Tumbling Frustration

A question about deburring, cleaning and polishing brass, copper and aluminum parts.  It's worth reading through the entire question as it provides an excellent description of what may small shops are doing and the frustrations they have.

Question: I primarily machine small parts out of 6061 aluminum, a small amount out of C360 brass and a handful out of C145 copper. I currently use a small vibratory tumbler to finish my parts. I use a small (about .25" square base) green plastic triangle to do the deburr and initial cleaning cycle. The problem with this, is it takes two to three days to run to get satisfactory results. I have tried running dry, but it takes even longer to achieve the results I want. I usually add a small cup of water to add some lubrication. But I have to take the tumbler bowl off and rinse it out and wash off the media every 6-8 hours because I get a thick black foam that fills the bowl and turns the media into a solid mass that just moves around the bowl. After this step, I clean out the tumbler and add a relatively fine red corn cob media to do a polish. But that also takes two to three days of tumbling to achieve a decent finish. I never add rouge or other polishing compound because I have no idea what to use. I just use the pre-treated corn cob media and replace it every 3 or 4 months. I am hoping to add a new machine to my shop and expect to produce several hundred parts a week that need polishing (I do not need to polish to a mirror shine, I merely need to deburr after the machining operations and provide a nice level of shine). Obviously, I would like to find a better way to accomplish this over what I have been doing. My parts are typically small - 1.2" diameter x 0.12" thick discs, 0.75" diameter x 0.25" thick slugs, 0.8" x 0.375" x 2" bars. Many of the parts have threaded holes and small slots (1/16" wide). I spend a lot of time hand cleaning the corncob out of the slots with a small pick. I have tried to do research to find a better solution, but it is one of those ask a dozen people, get 2 dozen answers types of questions. Some people tell me I need flow through on my first step. Some recommend ceramic some plastic and some steel shot. Corn cob and walnut shell seem to be recommended for finishing - some say pre-treated and others say to add rouge. I am at a loss now as to which direction to proceed so I figured it was time to seek more professional advise. Would you be able to help me find the best solution to my problem? I do not have a large space to set up multiple large tumblers, I need a compact solution with at most, two tumblers.

Response: I will try to address your questions and offer a simple and reasonable recommendation.

1. Plastic Media, Ceramic Media and other types of deburring media like this require a wet process using an appropriate tumbling compound (i.e., soap/lubricant). Assuming the proper shape and size of media is being used, the cycle times on the types of metal you noted (aluminum, brass and copper) should not be more than a couple of hours. The cycle times could be longer if you have very heavy burrs or need to get a highly rounded edge. The foam is probably a result of not using enough water and no compound. This is common with Plastic Media with no flow-through system for the solution.
2. Given the surface finish requirement of a 'nice shine' and parts descriptions, I would recommend using a 3/8" x 3/8" KSV Synthetic Cone Media and a solution of Kramco 1030 Mild Acid Cleaner. This media is a light, deburring media that will result in a smooth, pre-plate finish. The Kramco 1030 will leave the metals you are using with a bright finish. This media choice may require a slightly longer cycle time than a more aggressive media (i.e., Ceramic Media) but the surface finish will be much better. In addition, a flow-system is recommended for the best results.
3. The polishing media you are using - treated corn cob grit - may be treated with a rouge or something similar. Cycle times with dry tumbling like this are often long, 24 hours or more, but can produce excellent, near-mirror finishes. Of course, if the parts have holes or slots that are similar in size to the grit, it is common for the media to get stuck. Using the recommendation in #2 above may eliminate the need for a second polishing step (depending on the specific finish desired).
4. An alternative polishing step could be with a polishing Ceramic Media or Precision Ceramic Media instead of the dry process. While this may not result in the near-mirror finish achievable with a Corn Cob Grit, the finish can be a smooth, polishing, bright surface. A solution of Kramco 2030 Neutral Burnishing Compound would be used in this step.

Water Spots on Copper

Copper is a soft metal that is easily oxidized and stained (note previous post re: green color of Statue of Liberty). Stamped copper parts need to be deburred and cleaned prior to further use in most manufacturing processes.



This picture shows a portion of a copper component of an electronics part. The stamping and cleaning process (part on the left) shows light burrs and clearly visible water spots and staining.

In order to get a clean part for proper electrical contact, the part on the right was vibratory tumbled. The customer wanted a short cycle time and was not interested in a polished or bright surface finish. A Ceramic Angle Cut Triangle was used in order to deburr the outside edges as well as the ID of the holes and other features. The media and parts were tumbled in a compound solution of the Kramco 1030 Mild Acid Cleaner to remove the oxidation and water spots. The end result was a clean, smooth surface finish that met the customer's requirements. The total cycle time in the tumbler was only 15 minutes.

Cleaning Phosphor Bronze

Bronze is primarily a metal alloy of copper and tin. The tin adds hardness to the copper and increases durability. Phosphor bronze includes the addition of up to 1% of phosphorous to the alloy. This addition improves strength, reduces the coefficient of friction of the metal and results in a finer 'grain'. The fluidity of the molten metal is also increased making this alloy easier to cast.

Just like pure copper metal (note green Statue of Liberty) and other bronze alloys, oxidation and stains can appear of the surface of the metal. This degrades the visual appearance required for certain part requirements. The electrical parts pictured below show tarnish, oxidation and water stains on the parts on the left.


Bronze Water Spots


Bronze Oxidation/Tarnish

The clean, bright parts on the right were vibratory tumbled with KP Ceramic Media and a solution of a mild acid cleaner, Kramco 1030. The mechanical action of the polishing media combined with the chemical cleaning of the compound resulted in a smooth, brilliant surface finish. This result was obtained in only 30 minutes of tumbling.

Cleaning Copper

Before

After

In addition to deburring and polishing, vibratory tumblers can be used successfully to degrease, clean and remove oxidation from parts. These copper component parts needed be cleaned prior to being used in re-building the larger equipment. The dirty part pictured containing some light soils and oxidation was vibratory tumbled with a fast cutting Ceramic Angle Cut Triangle Media (KXF bond) and a mild acid cleaner (Kramco 1030) to restore the part to a clean metal finish. The combination of media and compound required only a 30 minute cycle time to achieve this finish.

Glass Bead vs. Crushed Glass Grit

We get a lot of questions about the difference between Glass Bead and Crushed Glass Grit. Below are some before and after pictures of a copper pipe after blasting with similar grit sizes of both of these materials. [Note: The pictures are not great but you should be able to see some clear differences.]

Welding burns & discoloration on surface

Glass Bead is a round particle that produces a very smooth, satin-like finish. This media can be re-used multiple times and is typically used in a blast cabinet. Crushed Glass Grit is an angular grit that offers very good strip rates but breaks down very quickly. Crushed Glass Grit is generally used in portable blasting equipment where collection and re-use is difficult.


Glass Bead - Bright, smooth, satin-like finish


Crushed Glass Grit - Lightly etched finish, very short blast time