This article was translated from German. Despite our care, some technical terms may be imprecise. The German version is authoritative.
As one of the most traditional methods of metalworking, filing remains an indispensable tool of the trade in contemporary watch and clock restoration. Whether for the subtle correction of part of an escapement or the precise fitting of a movement component, skilled command of this instrument is the foundation of professional results. Anyone who understands the physics of material removal can develop their way of working from an intuitive action into a controlled, precise technique.
The wedge principle: the physics of cutting edges
Every single tooth of a file acts as a microscopically small wedge. This principle of metal cutting, which is also found in milling cutters and turning tools, is defined by three decisive angles: the clearance angle, the wedge angle and the rake angle.
Clearance angle: It ensures that the cutting edge can enter the material without rubbing, by creating space below the edge.
- Wedge angle: It determines the stability of the edge and how it penetrates. An acute angle favours penetration, while a more obtuse angle mechanically stabilises the cutting edge.
- Rake angle: The geometry of the rake angle determines the type of removal: while a negative orientation removes the material by scraping, a positive geometry produces an active cutting action.
Precise filing is therefore always a play on these geometries, in order to shape the surface of the workpiece exactly to the requirements of precision engineering.
Classification: choosing files by standard and need
To work efficiently in everyday workshop practice, a sound distinction between tools is essential. While workshop files – often called bench files – are used for coarser roughing work, precision files have their place in precision engineering and horology. In the absence of universal standardisation, these tools are based on the individual manufacturing specifications of each maker.
The following criteria are decisive when choosing:
- Cut: The cut number indicates the fineness; the number of cuts per centimetre also depends on the length of the file. Note that “cut 3” on a German file is not identical to the Swiss cut of the same designation.
- Cross-section and point: From flat shapes through round and square files to special forms such as the crossing or barrette file. The distinction between blunt flat and pointed flat models is also decisive for reaching awkward places.
- Material and hardness: High-quality tool steel is the standard; special models such as the “Valtitan” files achieve an exceptional tool life thanks to their high Rockwell hardness (up to 72 HRC).
The art of the clean filing movement
An accurate, flat filing result is no accident but the result of disciplined posture and hand control. For secure guidance, the right hand grips the handle while the left hand steadies the tip of the file with the thumb, index finger and middle finger. Whether you work sitting or standing, a steady, straight movement is decisive.
The secret of flat surfaces lies in using the full length of the file. Beginners often tend to use only a short section of the file, which inevitably leads to convex and therefore unusable surfaces. In addition, pressure is applied only on the forward movement. At the end of the stroke the file is stopped and only then lifted vertically off the workpiece. Anyone who lifts it off or sets it down while it is still moving rounds the edges, and the surface becomes convex.
Precision through proper separation by material
An important tip for workshop practice: it is advisable to keep files strictly sorted by material, because brass deposits clog the spaces between the teeth much more quickly than steel does. Consistent separation, or individual marking of the file faces, ensures that the teeth keep their cutting power for a long time and that you always work on the component with an optimal tool.
Mastering these craft fundamentals is the first step towards mastery. When the coordination of the filing movement merges with an understanding of the geometric demands of the material, you lay the foundation for the precision that distinguishes a high-quality movement.
Featured image: illustrative image, created with AI.
