All About Tubbing
Q. Can you explain tubbing and when to use it? A.W.
A. On Google you will find that you can get a ticket at Mammoth Lake for tubbing in your birthday suit.
Tubbing of the mass finishing sort, however, is well known in the jewelry industry, and in a few other fields. It involves use of a hexagonal or octagonal tumbling barrel constructed of perforated plate. The barrel is usually not lined. The tumbler is operated completely submerged in a tank of solution. Tumbling speeds and barrel loading are about the same is in regular tumbling operations.
Underwater tumbling has several advantages: Depending on their specific gravity and surface area, delicate parts are partially buoyed up by the water. Particulate matter removed from the parts or media quickly drops to the bottom. Oils are coalesced or otherwise separated while the process is ongoing.
Tubbing is an excellent method for delicate parts and to prevent part-on-part damage. If contaminants might be impregnated into the surface by conventional tumbling or other mass finishing methods, this problem is solved. If fine finishing is desired, and if that finish would be damaged by either part-on-part impingement or scratching by stray metal or other particles, this process may be the best choice.
Removal of oil and grease is sometimes done in mass finishing; tubbing offers several advantages over other processes, and even over conventional parts washers, for this purpose.
I am familiar with a degreasing operation that uses a series of six tanks with an automated system to move barrels full of parts from one tank to the next and tumble them in each tank. The first five tanks in this system are solution and rinse tanks; the sixth is a dryer tank. Each tank has an individual temperature control, adjustable dwell timer, and solution concentration system. The first three tanks have oil coalescing systems.
There is no media, so media separation is not necessary. Barrel loading and unloading stations are off-line and manually operated because the parts are delicate and automated handling is not satisfactory. The last tank, the dryer tank, blows a large volume of heated air over the rotating barrel, drying and draining the parts. Energy-efficient regenerative blowers heat the air. Removed oil is recycled.
Cleaning solution is a water-based, biodegradable degreasing compound developed specifically for this process. There are no solvents or harsh chemicals such as hydroxides or silicates. The first tank uses the compound at 2% concentration with a pH under 8.5. The second tank uses a 1% solution, and the third, fourth, and fifth tanks are fresh water rinses. The first two tanks are operated at 120°F and the rinse tanks at ambient temperature. With the oils removed, the effluent is treated as non-hazardous waste.
Tubbing is also used for deburring and burnishing. In this application it can be done using media or part-on-part. The media can be ceramic or metallic; stainless steel balls are a popular choice for jewelry finishing. The advantage of media is that parts can be burnished or deburred in areas not accessible to part-on-part tumbling. Even when media is used, there is very little damage to delicate surfaces because of the cushioning and floating effect of being submerged. Burnishing requires a very clean surface, and tubbing can be the cleanest of the wet mass finishing processes.
Tubbing has many advantages, but they are offset by factors that have considerably reduced its popularity. When tubbing is used for deburring, cycle times can be excessively long. A deburring job that might take 20 min in a conventional bowl vibrator can take 2–4 hr in a tubbing operation. The capital investment and floor space required for tubbing is greater than that required for the same production using more conventional mass finishing processes.
Surface finish types for commercially supplied stainless steel sheet are detailed in various standards. ASTM A480-12 and EN10088-2 are two; BS 1449-2 (1983) is still available, although no longer active. These standards are very similar in that they define eight grades of surface finish for stainless steel. Grade 7 is “buff polished,” while the highest polish—the so-called mirror polish—is designated Grade 8
This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.
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