Cartridge Brass - Study (Part I)

Dirty Cartridge Brass

Cleaning cartridge brass (see 'dirty' brass above) has traditionally been accomplished using a vibratory or barrel tumbler with a dry media such as Walnut Shell Grit or Corn Cob Grit. Both of these medias will do a good job of cleaning the dirt and oxidation off the brass but typically require a long cycle time (12-24 hours or more).

Alternatives to this long tumbling time are available that will result in clean, bright brass is as short a time frame as 10-15 minutes. These alternatives also require a vibratory or barrel tumbling system but with different media and use a wet (chemical compound solution) process. This post and the next will review the results using different tumbling methods as well as comparisons of different media.

The basic process involves (vibe or barrel) tumbling the brass with a polishing media. A typical ratio is about 3 parts of media to 1 part of brass by volume. The solution would be added to the level of the load in a barrel tumbler. In a vibratory tumbler, the solution could be circulated through the media/parts load or could be added as a small amount (usually about 5% by volume of the load size) in a batch process. The selection of compound (i.e, Kramco 1030 Mild Acid Cleaner) is critical to achieving good results. Obviously, it is important to ensure that the equipment being used is designed for a wet process.

Vibratory vs Barrel Tumbler

This picture shows the results using the exact same media type, parts/media ratio and solution but with different tumbling system types. The parts on the left were vibratory tumbled for 15 minutes with a recirculating solution of Kramco 1030 compound. The brass on the right were barrel tumbled also for 15 minutes with the same solution filled to the level of the load.

It is clear that both tumbling processes did an excellent job of cleaning the brass and that the results are very similar. Based on this test, it seems that either a barrel tumbling system or a vibratory tumbling system can be used to clean cartridge brass. The decision comes down to the type of equipment available and the desired in-house handling process.

The next post will compare different types of media all in a barrel tumbling process.

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.

Cleaning Motorcycle Parts

Question: I have a small vibratory tumbler (designed for gun casings). I would like to use it to clean and polish small parts from motorcycles as I restore them. This would include nuts, bolts and washers (steel, stainless steel, brass) and some aluminum parts. I assume a variety of media would be needed -- can you please let me know which media I should get?

Response: Small motorcycle parts can be cleaned using either a wet or dry process. Vibratory tumbling the parts with Corn Cob Grit in a dry process will remove the soils (oil, grease, etc.) and leave the parts clean. Depending on how dirty the parts are and what type of surface finish is required, a dry tumbling process can be up to 6 hours or even longer.

A wet process using a Ceramic Media such as a 3/16" x 3/8" KM (general purpose bond) Angle Cut Cylinder with a general purpose compound such as Kramco 1010 will degrease the parts, remove light rust (if any) and leave the parts clean. This process is typically a maximum of 1-2 hours. It is important to make sure that the vibratory tumbler being used is able to handle the weight of Ceramic Media.

Selecting Dry Tumbling Grit

Polishing or burnishing soft metal (brass, copper, aluminum, etc.) parts with Corn Cob Grit in either a barrel or vibratory tumbler is a popular process. Selecting the proper grit size for small parts or parts with complex geometry can be a real challenge.

The initial testing used for this part (only portion of part shown) used the same medium grit size as is used for larger, less complex parts also being tumbled by the customer. Due to the size of the many slots, holes and grooves in this part, this medium grit size media got stuck in many areas making the cleaning process timely and expensive. There are two options.

Using a larger grit size to prevent the media from getting stuck in the first place will address the problem and still provide the required burnishing action. Of course, as the media wears and breaks down, there is an increasing risk of media getting stuck.

Using a very fine grit will prevent the problem completely. Unfortunately, finer media sizes (i.e., lighter particles) can sometimes not provide the force/work necessary to achieve the desired surface finish. In addition, finer media can 'pack' into the interior areas of the part - like the inside diameter of the part pictured - making the media removal just as difficult.

There is no right answer to this problem. Balancing the required surface finish with the effort (time, labor, etc.) to achieve that finish will determine the appropriate process for each situation.

Dry Tumbling with Corn Cob Grit

Stamped, cut, machined and otherwise processed parts often contain cutting oils and lubricants and need to be cleaned. These parts also have burrs and sharp edges that may need to removed.

