Inserter

A machine count without inserters omits the transfer layer that makes a line work; compare arm speed/reach for high-rate recipes.

When it fits: Add it at every required handoff, then upgrade only where transfer rate is measured as the bottleneck.

The tradeoff: One source recipe run makes 1 and lists 1 Electronic circuit, 1 Iron gear wheel, 1 Iron plate at Assembling machine 1 / Assembling machine 2 / Assembling machine 3; 1 recipe variant(s) for this output are grouped together. Verify access where the pack is incomplete. Each junction consumes an inserter and power; a belt can still back up if the arm cannot clear its output.