Polishing Stainless Steel

Stainless Steel Polishing

Machined stainless steel parts typically have sharp edges, burrs and a matte surface finish. Vibratory tumbling can be used to get a polished finish sometimes even down to just a few Ra surface profile. This is often a multi-step process.

The part on the left is the 'raw' part ready for deburring and polishing. The first step is to deburr and round the edges. This was accomplished using a KM general purpose Ceramic Media. This was a wet process using a solution of Kramco 1010 General Purpose Compound. The result at this point (not pictured) was a matte, tumbled finish with rounded edges.

The second step needed to get the pictured polish (on the right) involved tumbling with a Precision Ceramic Sphere and a solution of Kramco 2020 Burnishing Compound. Generally, a rounder media will produce a smoother finish than an angular media.

This part could be further polished with other tumbling media or hand buffed to a near-mirror finish.

Polishing Stainless Steel

After heat treating and laser cutting stainless steel parts, the edges generally need to be deburred and the surface needs to be cleaned from scale and discoloration prior to polishing. A two-step vibratory tumbling process is normally used to achieve a uniform surface finish.

The part on the far left in the picture shows the burrs and slight discoloration on the edges. The first step was to tumble the parts with a Ceramic Media to get a the uniform, tumbled, matte finish shown on the part in the center of the picture. For this particular part a 2 hour cycle time with a general purpose tumbling compound resulted in the finish seen.

The second step (far right in picture) was also a 2 hour vibratory tumble with a Precision Ceramic polishing (porcelain-type) media and a high lubricity tumbling compound. The surface is improved to a more uniform and brighter finish and the part is ready for a clear coat or further polishing as needed.

Dry Tumble Deflashing

Molded plastic parts often have flashing or excess material that needs to removed from the edges of the part (see part on left of picture). This can be accomplished in a variety of ways including a wet process using vibratory and barrel tumbling with an abrasive media (i.e., Plastic Media) or with an abrasive blasting process to blast the flash off the edges.

If the part is a hard plastic with a relatively simple geometry and the surface finish needs to be maintained, tumbling the part with a polishing media is ideal. The mass of a polishing media will knock the flash off the edges and will not change the surface finish since it is a non-abrasive media.

The part on the right was barrel tumbled with a KP Ceramic Angle Cut Cylinder that was small enough to pass through the ID of the part. These results were obtained in only 15 minutes. The parts were retrieved from the process clean, dry and ready for the next step in the production process.

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.

Two-Step Polishing

The far left in the picture shows a laser-cut, stainless steel part with sharp edges and heavy burrs. The goal is to clean up the edges and leave a smooth, polished surface finish. The part will need to be deburred and then polished. This will be a two-step process as an abrasive process like deburring will typically leave a smooth but matte surface finish.

In this situation, the part was first vibratory tumbled with a moderately aggressive Ceramic Media to remove the burrs and discoloration. This process took a couple of hours to result in the smooth, rounded edges and tumbled, matte surface finish shown in the middle part.

The second step used a polishing Precision Ceramic Media to improve the surface finish and brighten the part. The part on the far right have a smooth, polished look after about 2 hours in the tumbler with the polishing media. Overall, a good result without any hand deburring and polishing or manual labor.

Deburring Steel Parts

Machined steel parts often need to be deburred. For large quantities of parts, both barrel tumbling and vibratory tumbling can accomplish this task saving labor hours and increasing throughput capacity. The specific details of the process must be chosen carefully to ensure that the parts are not only properly deburred but that the other surface finish specifications are met as well.

The machined part on the left in this picture has a bright surface and sharp edges. The goal was to deburr the edges with minimal impact on the surface finish. The initial result (part on right) of tumbling with Ceramic Media and a general purpose compound did deburr the part but it also resulted in a dull, matte surface finish.

This part will need to be tumbled in second step using a polishing media and a different compound to bring the desired finish back. Having a clear understanding of the ultimate surface finish required and that multiple steps are often needed is the first step in finding the best solution to finishing parts.

Wet Barrel Tumbling Guidelines

Below are some quick guidelines for wet barrel tumbling for both deburring and polishing operations: 

• Fill the barrel about half full with media and parts.
• Use a ratio of 3 or 4 to 1 (by volume) of media to parts.
• Dilute the tumbling compound to use a solution of about 2 ounces per gallon of water.
• Add enough solution to level of the load.
• Change the solution with each batch to keep the parts, process and media clean.

Of course, these are strictly guidelines and modifications based on any specific application or process is often needed (and recommended). 

Complex Part Geometry

There are often many tumbling media options for deburring or polishing parts. Different shapes and sizes of media can be used to round edges, polish into corners and provide a uniform and consistent surface finish. Part shapes and sizes that are relatively 'open' make the media selection much easier.

Sometimes parts are so complex that there may be only one option for tumbling media. Finding a media that will be small enough to get into tighter areas of a part but not get stuck in other areas can seem like an impossible task. Selecting a shape that will pass through holes without getting caught in interior areas of the part are a frustrating task. It is amazing how intelligent tumbling media can be and how it seems to always find a way to gang together and cause problems. Simply evaluating a part based on a drawing (or even holding with various media on hand) is not enough. Testing the part with different media options is critically important.

