The Finished Part Blog

TR90 plastic is a newer type of polymer plastic that has many advantages over more traditional plastics used in eyeglass frames. TR90 is lightweight, impact and heat resistant (to 350 degrees!) and is resistant to deformation. This plastic can be manufactured with bright colors and patterns. It is often referred to as plastic titanium or memory plastic.

TR90 Plastic Eyeglass Frames

Since TR90 plastic is clear, it is often desirable to create a lightly matte finish with slightly rounded edges. This will create a better surface for printing and will create a more comfortable consumer product.

The eyeglass arm part in the top of the picture is the clear molded part. This part was vibratory tumbled with a small general purpose, pyramid shaped Plastic Media and a mildly acidic compound solution for 2 hours. The part on the bottom is ready for printing with frosted surface finish and deflashed, rounded edges.

Many types of parts are molded and machined from acrylic plastic. Once the parts are made they often have a dull surface finish or have machining marks that need to be removed.

The part on the left in the above picture has a ‘rough’, matte surface finish. The goal is to get to a bright, polished finish seen on the right. While there are many options for abrasive tumbling plastic parts including both wet and dry processes, this part was polished using a two step, dry tumbling process.

The first step involved barrel tumbling with a Hardwood Media and Shynolyte Pre-Polishing Cream. This step provided a highly uniform, matte surface finish that was ready for polishing. The second step utilized the same type of Hardwood Media and Microlyte Polishing Cream producing the finish seen in the picture. Each step was about 20 hours which is typical for dry tumbling.

Polishing various types of metals can often be a challenging process using just mass finishing. This difficulty is often compounded with softer metals such as copper, aluminum and brass. The picture above shows the before and after of a brass plate. The plate on the left shows heavy oxidation and light scratches. After just 4 hours in a barrel tumbler with a Stainless Steel Round Ball and a neutral pH compound, the part was cleaned and polished to a near-mirror finish. This part is now ready to be hand polished to a mirror finish, if necessary.

 

Stamping of stainless steel parts will result in a burr (typically) on the back edge of the part. This burr will need to be removed in order to provide a smooth, clean edge on the part. Welding of stainless steel will cause discoloration (generally) due to the heat. This discoloration will need to be removed to result in an appropriate cosmetic finish.

These two tasks can sometimes be accomplished in one process. Vibratory Tumbling with a Ceramic Media will remove the burr and some of the discoloration but may not be aggressive enough return the steel to its original color. Abrasive blasting may not deburr or round the edges enough but will almost definitely remove all the discoloration.

Sometimes both of these processes are needed to get the desired result. The parts pictured (before and after) were first vibratory tumbled with a Ceramic Angle Cut Cylinder media. This deburred part was then blasted with Glass Bead to achieve the required surface finish.

The terms Mesh and Grit are often misunderstood and confused. When it comes to discussing these terms in regard to abrasive blasting or similar, the terms are used interchangeably. A 60 mesh Aluminum Oxide can also correctly be termed a 60 grit Aluminum Oxide.

The differences arise when trying to determine the exact particle size of a 60 grit (or mesh) product. Each of the terms would be more correctly defined as follows:

Mesh is a count of the number of evenly spaced openings in one square inch of a screen (i.e., screen size). Mesh is basically a specific size or dimension (see Mesh and Micron).

Grit is generally defined as an angular media used to abrade or cut a work piece. Grit is a specified range of particles for a given product.

In practical terms, identifying a specific abrasive product with the term 60 Mesh would normally indicate that the product has an median size of the openings on a 60 mesh screen. The term 60 Grit more accurately identifies the particle size distribution of the product.

Take a quick look at this specifications sheet for 60 Mesh Aluminum Oxide Grit. The Screen Size is the Mesh and the overall Particle Size Distribution is the Grit.

Whether calling an abrasive product Mesh or Grit ultimately makes no practical difference.

Deburring Small Parts

Often times, the machining of small parts can lead to very heavy burrs and difficult areas to finish. The steel gear pictured above has a diameter of about 1/4″. The machining to make the gear teeth left very heavy burrs on the edges (see part on left). The burrs can be seen as the excess material that is hanging off the edges and rolled into curls. All this steel needs to be removed before the part is functional.

