Combining drills?

[Streetboat]

Okay, sooooo it doesn't seem to work. Here's how I have it set up:

I have the 6 items, 3 drill pieces per drill. Each one is labeled according to its part, like 'drill_bit' and 'drill_bit_plastic', 'drill_crank' and 'drill_crank_plastic', and 'drill_part' and 'drill_part_plastic'. Each time one is picked up, a GlobalVarInt is added, adding up to 3 total, according to which type is picked up. And in my inventory.hps, I have an AddCombineCallback (or whatever it's called, I'm not at my computer right now) for each part, but it doesn't register. It just says 'combination doesn't work' when I combine them in the inventory. Hulp? I have no idea how Frictional made it work.