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Machined steel parts often need to be deburred. For large quantities of parts, both barrel tumbling and vibratory tumbling can accomplish this task saving labor hours and increasing throughput capacity. The specific details of the process must be chosen carefully to ensure that the parts are not only properly deburred but that the other surface finish specifications are met as well.
The machined part on the left in this picture has a bright surface and sharp edges. The goal was to deburr the edges with minimal impact on the surface finish. The initial result (part on right) of tumbling with Ceramic Media and a general purpose compound did deburr the part but it also resulted in a dull, matte surface finish.
This part will need to be tumbled in second step using a polishing media and a different compound to bring the desired finish back. Having a clear understanding of the ultimate surface finish required and that multiple steps are often needed is the first step in finding the best solution to finishing parts.
Machining and work-hardening steel creates burrs and scale that needs to be removed. The part in the middle of the picture above shows the 'excess' material in threads as well as the darkening caused by scale formation in the heat treating process. While barrel or vibratory tumbling can be used to remove these burrs and create a smooth surface finish, threaded areas often need to maintain the square edges and dimensionality on the part.
The other two parts were blasted with different types of abrasive blast media to show how each media can accomplish the deburring task but create a totally different surface finish. The part on the left was blasted with a Medium Glass Bead (70-100 mesh) and resulted in a satin-like surface finish with clean edges. The part on the right was blasted with 120 Mesh Aluminum Oxide Grit and has a more matte, gray surface finish.
While Glass Bead can often result in a more aesthetically pleasing finish, the micro-profile achieved with Aluminum Oxide Grit is often required for future processing of the parts (i.e., coatings application).
When laser cut (or saw cut), steel parts can develop heavy slag on the edges (see picture above). The slag is developed as the metal is melted during the laser cutting action and then re-forms on the edges on the 'bottom' of the part. The excess metal can be removed by hand grinding; however, if the parts are small or there are many parts to be cleaned, hand finishing is not efficient.
The finished part above was barrel tumbled part-on-part to remove the slag and excess metal on the edges. A small addition of 36 Mesh Aluminum Oxide Grit increased the abrasiveness of the tumbling action allowing the process to go from the before to the after in only 30 minutes. While the surface finish is rough after the tumbling process, the parts can be painted or sent through additional steps using other tumbling media to smooth out the finish and possibly polish the part.
Before
A quick before and after result of Fine Glass Bead blasting on steel parts. Before blasting part shows scratches, machine marks and 'pock' marks on the surface. Blasting with Fine Glass Bead resulted in a smooth, uniform finish ready for polishing, coating or other types of surface finishes.
After Glass Bead Blasting
Welding steel often creates heat discoloration and welding burns. Prior to finishing many parts with processes such as powder coating, painting, clear coats or simple final assembly, the removal of the discoloration and preparation of the surface is necessary.  Elements were welded onto this steel tube prior to powder coating. In addition to the discoloration (blue streak seen on the left side), this part started to rust in storage. The powder coating process specified that the part be clean and smooth. The right side of the part was blasted with Medium Glass Bead to remove the oxidation, discoloration and provide a smooth, lightly peened surface finish.
 Removing heavy rust (picture above) off steel can be accomplished successfully using an abrasive blasting process. While effective, the etched finish can be a challenge to polish. A vibratory tumbling process will remove the oxidation and leave a matte surface finish which can be subsequently polished. These steel tubular parts were cleaned and polished in a 2-step vibratory tumbling process first using a general purpose Ceramic Media to remove the oxidation and any pitting caused by the rust (picture below left). The second step involved a Precision Ceramic Sphere to smooth out the surface finish and produce a polished surface finish (picture below right). 
Cutting steel tubing can produce heavy burrs on the edges and if done wet can lead to rusting on the surface of the parts (picture below).  Rusted Steel Tube Prior to polishing the steel, the part must be deburred and cleaned. The oxidation (rust) will lead to pitting on the surface so an appropriate abrasive media must be used to create a smooth surface finish. Vibratory tumbling the parts with a general purpose Ceramic Media is typically aggressive enough to deburr the edges and remove any pitting from the rust (see left on picture below).  Clean (L) and Polish (R) After the tube was tumbled with a Ceramic Cylindrical Wedge (a good shape for round tube parts), the steel tube was polished with a polishing Precision Ceramic Sphere. This porcelain-type round ball will significantly improve the surface finish and produce a bright polish on the part (see right on picture above). This part is now ready for producing an even higher, mirror-like finish using further mass finishing steps or hand buffing.
Often stamped or cut parts have sharp edges that need to be deburred and rounded to meet the final part specifications. This hardened steel part has a square edge that needs to be rounded. The simplest process would be to use a grinding tool to round the edge. Unfortunately, if the part is small (as in this case) or a perfectly consistent edge is required, grinding by hand is not feasible. Barrel or vibratory tumbling with an aggressive ceramic media can accomplish the task. Using a media that is bonded with an aggressive, abrasive grit and breaks down quickly can round the edges in short cycle times. 
