A lancet is a very specialised surgical knife, it's not a standard scalpel (the French for scalpel is
le scalpel ).
It's for making very small incisions, punctures, etc. Something like that.
Hachoir can mean mincer (just type it into Google Images and you'll see lots of images of mincers) but here it means what we'd call a meat cleaver.
Edit: Oh, somebody already said that, sorry.