Thingomatic uses metric hardware for all its fasteners. Metric bolts are described in terms of the diameter of their shafts. For instance, an "M3" bolt is one with a shaft that is 3mm in diameter. Likewise, an M8 bolt has an 8mm shaft diameter. M3 nuts are nuts that fit M3 bolts.
The Thingomatic uses bolts of several lengths. They're described by the length of the bolt excluding the head. For example, a 16mm bolt is 16mm from the tip of the screw to the underside of the head. Sometimes you'll see a bolt described, for example, as M3x16. That means an M3 bolt with a length of 16mm, excluding the head.
Almost every one of the nuts and bolts you'll use will be M3x16 - the Thingomatic comes with 200 M3x16 nuts and bolts, but you'll end up with quite a few left over.
Bolt Together Action: The Nuts and Bolts
Nuts and bolts hold the MakerBot together. You put the nut in the t-slot, put the bolt in the hole and twist it with your fingers. Once you've got it hand tight, give it a small twist with the allen key to make sure it stays tight.
The Thingomatic uses two main lengths of M3 bolt. The 16mm bolts are used to hold the wooden parts together; the 10mm M3 bolts are used to fasten the NEMA 17 stepper motors to the bot. Be sure not to use the 16mm bolts to attach the motors— they can enter the motor housings and cause them to jam!
M3 nuts are not manufactured to very tight specs, so some may be slightly smaller or larger than others. Additionally, wood always has a little variability. If you're having trouble fitting a nut into a t-slot, try rotating the nut a sixth of a turn, or inserting the nut from the other side of the t-slot and see if you have an easier time of it. If you're really stuck, try lightly sanding or filing the slot.
Sometimes the t-slot is actually a touch too wide, and won't hold the bolt in place until you're ready to screw in the bolt. In these cases, you can add a dot of white glue or super glue to keep it from falling out until you're ready to screw in the corresponding bolt.
MakerBots vibrate, so you're going to want to occasionally check the nuts and bolts and tighten them down if they loosen up. Most bolts, like those in the outer frame, may never need tightening; others, like those that hold the build platform down, may need to be checked every few months or so.
Slots and tabs
The slots and tabs in the frame should match up well. If you've screwed a couple of parts together, and are having trouble fitting the tabs on a third part that connects to both, try loosening the bolts first.
Next Step: About Timing Belts