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.

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 resistant 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 smooth 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.

Why a Steel Scrap Surcharge?

Steel Grit and Steel Shot primarily use scrap steel as the source raw material. The scrap is melted, the chemistry is modified and then solidified into small particles using a high pressure stream of water. Back in the early 1970's (as well as today) the scrap steel market was very volatile with significant monthly swings in prices. This made long term purchasing difficult for large shot and grit users.

A pricing scheme for steel shot and grit was developed that fixed the base price and added a surcharge to that base based on the ups and downs of the market. The Iron Age Scrap Price Bulletin has been used as the standard for setting the monthly steel scrap surcharge.

In the last 30+ years the pricing scheme has become the accepted standard. As such, the base price of Steel Grit and Steel Shot remains consistent and the applicable steel scrap surcharge is applied at the time of shipment. It's a strange but fair system that gives everyone - suppliers and purchasers - an opportunity to get the best value.

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.

Getting Aggressive with Tumbling

Barrel tumbling is a time tested method to deburr, polish and create a desired surface finish. Vibratory tumbling is a new (well...newer) technology that also works great and has replaced barrel tumbling for deburring and surface finishing for lots of reasons. The aggressiveness of a vibe is largely limited by the tumbling media being used.

Welcome back - barrel tumbling. Extremely aggressive processes can be achieved in barrel tumblers. Rough cut, laser cut, parts with lots of slag and scale, heavily rusted parts can be tumbled successfully part-on-part in a barrel tumbler. The weight of each part acting on the others creates an environment in the barrel that will clean these parts in a way that a vibe never could. Adding a grit to the process will add to the cutting action.

Sure, it's a loud process. It can even be a little dirty. But the results speak for themselves.