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.

17-4 Stainless Deburring

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.

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.

Increasing Tumbling Abrasiveness

There are a number of strategies that can be used to increase the abrasiveness of both barrel and vibratory tumbling operations. These can be used when a boost to more aggressive surface finishing. deburring or edge rounding is needed.

• Increase the rpm (barrel tumbler) or the amplitude (vibratory tumbler).
• Use a denser/heavier media (i.e., switch from Synthetic Media to Ceramic Media).
• Switch to a more aggressive bond/formulation of tumbling media.
• Reduce the amount of water and/or compound in the process.
• Add an abrasive such as Aluminum Oxide Grit or similar (best with a barrel tumbler).
• Adjust the load level for a more aggressive tumbling action.

Identifying the critical issues with your part will determine which strategy(ies) are best to employ. Of course, all these strategies work just as well in reverse to decrease the abrasiveness.

Choosing Tumbling Media

Choosing to proper tumbling media for a specific part can be challenging enough. When you are producing a variety of parts, the decision can be overwhelming. The following steps will provide a guideline for moving through this process as effectively as possible:

1. Identify the results you need to accomplish - deburring, polishing, surface profile.
2. Calculate the desired cycle time and the volume of parts you need finish per unit time.
3. Determine the type of equipment you need (i.e., barrel vs. vibratory tumbling; wet vs. dry, etc.).
4. Review the part size and geometry for all the parts that need to be finished.
5. Select the proper media and compound to achieve the desired results with these parts.
6. Confirm the process and media selections with testing.

And just to make the decision more difficult, there are many surface finishing options such as abrasive blasting, high energy tumbling, shot blasting and others that may result in as good a process or even better.

Cascading Vibratory Tumbler

The key to achieving the optimal result in a vibratory tumbler is to make sure that the vibe is properly filled. Processing parts in vibratory tumblers is most efficient when the media and parts are able to tumble in addition to vibrate. In a tub-style vibe, this cascading action happens from one side of the tub to the other.

The video below demonstrates how a low media level simply vibrates the load and any tumbling that does occur is from the middle out to the sides. As media is added, it is clear to see how the media load begins to tumble from the right to left.

Choosing Tumbling Media (2)

In addition to choosing the proper type of tumbling media, the shape and size of media is also critical to achieving the proper finish. Given the multitude of tumbling media options, the size and geometry of the parts being tumbled will narrow down the choices considerably. Optimizing a process requires using a good quality media that is appropriate for the job at hand.

The most popular Ceramic Media shapes are Angle Cut Cylinders and Angle Cut Triangles. The angled edges help the media to produce a uniform finish across the entire part including inside corners, holes and slots. Cone shapes are popular choices when using Plastic and Synthetic Media. Generally, larger media will work better with larger parts. Smaller media will have a tendency to impact surface finish more than edges.

Aggressive media (whether large or small) will shorter the cycle time to deburr or round edges - and even for polishing (aggressive = dense) operations. Aggressive, abrasive media will break down more quickly than general purpose type media so replacement cost and the waste stream are an additional consideration.

The high quality tumbling media manufactured by Vibra Finish provides the balance between options (size, shape and type), cost (economical) and quality (uniform bonds, even break-down).

Optimizing Mass Finishing

Here is a quick (and certainly not comprehensive) checklist to assist in optimizing your mass finishing process:

Equipment Issues
Is the lining in good condition?
Are the parts tumbling and cascading properly?
Does the solution drain properly?
Are the drain holes plugged?

Tumbling Media
Is the media getting stuck in the parts?
Has the cycle time increased since the media was first used?
Does the media break down properly or is it 'glazed'?
How much has the media broken down since it was new?

Solution/Compound
How often is the solution changed?
Is the 'bucket' clean or over-loaded with residue?
Does the bowl/tub/barrel have a build-up of media residue?

The Right Way to Vibe

Vibratory tumbling can be a successful method for deburring, polishing and surface finishing. When a vibe process is operating correctly, the load (blend of tumbling media and parts) will not only vibrate against each other but the entire load will tumble or cascade from the outer wall to the inner wall in a bowl and from one side wall to the other in a tub. This circulation provides maximum energy transfer to the load. If the vibe is not filled properly, the load will typically tumble from the middle toward the sides forming a high point in the middle of the load and lead to poor and inconsistent results.

