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.

Body Jewelry Tumbling

We received a post a couple of years ago about polishing body jewelry in a vibratory tumbler. While very similar to other jewelry there was a concern about the size of the parts and the threads at the ends. The recommendation was to polish the parts using a Precision Ceramic media to be able to get into the finer areas of the part without damaging the threads. Steel tumbling (while also effective) might roll the thread edges. A simple but effective result.

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.

Abrasive Blast Polishing?

I wish there was a way to polish metal, plastic and wood surfaces using abrasive blasting. Unfortunately, using abrasive media (no matter how soft or round) just doesn't produce a polished finish. Glass Bead will result in a uniform, satin-like finish on some metals and Steel Shot can 'almost polish' hardened steel surfaces.

So far, we have not identified a media and process to consistently create a high polish using a blasting process - especially on plastics and wood.

Have you had any success in this area?

Polishing Eyes

An interesting thread (edited slightly) on the ShopTalk Forum:

Question: We are looking to for a way to polish parts made from a clear urethane casting resin, shore hardness 85D. We currently do all hand polishing and the result is a high glass type polish and looks good.

Response: Urethane is designed to be very resistent to abrasion and does not polish the way most plastics do. Most of the work must be processed in a wet operation with a ceramic or porcelain polishing {Precision Ceramic} media in order to provide enough pressure to abrade the urethane. If the starting finish is rough, the {first step using general purpose} ceramic and abrasive (i.e., pumice) will smoothe out the surface. The "pre-polish" would be done in the porcelain media using a fine Aluminum Oxide powder - 100 to 150 grit. Start the water level very low (30% ) and then after 6hrs, raise the water level to even with the top of the load. Run overnight. The final high polish would be achieved using a dry process with wood pegs and {Microlyte} polishing cream . It may take 6-12 hrs to achieve the final polish.

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 corrossion-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 corrossion 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.

Mass Finishing Basics (Part 1)

There are two basic types of mass finishing - barrel tumbling and vibratory tumbling. Each of these mass finishing methods have pros and cons. There are also a number of sub-varieties which we will discuss in future posts.

Barrel tumbling might be more familiar if described as rock tumbling. Inside the tumbler the parts and media are lifted by the corners of the barrel and allowed to bump, scrape and slide against each other. The media used will depend on whether you are trying to deburr or polish your parts. Barrel tumbling is also good for part-on-part tumbling (another future post - so many posts and so little time). While typically used wet, barrel tumbling can also be a dry tumbling process. The big down side to barrel tumbling is the loading and unloading of parts.

Vibratory tumbling is a little different. Similar to barrel tumbling the parts and media are sliding against each but in more of a 'filing' motion. The tumbling action is occuring in 100% of the load with a vibe versus only 20-30% of the load in a barrel tumbler. This allows a vibrator (you know what I mean) to typically offer shorter cycle times. Vibratory tumbling can abrade and smooth a surface with minimal affect on the edges while barrel tumbling loves to round the edges. Vibratory tumblers can offer very simple and even automatic separation of parts and media.

Generally, barrel tumbling is good for durable parts that require heavy deburring or high polishing and for processes that use a lot of weight (i.e., tumbling with steel media). Vibratory tumbling is great for general purpose deburring, surface finishing and for easy loading/un-loading of parts.