OpenNest/docs/support-tabs.md

Support grid and support tabs

A cut part drops onto the machine's support bed. If its center of gravity is not over the points it rests on, it can tip into the head's path or fall through. OpenNest models the bed, checks every part against it, and can tab only the parts the bed cannot hold level, at the perimeter point where one tab steadies the part most.

The support bed

OpenNest.Support.SupportBed is the machine's bed, fixed to the machine and measured from the plate origin into the sheet; a sheet in another quadrant sees it mirrored. The built-in layout is the laser grid, SupportGrid: one contact point every PitchX along X and PitchY along Y, the first at (OffsetX, OffsetY). Lengths are in the nest's units. Straight slats, and curved slats imported from a CAD layout (as on the plasma table), are planned as further SupportBed subclasses: a layout only reports its contacts under a part's material.

Every bed also carries:

• RegistrationTolerance: how far a sheet may sit from its nominal place. The bed is checked shifted by up to this much along X, Y and both diagonals, and the worst result is kept.
• StabilityMargin: how far inside its contacts the center of gravity must lie to count as supported, allowing for slag, gas pressure and worn contacts.

The bed belongs to the nest (Nest.SetSupportBed); every plate follows it, including plates added later. It is display, analysis and tab placement only: never material, spacing or a keep-out. Nest > Support Grid... edits it; Save as Machine Defaults writes %APPDATA%\OpenNest\support-grid.json, which new nests receive while it is enabled. The nest file stores it in plateDefaults.supportBed (see nest file format).

The support check

SupportAnalyzer.Analyze takes a placed part's material (SupportMaterial: outline minus holes, chorded at 0.001; leads, rapids, scribes and break cuts are not material, and a tabbed cutout's slug counts as part since it stays attached). Plate is of even thickness, so the center of gravity is the area centroid. The part rests on the bed contacts inside its material; the margin is the signed distance of the center of gravity inside the convex hull of those contacts (negative outside). Fewer than three contacts that are not in line can only balance a part, so their margin is at most zero; no contacts at all is negative infinity. A part is supported when its worst margin is positive and at least the stability margin.

A perimeter tab holds the part at the tab, so a tab point counts as one more contact. PartSupport.MarginWith(point) is the worst margin with a tab there, and BestTabPoint the outline point (vertices plus 256 even samples) that maximizes it. AcceptsTabAt(point) accepts any point that reaches the stability margin, or, when no single tab can, only points as good as the best one found. A tab's bending strength is ignored: it is a support point, not a cantilever.

A tab never goes in an inside corner, where its nub cannot be ground flush. An inside corner is a run of consecutive inward-turning outline vertices that turn at least 30 degrees together, so a chorded fillet counts while a shallow bend does not. Tab points keep the analysis's inside-corner clearance from those vertices (Plan Cutting and the overlay use twice the tab size; 0 keeps a tab off the corner only), and AcceptsTabAt rejects closer points. Equally good points usually run along a straight edge; of those, BestTabPoint is the one farthest from an inside corner. Only when every outline point is too close does the clearance give way.

View > Draw Support shows the grid (while its points are at least 6 pixels apart), each part's contacts, and its center of gravity: green when supported, red when it can tip, with an orange diamond on a red part's best tab point (keeping the plate's tab-size clearance).

Support tabs in Plan Cutting

Cutting Settings > Tabs > "Tab parts the support grid cannot hold level" sets CuttingParameters.SupportTabs. With it and a plate support bed, CuttingPlanService capture analyzes each regenerated part (SupportTabCapture). A part the bed cannot hold level gets a tabbed perimeter (PreparedContours.WithSupportTab), using TabConfig for the tab size; all other parts keep the plate settings, so supported parts are not tabbed.

The emitter tabs a perimeter at its entry: the cut stops short of the entry by the tab size. A support-tabbed perimeter's automatic entries are therefore limited to catalogue points the analysis accepts, plus the closest native point to its best tab point (AutomaticEntryKind.SupportTab, never on a reflex or cusp vertex). Ranking, lead checks and replay are unchanged.

If none of those entries has a lead-in that fits, the part is still tabbed. The search falls back first to every catalogue entry clear of inside corners, then to every entry (SupportEntries), each ordered by how well the tab steadies the part; of the entries whose lead-in fits, it keeps every one that holds the part level, or else only the steadiest. The open-scrap pierce fallback then tries the last of those catalogues.

Replay notes each planned part whose tab, at its perimeter entry, cannot hold it level (no single tab can, or no lead-in fits where one would) or sits in an inside corner (SupportTabCapture.Notes). The notes are advisory: a Ready plan keeps its status and lists them in Plan Cutting's warnings, and the console and MCP summaries print them under the plate.

Not covered yet:

• A drawing pin on the perimeter wins: its entry and tab are kept as pinned.
• Locked or ineligible parts keep their programs; the older Assign Lead-ins command and Auto Tab sizes ignore the bed.
• Only one perimeter tab per part. A part that needs more (one tab cannot reach the stability margin) gets the best single tab and a note.
• Pre-post verification does not yet warn about unsupported, untabbed parts.