The steel parts pictured on the left are dirty and have light burrs that need to be removed. A typical process would simply demand a mass finishing, tumbling process with a Ceramic Media to degrease the parts and remove the burrs in a wet process. The challenge with these parts is the shape - long, flat parts. In a wet process these parts will stick together to form 'sandwiches' and will not be cleaned and will resist deburring.

The solution is to dry tumble the parts with Corn Cob Grit. The Corn Cob Grit media will remove and absorb the soils (note clean, brighter parts on right). In a barrel tumbler, with a high ratio of parts to media, the part-on-part tumbling action will remove the burrs. The Corn Cob Grit can be separated from the parts efficiently with a simple wire mesh screen. The dry tumbling process also eliminates the need to dry the parts so they are immediately ready for any further processing.

Finishing Aluminum Parts

Deburred
Deburring and polishing aluminum is almost always a multi-step process. The aluminum part pictured above was deburred using an aggressive Ceramic Media in order to achieve a heavily rounded edge. Using an aggressive media, of course, makes the polishing process much more difficult since the surface finish after deburring will be a rough, matte finish.

Polished

After tumbling with a Precision Ceramic Media (2nd picture above), the surface finish is much smoother and brighter. While this would certainly not be considered a mirror finish - which is extremely difficult to achieve on aluminum with a tumbling process alone - the part is ready to be painted or anodized to produce a beautiful result. In addition, the part can be further burnished using a dry tumbling process with a media such as Walnut Shell Grit, Corn Cob Grit or even a Hardwood Media and a polishing cream.

Cartridge Reloading

Manufacturers of ammunition have been working overtime to try to meet the recent surge in demand. Due to lack of supply, many shooters have been doing more and more re-loading. The traditional process of dry vibratory tumbling with Walnut Shell Grit or Corn Cob Grit continues to be the primary method for cleaning the cartridge brass. While this process works well for newer brass, heavily oxidized brass can be more 'stubborn' using this process.

Using the same dry vibratory tumbler (assuming it is water-tight) with a Precision Ceramic Media can clean even severely oxidized and soiled brass. In addition, the cycle time can be as little as 15 minutes versus the 12-24 hours for dry tumbling. The key is to use the proper shape and size media for the brass you need to tumble in combination with a cleaning compound such as Kramco 1030.

To see a great before and after example, click to this Finished Part post.

Fluted Wood Columns

Question: I am needing to strip 80 years worth of paint from some fluted wood columns. The columns are in good condition and worth saving if I can remove the paint effectively. What media and grading do you recommend? How much media do I need?

Response: Corn cob grit will be the best option to protect the wood from being etched or 'raising' the grain. If the paint is too difficult to strip using this media then a Fine Walnut Shell Grit will be a better option taking care to protect the wood surface. Depending on how difficult it is to strip the paint I would conservatively suggest 2-3 pounds per square foot will be required.

Slightly edited from the Shop Talk Forum.

Abrasive Blasting Media - Corn Cob Grit

A natural, organic blasting media like Corn Cob Grit provides unique properties and performance. Corn Cob Grit blasting media is extracted from the 'woody ring' core of the cob (it is not the entire cob ground up). The wood-like material is a very soft (Mohs 4) medium that also includes an ability to absorb moisture.

Corn Cob Grit is ideal for blasting delicate surfaces. It will strip paint, coatings and soils off many surfaces without impacting (etching, etc.) the substrate. Log home blasting (soft media; effective strip/clean rate), mold remediation (absorbency; strip rate; organic) and paint stripping (soft media) off wood surfaces are three predominant applications for blasting with Corn Cob Grit. Compared to high pressure washing, blasting with Corn Cob Grit leaves the surface dry and ready for immediate application of a sealant or coating.

While Corn Cob Grit can be recycled as many as 4-5 times, it is important to make sure that the collected media is clean so that damage to the blasted surface does not happen during re-blasting. Available in a variety of grit sizes, Medium and Fine are certainly the most popular.

Interesting fact: Corn Cob Grit is used for absorbent/drying applications such as soaking up wet ball fields and oil spills.

