High-Energy Mass Finishing

Article From: Products Finishing, from AAC Engineered Systems, Inc.

Posted on: 7/1/1999

New rules for a new manufacturing millennium...

CLICK IMAGE TO ENLARGE (+)

Spindle finishing.

Spindle finishing.

Centrifugal barrel finishing.

Centrifugal disc finishing.

Centrifugal disc finishing.

In a segment of the metal finishing market that has long been dominated by tumbling barrels and vibratory finishers, high-energy mass finishing has started to come of age. While the four methods detailed here are often presented as highly specialized "niche" products, high-energy deburring machinery is finding new converts and wider applications as customers demand faster delivery, improved quality and higher degrees of accountability. As automation, material handling systems and machinery designs have improved, high-energy finishing systems are often better suited to today's advanced production machinery.

What is high-energy finishing?

Simply stated, high-energy finishing describes any form of abrasive media-based parts finishing that uses more than the force of gravity to deburr, radius, smooth, burnish, descale or otherwise prepare parts. It might be easier to define what high energy is not. It is not tumbling barrels: open- or close-ended, conical, octagonal or cylindrical. Tumbling barrels have been the workhorses of mass finishing for hundreds of years. They work on the simplest of principals. Mix a part long enough with the right abrasive media, and the burrs or imperfections will eventually wear off.

It is not vibratory bowls or tubs. Vibratory deburring made its debut in the U.S. in the 1940s. A vast improvement over traditional tumbling, vibratory systems nevertheless rely on the force of gravity as they shake parts in a slurry of abrasive media and water. Do not be confused by the term "high-energy vibratory." These are simply vibratory systems with higher frequency motors.

The four basic forms of high-energy deburring are centrifugal disc, centrifugal barrel, spindle and drag finishing. Each of these will be detailed, but first we must ask an important question.

Why High-Energy?

The need for speed. The first and most obvious advantage of high-energy finishing is process speed. Compared with vibratory finishing, which became the gold standard in mass finishing in the 1950s, high-energy systems churn out finished parts at a blistering pace (See Table I).

One thousand parts in. Only 997 out? Today's customers demand not only high quality but a level of total batch accountability that until recently would have seemed absurd. Requirements for 100% inspection have given a new meaning to the "one bad apple" cliché. Unless a vibratory bowl or tumbler is emptied completely between every cycle (a potentially lengthy and tedious process), there is no guarantee that every part that went in came out. This can result in a few parts "taking an extra ride." Or two. Or three. The next extra ride they may take is on the truck back to the manufacturer for rework or a trip to the scrap heap. High-energy systems by nature are completely unloaded and reloaded between every processing cycle. In fact, with spindle and drag finishing, each piece is finished individually ensuring total accountability.

I would like those parts ASAP and JIT, OK? Rarely have three little letters changed an industry so profoundly (well, except maybe for IRS, but that is different.) JIT, Just-In-Time, inventory systems mean that customers no longer want to see a truckload of parts show up at the dock for the month's production. They want today's parts today. Then tomorrow's parts tomorrow. That leaves the average component manufacturer with two choices. He could inventory parts for his customers, which is expensive and inefficient. The more logical solution is to produce today's parts today and tomorrow's parts tomorrow. That translates to producing larger numbers of smaller batches faster. High-energy finishing with its ultra-short time cycles and smaller batch orientation often fits that requirement perfectly.


TABLE I—High-Energy Finishing Systems and Applications
Type Description Compared to Vibratory* Advantages Disadvantages Application Profile Examples
Centrifugal
Disc
Rotating disc spins parts/media mix within stationary chamber in a "hurricane." 5-10x faster Offers the most material handling/automation options, can run unattened, allows in-process inspection, clean process, easily automated, quiet. Potential for part-on-part impingement, some limitations on media selection, cannot run very thin parts (under 0.010 inch), not usually suited to large/heavy parts. Small parts produced in high quantities where moderate part-on-part contact is not an issue. Manufacturers desiring automated systems. Small stamped or fine-blanked parts, powdered metal, investment castings, small forgings, bearings.
Centrifugal Barrel Closed barrels mounted on rotating turret. Barrels counter-rotate to create high g tumble. 5-15x faster Fastest batch process available, flexible (very aggressive deburring or delicate parts handling), barrels can be divided to eliminate part contact, quiet. High load/unload labor, closed process, does not allow flow-through processing, not easily automated, too aggressive for some parts. Parts requiring heavy burr, parting/mold line or other material removal; parts requiring relatively low microfinish where moderate labor involvement is not an issue. Forgings, castings, stampings, medical devices and implants, dental components.
Spindle Finishing Workpiece(s) is attached to a fixture(s) or moveable spindle(s), then immersed into rotating media mass. 10-20x faster No part-on-part impingement, can operate continuously (with off-line fixturing), can produce extremely fine finishes, can selectively finish complex part surfaces. Not for delicate parts, high labor due to need to fixture. Larger parts requiring fine or "no nick" processing where hand labor is too inconsistent or costly. Larger castings, forgings, complex shapes.
Drag Finishing Workpiece(s) attached to rotating fixture(s), immersed and pulled through stationary media mass. 20-40x faster No part-on-part impingement, can produce fine finishes, can selectively finish complex part surfaces. Not for delicate parts, comparatively high labor cost due to need to fixture, batch processing only (cannot run continuously). Larger parts requiring fine finishing, where part contact is unacceptable. Also, where labor involvement is acceptable, but hand finishing is inconsistent. Large castings, forgings, complex shapes.
*Using vibratory bowls as a standard, a general measure of the in-process cycle time experienced with each technology. Actual machine productivity should be compared on a "floor-to-floor" cycle time basis (process cycle time plus any time spent unloading and subsequent reloading between process cycles).

