A return loop on a line: a second pass for the other side or rework

Built in A field or setting does it directly.

When

They leave the line at its end or at a buffer, are taken back (by hand, by a crane, by a truck or by a return conveyor) and enter the line again at an earlier station.

How

• The product stays as it is. Each side's work is its own part with its own pose (top A, bottom B). Rework is rework_probability and rework_from on the check, as in Inspection, sampling and rework loops.
• The station that sends units back gets return_to: the id of the machine whose station receives them (Hardware → Returns units to). Here the inspection sends boards back to paste.
• The move back is not a station. Whoever moves it does it: return_transfer (Hardware → Moved back by) is free by default (a truck, a worker or a cart: the project's carriers, Project settings → Free moves), or routed_transport for a return conveyor. return_move_s ± return_move_variation_s (Move back) states its time instead (secure, move, place). Then the sending station is free at once.
• A board due to be flipped (its other side next) is flipped on the way: return_flip_s (Flip on the way), else the flip task's own time. The flip needs no machine. A flipping machine of your own on the loop still flips, and it may be the one that sends back.
• On a drawn line, link the sending station back to its return_to station, and on to the rest of the line. The canvas draws that link as the way back: a dashed lane over the line, labelled with the move time ("Way back · 5–10 min"), never a conveyor.
• The sender may stand beside the line: a carriage table on the line takes a unit across to it (a buffer rack, a check place) and, when it does not go back, down onto the line again. Link the table to the station and the station back to the table; a route may pass the table twice in a row, and a unit with nothing to do there goes straight on.
• A unit goes round as often as its work needs (the other side, each rework round), and its route takes the fewest rounds. Each round passes a station once. How often a check may send a unit back is its own rework_max (Product table → Rework max, default 10). A unit is reported (RETURN_LOOP_UNROUTABLE) only when no route can do its steps in order.
• A new unit enters only while that leaves one place in the loop free, so a loop that is kept full cannot lock itself.
• Build layout builds the loop itself. The stations after the flip fold onto the same machines before it, if each shared station still meets the takt with both passes, and the flip station it would have added is left out. Otherwise it keeps a second set of stations and says why (RETURN_LOOP_NOT_ON_CANVAS). It never adds a machine of its own for the loop. A line with rework and no way back reports REWORK_UNROUTABLE.
• Work after the last side that does not care which side is up (packing, finishing for one- and two-sided products alike) gets pose: "ANY" (Products → component → Side → Any side). Then both products share it, with no flip before it.

Check

A board every 4 minutes goes through paste, placement, reflow and inspection, is sent back from the inspection and flipped on the way (a minute to carry back, 20 s to flip), and goes through the same four stations again. 2% fail each inspection and go round once more without being flipped. Every board finishes both sides, none is lost, and each station works two passes per board.

Capabilities it covers

• Re-entrant manufacturing flow
• Loops with a limited number of rounds
• Rework loop
• Rework at original station

All capabilities and guides