VOL. 16 March ISSUE YEAR 2015
in Vol. 16 - March Issue - Year 2015
Vibratory Finishing flexes its muscles - processing of big, heavy parts
Photo 1: Large tub vibrator - bowl dimensions 4,000 x 1,500 mm (157 x 59
Photo 2: Processing of a bearing ring in a large rotary vibrator; OD = 3,000 mm (118
Photo 3: Rotary vibrator with special work piece holder for finishing large ring-shaped gears with inner teeth
Photo 5: Large A/C fan wheel mounted to the work bowl of a rotary vibrator
Photo 4: Landing gear for commercial aircraft; length > 2,500 mm (98
While in shot blasting it is not uncommon to handle large, heavy components with weights of several hundred kilos or even several tons, mass finishing generally is used for treating small to midsize mass-produced parts. But mass finishing is an evolving technology that constantly reinvents itself. Not too long ago, deburring of tiny wristwatch components in vibratory finishing equipment was considered unthinkable, but thanks to the development of extremely precise and small grinding media, mass finishing has become the preferred surface finishing method at many wristwatch manufacturers. On the other side of the spectrum, the mass finishing technology has lately expanded to treating very big, heavy parts weighing more than 3,000 kg.
One would have expected that surface finishing by hand disappeared from the industrial landscape a long time ago. But this is not so at all: Manual deburring and surface smoothing is still the prevailing method for finishing the surface of very large and heavy components. But thanks to the development of bigger finishing machines with considerably stronger vibratory drive systems, it has now become possible to mechanically finish the surface of high-value, large components with diameters of over 2,300 mm (over 90") and weights of over 3,000 kg. And with new process technologies such as the Isotropic SuperFinish (ISF) method, the surface roughness readings on these parts can, for example, be reduced to as low as Ra = 0.1 £gm.
Powerful and big - custom engineered finishing equipment
Treating such large, heavy components poses a considerable engineering challenge. The finishing equipment does not only have to be bigger and sturdier, but it also requires considerably more powerful vibratory drive systems. Owing to metallurgical advances, improved welding methods and utilization of the latest bearing technologies, a renowned equipment manufacturer was able to develop finishing equipment that can not only handle these oversize components but also provide the required vibratory energy to achieve the desired finishing results.
The size and power of these machines is astonishing: The work bowl dimensions of the largest tub vibrator are 4,000 mm (157") by 1,500 mm (59"). Its vibratory drive system delivers a whopping 37 kW (see Photo 1). And the largest rotary vibrator (see Photo 2) has a diameter of 3,000 mm (118").
Single piece processing
Of course, the size and weight of such large components only allows finishing one piece at a time. Even if their size would permit placing more than one work piece into the finishing machine, the tumbling of such heavy parts against each other would cause them to be severely damaged. Depending on the work piece shape and processing goal, a single part can either move freely within the media, or it must be mounted on special work piece holders. For example, certain ring-shaped gears with the gear teeth on the outside can tumble in a large rotary vibrator without requiring any special fixtures. However, ring-shaped gears with inner teeth must be mounted on special work piece holders in order to achieve the required surface finish (Photo 3). Occasionally, in the case of extremely heavy work pieces, these may have to be treated by being suspended from a crane.
Excellent and absolutely repeatable finishing results!
Manual surface finishing obviously poses many problems. It is extremely slow, sometimes taking several days, requires highly skilled labor and is, therefore, very expensive. And, of course, like with any other manual work, the results are generally somewhat inconsistent, even occasionally requiring scrapping of parts. For work pieces costing several thousand dollars to produce, this can be quite costly.
On the other hand, mechanical surface finishing eliminates all these problems. Once a process is established, the achieved finishing results are very consistent and absolutely repeatable. But above all, the end finish is most likely considerably better than what can be achieved with manual finishing.
For example, even after undergoing a machine grinding process, large gear components for rolling mills made from high alloy steel still have a surface roughness of Ra = 0.4 to 0.5 £gm. At this stage the metal surface contains so-called "roughness peaks" which can disrupt the oil film, thus preventing reliable lubrication. With the ISF process (Isotropic SuperFinish), also known as chemically supported vibratory finishing, these "roughness peaks" are completely removed, resulting in a considerably larger part-on-part contact area. At the same time, the small indentations ("valleys") on the part surface, indispensable as lubricant reservoirs, are maintained. The outcome is a perfect surface finish that could never be achieved by manual grinding and polishing.
From wind power gear boxes to super large bearings
Due to the excellent surface finish and the consistent and absolutely repeatable finishing results, in recent years, vibratory finishing of large heavy parts has become quite popular and is now utilized in many industries like aerospace, bearing manufacturing, power generation, heavy equipment manufacturing and steel production. Here are a few typical examples:
Landing gear components for commercial aircraft (Photo 4)
After machining, the parts with lengths of over 2,500 mm (8 ft.) and weighing half a ton require an intensive edge breaking (radiusing) before final assembly. Mounted onto a special circular work piece holder, they are finished in a special heavy-duty tub vibrator one piece at a time.
Such rings with a diameter of up to 2,300 (79') mm can be processed in special rotary vibrators. They require a surface finish of approximately Ra = 0.5 £gm. Depending on the initial surface roughness, the required finish can be achieved in about 2 hours, compared to several days with manual smoothing and polishing.
Large ring shaped planetary gears for windmills
Weighing more than three tons, for the time being, these are the heaviest components that can be mechanically finished. They are processed in rotary vibrators with a work bowl diameter of 3,000 mm (118"). The ISF process, generally used for such applications, requires 4 - 6 hours to achieve the required surface finish of Ra < 0.75 £gm and > 0.25 £gm.
Fan wheels for large A/C units
These aluminum sand castings have a diameter of up to 900 mm (35") and an initial surface roughness of Ra = 60 £gm. After de-sanding, the raw parts are placed straight into a rotary vibrator without center dome where they are directly mounted to the work bowl (Photo 5). The resulting direct transfer of the vibratory energy to the work piece produces not only a smooth finish on the outer but also on the inner surface areas. After a processing time of about 6 hours, the surface roughness has been reduced from Ra = 60 £gm to Ra = 4.0 £gm!
A glimpse into the future
The advantages of mechanical vs. manual surface finishing of large, heavy components are so obvious that the manufacturers of vibratory finishing equipment will no doubt be pushing the envelope to develop even bigger and more powerful machines. Rotary vibrators with an OD of 4,000 mm (157") able to handle work piece weights of up to 5 tons are already on the drawing board. And it is safe to assume that even bigger and more powerful machines will eventually enter the market. It is about time that manual surface finishing finally becomes a thing of the past!
by Eugen Holzknecht
Contributing Editor MFN and
Rösler Oberflächentechnik GmbH