The tumbling media transmits the vibration energy to parts. The interaction of the media and the parts achieves the desired finish. Typical media to part ratios (by volume) of 3 to 1 or higher will keep the parts separated in the load and minimize part-to-part interaction or impingement. Low media levels or a low ratio will not allow the media to properly 'carry' the parts in the process.

Parts Tumbling Checklist

As the economy seems to be improving and production is starting to show signs of life again, now is a good time to review your barrel or vibratory tumbling process to optimize efficiency. Here are a few issues to consider:

• Perform maintenance on machine - check electrical system, grease bearings, clean bowl/tub and check lining
• Check separating screen deck (if any) for cracks and secure to vibe
• How old is tumbling media and how much has it worn down?
• Make sure load is still tumbling properly and bowl or barrel is filled to appropriate level
• Compare parts finish to previous batches to confirm results continue to be consistent
• Review recirculating system and age of compound.
• Confirm proper solution flow rate and clean nozzles and pumps.

Adding some or all of these checks as part of a periodic maintenance program will keep your process consistent, efficient and productive.

Deburring Small Slots

Choosing the proper tumbling media is critical when the geometry or configuration of a part provides a specific challenge. Elements like holes, grooves and slots often can only be deburred with a unique media shape and size.

The slot in this stainless steel part is only 0.050" wide. Choosing a media that is too large may not 'attack' the burr at all but simply roll it inside. A media that is very small (like an abrasive grit) may get into the slotted area but not with enough mass to deburr the edge or could get stuck inside the part.

The long, thin shape of this slot can be deburred using a media with a angled edge. A ceramic angle cut cylinder (ideally 45 degree) with a diameter less than 1/4" will deburr the inside edge of the slot without getting stuck in the part. Alternatively, a small precision media such as a 4 mm (0.15") abrasive triangle will be able to fit inside the slot.

The part geometry and the tumbling demands (deburring, polishing, etc.) will determine the media that will best provide the desired results. Take a close look at your parts as there may be alternatives that will provide better, faster and even more cost effective results.

Benefits of Steel Tumbling Media

Given the investment required to purchase Steel Tumbling Media, why use it? It basically all comes down to weight and wear.

Steel media is much heavier than other types of tumbling media. As the parts are forced through the media the high pressure results in reduced cycle times. The pressure or compression on the part helps to work-harden the surface. In many cases, tumbling parts in steel can eliminate shot blasting and lead to parts with increased wear resistance. Work-hardened parts have much less porosity increasing corrosion resistance.

Many parts ready for plating that appear to have a very smooth finish actually have microscopic imperfections in the surface. The weight of the steel media 'flattens' irregularities in the surface and produces a noticeably better plated finish - especially with thin depositions such as nickel.

Steel is a considered a non-consumable tumbling media and does not wear down in the tumbling process. This means much longer lifetime than traditional media (100x or more) and no media residue waste from the media.

Steel media should be considered a long term capital investment not a consumable media expense.

9 Tips for Tumbling Media Selection

9 quick guidelines for selecting the proper tumbling media:

1. Large media will cut faster than small media of the same bond (formulation)
2. Small parts, small media; large parts, large media
3. Flat (vs. round) media will do more work on edges
4. Large media is generally less expensive than small media
5. Soft metals usually demand lighter cutting and lighter weight media
6. Small media will produce a finer surface finish
7. Rounder shapes have more impact on the surface (vs. edges) than flatter shapes
8. Denser (heavier) media will lead to a higher polish
9. More aggressive bonds break down more quickly than less aggressive bonds

What is Tumbling Media?

Preformed tumbling media used in both vibratory and barrel tumblers are simply the means to achieving a desired surface finish. The type of media formulation or BOND used can vary widely for each process or application.

Tumbling media is a blend of abrasives and binders. Ceramic Media is generally a clay or porcelain binder while Plastic Media and Synthetic Media are typically made from urea formaldehyde or polyester polymers. These binders are used to hold (bond) and shape the abrasive grit blend needed for a particular media type. Aluminum oxide and silicon carbide are the most popular abrasives used.

The specific type of binder and abrasive used along with the ratio of binder to abrasive will determine the impact of the media on the part. A high abrasive content will lead to quicker break down of the media and a more aggressive operation. Low or no abrasive content will result in a media that is very soft/gentle on the part and used for polishing and fine finishing.