The picture above shows a milled slot on the interior diameter of the part. While this type of geometry can be challenging enough, there is a small interior hole at the end of the slot. The media selection to deburr the slot without getting stuck in the hole can be maddening. The pictures below show different features such as exterior threads, small channels and slots of varying widths and depths. While any of these features can be a challenge, all this geometry pictured was located on a single part. For this particular aluminum part, it worked out that an angle cut tristar shape deburred all the critical areas.

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.

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.

Polishing Machined Aluminum

Taking machined aluminum parts and polishing to a smooth, uniform surface finish is often a multi-step process. Each step is designed to accomplish a different task. The part above has burrs, sharp edges and clear machine marks. Vibratory tumbling is the preferred process.

Step 1 - The edges are softened and the machine marks are removed using a Plastic Cone tumbling media. This media is selected due to it's lower density (vs. Ceramic Media) and lesser aggressiveness. The resulting surface finish is smooth and uniform. This part could be anodized to a matte finish, powder coated or painted with excellent results.

Step 2 - In order to brighten the part and prepare for anodizing to a higher luster, the part was tumbled with Polishing Precision Cearmic Spheres. While the finish is bright and smooth, the proper shape must be chosen to ensure that a uniform surface finish is achieved based on the geometry of the part.

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.

Mirror Surface Finishing

Achieving a near-mirror finish on many metals using only mass finishing such as vibratory tumbling or barrel tumbling can be a challenging process. The combination of tumbling media, equipment and compound will determine the results. And these results will vary depending on the type of metal being polished.

This stainless steel part has a very flat geometry with minor scratches and imperfections. Generally, the heavier and rounder the tumbling media, the better the surface finish. Additionally, a tumbling process that produces a smooth, even tumbling action will produce the best results.

The part was barrel tumbled to give a nice rolling action of the media against the part. A Stainless Steel Round Ball (Sphere) in combination with a highly lubricating, tumbling compound (Kramco 910) kept the process clean and resulted in a near-mirror finish on this stainless steel part. The total tumbling time get from the before (top picture) to the after (bottom picture) was 1 hour.

Polishing Firearm Parts

There are two types of finishes needed for firearms. Most (military and hunting) require a matte or dull finish to minimize reflection and give good 'grip' properties. Firearms used for show pieces or when reflection is not an issue will often need a higher polish or near-mirror finish on certain areas of the gun.

The part above is an as-produced finish. The surface is relatively smooth but dull. To process large quantities of these parts, a single-step process can be used to significantly improve the finish. The same part below was barrel tumbled for 18 hours (overnight) in Stainless Steel Ballcones and a solution of Kramco 910 Burnishing Compound to produce much brighter, mirror-like finish.

Grinding & Polishing Steel

The two pictures below show both the handle area and the blade point on a steel part that has been prepared for cleaning and polishing. The before (left) shows the grinding marks as well as some oxidation (rust) on the surface.


Steel Blade Point


Steel Handle

In order to prepare the steel part for polishing to a mirror finish, a two-step vibratory tumbling process was used. The first step used a fast cutting (KSF Bond) Ceramic Media with a neutral pH compound to remove the machining and grinding marks, clean the surface and create a smooth, matte finish. The second step to achieve the finish on the right was achieved with a polishing (K-Polish Bond) Precision Ceramic Media to smooth out the matte finish and prepare the part for polishing.

Additional vibratory tumbling steps using a harder and heavier media such as Stainless Steel or hand polishing with a buffing wheel will result in a mirror finish.

Pewter Polishing - Vibratory Tumbling

Many small-run, decorative pewter manufacturers either hand polish or have smaller equipment for polishing cast pewter parts. While we have detailed pewter polishing in a previous post using steel tumbling media, small vibe tumblers generally are not able to handle the weight of steel media.













Before (first) - Polished (second)

These lead-free pewter parts showed rough and dull surface finishes. While the objective was not to completely remove the flashing and produce a mirror-like surface finish, these parts did need to be cleaned and polished.

A two step process was used in a small (4 gallon capacity) vibratory bowl. The first step used a general purpose Ceramic Media and a mild acid compound to remove oxidation on the surface and soften/remove the flashing. This resulted in a smooth, matte finish. Instead of using steel tumbling media in the second, polishing step, a Precision Ceramic polishing media was able to bring up a nice shine on the parts in a relatively short cycle time of 2 hours. A neutral pH compound was used to keep the parts clean and bright.

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.

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.

Rock Tumbling

Rock tumbling and stone polishing is a popular hobby. The most important ingredient to achieving a great finish is time. Polishing is typically a 4-step process involving progressively finer and finer abrasives. Each step is done with a clean barrel, fresh water and a virgin abrasive. The Creetown Gem Rock Museum (in the UK) has a good outline for polishing stones.

The first three steps require a coarse grit (80 mesh) followed by a medium abrasive (220 mesh) and finally a fine grit (400 mesh). Aluminum Oxide or Silicon Carbide are the preferred media due to their durability and hardness.

The fourth step demands a very fine, hard media that will maintain it's 'cutting' ability for the entire cycle time. Cerium or tin oxide are the abrasives of choice as they will maintain their sharp edges as they break down over time. Sawdust can be added to this step to ease the impact and eliminate chipping of the stones.

Hobby size barrel tumblers will generally have capacities of just a few pounds of stones. Tumbling times for each step is about 1 week. Larger, industrial-grade tumblers such as the MT Series Barrel Tumblers will allow for more weight to be tumbled at one time. This often leads to reduced cycle times and higher production rates. Regardless of the equipment, patience is the key to beautifully polished rocks and stones.

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.