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Home » The Finished Part Blog
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| 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.
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.
There are four major types of surface finishes that are designated for stainless steel. While many of these finishes are somewhat subjective, below is a guideline for the most common finishes:
#3 Coarse Finish -- Usually a preliminary, coarse surface finish. Also referred to as rough grinding. Used for removing gates from cast parts, heavy burrs, weld splatter. Often accomplished with a disc or belt grinder but parts can be tumbled part-on-part with an abrasive grit to achieve a #3 finish.
#4 Brushed Finish -- Defined as a directional or uniform surface finish. Removes surface defects in the surface but accepts fine polishing lines. A #4 Sanitary Finish is a slightly smoother, polished finish commonly designated in the food, dairy and medical industries. Tumble polishing can be used to accomplish a close approximation to a #4 Finish.
#6 Fine Finish -- A polished, softer version of a #4 Finish. Often less reflective than a #4 Finish. No polishing lines or surface defects visible.
#8 Polished Finish -- All surface defect are removed. Mirror-like finish depending on the quality of the metal. Highly buffed surface.
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| 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.
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| 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.
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| 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.
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.
Ceramic Media is the most commonly used tumbling media for deburring and surface finishing on steel. This is due to the fact that Ceramic Media is heavier and more aggressive than most other tumbling media and that steel is harder than many other metals (i.e., aluminum, copper, brass, etc.).
The 17-4 stainless steel part on the right in this picture has sharp edges and machining lines that need to be removed. In as little as 1 hour, a general purpose Ceramic Cone in a vibratory tumbler resulted in a uniform, matte surface finish and slightly rounded edges.
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.
Stainless steel parts are often heat treated to anneal (harden) the metal and relieve stress in the parts. While most stainless steels are resistant to oxidation at low temperatures, the various metal in the steel formulation (iron, chromium, molybdenum, etc.) can form oxides at higher temperatures. This oxidation causes the discoloration often seen on steel after heat treatments.
The knife blade picture above was heat treated to create a harder, more durable product. The part on the left shows the telltale blue, green and black discoloration due to elevated temperatures. Prior to further processing such as polishing and sharpening, the blade needs to be restored to it's original surface finish and coloration.
The part on the right was abrasive blasted with a Medium Glass Bead at 80 psi to result in a satin-like, smooth surface finish. This part is now ready for the next step in the process.
Deburring stainless steel parts is a well established process that can be accomplished in a variety of different ways. Vibratory tumbling with a ceramic media is a common method. Cycle times are typically 1-2 hours or less. The challenging aspect (read: art) begins when the parts don't want to behave nicely.
Flat parts or stubborn burrs or scale or oxidation can all create problems when trying to deburr and surface finish a part. The part pictured above (left) is a hardened stainless steel, laser-cut, flat part. Sharp edges, burrs, heat scale and oxidation were all present on this part. Testing in a vibratory tumbler and in a barrel tumbler with a general purpose ceramic media did not provide the necessary deburring and edge rounding. The geometry of the part also lead to the parts sticking or sandwiching together in the tumbler.
In order to get the clean surface finish and proper deburring and edge rounding (right), the part was barrel tumbled with a fast cutting, dense KDF Ceramic Media. To prevent the parts from sticking together, an additional fine media was needed to break the surface tension between the parts and allow them to tumble freely in the barrel. The cycle time for this part was 4 hours.
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.
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.
Stainless steel can be polished to mirror surface finishes. Sometimes, the surface needs to be etched in order accept a coating or prepare the part for a process that requires a little 'grab'. While tumbling in a vibratory system or a barrel tumbler will create a matte finish, the surface is generally not rough enough for many coatings. Also, when the parts are small, it can be difficult to separate the media from the parts in a tumbler.  Stainless Steel Etching When an etch is required and the parts are small, a tumble blasting process is often an ideal solution. The stainless steel parts above (before and after) are less than 1/2" in any dimension. These parts were tumble blasted with 60 mesh Aluminum Oxide Grit for only 20 minutes. Depending on the size of the blasting basket, batch sizes as large as 3 cubic feet can be processed at one time.
Tumble blasting is a process that allows for a batch or load of parts to be abrasive blasted automatically. Tumble blasting is beneficial when a large quantity of 'smaller' parts make it difficult or labor intensive to blast one at a time. This process also removes the inconsistency of manual blasting as each part in every batch will result with the same finish on the entire surface area of the part. The most successful part sizes are generally less than 4 inches and batch weights are generally less than 150 pounds. The tumble blaster specifications and basket size will determine what combination works optimally.  The stainless steel parts pictured above (before on left) are about 3" in size and weigh less than 2 pounds each. The parts were tumble blasted for less than 10 minutes with Coarse (#6) Glass Bead to remove the heat treat discoloration and achieve the finished surface on the right. The geometry of these parts are ideally suited for this process since they will tumble over each in 3 dimensions allowing all areas of the part to be exposed to the blast stream.
The process of hardening steel using heat treatments, annealing and quenching can produce scale and blackening on the surface. In addition to cleaning and polishing the surface, steel parts often also need to be deburred.  The picture above shows a two step process to clean and deburr the part followed by a polishing step to smooth-out the surface and produce a pleasing surface finish. The part on the left is the unfinished, hardened stainless steel part. Step 1 (middle part) required a vibratory tumbling with a general purpose Ceramic Angle Cut Cylinder to remove the 'burn' marks on the surface and to deburr and round the edges. The result was a matte, tumbled surface finish. The second step (far right) was designed to further clean the surface and result in a smoother part. A polishing Precision Ceramic Sphere was perfect for this part since all the features of the part (i.e., no inside corners) were conducive to a round media.
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.
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