All tumbling media are designed to break down or erode during use. This is necessary to perform the 'work' on the part. Generally, the quicker a media breaks down, the more aggressive the bond. The ideal media for any process will provide the minimum erosion (longest lifetime) but must be balanced with a reasonable cycle time to achieve the desired surface finish.

Screen Separating Basics

There are a couple of ways to separate parts from media. Basically, a screen is used to separate out smaller items from larger items. The best separation is achieved when the media and parts (or whatever is being separated) are different enough in size and shape to make separation simple.

A tubular rotary screen separator will tumble the items to be separated while the load passes through the screen tube and the smaller items are separated. This type of process can be used for separating almost anything and works great for parts that 'hold' the media and need to be over-turned to be emptied. A tubular rotary screen separator can also process high volume loads (hundreds of pounds per minute if necessary). Multiple tubes in sequence can be used for multiple separations.

A flat vibrating screen is a good general purpose method to separate parts and media. Loads are simply emptied onto the vibrating screen and the smaller pieces fall through the screen. This is more of a batch process and ideal for small load sizes. A flat vibrating screen can be incorporated into vibratory tumbling systems for automated separation of media and parts in process.

The biggest challenge with screen separating is identifying a screen opening (size and shape) that will quickly and efficiently separate the load. If parts and media (or whatever is being separated) are very close in size and shape the optimal results will be much more difficult to achieve.

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'.

Mass Finishing Basics (Part 3)

Vibratory tumblers come in all different shapes and sizes. Bowl style vibes include both round bowl and oval-bowl shapes. Tub style systems look kind of like bathtubs. In general, vibes are produced in sizes as small as 1-2 gallons up to 150 gallons (and much larger). Both systems force the tumbling media and parts to vibrate and rotate around the vibe.

Tub style vibes force the parts (and media) to vibrate and rotate around the center axis of the tub. This creates a similar sliding and rolling action to a barrel tumbler but adds the additional vibratory energy working against the parts resulting in shorter cycle times. Since tub style vibes can be very long in length and diameter extremely large large parts can be tumbled.

Bowl vibes add circular movement around the bowl in addition to the typical tumbling and vibrating. Since the entire load is moving around the bowl, this system allows the option for a separating screen (either above or below the load) to automatically remove the parts from the media. Unlike tub style systems, bowl vibes can only process parts smaller than the bowl channel width.

Mass Finishing Basics (Part 2)

The two types of barrel finishing systems are rotary barrel tumbling and centrifugal barrel tumbling. Both mass finishing techniques use tumbling media, water (usually) and compounds to tumble parts inside a closed barrel. The similarities end there.

Rotary barrel finishing is an abrasive process that involves a sliding and rolling motion of the media/parts load. Filling the barrel 50-60% full allows the upper layer of the load to slide down (due to gravity) as the load is turned-over. Changing the load size as well as the ratio of media to parts to water to compound will create more or less aggressive operations. This ancient mass finishing technique is a highly economical, time-tested technique.

Centrifugal barrel finishing creates more sliding action at higher rates as the individual barrels spin independently of the turret rotation (think of the carnival ride with individual cars that spin while the entire carousel of cars are spinning) . The high centrifugal force created inside the barrel compacts the media/parts load and results in low part to part interaction. These high forces also lead to lower cycle times and very smooth, consistent surfaces finishes.

Tumbling with Steel Media

Polishing all types of metal with steel media is an established procedure for many industries. As the price of steel increases (especially stainless grades), many companies are looking at barrel and vibratory tumbling with carbon steel instead of stainless. The BIG disadvantage of carbon steel is rust. Of course, the big ADVANTAGE is cost.

Keeping carbon steel clean and corrosion-free is critical to getting an acceptable surface finish on the parts. Using the proper chemical compounds is the answer to this problem. Even though acid compounds will return the rusted steel to near original condition, this should be avoided as this can actually make rusting problems worse.

Steel media should first be restored with a cleaner (like Kramco 750) containing sequestrants, detergents and be pH buffered to remove the corrosion and prevent flash rusting. The second step is to treat the shot with a rust inhibitor that will both prevent rust formation and not contaminate the parts {like oil based rust inhibitors}. Simply tumbling the steel media with a non-durable rust inhibitor (like Kramco 510) for a few minutes, draining the solution and then letting it air dry will keep the shot clean and bright even during longer term storage.