ASC file is a 3D model file in text format, while STL (standard mosaic language) is a commonly used triangular mesh format. Therefore, converting ASC files into STL requires three-dimensional geometric calculation and data reconstruction.

Here are some tools that can convert ASC files into STL:

1.MeshLab: This is a free and open source 3D modeling software, which supports multiple input and output formats and provides powerful editing, repairing and optimization functions. You can use MeshLab to open ASC files and export them to STL format.

2.FreeCAD: This is also a free and open source CAD software, which supports various input and output formats and provides many advanced modeling tools. You can use FreeCAD to open ASC files and export them to STL format.

3.Blender: This is another free and open source 3D modeling software. It not only supports importing/exporting many common 3D model formats (including ASC and STL), but also provides many advanced functions, such as rendering and animation.

4.Online-convert.com: This online conversion website can help you upload ASC files to its server and automatically convert them into STL or other required formats.

Please note that before any online operation, please make sure that you have backed up the original data to prevent accidental loss.