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5/1/1996 | 11 MINUTE READ

Centrifugal Disc Finishing

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Ten questions you should be able to answer...


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It is not the latest Internet on-ramp. It has no relationship to anything that was "developed by a team of NASA scientists for the space program." Still, it does share something in common; it is an emerging technology for the mass-finishing industry. It is centrifugal disc finishing.

Remember the earliest days of personal computers? No one quite understood them, or if having one was necessary. Now, it is hard to find a desk without one. Like many other emerging technologies, centrifugal disc finishing has spent a long time in development.

A recent, "non-scientific" survey of manufacturing professionals at an industry trade show revealed that about only five pct had any idea what centrifugal disc finishing (CDF) was. More surprising, the question was asked while the attendee was standing less than eight ft from a unit.

The following are the 10 most frequently asked questions about centrifugal disc finishing (CDF).

1. Where did centrifugal disc finishing originate?

Some say that mass finishing began with the ancient Roman armies. Riding and marching to battle, the centurions carried their swords, shields and armor in burlap bags containing sand, leather and river stones. As they hiked, their armaments were burnished to a high shine. Exploiting the result, Roman armies favored early morning attacks, on an open hillside, facing east. Even a small battalion appeared as an enormous force.

This points out two interesting facts about today's metal product finishing industry: 1) metal product finishing has changed much since then; 2) it really has not changed much at all.

The fundamental problems of shining dull metal, smoothing rough metal and softening sharp edges have not changed. The basic solution of parts, abrasive finishing media, and water is almost identical. What has advanced is the machinery used.

The right tool for the job. Until about 1950, the primary tool for mass finishing was the rotary tumbling barrel. These slow-moving, open-ended tumblers may date back as far as feudal China, reportedly first used in making gunpowder. They were and still are effective, but slow. Mass finishing got a shakeup in the 1950's when vibratory finishing first appeared.

Debuting in Europe, this method uses an eccentric weight and rotating shaft to forcibly shake the parts-media mass in a tub. Vibratory finishing shortened time cycles. It became the unquestioned standard for mass finishing, where it remains in many manufacturing facilities today.

The high-energy revolution. In the mid-1960's, the rules changed. Finishing machinery manufacturers discovered and harnessed "high-energy." This was a major improvement upon the gravity-dependent tumbling barrels and vibratory systems. In centrifugal barrel finishing (CBF), operators place parts and media into a closed barrel attached to a processing turret. As the turret rotates at high speed, the barrels counter-spin against the turret's rotation. This action delivers high levels of centrifugal force, up to 25 g's (1g = one time the force of gravity). Parts were now finished in minutes. With CBF, finishers could exert tremendous finishing forces on a part while also tightly controlling the finishing operation.

Frustrated with the material handling barriers of centrifugal barrels and the lengthy time cycles of other methods, finishers needed a new approach. They wanted something that could offer the "best of both worlds," high process speed and control plus material handling that fit in with manufacturing automation.

Debuting in the U.S. in the early 70's, the centrifugal disc was the beginning of the answer. Centrifugal disc's answers often raised more questions.

2. What is Centrifugal Disc Finishing (CDF)?

The basic technology is just about as simple as your kitchen blender. Rather than blades crushing ice inside a pitcher though, a round rotating disc resides at the bottom of a round stationary chamber. Fill this chamber to its working capacity with your parts plus any or all of the following: finishing media, water and liquid or dry compounds. When you energize the machine, the disc's rotation forces the entire mass to rise quickly upward and outward. The walls of the upper processing chamber slow the mass, effectively "braking" it, turning it over upon itself to tumble down and inward back to the disc to begin again. The mix takes the shape of a vortex of parts, water and media. The centrifugal force developed by the disc adds weight to the parts and media. The result is a highly aggressive parts-finishing energy.

Depending upon the size of the machine (smaller machines require higher speeds than larger ones) this action will take place at anywhere from 60 to 250 rpm or more. In many machines, the rpm speed is variable, allowing the operator to vary processing forces from one part or process to the next, while maintaining the optimal "shape" of the finishing mass.

3. Why should I consider a CDF system?

As the real estate business has the "three L's," (location, location, location) the answer to this question is in the "three R's" (results, results, results). Generally, a centrifugal disc will finish parts eight to 10 times faster than a conventional vibratory bowl or tub. A two-hr vibratory cycle could be done in 12 to 15 min. Viewed another way, a two cu ft centrifugal disc could do the work of a 20 cu ft vibratory system in about one-half the space.

Speed is not the only issue. Response and flexibility have become the new watchwords for component manufacturers. JIT delivery and cell manufacturing demand it, with their emphasis on greater numbers of smaller orders.

The centrifugal disc is flexible. By varying media, water, parts/media ratios, machine fill level, disc speed and cycle time, a CDF system allows effectively infinite finishing results for different parts and finishes. The rapid time cycles allow a complete changeover when the current processing cycle ends.

4. What is the benefit for my processes?

CDF offers efficient material handling and process control. Material handling is critical to maximizing CDF output. (After all, what good is a five-minute processing cycle that takes a half hour to separate and reload?) Only one thing guarantees no parts mixing between finishing cycles, emptying the contents of the system; media and all.

In CDF, the process chamber empties quickly and completely between cycles. With integrated screening, conveying and loading, a new process batch, with different parts and even different media, can be loaded in an empty machine in minutes. With a quick adjustment of cycle time and disc speed, an entirely new process can be set up and running quickly.

