lathe chuck face stop

[Toolmaker51]

Well Mr Davies, I think homeengineer will agree, this suits primarily lathe work in a chuck, but could help when that chuck is bolted to a mill table etc.
When duplicate parts are needed, it is common to insert a stop inside the lathe spindle, providing they [required parts] of sufficient length to touch stop and protrude from chuck jaws.
Plenty of instances occur when said stop will not reach shorter pieces. The homeengineer's tool uses face of chuck instead to register so small items contact center of spider, which can have a extension [ie a screw] to reach farther yet.
There is point shorts will not chuck reliably parallel and need aligning by other means. Generally this is when they get shorter than equivalent diameter, "under-square" so to speak.