Calibre does a useable job of converting Word Documents to epub. It is just that it puts in a lot of things that are special to calibre. This tends to make it difficult to understand, since the special things tend to be labeled calibre1, calibre2, calibre3, etc, instead of what kind of function they are performing. It can make it hard to understand to edit it if it needs more work.
You can save Word documents as HTML which Sigil imports for further editing. Toxaris has done a Word macro that improves the HTML that Word outputs. It is worth looking into.
If you are looking for something automatic, you are likely to be disappointed, especially if the book is heavily formatted with columns, drop caps, pictures, etc. For heavily formatted books, it is often best to output it as PDF, which allows precise formatting.
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