Scraping (procedure)

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Scraping is a manufacturing process carried out manually or by machine . In the classification of manufacturing processes according to DIN 8580, it counts for machining with a geometrically defined cutting edge and is used, in particular, in machine tool construction for the production of guides, fitting surfaces and for the production of oil pockets in sliding surfaces. In the VDI guideline 3220 it is defined as: "Cutting with preferably single-edged, not constantly in engagement, tool moving in one main direction to improve the shape, dimensions, position and surface of pre-machined workpieces." Accordingly, scraping is only possible after other methods such as used for turning or milling . As these procedures have become more accurate, they have in many cases made scraping unnecessary.

Procedure

The aim of the process is to remove the areas raised in relation to the ideal plane (due to waviness , warpage, etc.) of a workpiece. Since all the load-bearing points of a surface, which are evenly distributed over the entire area, are located on one level, the best possible even distribution of the load can be achieved. The position of the raised areas and the quality of the surface are determined by spotting with spotting paint on a spotting plate or with a spotting ruler. The color remains on the raised surfaces or in the depressions, depending on the method used. After the surface has been leveled, burrs are smoothed with fine sharpening stones . Flatness tolerances of 0.001 mm can be achieved.

By changing the direction of the scraping tool in a crosswise manner, a typical scraping pattern is created, which is partly produced solely for its optical effect ("pattern scraping"). At the slow disappearance of the pattern due to wear can wear to be estimated.

By scraping, indentations are deliberately created in the work surface, which, as so-called oil pockets, enable the lubricating film to be maintained for a longer period of time, including in hydrodynamically lubricated plain bearings .

The importance of scraping is judged differently. It is often used in machine tool construction and the used machine sector (" retrofit "). New machines are no longer manufactured with scraping because of more efficient and precise alternative processes.

If the quality of a surface is determined with a dial gauge , it is also possible to tilt the surface to be processed. One then speaks of scraping for angularity.

variants

A distinction is made between hand and machine scraping. According to the surface generated, a distinction is made between face scrapers for producing flat (flat) surfaces and round scrapers for round surfaces, especially on the circumference of cylindrical parts. A distinction is also made between shaving for rough machining (so-called roughing ) and scraping for fine machining ( finishing ).

Tools

Hand scraper

As with all cutting tools in charge of the material removal has cutting edge a wedge shape. In order to prevent the tool cutting edge from penetrating too deeply into the material, the sum of clearance angle and wedge angle is kept greater than 90 °. This creates a negative rake angle . When scraping flat surfaces, a flat scraper is used first, and a scraper is used for scraping. For curved surfaces, e.g. B. with plain bearings, one uses triangular scraper and spoon scraper.

Hand scraper

The hand scraper is used to process mostly flat, in exceptional cases also cylindrical surfaces. The hand scraper is in its original form a very old tool, which was used to work on wooden trunks as early as the Stone Age.

In the course of industrialization in the 19th century, the hand scraper was then often used to process machine beds, spindle sleeves or sliding surfaces.

The hand scraper usually consists of a wooden handle, a handle and a clamp to hold the replaceable scraper plate. The scraper plate is made of high-performance high-speed steel or carbide and must be sharpened regularly with special fine- grain grinding tools.

Electric scraper

Electric scrapers shorten the processing time. An electric motor generates an oscillating stroke movement. The stroke length (amplitude) and the stroke speed are usually adjustable. However, electric scrapers are much heavier and cause strong vibrations.

See also

Other processes for fine machining are filing , grinding , honing and lapping .

Scraping as a cleaning method

In contrast to the above-discussed scraping as a machining production process , the material of the workpiece should not be removed when scraping to clean surfaces . The blade of the scraper can therefore also consist of a softer material than the workpiece to be cleaned. In this case, the rake angle is no longer important, so that hard materials such as glass and glass ceramic can also be cleaned with sharp blades , ie with a very small wedge angle .

In many cases, simple plasterer or painter's spatulas or scraping tools with the option of clamping disposable blades are used to clean surfaces from coatings or contaminants .

There are a variety of hand and power tools that can be fitted with various types of blades for removing paint from wood. In order not to inadvertently penetrate the wood surface with the scraper blade when removing coatings from softer types of wood, the blades are preferably used pulling (the so-called scraper is a tool that is also used for cutting if the cutting edge is appropriately prepared).

To make it easier to remove coatings, they are often softened by chemical paint strippers or by heating before they are scraped off .

Individual evidence

  1. ↑ Uwe Heisel, Fritz Klocke , Eckart Uhlmann , Günter Spur : Handbuch Spanen , Hanser, 2014, p. 521.
  2. ↑ DIN 8589, Beuth-Verlag
  3. ↑ Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen , Hanser, 2014, p. 522.
  4. ↑ Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen , Hanser, 2014, p. 521.
  5. ↑ Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen , Hanser, 2014, p. 521.
  6. ↑ Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen , Hanser, 2014, p. 522.