 


One at a time? In an almost perverse evolution of 100% inspection and cell manufacturing, many manufacturers are exploring "one-piece-flow" production. This process requires straight-line part production that never "batches" parts. The advantage of this approach is obvious. Every part is accounted for, and rejects are identified immediately without affecting an entire run of production. The downside is if you are only doing one part at a time, you had better do it fast! In cases like this, high energy may be the only answer.

Automate or evaporate. These were the words of a past president of General Electric. Unfortunately, the conventional mass-finishing and deburring world historically has not kept pace with the new wave of manufacturing automation systems. Consider this, if a finishing system is going to process parts 10 times faster than your current equipment, then it must be loaded ten times more often. If the system cannot help the operator go "floor-to-floor" with ease (or preferably perform material handling functions by itself), then the system's speed itself becomes counterproductive, a victim of its own success. Fortunately, many high-energy systems offer high-volume manufacturers computer controls, onboard material handling and upstream/downstream parts handling and metering systems. These integrate easily with automated machining centers, stamping presses, compacting presses and other production tools.

Wagging the dog. Deburring departments have historically been "tail of the dog" operations. It is where production problems go to disappear. Indeed, when finishing cycles are measured in hours or even days, a half-hour time increase does not get anyone's attention. However, when cycles are only 10 minutes, an increase to 15 minutes is huge. Therefore, instead of taking half a shift or several days to detect a production problem, high-energy finishing reveals it in minutes. Manufacturers experienced in the use of high-energy systems use their finishing department as a leading and "real-time" indicator of tooling wear, incoming material quality and operator performance.

centrifugal disc finishing
Centrifugal disc finishing.

Centrifugal disc

Picture an industrial-strength kitchen blender and you pretty much have it. The centrifugal disc approach uses a concave disc positioned at the bottom of a round work chamber to create an aggressive torroidal action. Rotating at speeds of 100-300 rpm, the disc forces the parts-media mass outward and upward where it encounters the side walls of the process chamber. The side walls "brake" the mass, causing it to tumble back down and inward. The result is a powerful sliding/tumbling action that finishes parts up to 10 times faster than vibratory deburring. Using multiple process controls such as disc speed, water flow (or dry processing with no water), parts/media ratio and open vs. closed processing, the centrifugal disc offers multiple processing variables to produce many desired outcomes. The technology does have some limitations regarding parts and media selection. Nonetheless, of all the high-energy technologies, it offers the most advantages in material handling, automation and programmability.

centrifugal barrel finishing
Centrifugal barrel finishing.

Centrifugal barrel

One g is the measure of mass at the earth's surface at sea level. Performing a 3 or 4g climb, an aerobatic pilot will experience blurred vision. At 9g, the average test pilot will lose consciousness. Now, imagine how 25g might feel. That is the secret to centrifugal barrel finishing. Closed barrels containing parts, media, water and compounds are fixed to a turret that rotates at high speed, creating tremendous lateral acceleration (up to 25g). Simultaneously, the barrels themselves are rotated against the turret's rotation, creating an extremely powerful tumbling action. The action within the barrels can be selected to produce different finishes by changing the relative rotation of the barrels to the turret. A 1:1 turret/barrel rotation will produce an aggressive grinding action. A 4:1 ratio would produce a gentler "micro-finish" tumble. Since the barrels are sealed, in-process inspection is impossible and changeover labor can be significant. Nevertheless, centrifugal barrels offer the fastest batch processing available.

spindle finishing
Spindle finishing.

Spindle finishing

A spindle finisher consists of two main components. First, a round tub containing an abrasive slurry that is fluidized by rotating it at speeds of up to 1,000 surface feet per minute. Second, the spindle, which is essentially a heavy rotating fixture upon which the raw part is attached. To finish the piece, the spindle lowers the part into the rapidly moving mass, slowly turning it at various angles and exposing the part surfaces to the grinding action of the media mixture. By changing the speed, media and mixture of the finishing mass, speed of spindle rotation, angle of attack, depth and time, finishes can be carefully controlled and easily changed. One major advantage of spindle finishing is the option of selective surface finishing. For example, suppose the leading edge of a casting required more work than the trailing edge. No problem, just point the leading edge into the abrasive flow for a longer time than the trailing edge. Or, suppose you wanted to completely deburr only the top 3 inches of an aluminum forging, leaving the remainder untouched. Simply set the spindle to a depth that exposes only the desired surfaces. While each part must be fixtured individually, machines can be set up with multiple spindles, allowing one to be loaded while the other runs. This allows continuous, if not true, batch processing.

