The vibratory grinding (trowalising) process
In industry jargon, tumbling is referred to as vibratory grinding or trowalising, a cutting procedure which is mainly used for the surface machining of metal workpieces. Poured as bulk material into a tank, these metal bodies are subjected to material abrasion caused by abrasive chips. During rotational or vibrational movements of the tank, the chips slide along the workpieces. In most cases, a water-based compound is added between the workpieces and the chips. This compound modifies the abrasive effect in such a way that it mainly acts on the workpiece edges.
The surface and the roughness of a ground workpiece strongly depend on the compound and chips serving as tools. They mainly determine the degree of abrasion of the workpiece material, which is referred to as the “deburring power”.
Furthermore, the machines used, and the types of movement applied, influence the grinding result, which can thus be strongly modified. Tumbling in accordance with the German DIN 8589 standard even enables much greater variability because this standard includes lapping and polishing in addition to the frequently used grinding procedure.
In industry jargon, the older procedure referred to as vibratory grinding is also known as Trowalising® (named after the Walther Trowal company (“Trommel-Walther” – “Barrel Walther”)): In 1931, Carl-Kurt Walther founded the first company to rely on the technology of “vibratory grinding in a barrel”. Based on in-depth research, this successful company consistently developed versatile application options of its original technology, enhancements of which are still being used for various purposes today.
These include changes with regard to the cleanliness, appearance, and shape of a workpiece. The shape of the object machined is modified by means of edge radiusing, grinding and deburring. Metal bodies can also be glossed and smoothed within the scope of high gloss compaction, or given a rather mat appearance by subjecting them to the corresponding machining process. Furthermore, descaling, as well as the removal of dirt particles, greases, and oils, provide for a clean workpiece.
These and other work steps cannot be manually executed on the same workpiece at the same time, which clearly demonstrates the benefits of these machine processing technologies. In addition, the large quantities produced by vibratory grinding or trowalising could never be manufactured as efficiently through manual machining. During trowalising, simple physical processes, as well as the intelligent pairing of workpieces and suitable vibratory grinding chips, make it possible to achieve excellent results in the shortest time possible. Extremely large quantities, especially of small- and medium-sized workpieces in large tanks, result in high resource efficiency.
Lined work tanks made from robust steel are used for all kinds of applications. The polyurethane or rubber lining of the tanks protects the working environment against noise emissions while at the same time ensuring that the workpieces do not suffer unintentional, random abrasion on the tank walls. As an alternative to steel, synthetic materials can be used to enclose the active space – usually polypropylene or polyvinyl chloride as a barrel wall material.
Wooden tanks are still being used in rare situations which, however, result in the abrasion of the workpiece. In addition, wood absorbs the material abraded, and the chip abrasion, as well as the compound. In particular, wood releases these substances back into the tank’s interior. For this reason, wooden tanks can only be used for the machining of the same material with the same tools (chips, compound).
During vibratory grinding or trowalising, these chips occur in vastly different shapes, independent of the tank material. In most cases, grinding or polishing minerals are embedded in these geometric bodies made from various substances in order to abrade material from the workpiece. As carriers of these minerals, chips can have various diameters in the range of one millimetre to eight centimetres.
The concentration and the type of abrading minerals in these variously shaped abrasive chips determine the material aggressiveness of the chips. The definitive intensity of surface wear on the workpiece can thus be controlled in order to achieve the desired level of smoothness.
Vibratory grinding chips are used in various versions on numerous workpiece types in order to fulfil the requirements of the most diverse applications. Chips can thus be manufactured from ceramics by combining clay with the corresponding grinding mineral. This mix runs through an extrusion line; then the product is cut into a large number of identically shaped chips. Finally, once they have been fired, these abrasive chips serve as hardened tools and are reusable many times over.
In contrast to ceramics, synthetic resin can be shaped in moulds after blending with grinding minerals. Once hardened, the moulded chips are immediately ready for vibratory grinding or trowalising.
In contrast to chips made from synthetic materials, abrasive chips made from steel also serve as a grinding additive. Following simple manufacture, abrasive chips made from steel remove rust from e.g. drill holes of various types during vibratory grinding or trowalising, but they are also suitable for other cleaning tasks. Intersecting holes, for example, can be efficiently deburred with steel chips, as is especially the case for drill holes which overlap each other.
As an alternative to manufactured substances, certain natural products can also serve as constituents of abrasive chips. Robust granulates made from resilient walnut shells, for example, facilitate the effective polishing of numerous types of tool surfaces.
The low workpiece abrasion during polishing, as well as particularly large burrs, must be removed while vibratory grinding is ongoing. This removal is ensured by the compound which also removes the occasionally high-volume chip abrasion from the work process.
Most mixtures contain additives which protect the workpieces against corrosion, whereas the compound often also degreases the bulk material. Even pickling tasks using acid compounds can be executed at the same time as trowalising.
In addition to chemical effects of this kind, compounds can support the vibratory grinding process mechanically, if required. For example: Compounds include separating agents in order to reduce adhesive forces between flat workpieces. Plastic beads in the submillimetre range, for example, keep metal sheets at a certain distance apart. This ensures that, during vibratory grinding, workpieces do not form complexes consisting of internal surfaces which would thus escape the grinding process. Furthermore, the time-consuming separation of workpieces attached to each other – as well as the risk of deforming or damaging them – is thus a thing of the past.
All in all, vibratory grinding or trowalising is a particularly sophisticated type of procedure which allows the highly efficient, versatile surface treatment of numerous workpiece types.