SCRAPERS

SCRAPERS

 Introduction:-

Scraping is used to produce a high degree of fit between two flat or two curved surfaces particularly where the surfaces can rub together in use.

After a surface is filed or machined as accurately as possible, it can be further improved by rough scraping, after which finish scraping is employed. Accurate scraping is used only to remove minute amounts of material.

It is essential that the cutting edge of a scraper be care­fully tempered, uniform and free from defects. In addition, the cutting edge must be sharp, hard and durable. This requires careful grinding followed by honing on a suitable abrasive stone.

COMMON TYPES OF SCRAPERS:-

There are five common types of scrapers, which can be divided into two main groups.

For scraping flat surfaces there are:

    • flat scrapers with straight rectangular blades.

• Hook scrapers with rectangular blades bent at right angles.

For scraping curved surfaces there are:

-          Half-round scrapers of segmental section which may be curved slightly toward the curved surface

Three-square or three-corned scrapers of triangular section often with each of the three faces hollow ground

Bull-nose scrapers in which the rectangular blade is forged to a disc-like end. (Fig 5)

Scrapers are made of forged blades, either of high-grade tool steel or of special alloy steel and provided with handles, either of wood or of knurled steel tube. The overall length of blade and handle may range from 150 to about 500 millimeters. A good scraper blade should have a slight amount of spring.

Flat scrapers are made of flat rectangular blades, which may range in width from 10 to 35 millimeters or more. The working portion of the blade is thinned to a thickness of about 2 mm to facilitate honing. Often this tapered portion of the blade is made with both wide faces slightly convex. The end of the blade is also slightly curved. This allows a very narrow portion of the center of the curved cutting edge to scrape a very small area and it eliminates the danger of the corners scratching the work.

Hook scrapers are formed in a similar fashion to flat scrapers, but a short portion of the blade is bent at right angles. Hook scrapers are used for scraping the center portions of large flat surfaces where it is not convenient to use a flat scraper. Also hook scrapers are used for finishing work where a light touch is required.