user-edit.php makes no update to wp_usermeta(mysql-database)

hallo!
first at all it is nice to be here. please excuse my english, mine native language is german.
i m very happy with my new wordpress, but i have to change the profil-user-interface, because i need some more fields in the gui.
so i added into wp-includes\user-edit.php this:

<!-- BC Feld zum testen Eingefügt wird im Userformular angezeigt -->
<tr>
	<th><label for="maschine"><?php _e('maschine'); ?> <span class="description"><?php _e('(required)'); ?></span></label></th>
	<td><input type="text" name="maschine" id="maschine" value="<?php echo esc_attr($profileuser->maschine) ?>" class="regular-text" /></td>
</tr>

It will show up perfect in user-backend.
I also added in mysql database -> wp_usermeta a meta_key <maschine>.
and for testing i put in a userid and into meta_value some string.
No i got the problem, in the gui the string shows up okay,but
if i write something different into the field <maschine>, in the gui it will not be saved back to database.
so im just a beginner, is there a simple way to “tell” wordpress to update not just original fields like <firstname>?
where i can modify some code, that wp update also mine own field maschine?
thank you for u patience.

nice greetings from austria
glorias