is almost solely used for dry tumbling. This barrel may have a metal skin and a hardwood lining that can be replaced as needed. A barrel 30′′ in diameter by 36′′ or 42′′ in length is considered normal. These barrels can be separated into two parts for more versatility. Dry barrels are frequently double-decked, with one barrel positioned above the other in the same frame. This saves floor space, which is especially useful when using many barrels. Dry tumbling barrel speeds are typically held between 28 and 32 RPM.
Several strategies have been developed over the years. Water was not used in the early tumbling processes. Smooth stones were combined with sand. The sand not only helped with the cutting, but it also provided an unusually large surface area for carrying the unclean residue. This prevented the residue from being embedded in the surface of the pieces. Parts can be successfully run if something is given to convey the dirt instead of water.
The inclusion of an organic material, such as corn cob grit or walnut shell grit, is more absorbent than sand, allowing more debris and even oils to be carried. Organic compounds are also excellent abrasive carriers.
To avoid damage, plastics require a soft medium, such as wood pegs. A soft media, on the other hand, does not cut very well. Adding pumice-treated corn cob grit to the load dramatically accelerates cutting. Pumice is silica derived from volcanic ash. It is an extremely sharp, lengthy, friable crystal. Because it is soft, even under the light weight of wood media, it will fracture into tiny sharp crystals. Harder abrasives will not fracture and will thus stop cutting when they become dull. As a result, pumice is the preferred abrasive for dry cutting.
Self-tumbling metal parts using ceramic or plastic media works well with pumice as an abrasive and corn cob grit as an absorbent.
Wood pegs or walnut shells coated with wax and abrasive can be used to obtain extremely high, dazzling surfaces. The abrasive used should be one micron in size or less. For this type of work, ready-made polishing creams such as Metaglos and Microlyte are available. Metal pieces can be finished in a tumbling barrel overnight or in a high-energy machine in an hour or less. This is how most wire eyeglass frames and a lot of gold jewelry are polished.
Plastic components, such as buttons and eyeglass frames, are typically run with wood pegs or, in some cases, exceptionally big corn cob grit. Plastic items can be polished to a finish similar to hand buffing with enough time and care. Because the media is quite light, it takes 10 to 15 hours to produce effects. Because there is no burnishing effect, the abrasives accomplish the entire job. A plastic eyeglass frame must go through three to four processes (progressively finer), and each step may take one day. Nonetheless, bulk finishing is far more cost effective than hand polishing.
Although the barrel-finishing cycle described here will suffice in most cases, some simple deflashing operations, such as those commonly performed with plastic compression molded parts, can be performed by simply tumbling the parts against each other in a screened barrel that allows scrap to fall out. Typically, no media is required for this type of procedure. The motion of the parts against each other removes the flash, while the holes in the barrel allow the waste to escape, keeping the parts clean. This method cannot be used to deflash vinyl molded pieces because the flash will bend rather than break. This can be addressed by freezing the vinyl. In a normal wet barrel, tumbling with dry ice, either liquid or solid, will suffice.
Parts
The number of components that will fit securely into a barrel is governed by the barrel size, the size of the item, the fragility, form, and weight of the part, and the desired end result.
Parts typically contribute for one-third of the overall barrel load. The number of components that can be placed in the barrel in relation to the amount of media is a trade-off between maximum economy and maximum finish. If there are too many pieces in the barrel, good surfaces will not be obtained. Stock removal, on the other hand, will be hindered if there are too few pieces in the barrel. The greater the finish, the fewer pieces in the barrel. The more pieces in the barrel, the more economical it is.
Simple shapes, such as balls and squares, can generally be barrel-finished without concern of harm. If two components are of equal size, it is possible to process a basic shape rather than a complicated one. This is also true when considering weight, because larger numbers of lighter goods can be barrel finished in a single operation as opposed to heavier things.
Loading a Barrel
Parts and media load heights in barrels should not be less than 45% or more than 60% of capacity. Load heights of 40% to 45% result in more motion but a weaker finish. To protect parts from impinging, the optimum load height is 50%, with about three parts media to one part. On large or brittle pieces, the ratio may need to be increased to 6:1. The action slows as the load height increases. Raising the load height softens the motion; reducing the height speeds up cutting but results in a coarser finish. However, if the barrel is overloaded to 80% with additional material and driven at a slower pace, it is sometimes feasible to tumble very huge portions.
Transporting Agents
Abrasive operations are used to remove tool marks and flash, smooth rough surfaces, and produce radii. It is recommended to apply a transporting agent in addition to abrasive granules when dry tumbling.
By acting as a buffer between the pieces, a carrying agent prevents them from harming each other and produces smoother surfaces. The carrying agent will also carry the abrasive into crevices that would otherwise be inaccessible. Corn cob grit, walnut shell grit, and wood pegs are examples of carrying agents.
Because corn cob grit and walnut shell grit are too light to provide enough friction, the majority of abrasive activities are performed with wood pegs. All transporting agents should be of a size that allows for easy portion separation and will not lodge in any holes or crevices. Carrying agents, as a result, come in a variety of sizes.
Media
Any substance that is added to the load of parts to act as a cushion, preventing the parts from colliding, and as a carrier for the compound is referred to as media. Normally, media is utilized in a three-to-one media-to-parts-by-volume ratio. Large or fragile parts can be protected by using more media and fewer parts per load.
The size and kind of media used in barrel finishing are determined by the material, size, and number of holes in the item. The media should be small enough to move freely through holes, crevices, and prongs, but not so small that it becomes lodged. The faster the cut, the larger the media; the finer the cut, the smaller the media.
