Slide grinding
The vibratory grinding is a separating method for surface machining of primarily metallic workpieces . The workpieces to be processed are placed in bulk in a container together with grinding tools (so-called chips) and usually an additive in an aqueous solution (compound) . An oscillating or rotating movement of the working container creates a relative movement between the workpiece and the grinding body, which causes material to be removed from the workpiece, in particular at its edges. The surface appearance of the workpieces, the roughness , the material removal and the deburring performance can be varied almost as desired by the machines and tools used (grinding wheel and compound).
Vibratory grinding is specified in DIN 8589 and is referred to there as sliding chips , because not always a grinding process, but also, depending on the process, lapping or polishing takes place. After the company Walther Trowal ( " Tro mmel- Wal ther"), which first industrially used the vibratory grinding in a drum, the vibratory grinding is also subject to certain legal requirements (licensing) under the brand Trowalisieren known. The following processing goals can be achieved by vibratory finishing:
- Deburring, edge rounding
- Shining, smoothing, polishing (also known as high gloss compaction)
- Descaling, cleaning
- Matting
- grind
- Degreasing, de-oiling
Working container
The work containers are made of steel, which are lined with polyurethane or rubber for noise and abrasion protection , or plastic is used, whereby polyvinyl chloride and polypropylene predominate. In rare cases there are containers made of wood, but the wood absorbs workpiece abrasion, abrasive grains and compound and thus only allows work with the same compound and workpiece material.
Vibratory abrasives
The grinding wheels are between 1 mm and 80 mm in size and can have very different shapes. Their content of grinding or polishing mineral determines their aggressiveness, the wear and the achievable surface smoothness of the workpieces. Metallic vibratory grinding media can be made of brass, copper, aluminum, bronze or stainless steel. The advantage here is the smaller possible dimensions, which allow grooves to act even in the smallest bores. They also embed the workpiece much better in the grinding wheel mass and passively protect against impact marks. Due to their mass (factor approx. 1.5 to 3 compared to ceramic), metallic grinding tools are usually able to build up higher grinding pressures. As a result, better surface results can be achieved, but production times can also be reduced. Since metallic abrasives have no or only very weak abrasive properties, a so-called powdery or pasty abrasive, similar to abrasives in scouring milk or toothpaste, is necessary. In the process, this abrasive is placed between the workpiece and the grinding wheel, where it performs its abrasive performance.
Abrasives are made from several types of material:
- made of ceramic ; the abrasive mineral is mixed with clay, then the mass is extruded and cut. The grinding tools are then fired.
- made of plastic ; Abrasive mineral and synthetic resin are mixed and poured into molds where they harden.
- from natural products ; for example walnut shell granules for polishing tools
- made of steel, copper, bronze ; for deburring and polishing, as well as for chip removal or rust removal and cleaning of bores of bores or grooves that tap into and offset from one another and recessed workpiece contours that otherwise cannot be reached.
Additives
The compound is used to absorb and remove the abrasion of the grinding body and the removal of the workpieces. Most compounds also contain substances for corrosion protection and degreasing. Pickling work can be done using acidic compounds. In the case of flat workpieces or sheet metal parts, a release agent is also added (plastic beads <1 mm) to prevent the workpieces from sticking together ( adhesion ).
Procedure
As a rule, vibratory finishing is a discontinuous process; a lot of parts and grinding tools are brought into the machine together, and the workpieces are removed after processing. But there are also processes that allow continuous operation.
The generation of the relative movement determines the type of process. There are six variants:
Barrel finishing
In drum slide grinding, a horizontal or inclined container, the drum, is rotated around its longitudinal axis. The speed of the drum has a decisive influence on the metal removal rate and the surface quality achieved. As the speed increases, the contents are taken along the wall in the direction of rotation and thus lifted off. The solid components slide above a certain speed down the slope. Below the center of this processing zone is a neutral zone of variable thickness in which the lowest speed prevails. With increasing speed, the material removal increases and the surface quality decreases. However, the speed can only be increased up to a certain point, otherwise the solid components at the lower end of the slope form a trough that has a braking effect. Furthermore, the neutral zone increases and moves uphill. If it increases further, there is a risk that parts will rebound as they enter the trough and damage the surface of the workpieces. From a slope angle of approximately 70 °, slide grinding is no longer possible.
