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 one of two layouts; a layout only reports its contacts under a part's material. Lengths are in the nest's units.

• The laser grid, SupportGrid: one contact point every PitchX along X and PitchY along Y, the first at (OffsetX, OffsetY).
• Straight slats, SupportSlats (as on the plasma table): each slat a strip Thickness wide running the full length (Axis X) or width (Axis Y) of the table, one every Pitch across it, the first centered Offset from the origin. A part rests on the region where its material crosses a slat, so its contacts are that region's corners: where the outline and holes cross each slat's two edges, and the outline and hole vertices on a slat. A part narrower than a slat rests on its own corners; one balanced across a single slat has a margin of at most half the thickness, which the stability margin rejects.

Curved slats imported from a CAD layout are not modeled yet.

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 Bed... edits it and picks the grid or slats; Save as Machine Defaults writes it to the nest's machine configuration (MachineConfig.SupportGrid: both layouts' values and which one is in use; settings saved before slats existed are a grid), which nests assigned that machine receive while it is enabled: File > New and BOM import assign the Tools > Nest Defaults machine (the first by name when none is chosen), and Nest > Machine (or the status bar's machine label) reassigns a nest and replaces its bed with the new machine's (none when that machine gives nests no bed). A machine with no bed saved yet uses the legacy shared %APPDATA%\OpenNest\support-grid.json. The nest file stores the bed in plateDefaults.supportBed and the machine in machine (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. A tab cannot lift the margin past the contacts' own hull edges, so a whole stretch of outline often ties; among samples within Tolerance.Epsilon of the best, the one nearest the line through the center of gravity across the long side of the outline's minimum-area bounding rectangle wins. The search itself is not narrowed, so a part held only near one end is still tabbed at the far end. 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. The tie-break above runs over the clear points only; points equally near the center line go to 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 or the slats, cut off to the sheet (while its points or slats 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 and lead checks are unchanged.

Plates without a support bed

With no bed there are no contacts to judge a part by. Cutting Settings > Tabs > "No bed: tab widths" sets CuttingParameters.NoBedTabMinWidth to NoBedTabMaxWidth: on a plate with no support bed, support tabs then tab every part whose width (the short side of its outline's minimum-area bounding rectangle, SupportAnalyzer.Width) is in that range, bounds included. A maximum of 0 turns the rule off. SupportTabSettings.Analyze makes this choice for Plan Cutting, hand-placed lead-ins and the pre-post review alike; a plate with a bed never uses the range.

SupportAnalyzer.AnalyzeWithoutBed gives such a part a PartSupport with HasBed false: no contacts, never supported, and every tab clear of inside corners steadies it, so the perimeter's automatic entries are every catalogue point clear of inside corners plus the best tab point, the clear outline point nearest the line through the center of gravity across the long side. Which of those wins is left to the usual ranking toward the next cut.

With support tabs on, no bed and no range, nothing is tabbed and the plan carries one advisory note saying so; its status is 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.

A drawing pin on the perimeter wins: its entry, leads and tab are kept as pinned, and the analysis is only carried for review. Replay and best-effort plans (SupportTabCapture.Review) warn with PostVerificationKind.PartSupport whenever an unsupported part's emitted perimeter is untabbed or its entry does not steady the part: its pin has no tab or a tab elsewhere, no steadying entry had a lead-in that fits, or no single tab can steady it. Like every planner warning, this makes the proposal UNVERIFIED, so applying it needs explicit acceptance.

A planned tab that sits in an inside corner, because no lead-in fits clear of one, gets an advisory note instead (SupportTabCapture.Notes; never for a pin): a Ready plan keeps its status and lists it in Plan Cutting's warnings, and the console and MCP summaries print it under the plate.

Lead-ins placed by hand

Plate > Place Lead-in cuts where it is clicked. Under "Tab parts the support grid cannot hold level", SupportTabSettings.ForManualLeadIn enables tabs on a copy of the settings when the plate's bed cannot hold the clicked part level, so the tab sits at the clicked entry. The overlay's orange diamond shows where it steadies the part; the pre-post review checks the result. Plan Cutting is the only automatic lead-in assignment.

Pre-post review

PartSupportReview runs in every pre-post verification and in Validate Nest. Each part whose settings (its own, else the plate's) ask for support tabs and that needs one (the plate's bed cannot hold it level or, with no bed, its width is in the no-bed range) is checked, however its program was made. The tab is the gap between the end and the start of the program's outside cut (the chain of cutting motions with the largest extent). A part with no such gap, or whose gap does not steady it (without a bed any gap does), gets a PartSupport warning at its center of gravity naming the best tab point; posting needs consent as for every warning. A part whose material cannot be read is reported as not checked.

Not covered yet:

• Locked or ineligible parts keep their programs in Plan Cutting; the pre-post review still checks them.
• 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 warning.