The mishna in Masechet Shabbat says:
A man may take a hammer to split nuts, a chopper to cut [a round of] pressed figs, a saw for sawing cheese, a spade to scoop dried figs, a winnowing shovel and a pitchfork to place [food] upon it for a child, a reed or a whorl to stick [food], a small needle to remove a thorn, and a sack [needle] to open a door therewith.
We are allowed to use tools on food which are not designed for food on Shabbat, like a saw to grate cheese
The topic of the mishna is Muktse.
But an additional problem may be of Tevilat Kelim if the tools are manufactured by gentiles.
Do commentaries or poskim infer from this mishna that tevila is not necessary for incidental use? The saw will remain a saw to work with wood. Two defects:
- 1 time (as a new cup which you want to use 1 time before its tevila)
- not essential (the contact with food will be for one time only and there is no reason to make tevila after this unic use).