The resulting surface finish is also different. In CDF, the parts/media vortex creates a powerful sliding action in the finishing chamber. Compared with the peening action of vibratory finishing, the result is often a smoother surface finish. Also, unlike its closest cousin, centrifugal barrel finishing, the open top of the CDF process chamber allows easy in-process quality inspection and "on-the-fly" process adjustment.

5. What media can I run in it?

If you can run it in other systems, you can usually run it in a CDF unit. Ceramic, plastic, synthetic, porcelain, sintered, fused and random nuggets, grits, steel and agricultural media all work successfully in a properly designed and maintained system. It can work for part-on-part applications as well. The aggressive centrifugal disc action allows some media combinations that are impossible in other systems, which can allow media of different weights to separate from each other during processing.

If you are responsible for cost management, the following will interest you. All things being equal, a centrifugal disc can use a less aggressive media to do the same work as other methods. "Less aggressive" generally translates to "lighter," and that normally means a lower price per cu ft. It also means lower overall attrition rates, and that delivers lower cost over the long run.

6. What kind of parts can I put into a centrifugal disc machine?

CDF finishing is not the cure for all finishing ills. A centrifugal disc system is most effective with high volumes of small compact parts (generally not delicate or large, heavy work pieces). A good guide is "smaller than your fist, lighter than a pound," with some exceptions. Several jet engine turbine blade manufacturers successfully use large CDF systems to finish massive parts, some a ft or more long, weighing five or more lbs. each.

The centrifugal disc works best on parts requiring external burr removal, edge-breaking, radiusing, overall material removal, smoothing and burnishing.

7. What sizes of CDF machines are available?

No industry standard exists for listing machine size, so you must become familiar with two terms: machine (or overall, bowl or total) capacity and working capacity. Total machine capacity is simply the volume of the entire process chamber/disc area. However, the disc's rotation forces the bowl's contents to rise and tumble into a vortex. Therefore, the total machine volume is not available for use. The term working capacity describes the maximum amount of parts and media that can run without any of it flying out of the bowl.

Using working capacity, CDF systems can be found in sizes ranging from one-half to about 10 cu ft. The right machine size will depend on a combination of throughput requirements, part size, desired finish and budget.

8. CDF is faster, so it must be expensive, right?

A CDF system, measured on a cu ft-for-ft basis, can cost five to seven times as much as a same-sized vibratory system. On the other hand, it could do 10 times the work in the same amount of time.

It is the same with consumables. Obviously, as you do more work, you will expect to use more finishing materials. For instance, vibratory systems use about one to two gal of water/cu ft/ hr, while CDF will use around five to six gal/cu ft/hr. Media will break down faster, but not ten times faster.

While direct comparisons may seem difficult, diverse finishing options can be analyzed using "cost per part"factoring. To derive this number, add machine purchase price, consumables used, waste handling, maintenance, labor costs and other overhead factors, then divide the sum by total parts throughput. As labor and fixed overhead costs continue to rise, CDF can often produce favorable cost per part results on the strength of its speed and flexibility.

9. What is all this fuss about "the gap?"

If you know anything at all about CDF, you have heard complaints, arguments or theories concerning the spinner/chamber gap. If one surface is going to rotate and the other remains stationary, something has to give. What "gives" is a very small space between the rotating disc and the upper process chamber.

This gap begins at a width of 10 to 20 thousandths of an inch or even less. With use, and often abuse, the gap tends to widen slowly throughout the machine's life. Eventually this can cause considerable trouble, such as media lodging, parts jamming and damage, or other machine failure. If you consider purchasing a CDF system, pay special attention to gap design and the manufacturer's ability to renew, replace, or adjust this or avoid trouble in the first place.

10. What features do I look for in a centrifugal disc machine?

Look beyond the purchase order. Buying a machine is only part of the battle. Centrifugal disc finishing is an aggressive process. The machine endures much abuse and turns a heavy load at high rotational speed. The by-products are significant torque forces and high stress. To handle this, the disc-drive system must be powerful and reliable. Overall machine construction must be sturdy and preferably contain as few moving parts as possible.

Under the heading of reliability comes maintenance. Quiz your sales engineer and make him show you the answers. How easy is the system to service? Are maintenance points easily accessed? Are potential problems easily diagnosed? When a component fails can I get a new one quickly and preferably locally? What routine problems might I encounter, and how do I recover from them? How much of the machine set up, routine and restorative maintenance can my own people do with current tools and knowledge?

Remember, in CDF parts are finished aggressively. The two main contact surfaces, the disc and upper chamber, work in a highly abrasive environment. Therefore, many conventional wear-resistant materials used in vibratory or tumble finishing are ill-suited to the task. Learn how long these surfaces are expected to last, and the cost and lead time of renewal or replacement.

Get good help. Know that you are not on your own. Seek out a manufacturer or sales professional who can help you with the research. Good ones can develop a custom process for your part, or improve a current one. They can suggest ideas to mechanize or automate your system and accurately recommend action based on solid technical knowledge. More than likely, they already have experience with related applications. Use their prior work to your advantage.

The finish line. Since you have made it this far, you deserve a bonus question. Question 11: Is centrifugal disc finishing right for me? Naturally, there is no firm answer. For many finishing professionals though, the answer is becoming "yes." As competition for customers becomes more fierce, and margins for production errors and waste shrink, CDF provides the flexibility that many manufacturers need. The only way to know is to try it. The results will speak for themselves.

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