At what angle should a belt sander be held when grinding a mirror for the first pass?

When grinding a mirror for the first pass using a belt sander, holding the sander at a 15-degree angle is ideal. This angle provides a balance between effective material removal and minimizing the risk of damaging the surface. Angling the sander slightly allows for better control and a more even grind, which is critical when working with delicate materials like glass. The slight tilt helps to ensure that the initial pass does not create deep grooves or uneven surfaces, facilitating a smoother and more refined finish in subsequent passes.

Using a much steeper angle, such as 30 or 45 degrees, can lead to excessive pressure on the surface, potentially causing chipping or unwanted distortion of the mirror. Additionally, holding the sander at a zero-degree angle is too flat and may not provide sufficient material removal or effective contact with the surface. Therefore, 15 degrees is the recommended angle for starting the grinding process.