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.

Powder Coat Stripping

Powder coating is used in applications where a tougher finish is required than a traditional liquid paint. The free-flowing, dry powder is typically applied electrostatically and then heat cured to form the bond. Powder coatings are very difficult to remove from the sub-surface as they are often thicker and more aggressively bonded to the surface than more traditional coatings.

Removing powder coating from a surface can be a challenging process. In addition to the thickness of the coating, the bond between the powder and the metal often requires more energy to break than the force needed to etch or potentially damage the sub-surface.

This powder coated punched hole wire mesh was stripped (right side) using an abrasive blasting process with 60 mesh Aluminum Oxide Grit. A larger grit size was also tested (which did strip the coating more quickly) but the metal surface became too heavily etched. Less aggressive media such as Glass Bead and Plastic Abrasive Media were unable to efficiently remove the coating.

Cleaning Paint Plugs

When painting large quantities of parts - especially on a production line - it is common to use plugs and masking materials to prevent paint from being coating on unwanted areas. These materials are often made of flexible materials such as silicone, urethane and rubber in order to make it easier to plug the holes or slots or whatever area needs to be protected. Often, these plugs are discarded after a few uses as the coating builds up on the surface.

Removing the paint off the plugs allows for a longer lifetime of the plugs and significant cost reductions. Many companies will simply roll the plugs on a hard surface to remove the paint or knock them with hammer or similar. Since the plugs are typically flexible, it is difficult (but not impossible) to use a mass finishing process such as vibratory tumbling to get the uneven forces (i.e., rolling on a surface) necessary to knock the coating off. In addition, vibratory tumbling is often done wet so the plugs would need to be dried prior to re-use. Of course, this type of process eliminates the hand cleaning one part at a time.

Abrasive blasting is another process that can be used to strip the paint. On hard rubber plugs and similar, this process works very well especially in a tumble blast system. On more flexible parts, the results vary widely as the media can simply 'bounce' off the surface. The success pictured above was a simple blast cleaning with Walnut Shell Grit to completely strip the paint.

The type and thickness of the coating and the flexibility of the part are important factors when trying to determine whether blasting or tumbling will properly remove the coating. Testing of the parts is typically needed to determine the viability of cleaning re-using the plugs.

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.

Wood Stripping with Walnut Shell

An interesting string about stripping a wood ceiling with Walnut Shell Grit on contractortalk.com. The before and after pictures in the initial post are terrific examples of what can be accomplished with the right blast media.

Some highlights from the job:

• Used Extra Fine Walnut Shell Grit at 60 psi
• Screened media after blasting to re-use multiple times
• Stripped both lacquer and stain
• Minimal etching - sanded areas where grain was raised

Thanks to 'Sand Pot' in Seattle for the post.

High Performance Engine Parts

Restoring cars, motorcycles and trucks using abrasive blasting is achieved best when the proper media is selected for different areas of the vehicle. Just as it is important to use an aggressive media on a heavily rusted frame or a non-warping media on thin panels, blasting engine parts also demands the proper media selection.

Glass Bead is a very popular media for blasting engine parts. This media will strip paint, oxidation and soils without etching the surface. Glass Bead leaves a smooth, peened surface finish. In some cases (especially high performance engines), the surface tolerances are extremely critical. Blasting with Glass Bead can result in clean parts but potential diminished engine performance.

Before

This engine part (above) required the removal of oxidation on the metal. Blasting the part with Glass Bead produced a very clean finish but was too aggressive on the surface finish. The blasted part (below) was blasted cleaned with an Extra Fine Walnut Shell Grit. This much softer media removed nearly all of the oxidation with no impact to the metal surface. This part is now ready for further processing to prepare it for the high performance required.

After

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.

Walnut Shell and Brick

Removing paint from brick can often be a process that requires a difficult combination of aggressiveness and delicateness. Brick (and concrete would apply as well) is a porous surface that does an excellent job of holding onto paint and coatings. The paint can be trapped in all the nooks and crannies of the surface making it very difficult to remove all the paint. As brick can be a decorative surface, it is important to not use a process that is too aggressive as it can strip away the brick material. This leads to an uneven and damaged surface that is unappealing.

Walnut shell grit offers the perfect balance of aggressiveness and safety on the brick surface. Using a finer grade of walnut shell grit will strip the paint from all the 'hidden' areas on the brick surface but will not blast away the brick itself. In addition, because walnut shell is an organic material, the process is environmentally friendly and the clean up is easy.

Walnut Shell & Cosmetics

The cosmetic and personal care industry use crushed walnut shell media as an exfoliate in facial, body and foot scrubs. Crushed walnut shell is a hard, fibrous material that is ideal as an abrasive. The grit is extremely durable, angular and multi-faceted, yet considered a soft abrasive with a MOHS hardness of 4-5.

