The type of screws that you are talking about are called machine screws. The threads are uniformally spaced and close together. There is no taper to the shank as in a wood screw or thread cutting ability as in a sheet metal screw. These are meant to be used with a nut or in a tapped hole.
The convention is to identify the screw as follows: Length X diameter-thread pitch-head type. For example: 1/2”X#5-40 cap screw where 1/2” is the length under the head, #5 is the wire gauge size (1/8” diameter), 40 is the pitch in threads per inch, and cap screw is a tall head with parallel sides and usually a hex (Allen) recess. The description can also include the material and/or the finish.
In the metric system, the description is similar: 5mmX2.5-0.45 button head/TORX recess. 5mm is the length under the head, 2.5 is the major diameter in mm (outside the threads), 0.45 is the distance between threads in mm, and button head is the head shape with a TORX (star) recess. You can also add that the screw is stainless or Zinc plated etc.
Imperial threads are labeled in threads per inch. Metric threads are reciprocal ( mm/thread). One system measure threads per length, the other length per thread. Wire gauge sizes in the US system is similar to shotgun gauges ie. 16 gauge is 16 Lead balls per pound. #5 wire gauge is 1 pound of steel wire for 5 feet of length. The higher the gauge number the smaller the diameter and the more feet per pound.
When making pivot parts, you do not want the blade to turn on a threaded post. You can put a bushing over the pivot screw, or you can use a T-nut. T-nuts that are slabbed (D cross section) let you adjust the blade squeeze with one screw. Bushings are squeezed between the handle slabs, so the bushing thickness is your blade squeeze adjustment.
If you have thread charts, thread gauges (combs), and a caliper, you can order fasteners online from industrial supply houses or knife supply outlets.
