OpenNest/docs/common-line-cutting.md

Common-line cutting

Two parts placed with straight edges facing each other, exactly one kerf apart, can share one cut: programs are nominal geometry and the controller offsets each contour by half the kerf (G41/G42), so both parts' facing edges are cut on the same centerline. Placing parts this way saves the part spacing along every shared edge, which matters on costly plate.

Release gate. Cutting a common line once is not yet confirmed on a machine. Before release it needs a 3/16 stainless coupon pair on the CI FIBER 4020: measure both parts' shared edges against nominal and check that compensation starts cleanly in the opened kerf and cancels where a remainder ends in it (see CI Fiber output). Until then, Plate > Arrange > Cut Common Lines Twice keeps the double cut.

Groups

Plate.CommonLines holds CommonLineGroups: the member parts (by reference) and the kerf they were placed at.

• Spans are derived, never stored. CommonLineGeometry pairs straight edges of the members' clean perimeters (leads excluded, holes and arcs never share) that face each other, lie the group's kerf apart within CommonLineGeometry.GapTolerance (0.0005) and overlap by at least MinSharedLength (0.25). Only that overlap is shared; chamfers and other edges are cut as usual. A moved or rotated member therefore breaks its group instead of leaving a stale span. The tolerances are in model units and sized for inch nests.
• Group rule: every member shares at least one span with another member, and the members form one connected group. Rows, pairs and grids all qualify.
• Removing a part from the plate drops it from its group. A group left with one part is removed.

Cutting each shared line once

By default Plan Cutting cuts every shared span once (Plate.CutsCommonLinesOnce: the plate has groups and Plate.CutCommonLinesTwice is off). Material always comes from the drawings, so a planned member's open program changes no utilization, quantity, overlap or spacing result.

• Order. CommonLineCutting.Order takes the members breadth-first from the group's first member along its spans. Each shared cut is owned by the member of its pair that comes first, so the owner precedes its partner (a captured dependency, enforced by search and replay; a kept order that cuts the partner first is a ConstraintConflict naming "the common-line partner"). The tour treats each such pair like a nested part and its host, so a group is cut as one run. A rule the search learns later may still separate members; the dependency always holds.
• Each member is cut as: its holes (and break cuts and tabbed cutouts, as for any part), then the shared cuts it owns, then its remainder: its perimeter minus every span it shares, in the perimeter's own direction, as one open piece per stretch between spans. Shared cuts are never all cut first: each is cut where the route passes its owner.
• Shared cut: an open cut on the centerline (LayerType.CommonLine), cut without kerf compensation. It is pierced in scrap on the span's own axis, the external lead-in's reach before the span, and entered with a straight lead-in through the end notch; it leaves the same way at the far end with a straight lead-out as long as the external lead-out's reach (none with no lead-out). Capture tries each end in turn: the lead-in, cut and lead-out may touch no material, and the pierce keeps the pierce zone. When no end fits, planning refuses with UnsupportedGeometry naming both parts and the span. Leads are never shortened.
• Remainder: each piece starts where a shared span ends. It is pierced in that span's kerf, on the centerline, the external lead-in's reach back along the span, and enters along a straight lead-in to the piece's start, so compensation starts inside the kerf the shared cut opened; the pierce meets nothing. The comment COMMON LINE REMAINDER CW (or CCW) after its rapid names the perimeter's winding, so posts compensate it on the perimeter's side. It has no lead-out: it ends in the kerf at the next span. A span shorter than the lead-in's reach, or an external lead-in with no length, refuses with UnsupportedGeometry.
• Accounting. The planner marks each member's shared spans on its owned clean copy as Tab spans (CommonLineCutting.MarkSharedSpans): material the member never cuts. Replay's contour accounting proves each stretch between them is cut whole, once, as for tabbed cutouts, and refuses a member that also cuts a span.
• Rapids. A shared cut is never a rapid obstacle and takes no part in the open-cut retention proof. When a member's program ends, its remainder pieces, joined across its spans, become one obstacle: the part is cut free then.
• Refusals. A broken group, and a locked member or one a caller left out of the plan, cannot be cut once: rejoin, unlock or include it, or cut the plate's lines twice. A drawing pin on a member's outside is not applied (a warning makes the plan UNVERIFIED); pins on its holes are. Members are never support-tabbed. Remainder pierces in the kerf are held to the pierce zone like any other, so a pierce diameter wider than the kerf refuses them.
• Apply installs each member's program (holes, shared cuts, remainder); nothing is added to the plate.

Cut common lines twice

Plate > Arrange > Cut Common Lines Twice (Plate.CutCommonLinesTwice, saved with the nest, undoable) keeps the first slice's behavior: each member cuts its whole perimeter, so every shared edge is cut twice, the second pass running in the kerf the first opened. It needs no post support. It costs one extra pass per shared edge, and a part freed by the first pass could tip under the head on the second. Changing the setting after planning makes the plan stale.

