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.

Removing Machine Marks

Machining aluminum parts to create the specified dimensions and geometry of the part leaves light etching, 'swirls' and general machine marks on the surface. Part of the finishing process to achieve the desired finish requires the removal of these marks. While hand grinding and polishing will work, a mass finishing process is certainly a better process

Swirling machine marks

The choice of media for a mass finishing process will depend on the required surface finish to prepare the part for the final specified finish. If deburring and heavy machine marks need to be removed, an aggressive media (i.e., Ceramic Media) may be required. If a less aggressive media is needed, a Synthetic Media can be used. Either of these cutting steps can be followed with a polishing media to smooth out the surface finish and remove the 'tumbled' look.

'Tumbled' surface finish

The surface finish above was achieved by vibratory tumbling the part with a general purpose Plastic Media (medium aggressiveness) to completely remove the machine marks on the part. The ultimate goal for this part was a matte, anodized finish so no further processing was required. The appropriate compound (Kramco 1060) was used to keep the part and process clean without darkening the aluminum metal.

Aluminum Edge Rounding

Deburring and edge rounding often go together. Aluminum is generally tumbled with less abrasive and lighter Plastic and Synthetic media to minimize wear on the edges. Sometimes a heavy deburr or edge round is specified. The aluminum part in this picture was vibratory tumbled for only 1 hour with a Cylindrical Wedge Ceramic media. I think the finished part on the right speaks for itself especially when placed next to a raw machined part.

Aluminum Pre-Anodize Finish

Creating an appropriate edge and surface finish on extruded and cut aluminum parts that need to be anodized requires both deburring and a smooth surface finish. Anodizing creates a protective oxide layer which can range from a matte to a high polish finish. A major factor in the aesthetics of the final anodized part is based on the pre-anodized surface finish.

Extruded and cut

This aluminum part was extruded and cut to size. The part shows an uneven, rough surface finish, sharp edges and burrs. There are also aluminum shavings and scratches that need to be cleaned and removed.

Ceramic Media Tumbled

Vibratory tumbling the aluminum parts with a Ceramic Media will produce a beautifully rounded edge and smooth, matte surface finish. The cycle time will be short (as little as 30 minutes) but the anodized finish will typically be a smooth, matte finish.

Synthetic Media Tumbled

Tumbling the part for a slightly longer time with a less aggressive Synthetic Media will also result in a deburred, slightly rounded edge but a much smoother, more uniform surface finish. A slightly longer cycle time with a 'softer' media will significantly improve the anodized finish resulting in a bright, polished part.

Choosing Tumbling Media (2)

In addition to choosing the proper type of tumbling media, the shape and size of media is also critical to achieving the proper finish. Given the multitude of tumbling media options, the size and geometry of the parts being tumbled will narrow down the choices considerably. Optimizing a process requires using a good quality media that is appropriate for the job at hand.

The most popular Ceramic Media shapes are Angle Cut Cylinders and Angle Cut Triangles. The angled edges help the media to produce a uniform finish across the entire part including inside corners, holes and slots. Cone shapes are popular choices when using Plastic and Synthetic Media. Generally, larger media will work better with larger parts. Smaller media will have a tendency to impact surface finish more than edges.

Aggressive media (whether large or small) will shorter the cycle time to deburr or round edges - and even for polishing (aggressive = dense) operations. Aggressive, abrasive media will break down more quickly than general purpose type media so replacement cost and the waste stream are an additional consideration.

The high quality tumbling media manufactured by Vibra Finish provides the balance between options (size, shape and type), cost (economical) and quality (uniform bonds, even break-down).

Choosing Tumbling Media for Deburring, Deflashing or Polishing

Choosing the proper tumbling media (vibratory tumbling or barrel tumbling) for any deburring, deflashing or polishing application requires balancing the aggressiveness of the operation with the surface finish specifications. As long as a rough, tumbled appearance is acceptable a more aggressive media can be used to remove burrs and 'smooth' a surface resulting in a shorter cycle time . If the surface finish is important (i.e., for anodizing or thin-coat plating), a less aggressive media and longer cycle time is necessary. Often a multi-step process is used with progressively less aggressive media to achieve a deburred and smooth part or even polished part.

Generally, Ceramic Media are used on harder metals like steel or titanium. Plastic or Synthetic Media are better for softer metals like aluminum, brass and copper. Within each type of media, a variety of formulations (or bonds) are available from ultra aggressive to general purpose to polishing.

Powder Aluminum Polishing

Aluminum powder products offer many advantages over other metal powders due to aluminum's inherent corrosion resistance, lightweight density, machinability and non-magnetic properties. After pressing (at lower pressures than other metals) and sintering (at lower pressures) the parts needed to be deburred and prepared for surface finishing/coating or anodizing.

The parts in the picture were vibratory tumbled with a light cutting Synthetic Media to remove burrs and create a uniform, matte surface finish (2 discs on left). A second step to polish the parts (2 discs on right) also involved vibratory tumbling but with a polishing Ceramic Media. Appropriate neutral pH compounds were used to remove and prevent oxidation of the surface. Cycle times were only 1-2 hours for each step.

What is Tumbling Media?

Preformed tumbling media used in both vibratory and barrel tumblers are simply the means to achieving a desired surface finish. The type of media formulation or BOND used can vary widely for each process or application.

Tumbling media is a blend of abrasives and binders. Ceramic Media is generally a clay or porcelain binder while Plastic Media and Synthetic Media are typically made from urea formaldehyde or polyester polymers. These binders are used to hold (bond) and shape the abrasive grit blend needed for a particular media type. Aluminum oxide and silicon carbide are the most popular abrasives used.

The specific type of binder and abrasive used along with the ratio of binder to abrasive will determine the impact of the media on the part. A high abrasive content will lead to quicker break down of the media and a more aggressive operation. Low or no abrasive content will result in a media that is very soft/gentle on the part and used for polishing and fine finishing.

All tumbling media are designed to break down or erode during use. This is necessary to perform the 'work' on the part. Generally, the quicker a media breaks down, the more aggressive the bond. The ideal media for any process will provide the minimum erosion (longest lifetime) but must be balanced with a reasonable cycle time to achieve the desired surface finish.

Polishing Aluminum

Bringing machined aluminum into a highly polished finish is a multi-step and sometimes challenging process.
The first step is to tumble (vibratory usually works best) the parts in an abrasive media such as a Plastic or Synthetic Media. Using as round a media shape as possible - cones vs triangles - will result in a smoother finish producing a better end result.
The second step is a polishing step using a Ceramic Media. Either an Angle Cut Cylinder or a Precision Ceramic Sphere will produce a smooth surface finish - again rounder is better.
In order to burnish the surface (if necessary), a third step can be to dry tumble the parts with a Walnut Shell Grit possibly treated with a fine abrasive such as jeweler's rouge.
It is important to make sure that the chemical compounds you use will not attack the aluminum. Depending on the final finish required, a small amount of hand polishing may be needed.

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'.