Cartridge Brass - Study (Part II)

Dirty Cartridge Brass Casings

In the last post we discussed the differences in performance between vibratory tumbling and barrel tumbling to clean brass cartridge casings. A wet process using a polishing media is a much quicker method to clean brass than dry processes using Walnut Shell Grit or Corn Cob Grit. The test results indicated very similar results in both types of wet tumbling systems.

This post will focus on three different media options for cleaning the dirt and oxidation off the brass. In all three cases, the same solution of Kramco 1030 Mild Acid Cleaner was used and all tests were conducted in a barrel tumbler with a tumble time of 15 minutes.

Carbon Steel vs Stainless Steel

The first test compared the differences between Carbon Steel Tumbling Media and Stainless Steel Tumbling Media. It is pretty clear from the picture above that the Stainless Steel was much more effective at cleaning the brass. While this difference would not be expected initially, the reason may have something to do with the characteristic of the Carbon Steel Media to oxidize and possibly the transfer of iron to the brass casings. This reason will be more clear after reviewing the results of the second test.

Polishing Ceramic vs Stainless Steel

Once the Stainless Steel Media was declared the winner in the first test, the next step was to compare this heavy, burninshing media with a less dense (and less expensive) media. The above pictures compares the results of a K-Polish Precision Ceramic Media to the same Stainless Steel Media used in the first test. The K-Polish Precision Ceramic Media is a high-alumina, porcelain-type media used commonly in many different tumble polishing applications. Both of these media types are extremely long lasting with the capability for 1,000+ operating hours.

As the results for the Stainless Steel Media and the Precision Ceramic Media are very similar, the decision on which media to use will hinge on the type of equipment being used (many vibratory systems can't handle the weight of steel media), the available budget and the overall desired process.

In conclusion, these tests would seem to indicate that excellent results can be obtained in very short cycle times using either a Stainless Steel Tumbling Media or a K-Polish Precision Ceramic Tumbling Media in either a barrel tumbler or vibratory tumbler.

Polishing Medical Parts

Once cut, machined and heat treated, stainless steel parts to be used for medical applications (see tweezer parts above) need to be deburred and polished to a high shine. The part in the top of the picture above is ready for surface finishing and some light edge rounding.

To remove the heat treat discoloration, the part was first blasted with a Coarse Glass Bead. This created a uniform, satin-like surface finish on the part. In this situation, a larger bead was used on the hard metal in order to keep the finish a bright as possible and make polishing a little easier..

After blasting, the part was barrel tumbled with a Steel Polishing Media to smooth out the finish, provide some light edge rounding and prepare the part for an additional polishing step. A final step such as a burnishing wheel or electro-polishing would create a near-mirror finish.

Auto Parts Polishing

Question: I am interested in starting a metal polishing business mainly for car parts. I am wondering if your wet tumblers would be best and what type of media would be best for polishing aluminium, rough cast intake manifolds? I have polished some by hand using sand papers, air grinders with various deburring bits and abrasive wheels, polishing wheels and compounds. This is very time consuming, dirty and tiring work. I am looking at an easier way! I am also interested in smoothing and peening other cast or forged metal parts for increased strength and durability!

Response: There are a number of different ways to clean and polish auto parts. Depending on the final finish required and the size, shape and geometry of the part, the final surface finish can range from a smooth, bright finish all the way to a near mirror finish.

If parts simply need to be cleaned prior to painting or coating, blasting with Glass Bead will result in a smooth, satin-like or matte surface finish. If additional polishing is needed, vibratory (or barrel) tumbling with a Polishing Precision Ceramic or Stainless Steel Media can produce much smoother, brighter finishes. The key with both processes is to select the proper media shape and size based on the part and areas of the part that need to be finished. The size of tumbling equipment is based on the size and quantity of parts that need to be finished.

Silverware Burnishing

Previous posts about the basics of silverware burnishing (Part 1 and Part 2) described the process through which silverware and flatware are restored in a barrel tumbling process. The basic outline described the flowing of the metal to 'fill' the scratches and the removal of the oxidation on the surface. Below is a picture showing an extreme example of what can be accomplished.

