Polishing Watch Hands Using Colloidal Silica
This study outlines the successful optimisation of a mirror polishing process for watch hands using colloidal silica. The primary challenge addressed was minimising surface round-off while using three different fixtures on a KemCol 15” polishing machine. The process incorporated Chem-H cloth, a Dycem-backed pressure weight, and Col-K (NC) slurry, with detailed experimentation on cycle times to determine the most effective polishing duration.
Polishing watch hands to a mirror finish requires high precision, especially when transitioning from a fine abrasive finish (e.g., 0.5 µm) to a final polish. The objective of this study was to determine an optimal polishing cycle that effectively removes micro-scratches from a prior diamond polishing step while minimising geometric distortion, particularly round-off, often introduced during final finishing.
Process Breakdown
Each of the three fixtures containing watch hands was placed concentrically within a 15” ceramic control ring. A Dycem-backed pressure weight was applied atop each fixture, providing consistent pressure and ensuring uniform contact with the polishing cloth. This setup served the dual function of loading and fixturing the parts during the process. Polishing was conducted using Col-K (NC), a non-crystallising colloidal silica suspension. To identify the optimal polishing cycle, the process was repeated across three time intervals with decremental durations. Between each test cycle, fixtures were rinsed with DI water, dried using compressed air, and inspected under a microscope for finish quality and geometry retention.
Equipment and Materials Used
- Polishing Machine: KemCol 15
- Polishing Cloth: Chem-H, 15” diameter
- Slurry: Col-K (NC), colloidal silica-based
- Control Ring: 15” ceramic
- Weighting System: Dycem-backed pressure weight
- Rinse: Deionised (DI) water
- Drying: Compressed air (air line)
- Inspection: Optical microscope
Before Processing
After Polishing
Results and Analysis
Surface Finish: All test samples showed complete removal of prior scratches from the 0.5 µm diamond polishing step. Col-K (NC) proved effective in achieving a smooth, mirror-like surface finish across all tested durations.
Round-Off and Dimensional Retention: 1 minute 30 seconds: Good finish, but observable round-off on edges of the watch hands, indicating over-polishing. 1 minute: Reduced round-off compared to 1:30, but slight edge degradation still present. 30 seconds: Excellent surface finish with minimal to no perceptible round-off, offering the best balance of finish and dimensional preservation.
This study demonstrated that Col-K (NC) is a highly effective slurry for final mirror polishing of watch hands, particularly when used with Chem-H cloth and a Dycem-backed pressure weight for controlled loading. A 30-second polishing cycle yielded the most desirable results, removing all surface scratches from the previous 0.5 µm polishing step while preserving the geometric integrity of the parts.
