A violin opening knife is a very thin, semi-flexible wide-bladed tool used to separate the parts of a violin. The glue used to hold such instruments together is brittle and will let go under stress. Apparently that is a feature, not a flaw.
For mine, I was too cheap and impatient to find a thin cheese knife, so I took a dollar store spackle knife and after removing the blade from the plastic handle and trimming it down, I sandwiched it between two pieces of mahogany with epoxy. When that was set up, I shaped the handle and further thinned down the blade on my belt grinder. N.B. he blade is not sharp, but just very thin to get between the joints of the instrument. The finish was boiled linseed oil.
[Below] mandatory glue up photo!
-- Ni faru ion el ligno!