How to force to center the table captions?

I recommend to use the caption package. It provides features for justification, also for centering, such as

\usepackage[justification=centering]{caption}

It offers many features, just have a look at its comprehensive documentation.

Note: If you do not want to change the justification for the whole document you can use \captionsetup{} within a particular figure and it will only change the captions within that environment. Other than centering you may also want to try centerlast.

\usepackage{caption}
...
\begin{figure}
   ...
   \captionsetup{justification=centering}
   \caption{. . . }
\end{figure}