On 102b, the Gemoro (also see Rashi on Mishna) says "V'Im Toka, Chayav Chatos".
It seems that if tools are used to fix an item, there is an Issur D'Oraisa. Why should it make a difference if tools are used?
Meir Eliezer Bergman
It is not that with tools, putting the parts together is intrinsically a Melachah, and without tools, it is not.
The reason that tools create a Chiyuv d'Oraysa is because they make the repair permanent, for which one is Chayav even regarding a K'li where the part is normally fixed temporarily (i.e. loosely).
Where no tools are used, the repair is generally temporary, in which case, where the part is supposed to be fixed loosely, it is permitted.
Even fixing it loosely is forbidden however, where the K'li is normally put together tightly, in case one comes to use tools and fix it tightly.
be'Virchas Kol Tuv