Vibratory slide grinding
Large vibrators cause the entire contents to vibrate and thus enable the processing of heavy or large workpieces that would remain in the mix during drum and centrifugal grinding, would get stuck or knocked open in the container. The content moves in a horizontal helical shape. The sliding occurs mainly during the negative amplitude due to the different masses between chip and workpiece. The machines are either pot-shaped for one-time fillings or screw-shaped for continuous processes.
Immersion sliding chips
This method is the only one in which single or multiple workpieces are held at the same time by means of a gripper and held in the flowing chip material. As a rule, the clamping device is located together with the compound in a pot-shaped, rotating container. The formation of a flag behind the workpiece, i.e. a chip-free zone, can be counteracted by moving the workpiece at the same time. The small number of workpieces that can be machined at the same time make this process economical only in exceptional cases. However, the short processing time and the elimination of reclamping and sorting of the workpieces have a positive effect.
Drag loops
Drag grinding systems are ideal for high-quality and damage-sensitive workpieces that have to be ground or polished without contact. A rotating carrier carousel, equipped with up to twelve rotating spindles, immerses the workpieces attached to the stationary chip mass. Carousel rotation and rotary movement of the spindles guarantee uniform workpiece processing. Variable immersion depths and speeds in the rotary movement enable up to 40 times as high material removal as with conventional vibratory grinding.
Centrifugal sliding chips
Centrifugal chipping was able to establish itself on the market mainly in two variants. Both processes can only be used to machine medium-weight workpieces.
The planetary centrifugal chipping is characterized by a rotor with several drums attached to the circumference, the direction of rotation of which is opposite to the rotor. The centripetal forces that occur reach about 15 times the normal force of gravity . The increased forces lead to a considerable saving in processing time compared to the drum sliding chip, which is similar in its operating principle. Unstable and hollow workpieces cannot be machined. Depending on the machine, the speeds of the rotor and the drums are permanently coupled to one another, or they can be set separately from one another, which enables better process optimization.
In the case of centrifugal plate chips , the mixture is located in a stationary, pot-shaped container, the concave plastic bottom of which rotates. Arch-shaped, radially arranged ribs on the bottom take the mixture with them, which rises on the container wall and then slides down to the center again, pushed inwards by the following stream. The advantages are, on the one hand, the shortened working time compared to drum sliding chips, resulting from the absence of neutral zones in the mixture, and on the other hand, unstable workpieces with sensitive surfaces can also be treated.
Pressure flow lapping
With pressure flow lapping (or flow grinding®), a grinding paste is pressed through the workpieces under pressure. This removes particles and burrs, rounds edges and significantly improves the surface quality. Surface quality in the interior of the workpieces up to Ra 0.1 is achieved through pressure flow lapping.
environment
The further treatment and disposal of the waste water produced (aqueous solution with compound, release agent and abraded materials) is becoming increasingly important. While in the past such wastewater was often not properly disposed of, attempts are now being made to recycle and reuse it in a closed cycle. The period of time over which such a cycle can be used without complete renewal of the water depends heavily on the type of vibratory finishing and the substances present in the process water (abrasion, treatment agents, oils, etc.). Common usage periods are between a week and a year. The water treatment usually takes place by sieving or centrifugation or chemical treatment ( flocculation ). These methods can also be used in combination.
Complementary procedures
Systems for vibratory grinding (especially in metalworking) are in most cases supplemented by cleaning and drying systems. The process can be largely automated through the use of conveyor belts and automatic separation of the processed workpiece and abrasive.
Application examples
With vibratory grinding, work such as deburring , edge rounding, grinding , smoothing , polishing , degreasing, descaling, rust removal , matting and compacting can be carried out.
When rounding the edges of forming tools, vibratory grinding has several advantages over manual processing:
- longer service life, therefore less downtime
- smoother surfaces or edges on the workpiece, therefore less reworking required
- Droplets can be removed from coated tools
In woodworking , after deburring and polishing (including sanding), the next step is to use the drum process to color. Cleanliness is essential here. Therefore, the dyeing and varnishing should be done in separate containers and also in separate rooms.
See also
Individual evidence
- ↑ Registration of the word mark at the German Patent and Trademark Office on December 6, 1951: Register number 635168
literature
- Helmut Prüller: Practical knowledge of vibratory finishing - guidelines for production planning and process optimization , Springer, 2nd edition, 2015.