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.

Tumbling with Steel Media

Polishing all types of metal with steel media is an established procedure for many industries. As the price of steel increases (especially stainless grades), many companies are looking at barrel and vibratory tumbling with carbon steel instead of stainless. The BIG disadvantage of carbon steel is rust. Of course, the big ADVANTAGE is cost.

Keeping carbon steel clean and corrossion-free is critical to getting an acceptable surface finish on the parts. Using the proper chemical compounds is the answer to this problem. Even though acid compounds will return the rusted steel to near original condition, this should be avoided as this can actually make rusting problems worse.

Steel media should first be restored with a cleaner (like Kramco 750) containing sequestrants, detergents and be pH buffered to remove the corrossion and prevent flash rusting. The second step is to treat the shot with a rust inhibitor that will both prevent rust formation and not contaminate the parts {like oil based rust inhibitors}. Simply tumbling the steel media with a non-durable rust inhibitor (like Kramco 510) for a few minutes, draining the solution and then letting it air dry will keep the shot clean and bright even during longer term storage.

Deburring Aluminum Tube

A client asked us for a process that would remove the burrs from an aluminum part consisting of an assembled tube within a tube. The part was made by inserting one tube into another and then bonding the assembly together. The outer tube was 4 inches larger in diameter than the inner tube. Holes were then drilled through both tubes of the assembly. The holes could only be drilled after assembling the two tubes together due to the critical tolerances involved. There were a total of about one hundred holes in each part. The challenge was to remove the burrs from the inner as well as the outer surfaces of both tubes.

The problem was solved by attaching the part to the vibratory tub in a fixed position so that it would move along with the tub, but still be able to rotate on its axis. By clamping the part down and forcing it to vibrate with the tub, the amount of energy transmitted to the part was greatly increased.

A deburring, ball-shaped Precision Ceramic Media was used to minimize the chance of having media get stuck inside the part. The tub was slightly under-loaded with media to increase the flow rate through the holes and to aid in rotating the part. The compound flow rate was kept at eight gallons per hour. A mild acid cleaning compound (Kramco 1030) was used to speed up the cutting rate and maintain good color on the aluminum.

Due to the elevated cutting energy, the part was completely deburred with a 0.005 edge break in a cycle time of ten to fifteen minutes in a tub-type three cubic foot capacity vibratory machine. The inner tube was deburred thoroughly, exhibiting a 0.003 edge break. The parts were originally being deburred by hand, which took over three hours per part. The vibratory finishing system developed not only dramatically reduced the cost of finishing the parts, but also produced a more consistent part than when finished by hand.