Polishing Pewter

Molded pewter, rough surface finish

Pewter parts can be cast into a large variety of shapes. The casting process often results in a rough surface finish along with burrs and flashing on the edges. Surface finishing is often done by hand to produce a smooth finish and especially a high polish on the parts.

2-step polishing process

Mass finishing can be used to achieve all of the required deburring and edge rounding and often can result in a bright, smooth polished surface finish.

The part on the right was vibratory tumbled with a Ceramic Media to remove the molding marks and soften the edges. The second step to polish to surface (part on left) involved barrel tumbling with Stainless Steel Media with an appropriate burnishing compound. You can even see the camera reflection. Additional polishing can be done by hand if necessary.

The mass finishing process will significantly reduce the amount of hand work required. Vibratory and/or barrel tumbling will also allow for difficult-to-handle small parts to be deburred and polished successfully.

Dry Tumbling with Corn Cob Grit

Stamped, cut, machined and otherwise processed parts often contain cutting oils and lubricants and need to be cleaned. These parts also have burrs and sharp edges that may need to removed.

The steel parts pictured on the left are dirty and have light burrs that need to be removed. A typical process would simply demand a mass finishing, tumbling process with a Ceramic Media to degrease the parts and remove the burrs in a wet process. The challenge with these parts is the shape - long, flat parts. In a wet process these parts will stick together to form 'sandwiches' and will not be cleaned and will resist deburring.

The solution is to dry tumble the parts with Corn Cob Grit. The Corn Cob Grit media will remove and absorb the soils (note clean, brighter parts on right). In a barrel tumbler, with a high ratio of parts to media, the part-on-part tumbling action will remove the burrs. The Corn Cob Grit can be separated from the parts efficiently with a simple wire mesh screen. The dry tumbling process also eliminates the need to dry the parts so they are immediately ready for any further processing.

Smooth Aluminum Finish

Aluminum is a relatively soft metal that is easily nicked, scratched and damaged by handling - both during the part manufacturing process and after. Restoring aluminum to a uniform and smooth surface finish can be accomplished with mass finishing.



The aluminum part on the left clearly shows the scratches and damage due to heavy handling. The part on the right is the result of a simple mass finishing process to create a uniform, matte surface finish. The part (aluminum cylinders) were vibratory tumbled in a wet process with a general purpose (KM) Ceramic Media and cleaning/lubricating compound for 2 hours.

Ceramic Media is typically used with harder metals such as steel; however, in this case, there was no need to further polish the part. If a higher polish is required, a lighter cutting Plastic or Synthetic Media can be used in a (potentially) longer cycle time. This step would be followed with a polishing step.

Mass Finishing Basics (Part 4)

The art of mass finishing really shows itself when trying to select a tumbling media. Media is the material ('stones') that parts are tumbled with in either a vibe or barrel tumbler. The types of media available are as varied as the parts that can be tumbled in a mass finishing process.

Ceramic Media is available in bonds or formulations that range from super aggressive, cutting to non-abrasive polishing. The most popular preformed shapes are triangles, cylinders and variations of these shapes. Ceramic media is typically used with hard metals such as steel. Softer metals such as aluminum can be tumbled with ceramic media but care should be taken so the process is not too aggressive. Precision Media is a high density version for specialty applications.

Plastic Media and Synthetic Media have a lower density and are thus less aggressive than ceramic media. These media are typically used with aluminum, copper, brass and other soft metals. Popular preformed shapes include cones, pyramids and wedges.

Steel Media is used primarily for polishing and burnishing operations in barrel tumblers. Some vibes can be set-up to handle the weight of the steel media which is 3-5 times as heavy as the other preformed shapes.

Other types of tumbling media include Hardwood Media (dry tumbling of wood and some plastic parts), Aluminum Oxide (surface finishing and etching), Walnut Shell (burnishing) and Corn Cob (burnishing) as well as many other specialty media.

Choosing the proper type, size, shape and bond is critical to making sure that the required finishing is achieved. Common problems include excessive media wear, poor and/or inconsistent surface finish, too much (or too little) edge rounding and media 'jamming'.

Mass Finishing Basics (Part 3)

Vibratory tumblers come in all different shapes and sizes. Bowl style vibes include both round bowl and oval-bowl shapes. Tub style systems look kind of like bathtubs. In general, vibes are produced in sizes as small as 1-2 gallons up to 150 gallons (and much larger). Both systems force the tumbling media and parts to vibrate and rotate around the vibe.

Tub style vibes force the parts (and media) to vibrate and rotate around the center axis of the tub. This creates a similar sliding and rolling action to a barrel tumbler but adds the additional vibratory energy working against the parts resulting in shorter cycle times. Since tub style vibes can be very long in length and diameter extremely large large parts can be tumbled.

Bowl vibes add circular movement around the bowl in addition to the typical tumbling and vibrating. Since the entire load is moving around the bowl, this system allows the option for a separating screen (either above or below the load) to automatically remove the parts from the media. Unlike tub style systems, bowl vibes can only process parts smaller than the bowl channel width.

Mass Finishing Basics (Part 2)

The two types of barrel finishing systems are rotary barrel tumbling and centrifugal barrel tumbling. Both mass finishing techniques use tumbling media, water (usually) and compounds to tumble parts inside a closed barrel. The similarities end there.

Rotary barrel finishing is an abrasive process that involves a sliding and rolling motion of the media/parts load. Filling the barrel 50-60% full allows the upper layer of the load to slide down (due to gravity) as the load is turned-over. Changing the load size as well as the ratio of media to parts to water to compound will create more or less aggressive operations. This ancient mass finishing technique is a highly economical, time-tested technique.

Centrifugal barrel finishing creates more sliding action at higher rates as the individual barrels spin independently of the turret rotation (think of the carnival ride with individual cars that spin while the entire carousel of cars are spinning) . The high centrifugal force created inside the barrel compacts the media/parts load and results in low part to part interaction. These high forces also lead to lower cycle times and very smooth, consistent surfaces finishes.

Mass Finishing Basics (Part 1)

There are two basic types of mass finishing - barrel tumbling and vibratory tumbling. Each of these mass finishing methods have pros and cons. There are also a number of sub-varieties which we will discuss in future posts.

Barrel tumbling might be more familiar if described as rock tumbling. Inside the tumbler the parts and media are lifted by the corners of the barrel and allowed to bump, scrape and slide against each other. The media used will depend on whether you are trying to deburr or polish your parts. Barrel tumbling is also good for part-on-part tumbling (another future post - so many posts and so little time). While typically used wet, barrel tumbling can also be a dry tumbling process. The big down side to barrel tumbling is the loading and unloading of parts.

Vibratory tumbling is a little different. Similar to barrel tumbling the parts and media are sliding against each but in more of a 'filing' motion. The tumbling action is occurring in 100% of the load with a vibe versus only 20-30% of the load in a barrel tumbler. This allows a vibrator (you know what I mean) to typically offer shorter cycle times. Vibratory tumbling can abrade and smooth a surface with minimal affect on the edges while barrel tumbling loves to round the edges. Vibratory tumblers can offer very simple and even automatic separation of parts and media.

Generally, barrel tumbling is good for durable parts that require heavy deburring or high polishing and for processes that use a lot of weight (i.e., tumbling with steel media). Vibratory tumbling is great for general purpose deburring, surface finishing and for easy loading/un-loading of parts.