 Question: I am looking at using media blasting to identify/mark items made from stainless steel sheet using stencils to allow shielding of areas I don't want to blast. The stainless steel (used in a food factory) is highly polished. I was hoping media blasting would give a different enough surface finish to make the marking easily visible but what media would be best to achieve this? Also, would a direct pressure or suction system media blasting cabinet be best? The factory air system produces up to 90 psi and supplied more than enough air to run lots of machinery. Response: You can use either Aluminum Oxide or Glass Bead to create the marking you described. Aluminum Oxide will produce an etched finish with finer grit sizes producing a smoother, lighter etch. Glass Bead will produce a smooth, peened finish with finer grades producing a more uniform, matte finish while coarser grades will leave a brighter finish. Either a suction system or direct pressure blast system could be used. The pressure system will typically shorten the cycle time, produce a more uniform surface finish and allow for more media re-use since you can blast at lower pressures.
The process of bluing steel creates a micro finish on the steel surface to prevent corrosion and rust. Putting this type of finish is very popular with guns as the process improves the surface finish, protects it and reduces the glare off the metal. While not nearly as durable as plating or painting, bluing is an inexpensive and simple surface finish.  There are a few different ways to blue a surface but all involve creating a very thin 'coating' of iron oxide on the surface. This coating prevents oxygen from further attacking the steel. This is similar to the way the green 'coating' (copper oxide) coating on the Statue of Liberty protects the underlying copper. Bluing gets its name from the coloring of the iron oxide (magnetite) the process creates. This is a different chemical compound than the reddish color typically associated with rust (also iron oxide).
 When steel parts are laser cut, heavy slag can develop on the part. These burrs are typically much thicker and heavier than a normal burr or sharp edge and can sometimes be a large a 1/8" thick. Removing this excess metal requires an extremely aggressive operation. For this piece, part-on-part barrel tumbling was required to knock off the extraneous material.  To keep the cycle time to a minimum (less than 8 hours in this case), a hard abrasive grit, Silicon Carbide, was added to increase the deburring properties as well 'even out' the surface finish. After tumbling the part showed no burrs or slag on the edges and had a uniform, matte finish. After this ultra-aggressive deburring step, the parts can then be tumbled (barrel or vibratory) to improve the surface finish for further processing.
Question: I'm interested in ceramic and glass sphere media for tumbling compressed gas cylinders to remove rust and organic contaminants. I want to clean these cylinders for oxygen service. My cylinders range from large (5' tall by 10" diameter) to medium (2' X 7") in steel to small (1' x 4") in aluminum. I'm primarily interested in doing a thorough job cleaning the cylinders without damaging them. Response: The following recommendations would be appropriate: - 5/32" x 5/16" KM Ceramic Angle Cut Cylinders Media - for the cleaning step
- 150 Mesh Aluminum Oxide Grit - to increase abrasiveness of tumbling process
- 5mm K-Polish Precision Ceramic Spheres - as a replacement for large glass beads (more durable and longer lasting)
- Kramco 1060 - Aluminum safe (even in concentrated form) cleaner and degreaser
 Wow! An impressive show by the Chinese with these 2008 Olympics. Many of the controls the government put in place to control pollution (stopping construction, closing factories, significantly reduced traffic) seemed to work - although a couple days of well 'placed' rain also helped. A couple of random thoughts now that we are beyond these Olympic games:
- China spent A LOT of money on these games. While impressive, most of this $$$ came from western countries buying less expensive Chinese goods (one individual purchase at a time) leading, of course, to China's continuing economic growth. Is this really best for our local and global economies?
- Construction and factories were shut down across China to reduce horrible pollution for two weeks. Now that the Chinese have seen what a blue sky looks like again, I wonder if there will be any continuation of this environmental policy. If the Chinese return to the pre-games norm, I suspect that pricing and supply pressures on many materials (Aluminum Oxide, Silicon Carbide and Steel among many others) will be back in full force along with the pollution it generates.
London 2012 has a very tough act to follow. I hope that the world has learned important lessons about balancing the environment, global trade policies and individual consumption to springboard our (collective) path towards a better future.
I posted some information a few weeks ago about using Crushed Glass Grit to strip/clean steel. Here are some before and after pictures using Coarse Crushed Glass Grit on a cast iron part to remove some moderate to heavy rust. The total blast time was less than one minute and the total surface area is about 1 cubic foot.  
Historically, steel has been blasted using silica sand. For obvious health reasons (potential silicosis hazard), silica is not a safe media to use. Many industrial contractors, municipalities and military facilities are now using Crushed Glass Grit to blast steel. Crushed Glass Grit will remove paint, dirt, soils and rust leaving a 'white' metal surface. Depending on the grit size used, a micro-finish can be achieved so that the steel can be immediately primed and painted with no further preparation. Produced from recycled bottle (mostly) glass, Crushed Glass Grit contains no free silica, heavy metals and is approved in many of the most stringent air quality districts. Crushed Glass Grit is also considered a green blasting media since it is 'neutral' to the environment.
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