Hand scraper

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Three different engineering hand scrapers

For the woodworking handtool, see card scraper .

A hand scraper is a single-edged tool used to scrape metal from a surface. This may be required where a surface needs to be trued, corrected for fit to a mating part, needs to retain oil (usually on a freshly ground surface), or even to give a decorative finish.

Surface plates were traditionally made by scraping. Three raw cast surface plates, a flat scraper (as pictured at the top of the image) and a quantity of bearing blue were all that was required in the way of tools. Methodology, skill, patience and tenacity was to be supplied by the tradesman.

The scraper in the center of the image is a three corner scraper and is typically used to deburr holes or the internal surface of bush type bearings. Bushes are typically made from bronze or a white metal.

The scaper pictured at the bottom is a curved scraper. It has a slight curve in its profile and is also suitable for bush bearings, typically the longer ones.

The task of scraping has been the bane of most engineering apprentices, however its versatility and appropriateness far outweighs the hard work it requires, or is perceived to require. Its effective use requires skill and concentration. An often overlooked advantage of scraping is the ability to take the tool to the workpiece. When the workpiece weighs several tons and towers head and shoulders over the worker, the apprentice's solution of chucking it on the mill is impractical, if not outright impossible. A skilled craftsman can wield a scraper and turn out work that is the envy of his peers; it just takes more time than the usual methods.

The man who scrapes is called a "hand". It is done by using a precision surface such as a surface plate or a straight edge as a standard (a straight edge in this context is not a ruler it is a miniature surface plate of extreme accuracy). A professional scraping tool will be a special made tool, not an old file. The standard is coated with a very thin coating of some material such as Prussian blue. The work piece and standard are touched together by gravity alone and the high spots on the work piece will be colored by the dye on the standard. These high spots are scraped off and the process repeated until there is an even spread of high spots which total about 60% or more of the surface area. If desired the surface can then be “Frosted”. A surface prepared in this way is superior to any machining or grinding operation, although lapping can equal it. Grinding and machining stresses the metal thermally and mechanically, scraping and lapping do not.

Although well done scraping provides an extra measure of accuracy, for most applications it does not matter. A ground lathe bed or milling machine ways are perfectly adequate for almost all types of work.

Appearance of a slideway scraped for improved oil retention

With precision ground surfaces, any oil film applied to the surface will lack the means to adhere to the surface, especially between two mating parts of exceptional finish. The oil film will be swept away leaving nothing but bare metal and the risk of seizure. Carefully scraping the surface will leave the original high quality surface intact, but provide many shallow depressions where the oil film can maintain its depth and surface tension. When a scraping is used for this purpose it is more accurately called "Frosting", "Spotting" or "Flaking" as opposed to actually fully scraping an accurate surface. Typically a scraped surface is scraped to highly accurate flatness and then "frosting" is applied over it for oil retention. The advantage of this oil retention "frosting" is debatable.

Scraping can leave an attractive pattern on the surface and was in fact, the physical evidence of a craftsmen built machine. It is therefore still treated as evidence of craftsmanship by the astute buyer, or the more market savvy machine builders. Even machines that have been ground and not scraped often have frosting applied. Presumably this is at least partly to fool the unsavvy buyer into thinking the machine was truly hand scraped. That this frosting is applied not only to way surfaces but also to table tops and other non bearing areas shows that it is ornamental.