Drag finishing

Have you ever looked at a farmer's plow? Over time, the cutting edge becomes bright and smooth. This is because the blade, as it cuts through the earth, is micro-finishing itself. Drag finishing works on the same principal. From a distance, drag finishing looks a little like spindle finishing. But, instead of spinning the media, the part is simply dragged through the stationary mass on a rotating fixture, creating tremendous surface pressures. The part makes a planetary orbit through the bowl while rotating around its axis. Drag finishing is used for all kinds of polishing operations, but each piece has to be fixtured, so it is more appropriate for large items than for small- or high-volume ones.

One advantage of drag finishing is that the media container has no moving parts or drive components. Therefore, it is possible to maintain several bowls of media of different cuts and finish qualities. This allows multiple-pass processing without changing machines or fixtures. For example, a first stage rough cut to deflash could be immediately followed by a second stage deburring to eliminate parting lines, then a final finish stage for low Ra surface conditioning. Like spindle finishing, parts must be fixtured individually, requiring a fair amount of labor. This can be reduced by designing machinery with multiple fixtures on a turret or by purchasing multiple fixture sets that can be pre-loaded off line.

Obviously, no single approach is right for everybody. However, by using some general guidelines, you might find that your situation fits high-energy finishing.

Evaluating The Technologies

Run the numbers. One way to evaluate your investment is by comparing competing technologies on a cost-per-piece basis. This involves dividing the total number of parts produced into the sum total of machinery cost (averaged over life of the machine), media cost, water use, power consumption, chemical use, machine expendables (such as motors, wear items, fixtures, etc.), labor costs and quality (which can be quantified by measuring the costs of customer rejects or rework).

Pick your application carefully. High-energy finishing is not for everyone. Just because you WANT faster cycle times does not mean that you can have them. Your part may prohibit it.

One big "don't." Do not evaluate price; only results matter. The most expensive machine might just be best. For example, a centrifugal disc may require an initial investment of five times that of a similarly sized vibratory bowl. Still, if it produces ten times the amount of finished parts at the end of the day, it makes sense. Likewise, a spindle finisher may look prohibitively expensive, but forced to choose between meeting a customer's finish specification and losing the contract, the decision becomes greatly simplified!

Where to get more information

The independent finishing sales representative is usually the first to introduce high-energy finishing manufacturers to potential customers. You probably have one nearby. Look for one with plenty of experience in the field. The finishing business has become a recognized specialty within the last 50 years. The closer a finishing supplier's age comes to that number, the better off you should be. However, even the most seasoned professionals will tend to suggest what they are most comfortable with — not necessarily what you are interested in. Ask about that agency's experience with high-energy systems sales and service. Many finishing representatives have decades of experience with conventional finishing but little to none with high energy.

A local facility with a job shop or test lab attached is not necessary, but it may help you evaluate new technology and processes. Also, a local supplier who stocks supplies such as media and compounds will be more responsive after the sale as you experiment, adjust and refine your processes. Finally, make sure they will come to you. Solving a problem that has never been seen is impossible. An hour spent walking your plant floor with a trained specialist may reveal ideas or opportunities that were previously hidden.

The road will lead eventually to the actual manufacturer. Any reputable one will have a test lab and offer free sample parts processing to begin your evaluation. As with your local representative, get to know the equipment builder. How long has it been around? How long has it been building this kind of equipment? Does it actually build the equipment or is it subcontracted or imported? What makes its designs different from its competitors? How much of the company's total business is represented by this type of equipment (i.e., is this the company's primary product focus or the "poor stepchild," likely to get little attention or service after the sale). How many installations does the company have? Can it supply references, preferably ones similar to your own situation? The answers to these questions will tell you more than you will ever get from the manufacturer's brochure or web site.

As delivery, quality and price demands continue to challenge metal formers and fabricators, they will need new answers to help keep pace. Throughput, response and quality issues that were once the provinces of defense contractors, medical device makers and the largest automotive suppliers are now part of the everyday operations of average manufacturers. For these reasons, high-energy finishing is on the rise. Once seen as highly specialized niche products, centrifugal disc, centrifugal barrel, spindle and drag finishers are finding new applications every day. High energy is still not for everybody. However, when it is right, it can be very right.



Suppliers | Products | Experts | News | Articles | Calendar | Process Zones

The Voice of the Finishing Industry Since 1936 Copyright © Gardner Business Media, Inc. 2014

Subscribe | Advertise | Contact Us | All Rights Reserved