The two primary criteria for finding the proper media are (1) small enough to get into the area between the teeth and not get stuck and (2) aggressive enough to remove the extraneous steel and burrs without overly changing the dimensions and tolerances of the part.

This part was deburred (see part on right) using a general purpose, deburring Ceramic Cone in a vibratory tumbler. This shape was chosen since the tip of the media will get between the teeth to remove the burrs. The general purpose bond or formulation (KM in this case) provided the best balance of deburring without being overly aggressive.

While this process did remove nearly all of the burrs on the gear teeth in only 1 hour, looking closely at the finished part will reveal some material still left on the edges. In some cases this metal needs to be removed as well and would require longer cycle times, more aggressive media and/or additional grinding steps.

The picture above shows the results of abrasive blasting of an aluminum part. This picture shows in clear detail the differences between three different blasting media. From left to right, the media used are as follows:

#5 Glass Bead – a bright, peened finish
80 Mesh Aluminum Oxide Grit – lightly etched, matte finish
#13 Fine Glass Bead  – very smooth, matte surface finish

Question: I’m looking for a batch method for smoothing small pieces of hardwood. The pieces would be roughly 1.5″ wide X 2.5″ long X 3/4″ thick. They would start as rough, irregular pieces. I would like to have them finished to the equivalent of 400 grit sandpaper or better. I would like to be able to do them in batches of 20 to 50. Would barrel tumbling work for this? Would they have to be tumbled in stages? If so, how many different grits would be required to achieve the final smoothness?

Response: I calculate that 50 parts would be 1/2 gallon of volume so you will need about 8 gallons of total capacity in a barrel tumbler (see MT Series). This is based on filling the barrel about half full with Hardwood Media and parts at a ratio of 3 to 1 by volume. The KP-2 or KP-4 Hardwood Pegs are commonly used for wood tumbling.

Depending on how rough the parts are initially cut, you may need to start with a relatively coarse 40 Mesh or 70 Mesh White Aluminum Oxide Grit. This could be followed with a 150 Mesh and then a 360 Mesh tumble. Generally, about 5 – 10 pounds of White Aluminum Oxide Grit would be added to 4 gallons of ‘load’ and the cycle will be 4 to 8 hours per batch. You will need to use different Hardwood Pegs for each grit and step. 

Deflashing UHMW Polyethylene

Ultra high molecular weight (UHMW) polyethylene is a plastic that has a density and hardness that is perfect for machining to whatever shape or design is needed. During this machining process, flashing and ‘hairs’ are created (see left side of picture). These parts need to go through a deflashing operation to produce a smooth, uniform surface finish (see right side of picture).

The part in the picture was vibratory tumbled for 2 hours with a general purpose Ceramic Media. This process removed all the flashing, rounded the edges slightly and remove the cutting and machining marks from the surface.

Some of the most common tumbling applications are just simple cleaning operations. The objective is to get a uniform surface on the parts. During processing issues like oxidation, heat treat discoloration, machining marks and burrs can all become an issue.

Cleaning Steel Parts

Tumbling steel parts in a Ceramic Media is the quickest and most efficient method to create a uniform, matte surface finish. The parts in the picture above were vibratory tumbled in a medium sized Angle Cut Triangle media for about 1 hour. Simple, quick and easy.

Brass Jewelry Polishing

The above picture shows a piece of brass jewelry directly after molding and shaping (left) and a finished polish (right). The process to get to this finish required two steps in a barrel tumbler.

Step 1 was a cut-down step with a Precision Ceramic K-Fast Cut media and a solution of a mild acid cleaner (Kramco 1030). The combination of the media and solution removed any oxidation, sharp edges and  scratches or imperfections in the surface. The surface finish was smooth and matte and ready for polishing.

The best polish was achieved in Step 2 using a Stainless Steel tumbling media and a neutral burnishing compound (Kramco 2030). The weight of the media working with the compound and cascading action of the tumbler resulted in a bright, smooth, near-mirror surface finish.

