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