Log Home Blasting

Here is a great picture of a do-it-yourself log home blasting project. The blaster being used is a relatively small KF100DM Portable Pressure Blaster with a 3/16" nozzle. Fine Corn Cob Grit is the media being blasted. The objective was to take off the old stain and sealant so that the logs could be re-stained and sealed. I believe the job was completed in one weekend.

July 4th BBQ

Take a look at your wood deck or concrete or stone patio while you are grilling over the weekend. Does the surface need some attention? A few tips to begin the cleaning and restoration process:

Wood Decks -- Dry abrasive blasting with Corn Cob Grit can strip dirt, soils, stains and coatings without damaging the wood or raising the grain. Avoid harder media such as sand (don't ever blast with silica sand anyway) or glass or coal slag as these will definitely etch the wood and can cause damage. The main benefit of dry abrasive blasting versus high pressure water is that the dry surface can be re-sealed immediately - a one weekend project instead of two or three.

Concrete or Stone Patios -- Stripping paint off concrete surfaces requires a more aggressive abrasive such as Crushed Glass Grit. Stone which can be softer than concrete might demand Crushed Glass Grit but Walnut Shell Grit may be a better option since it is not as aggressive and stone is easier to clean (typically no paint or coatings to remove).

Final tip -- Enjoy the BBQ first, then start the work!

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'.

Wood Stripping

Question: I’m renovating a house built in 1917 and I would like to remove the years of built up paint off the 6 panel doors and trim that was used in the house. There are 15 doors and probably a 1000 lineal feet of various finish boards. Which Machine and Media should I purchase? Approximately how much media will I need?

Response: Either the KF110DM or the KF65DM will work with the Corn Cob Grit or Walnut Shell Grit. The Walnut Shell Grit will offer a more aggressive stripping action but the surface may require some sanding depending on the surface finish you require. Corn Cob Grit will be more ‘sensitive’ to the wood surface but will not strip as quickly.

The quantity of media required will depend on how aggressive the blasting that is required. Multiple layers of durable paint will demand more media; however, both Corn Cob and Walnut Shell are re-usable. If you are blasting in a clean area you can pick up the media and re-blast it a few times – just be careful to not pick up anything that could be blasted and damage the wood surface.

Corn Cob Grit for Everything!

Just a few applications and advantages of Corn Cob Grit:

• Log home blasting - won't damage logs or etch wood
• Spill clean-up - absorbs more than 100% of it's weight in oil and water
• Burnishing - used in barrel and vibratory tumblers to brighten metal and plastic parts
• Degreasing - absorbs cutting oils, grease and soils for cleaning parts
• Biodegradable - 100% organic, natural, no health hazards
• Abrasive - it's soft, but can be used in abrasive hand soaps
• Playing field absorbent
• Reusable in blasting applications - lowers operating costs and can even extend life of other more expensive media
• Parts drying - vibe and barrel tumbling to dry parts after deburring and/or polishing steps

Dust Control

We recently had a post on our ShopTalk Forum about how to control dust during vibratory tumbling. Either Walnut Shell Grit or Corn Cob Grit can be used quite successfully to burnish parts with a dry tumbling operation.

As these organic media break down (or if a fine grit is used initially) the vibrating action can send dust all over the place. Adding water to reduce the dust is not really an option (muddy Walnut Shell doesn't burnish very well).

Something as simple as draping a rubber mat over the tub or fitting a similar mat over a bowl vibe will keep the dust to a minimum. Using a thick, heavy-duty sheet of rubber of foam and bracing it to the machine will give good dust control and keep the mat from vibrating off.

These 'covers' also have the added benefit of reducing the noise level of the process.

Corn Cob Grit Hard as Iron!

All solid materials can be measured for hardness on the Mohs scale. The scale was created in 1812 by Freidrich Mohs and is based how easily one substance can scratch another. Diamond is the hardest rated at a 10 while talc is a very soft 1.

Suprisingly, Corn Cob Grit and iron have the same hardness at about 4.5. Even more amazing is that Corn Cob Grit is harder than brass (3-4), copper (2.5-3) and aluminum (2-3).

The really cool thing is that blasting with Corn Cob Grit won't frost or pit glass and can be used to absorb grease, oils and water from metal surfaces. Corn Cob Grit is also great for blasting on wood surfaces (more to come on that). I wouldn't recommend using iron grit near glass or on wood!