Silverware Burnishing Test Results

• Far left - Before sample with heavy oxidation, scratches and some pitting on the surface
• 2nd from left - Tumbled in Steel Media and a polishing compound. Sometimes testing is required to determine if the combination of media and compound will discolor the surface even though the process removed the scratches and pitting.
• 3rd from left- Tumbled in steel media with an alternative compound. While it is difficult to tell, both the 2nd and 3rd pieces are similarly polished The proper compound (for this application) clearly resulted in a brighter, polished finish.
• Far right - Tumbled with the same compound as the 3rd part but with a Precision Ceramic polishing media. The surface is clean and bright but not quite as good as the Steel Media. This process would be best if the tumbling equipment being used is not designed for heavy media.

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.

Silverware Burnishing Basics - Part 2

The purpose of silverware burnishing is to both physically and chemically clean and smooth the fine surface 'grains' of the metal. This is accomplished through the use of a barrel tumbler and the appropriate tumbling media and compound.

Stainless steel burnishing balls (media) are used to force the metal to 'flow' and fill the scratches on the surface. As the entire load is rotated in the barrel, the media lifts up the back side of the the barrel and cascades towards the front. As the parts are carried with the media in the sliding area, the weight of the burnishing balls impact with the surface of the silverware. This contact forces the metal to fill the low spots and even out the high spots.

In combination with the physical polishing, the water and burnishing compound help to remove the tarnish (silver sulfide) and keep the process lubricated. Using the proper compound (and changing the solution regularly) will limit damage to the silverware in process and keep re-deposition to a minimum.

The combination of chemical cleaning and microscopic, physical manipulation leads to a bright and polished surface finish.

Silverware Burnishing Basics - Part 1

Silverware burnishing is a process that combines surface hardening and polishing without the use of abrasives. On a microscopic scale, a metal surface is actually composed of tightly packed individual 'grains'. The basic theory of burnishing is that these surfaces can be hardened and shined through the manipulation of these grains.

Over time and normal use, silverware is scratched and tarnished with a thin residue of silver sulfide. This results in reducing the polished surface finish and texture. The burnishing process is designed to restore the silver surface through a barrel tumbling process. This process forces the metal to 'flow' and physically fill the scratches. At the same time, the burnishing process chemically removes the tarnish.

This combination of chemical cleaning and physical compaction results in a shiny, hardened surface that improves the surface appearance and increases the lifetime of the silverware.

Quick Polish on Aluminum

Aluminum cast parts typically have a dull, matte finish after coming out of the casting process. The part on the right shows this 'raw' surface finish. A quick method to significantly improve the aesthetic finish is to tumble the part with a stainless steel media in a barrel tumbler. The 'finished' part on the left was barrel tumbled for only 15 minutes with a Stainless Steel Round Ball in a wet process with a burnishing compound. This quick polish resulted in a part with a higher quality finish without significantly added processing cost.

Note: Use stainless steel media with aluminum or other soft metals to minimize the risk of iron contamination and oxidation/rusting during storage.

Stainless Steel Tumbling

Before and after of machined aluminum part tumbled with Stainless Steel Media. The 'raw' showed machine marks, scratches and burrs on the edges. After vibratory tumbling with steel media, the part came out smooth, bright and ready for anodizing. Improved polishing could be achieved by additional tumbling with Corn Cob Grit or Walnut Shell Grit or hand buffing to create a near mirror finish.

Benefits of Steel Tumbling Media

Given the investment required to purchase Steel Tumbling Media, why use it? It basically all comes down to weight and wear.

Steel media is much heavier than other types of tumbling media. As the parts are forced through the media the high pressure results in reduced cycle times. The pressure or compression on the part helps to work-harden the surface. In many cases, tumbling parts in steel can eliminate shot blasting and lead to parts with increased wear resistance. Work-hardened parts have much less porosity increasing corrosion resistance.

Many parts ready for plating that appear to have a very smooth finish actually have microscopic imperfections in the surface. The weight of the steel media 'flattens' irregularities in the surface and produces a noticeably better plated finish - especially with thin depositions such as nickel.

Steel is a considered a non-consumable tumbling media and does not wear down in the tumbling process. This means much longer lifetime than traditional media (100x or more) and no media residue waste from the media.

Steel media should be considered a long term capital investment not a consumable media expense.

Body Jewelry Tumbling

We received a post a couple of years ago about polishing body jewelry in a vibratory tumbler. While very similar to other jewelry there was a concern about the size of the parts and the threads at the ends. The recommendation was to polish the parts using a Precision Ceramic media to be able to get into the finer areas of the part without damaging the threads. Steel tumbling (while also effective) might roll the thread edges. A simple but effective result.

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'.

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 corrosion-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 corrosion 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.