The surviving craftsmanship in machine tool production – hand scraping

Nowadays, when everything needs to be smart, digital and as fast as possible, conventional technologies like hand scraping can seem a bit old fashioned and unnecessary. But the truth is that this combination of art, science and craftsmanship can not be replaced by ultraprecision machining techniques performed by smart CNC machines.

Even with the most precise machine tools, slight errors can occur in plane or curved surfaces. These errors appear as high spots on the surface which need to be hand scraped in order to improve surface finish quality. This is especially important in the case of mated and bolted surfaces, like in some parts of the foundation of modern metalworking machines.

When fitting two flat surfaces together, in order to make sliding elements, the purpose of hand scraping is to generate oil retention pockets that help to create a thin film of oil between the sliding surfaces. This largely improves stability, accuracy and prolongs the lifetime of the machine. Hand scraping is also crucial for bolted surfaces to provide perfect fit and it is also used to produce ornamental effects on exposed surfaces.

In order to correct errors, the part to be hand scraped is in the first step applied to the surface it needs to be fitted to. To make high spots more visible, some kind of marking material is applied to the fitting surfaces. Removing these marked spots will provide more evenly distributed bearings, effective oil retention, no rocking and more precise sliding movement.

The hand scraper will repeatedly remove bearing marks and also creates the pockets for the perfect gliding movement. Using the special scraping tools, thickness off the material removed is usually between 1 and 10 µm.

while many OEMs seek to perform the same thing using machines by machining oil pockets into flat surfaces, or simply replacing the components that usually need to be hand scraped, there is still no technology available to achieve the geometrical precision that hand scraping provides. In a world of modernization and digitalization, conventional techniques, like hand scraping, are still inevitable in achieving high machining accuracy and long-term reliability.