More aggressive formulations will use particle sizes as large as 20-40 mesh. The soft abrasive nature of walnut shell is still viable in flour sizes at 200 mesh and smaller for sensitive, fine scrubs.

The cosmetic grade of walnut shell is either 'natural' or 'irradiated' and has rounded edges (relative to an abrasive blasting grade) for a smoother feel. The irradiation process is performed with gamma rays and sterilizes the walnut shell to provide an indefinite shelf life (no micro-biological growth). 'Natural' product may require an additive in the formulation.

Stainless Steel Deburring and Polishing

Question: I modify stainless steel flatware and dinner service pieces for other uses. In the process of modifying I use heat to bend, cutting wheels, grinding wheels, drilling and some sawing. The pieces are blued from the heat and have grinding marks, cutting marks and holes with burrs. I am presently de-burring and removing grinding and cutting marks by hand. After cleaning up the pieces I polish them to a high shine by hand. I have recently obtained a vibrating tumbler. What suggested media can be used to do the clean up and the polishing? How long would it take to tumble in these processes?

Response: The surface roughness of your parts will determine what media to use. Typically the first step would use a preformed ceramic aggressive cut media (the larger the better) for material removal. The second step would be a general purpose ceramic. Both operations should be done with a nonferrous deburring liquid compound diluted to a proper user strength. The operation should be wet with minimal water or suds. Cycle time are typically 1-4 hours for these deburring steps.

For a bright finish a precision ceramic polishing or steel media can be used. A compound with good lubricity is recommended for this third step. Processing time may be 4 hours with steel media and up to 8 hrs with precision ceramic. For a higher polish and smoother finish, the parts can be tumbled in a pretreated walnut shell grit for 8 to 24 hours for a jewelry quality finish.

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 - Walnut Shell Grit

Walnut Shell Grit is a fascinating material for blasting. The hardness of Walnut Shell puts it in between the soft medias like Plastic and soda and the more aggressive, common media like Crushed Glass Grit and coal slag. The combination of this hardness (MOHS - 4.5) and the angular shape provides for a decent strip rate with little to no affect on the sub-surface being blasted.

Applications such as aircraft, automotive and boat stripping can be accomplished successfully without risk of etching or warping the metal or fiberglass. Removing paint from brick, steel, aluminum and even wood can be accomplished with very low probability of damage to the underlying substrate. And if kept clean, Walnut Shell Grit can be re-used a number times increasing its value as a blasting media.

Sourced from the leftovers after extraction of the walnut meat, the shells are crushed and graded to a large variety of size distributions. This organic and biodegradable media is ideal in environments where safety and waste disposal are an issue. While the larger grit sizes (+60 mesh) are used in blasting, the finer 'flour' grades are used in a variety of applications including cosmetics and personal care products.

Tumbling with Abrasive Grit

Tumbling parts to achieve a deburred, deflashed or rounded edge is a common process. Both vibratory and barrel tumblers are used for this purpose. Choosing the proper media type is critical to achieving the desired finish.

Most applications require the use a preformed tumbling media. These media contain abrasives that are released into the tumbling process to remove the burr, etc. The (relatively) large mass of the media assists in this process by acting as a sanding block of sorts on the edge and surface of the part. After this abrasive process, parts can be tumbling in other media (including grits) to achieve a smoother or polished surface finish.

Tumbling parts simply with an abrasive grit such as Aluminum Oxide, Silicon Carbide or Walnut Shell does not typically achieve the deburring, deflashing or edge rounding required. While the grit may be very sharp, hard and abrasive, there is very little force acting against the edge of the part. The surface finish will probably be affected (roughening, smoothing or even polishing) but the edges will be left mostly untouched.

There are some exceptions to this 'rule' (see this post) but generally preformed tumbling media is superior to grit media for deburring, deflashing and edge rounding.

Polishing Aluminum

Bringing machined aluminum into a highly polished finish is a multi-step and sometimes challenging process.
The first step is to tumble (vibratory usually works best) the parts in an abrasive media such as a Plastic or Synthetic Media. Using as round a media shape as possible - cones vs triangles - will result in a smoother finish producing a better end result.
The second step is a polishing step using a Ceramic Media. Either an Angle Cut Cylinder or a Precision Ceramic Sphere will produce a smooth surface finish - again rounder is better.
In order to burnish the surface (if necessary), a third step can be to dry tumble the parts with a Walnut Shell Grit possibly treated with a fine abrasive such as jeweler's rouge.
It is important to make sure that the chemical compounds you use will not attack the aluminum. Depending on the final finish required, a small amount of hand polishing may be needed.

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.

Motorcyle Parts

Question: I want to clean/polish an aluminum motorcycle engine. Would walnut shell be the best media for the job? I want a clean, lightly polished look typical of stock bikes.

Response: Walnut Shell Grit will clean the surfaces relatively well but will not have an impact on the surface finish. A Medium-Fine or Fine Glass Bead will clean the metal and leave a satin-like finish on the surface.

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.