Each step was about 2 hours in a relatively small diameter barrel tumbler.

While it seems a bit counter-intuitive, a concrete canoe can float. Using materials like Glass Beads and Pumice to create a more buoyant material, these student groups have worked with us over the years to great success (and failure).

Out of Concrete and Drudgery Come Canoes That Float – NY Times; June 23, 2013

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.

An interesting thread about deflashing and surface finishing 3D printed parts has been started on the ShopTalk Forum. As this 3D manufacturing process becomes more popular and mainstream, surface finishing using both barrel tumbling and vibratory tumbling will continue to be an issue.

Continuously painting parts using an automated process often involves hanging the parts from a moving track using paint hooks. These hooks invariably will get a build-up of paint and need to be stripped. Abrasive blasting with the proper media will strip the paint and allow the maximum usage lifetime of the hooks. Using a tumble blaster will allow for automated blasting of many parts in a batch and minimal downtime for the hooks.

The two pictures below show the part both before blasting (painted black) and the hooks after blasting with either 60 Mesh Aluminum Oxide Grit or 12-16 Mesh Melamine Plastic Abrasive Blast Media. A batch of these parts were tumble blasted for only 6 minutes at 60 psi.

Aluminum Oxide Blasted (right)

Melamine Blasted (right)

Both media types removed the excess paint and left the parts ready for re-use. An initial review seems to appear that the Aluminum Oxide Grit did a better job of removing the paint.

Upon a closer analysis, the Melamine actually provided an overall better result as can be more clearly seen in the comparison picture below. The Melamine Plastic part on the left does have a small amount of paint left on the surface but enough exposed metal to allow for the hook to perform properly and even could be used in a  electro-coating process, if necessary.

The Aluminum Oxide blasted part is also clean but this media tended to etch the metal and will ultimately result in fewer usable cycles and/or failure of the hook.

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.

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.

Equipment like coffee machines, hot water heaters, boilers and other commercial and industrial equipment that use hot water eventually shows a build-up on the parts. This residue is the deposition of the salts and minerals in the water on the surfaces.

If not removed, this scale can become quite thick and affect the performance of the equipment  The heat will not transfer efficiently from one area to another and holes and pipes will start to get blocked. Stripping down the scale, especially on smaller parts and in internal areas can be accomplished quite efficiently with vibratory tumbling.

Hard Water Scale Removal

Tumbling the parts with the proper media and compound can make a huge difference. Common sense would be to use as abrasive a media as possible to strip off the scale. But this is not always to best way.

The part on the left was tumbled with an abrasive Ceramic Media and a general purpose tumbling compound. This is a traditional deburring process and did knock-off some of scale (see oblong area in the middle of the part). The bulk of the residue wasn’t touched and if the part was left too long in this type of process, the threads, edges, holes, etc on the part may have been worn down so that the part was out of tolerance.

The part on the right (which looked the same as the one on the left before tumbling), was tumbled with a polishing Ceramic Media. While this was a much less abrasive process, the appropriate compound, Kramco 1030 Mild Acid Cleaner, was used to soften, dissolve and then remove the scale. Because this was a polishing media, the part came out bright and clean with no wear on critical areas of the part.

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.

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.

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.

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.

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). 

Machining and work-hardening steel creates burrs and scale that needs to be removed. The part in the middle of the picture above shows the ‘excess’ material in threads as well as the darkening caused by scale formation in the heat treating process. While barrel or vibratory tumbling can be used to remove these burrs and create a smooth surface finish, threaded areas often need to maintain the square edges and dimensionality on the part.

The other two parts were blasted with different types of abrasive blast media to show how each media can accomplish the deburring task but create a totally different surface finish. The part on the left was blasted with a Medium Glass Bead (70-100 mesh) and resulted in a satin-like surface finish with clean edges. The part on the right was blasted with 120 Mesh Aluminum Oxide Grit and has a more matte, gray surface finish.

While Glass Bead can often result in a more aesthetically pleasing finish, the micro-profile achieved with Aluminum Oxide Grit is often required for future processing of the parts (i.e., coatings application).