Simple Scraper
A scraper is a hand cutting tool used to remove small amounts of material from metal surfaces. Scraping is mainly done to produce smooth and accurate bearing surfaces. It is commonly used in fitting shops for finishing surfaces that require high precision.
Types of Scrapers
Flat Scraper
The flat scraper has a straight cutting edge and is used for scraping flat surfaces.
Uses
- Finishing flat machine surfaces.
- Removing high spots from flat metal surfaces.
- Improving accuracy of bearing surfaces.
Half Round Scraper
The half round scraper has a semicircular cross-section and is used for scraping curved or concave surfaces.
Uses
- Scraping curved surfaces.
- Finishing internal cylindrical surfaces.
- Removing high spots from curved bearings.
Triangular Scraper
The triangular scraper has three cutting edges and is used for scraping corners and narrow grooves.
Uses
- Scraping internal corners.
- Cleaning narrow slots.
- Finishing grooves and keyways.
Hook Scraper
The hook scraper has a curved cutting edge and is specially designed for scraping internal surfaces such as bearings.
Uses
- Scraping internal bearing surfaces.
- Finishing curved holes.
- Removing small irregularities from internal surfaces.
Blue Matching of Scraped Surfaces
Blue matching is a method used to check the accuracy of scraped surfaces. A thin layer of marking blue (Prussian blue) is applied on a reference surface. When the workpiece is placed against this surface, the high spots on the workpiece become visible due to the transfer of blue color.
Flat Bearing Surfaces
For flat surfaces, marking blue is applied on a surface plate or master plate. The workpiece is rubbed against the plate and the blue marks indicate the high spots. These spots are removed by scraping until the surface becomes uniformly smooth.
Curved Bearing Surfaces
For curved surfaces, blue matching is performed using a reference curved surface such as a shaft or cylindrical master. The high spots are marked with blue and removed by scraping.
Testing Scraped Surfaces
Testing scraped surfaces is important to ensure proper contact and smoothness. Normally a master surface plate is used for testing, but sometimes testing is done without a master plate.
Testing Without a Master Plate
When a master plate is not available, the following methods can be used:
- Visual Inspection: Check the uniformity of scraping marks.
- Using Straight Edge: A precision straight edge can be used to check flatness.
- Light Gap Method: Hold the surface against light to detect uneven areas.
- Contact Method: Rub two surfaces together and observe the contact points.
Importance of Scraping and Testing
- Ensures accurate fitting of machine parts.
- Improves bearing contact surfaces.
- Reduces friction and wear.
- Increases machine efficiency and life.
Conclusion
Scrapers are important finishing tools used in mechanical workshops to produce precise surfaces. Different types of scrapers such as flat, half round, triangular, and hook scrapers are used depending on the shape of the surface. Blue matching and proper testing methods help ensure that the scraped surfaces are accurate and suitable for machine operations.