Media Suggestions
Typically, wood pegs are used in “dry tumbling,” although denser or coarser media, such as walnut shell or corn cob grit, can be utilized for faster cutting action if the finish is unimportant.
When uneven shape and size are not important, aluminum oxide is utilized for deburring and honing. Aluminum oxide is both inexpensive and long-lasting.
Preformed plastic media is a lightweight, durable medium that is ideal for finishing aluminum, die-cast, and sensitive parts. It will have little or no impact on the parts.
Preformed ceramic media is an abrasive media that is utilized where size and shape consistency is required. It is faster to cut than aluminum oxide, but it is also more brittle.
Steel burnishing balls and forms are only utilized in burnishing and brightening operations. They will soften light burrs but will not eliminate metal. With the use of a burnishing compound, very high finishes can be attained. For ordinary ferrous work, use three to five parts medium to one component work pieces by volume (3:1 for steel, 5:1 for magnesium or aluminum). The finish will be finer if the ratio or media is increased. Steel burnishing shot should not be used or mixed with abrasive media.
Only deburring or burnishing compounds and water are used in self-tumbling (tumbling components without medium). Self-tumbling is achievable if the pieces are not too delicate or detailed in shape, and the burrs are fully exposed. It is more cost effective since more parts can be loaded into the barrel. Another advantage is that there is no separation issue.
Compounds
The following consideration is an abrasive compound. This is the ingredient that specifies the type of operation to be carried out. A grinding action will occur if the media is treated with a cutting compound; a smooth shine will appear if the media is treated with a polishing cream. Do not combine acidic and alkaline substances, since this may result in unwanted pressure in the barrel.
Pre-Polishing & Cutting (Dry Tumbling)
Our Dry Abrasive Cream and Dry Cutting Cream cut smoothly without caking or blocking recesses. Because the pumice in the cream breaks down after around 20 hours of running time, this abrasive compound must be removed from the barrel after each 16 to 24 hour run. If the broken down compound is not removed, it will foul the plastic and barrel and slow the action of any new compound applied. By running the barrel with a screen door, the compound can be removed automatically.
A pumice and compound mixture should not be used in some cases because it will block holes or recesses in the parts. In this case, apply Shynolyte Pre-Polishing Cream directly to the wood pegs.
Although the abrasive process produces a smooth surface, it is not smooth enough to accept a polish, thus more abrasive procedures are required.
Dry Tumbling (Polishing)
This is the most important step in the polishing of plastic items. A acceptable finish cannot be obtained if the surface of the plastic pieces is too rough.
In general, a cream is used as a polishing agent and is applied directly to wood pegs. Waxes and/or abrasives are used in finishing creams. The disadvantage of a wax-based cream is that the wax gets removed during handling, so removing the polish. It is best to polish with a wax-abrasive cream, such as our Microlyte Polishing Cream or our Metaglos Polishing Cream for metals. Waxes can be combined with an abrasive to act as a lubricant and provide some protection during handling.
Before placing plastic pieces in the barrel for polishing, they should be somewhat clean. If there is not enough cream in the barrel, the wood pegs will scratch the parts. Cream and dirt buildup on the wood pegs must be avoided. When there is a lot of accumulation, the plastic portions scrape the cream and dirt off the pegs, resulting in a powder. This powder then renders any lubricating agents ineffective, and the pegs scrape the plastic parts. Placing too many components in the barrel will also result in this scenario, as well as damage from parts hitting with one another.
Speed
The RPM of the barrel is critical. Too modest speeds will not provide enough friction between the pieces. Depending on the work to be done and the size of the barrel, the greatest results can be obtained around 28 to 30 RPM. Higher speeds produce a faster action but a weaker finish. Slower rates take longer to complete the task but are safer for large or fragile pieces. When deburring parts, start at a low RPM to cut the burr rather than rolling it over. When the danger has gone, the RPM can be increased. A higher RPM can be employed for burnishing.
Smaller barrels necessitate greater speeds to achieve the same surface feet per minute as larger barrels. Faster speeds for deburring parts may be desirable, but they may overcome gravity and disrupt the constant even slide zone within the barrel, resulting in part impingement, pitting, or damage to the components being processed. Instead of tumbling, the pieces may become airborne and be pummeled or bombarded with heavy particles, resulting in impact damage. This may be a reasonable procedure where the part surface polish is unimportant and shorter cycle durations are preferred, but it is not the optimal use of the equipment. Continued use at high RPM will limit the life of the media by breaking it up faster, or it may damage the work chamber’s inner walls or liner.
Slope and Slide
The ability of the pieces and media to slide down a gravity-created slope determines the efficiency of the typical barrel system to deburr or polish. If the slope is broken up too quickly, the components may become airborne, causing part damage and/or rendering the barrel system ineffective. A slow speed does not harm the parts but increases the cycle time. When quicker speeds are desired, additional water and chemical compounds should be added to the barrel to increase mass cohesion; this will also soften or buffer the impact or hammering effect of the media on the pieces.
The desired sliding action will be compromised if the barrel turns too quickly. Parts will be taken to the barrel’s top and dropped. This has the potential to harm the plastics. The workload slide is determined by the diameter of the barrel, the RPM of the barrel, the load level height, and the water level height. The maximum sliding is obtained at roughly 150 surface feet per minute (SFPM) with a 60% load level and a water level one inch below the load level. A low SFPM at the start of the tumble cycle will prevent certain burrs from rolling over. After ten or fifteen minutes, the SFPM can be increased. A low SFPM will also aid in the protection of delicate parts. Too much SFPM causes the load to fly around in the barrel, causing impingement and pitting.
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