Desktop

• Plate > Arrange > Join Common Line slides the selected parts together one kerf apart (CommonLineJoiner). The unit nearest the origin stays put. Each other part moves along the normal of its best facing edge pair (the longest overlap, then the smallest move) to the part it is closest to joining. Facing edges up to four part spacings apart (at least 1) qualify. A part already in a group brings its whole group, which moves as one and merges into the new group. The kerf is the plate's cutting kerf for the nest's material and thickness (the machine's, see machine cutting defaults). With no kerf, or any part left without a facing partner, nothing moves.
• Plate > Arrange > Remove Common Line removes the selected parts' groups. The parts stay put and are then held to the part spacing again.
• The plate draws each shared span as a teal dashed centerline. A member that no longer shares a span is outlined in red.
• Moving one member moves only that part and breaks the group. Rejoin, or remove the group.
• Join, Remove and Cut Common Lines Twice are undoable like any other edit.

Automatic placement

Auto Nest, the server's nesting worker and Fill Area can place parts on common lines themselves.

• Who may pair. Only drawings with Allow common line set in Drawing Properties (Drawing.AllowCommonLine, off by default: a shared edge is cut from the centerline, which some customers' edge-quality requirements do not accept). A drawing pairs only with copies of itself.
• The level is the nest's Nest.CommonLineUsage, chosen in the Auto Nest dialog (saved with the nest) and seeded for new nests from the nest defaults (defaults.json, Tools > Nest Defaults):
  • Off (the default): engines and fills run exactly as without common lines; the pipeline asks the engine nothing more.
  • When it adds parts: a common-line layout is kept only when it places more parts than the best layout without common lines, or (whole jobs) the same parts on fewer sheets. A tie keeps the layout without common lines.
  • Whenever available: part count still ranks first and sheets second, so common lines never cost a part or a sheet. With both equal, the common-line layout is kept when it is better on one of material (a lower salvage-credited cost, then a larger compactness offcut: a shorter used length or a larger remnant) and cut length (perimeters less each shared span once), and no worse on the other.
• The kerf is the plate's cutting kerf for the nest's material and thickness (the machine's, see machine cutting defaults). With no kerf, or a kerf not smaller than the part spacing, nothing pairs.
• How. CommonLinePair builds a compound of two copies sharing the part's longest straight edge (the second turned half a turn, so that edge faces its own copy one kerf apart; shorter edges are tried when the turned copy would come closer than the kerf elsewhere). The compound's outline is both perimeters (arcs circumscribed) and the kerf between them, without holes. The pair needs a part that may turn half a turn: automatic rotation, or a rotation limit that allows the 180° equivalent.
• Whole jobs (NestPipeline, every engine, built-in or plug-in): after the ordinary solve, CommonLinePipelinePrepass offers each eligible part as pairs plus its odd copy (all copies paired, then about half, so rows that fit an odd count are tried too) and solves again. Placed pairs expand into their two parts and one NestJobCommonLine group each on the result sheet (NestJobPlateResult.CommonLines: placement positions and kerf). The expanded layout is checked by NestLayoutCheck again, and CommonLineRanking chooses at the job's level (NestJobOptions.CommonLineUsage, CommonLineKerf, NestJobPart.AllowCommonLine). Committing adds the groups to Plate.CommonLines. Engines themselves are unchanged, and FillScore and every engine comparer still rank their own layouts.
• Fill Area (PlateFillService.FillItem with a level and kerf) also fills the area with pairs and keeps that fill when it places more parts, or, at "Whenever available", as many and the strategy's own ranking does not prefer the plain fill. Other interactive fills, the console and MCP fills do not pair.
• Benchmark: --common-lines off|when-it-adds-parts|whenever --kerf <k> runs each job through the pipeline with every part allowed to pair; off is the baseline to compare the levels with (see engine benchmarking).

Checks

• Part spacing (the pre-post review and the desktop's live label): any two members of one group are held to the group's kerf instead of the plate's part spacing. Everything else, including spacing to non-members and the plate edge, is unchanged.
• Engine results (NestLayoutCheck, the benchmark's and the pipeline's validation): the exemption comes only from a result sheet's explicit group list, never from proximity. A group must be allowed by the job (a level on, the job's kerf, members whose parts allow common lines, each placement in one group at most), smaller than the part spacing, and intact (CommonLineGeometry.Check).
• Common lines (pre-post, a warning that needs consent like any other):
  • a member that no longer shares a span one kerf from another member, or a group split in two;
  • a group whose kerf differs from the plate's cutting parameters' kerf (when the plate has them) by more than GapTolerance. A group at the wrong kerf would leave a web or cut a sliver. Rejoin it at the current kerf;
  • a lead-in, lead-out or shared cut of a member that touches or enters another member's material. A lead on a shared edge would cut the partner one kerf away. An unreadable lead check is incomplete, never clear;
  • on a plate that cuts its lines once, a span no member's program cuts on its centerline (for example a plate never planned, which would cut it twice), a span cut more than once, and a span cut after the partner whose remainder ends in it;
  • on a plate set to cut its lines twice, a member program that still cuts one once.
• The rapid check treats members' remainders and shared cuts as planning does.

Posting

A program that cuts a common line once (CommonLineCuts.IsSinglePass: CommonLine motions or remainder comments) posts only through a post that implements ICommonLinePostProcessor. The desktop's Post command and the console refuse any other post before verification (CommonLineCuts.EnsurePostable), and the built-in posts refuse it themselves; no post falls back to cutting the line twice. Set the plate to cut its lines twice and plan again instead.

• Cincinnati CI Fiber: supported.
• Cincinnati CL and Gravograph IS: refuse.

File format

See nest file format.