| A hand scraper is a single-edged tool used to | | | | together by gravity alone and the high spots |
| scrape metal from a surface. This may be | | | | on the work piece will be colored by the dye |
| required where a surface needs to be trued, | | | | on the standard. These high spots are scraped |
| corrected for fit to a mating part, needs to | | | | off and the process repeated until there is |
| retain oil (usually on a freshly ground | | | | an even spread of high spots which total |
| surface), or even to give a decorative | | | | about 60% or more of the surface area. If |
| finish. | | | | desired the surface can then be |
| | | | “Frostedâ€. A surface prepared in |
| Surface plates were traditionally made by | | | | this way is superior to any machining or |
| scraping. Three raw cast surface plates, a | | | | grinding operation, although lapping can |
| flat scraper (as pictured at the top of the | | | | equal it. Grinding and machining stresses the |
| image) and a quantity of bearing blue were | | | | metal thermally and mechanically, scraping |
| all that was required in the way of tools. | | | | and lapping do not. |
| Methodology, skill, patience and tenacity was | | | | |
| to be supplied by the tradesman. | | | | Although well done scraping provides an extra |
| | | | measure of accuracy, for most applications it |
| The scraper in the center of the image is a | | | | does not matter. A ground lathe bed or |
| three corner scraper and is typically used to | | | | milling machine ways are perfectly adequate |
| deburr holes or the internal surface of bush | | | | for almost all types of work. |
| type bearings. Bushes are typically made from | | | | |
| bronze or a white metal. | | | | With precision ground surfaces, any oil film |
| | | | applied to the surface will lack the means to |
| The scaper pictured at the bottom is a curved | | | | adhere to the surface, especially between two |
| scraper. It has a slight curve in its profile | | | | mating parts of exceptional finish. The oil |
| and is also suitable for bush bearings, | | | | film will be swept away leaving nothing but |
| typically the longer ones. | | | | bare metal and the risk of seizure. Carefully |
| | | | scraping the surface will leave the original |
| The task of scraping has been the bane of | | | | high quality surface intact, but provide many |
| most engineering apprentices, however its | | | | shallow depressions where the oil film can |
| versatility and appropriateness far outweighs | | | | maintain its depth and surface tension. When |
| the hard work it requires, or is perceived to | | | | a scraping is used for this purpose it is |
| require. Its effective use requires skill and | | | | more accurately called "Frosting", "Spotting" |
| concentration. An often overlooked advantage | | | | or "Flaking" as opposed to actually fully |
| of scraping is the ability to take the tool | | | | scraping an accurate surface. Typically a |
| to the workpiece. When the workpiece weighs | | | | scraped surface is scraped to highly accurate |
| several tons and towers head and shoulders | | | | flatness and then "frosting" is applied over |
| over the worker, the apprentice's solution of | | | | it for oil retention. The advantage of this |
| chucking it on the mill is impractical, if | | | | oil retention "frosting" is debatable. |
| not outright impossible. A skilled craftsman | | | | |
| can wield a scraper and turn out work that is | | | | Scraping can leave an attractive pattern on |
| the envy of his peers; it just takes more | | | | the surface and was in fact, the physical |
| time than the usual methods. | | | | evidence of a craftsmen built machine. It is |
| | | | therefore still treated as evidence of |
| The man who scrapes is called a "hand". It is | | | | craftsmanship by the astute buyer, or the |
| done by using a precision surface such as a | | | | more market savvy machine builders. Even |
| surface plate or a straight edge as a | | | | machines that have been ground and not |
| standard (a straight edge in this context is | | | | scraped often have frosting applied. |
| not a ruler it is a miniature surface plate | | | | Presumably this is at least partly to fool |
| of extreme accuracy). A professional scraping | | | | the unsavvy buyer into thinking the machine |
| tool will be a special made tool, not an old | | | | was truly hand scraped. That this frosting is |
| file. The standard is coated with a very thin | | | | applied not only to way surfaces but also to |
| coating of some material such as Prussian | | | | table tops and other non bearing areas shows |
| blue. The work piece and standard are touched | | | | that it is ornamental. |