There are a total of 23 Machine Chests. Machine Key is dropped by Key Orbling in Wormhole - Human Realm.
Machine Chest #1
Item: Nanomic Blade
Location: Secret Base, near the east entrance door to the path to wormhole.
Machine Chest #2
Item: Sealed Grimoire
Location: Vibra - Town Center, Leo's House, Attic.
Machine Chest #3
Item: Particle Gun
Location: Secret Base, inside the Guest Room next to the Throne Room.
Machine Chest #4
Item: Blast Ring
Location: Secret Base, inside the Guest Room next to the Throne Room.
Machine Chest #5
Item: Wizard Belt
Location: Machine City - Security Gate Area, just outside the Secret Base, it's hidden behind the wall.
Machine Chest #6
Item: Ruby XL
Location: Midi Toy Box, hidden room.
Machine Chest #7
Item: Judgement
Location: Midi Toy Box, hidden room.
Machine Chest #8
Item: Thanatos Globe
Location: Ancient Mountain Path - Abandoned Clifftop Huts.
Machine Chest #9
Item: Trinity Light
Location: Vibra - Town Center, Leo's House, in Lab Room.
Machine Chest #10
Item: Valiant Belt
Location: Maxi Toy Box, Path to the Maxi Toy Box. Just Teleport to "Maxi Toy Box - Underground - Cavern".
Machine Chest #11
Item: Ori Mail
Location: Maxi Toy Box, Path to the Maxi Toy Box. Just Teleport to "Maxi Toy Box - Underground - Cavern".
Machine Chest #12
Item: Fool's Staff
Location: Maxi Toy Box, Path to the Maxi Toy Box. Just Teleport to "Maxi Toy Box - Underground - Cavern".
Machine Chest #13
Item: Cyclone Ring
Location: Maxi Toy Box, Path to the Maxi Toy Box. Just Teleport to "Maxi Toy Box - Underground - Cavern".
Machine Chest #14
Item: Solid Knuckles
Location: Thaumatech Factory - Rooftop.
Machine Chest #15
Item: Quark Edge 2.0
Location: Thaumatech Factory - Rooftop.
Machine Chest #16
Item: Rune Blade
Location: Thaumatech Factory - Rooftop.
Machine Chest #17
Item: Morganite L
Location: Thaumatech Factory - Entrance Floor.
Machine Chest #18
Item: Lantern of Corruption
Location: Thaumatech Factory - Entrance Floor
Machine Chest #19
Item: Pearl XL
Location: Thaumatech Factory - Top Secret Storage, just before the Omega.
Machine Chest #20
Item: Onyx XL
Location: Thaumatech Factory - Top Secret Storage, just before the Omega.
Machine Chest #21
Item: Diamond XL
Location: Thaumatech Factory - Top Secret Storage, just before the Omega.
Machine Chest #22
Item: SP Capsule x2
Location: Thaumatech Factory - Top Secret Storage, just before the Omega.
Machine Chest #23
Item: Opal XL
Location: Thaumatech Factory - Top Secret Storage, just before the Omega.