As a follow-up to one of my recent posts, I received this question…
Regarding the screw – Most screw manufacturers will recommend that you use an outside micrometer. Since the flights do not match up, you should lay a block gauge block across a couple flights on one side of the screw and deduct the thickness of that block from the overall measurement. For consistency, you should note the distance, from the end of the screw, that the micrometer contacts the screw. As a result you should have a table with lengths and corresponding diameters.
Regarding the barrel – Using an inside micrometer, most companies will follow a similar procedure as with the screw. The diameters should be take at specific distances down the barrel and be listed in a table of lengths and diameters.
You should avoid using a surface plate with a height gauge to measure the screw wear… This will often mask the areas of the screw with high wear.