The customer’s DXF opens fine in a viewer. Then you drop it into the nester and one of three things happens: it refuses the part, it nests a rectangle around it, or it produces a contour with a 0.003" gap that the laser will cheerfully cut as two separate paths with a pierce at each end. The file looked right. It wasn’t.
These are the seven problems that show up most often, roughly in the order you’ll hit them, with how to spot each one and what fixes it.
1. Open Contours
Two endpoints that should meet but don’t — a 0.001" gap from the CAD export, a line that overshoots the corner, a segment that never got drawn. A nesting engine needs closed loops to know what’s part and what’s hole, so an open outline either gets rejected or, worse, gets read as a hole that isn’t there.
Spot it: the contour count doesn’t match what you expect, or the part previews with a missing edge or an unfilled interior. Fix it: auto-close with a tolerance appropriate for sheet metal — usually 0.005" to 0.010" — then join the pieces into single closed polylines. Anything the tolerance can’t close needs a human to look at it, because that gap may be a real design mistake.
2. Duplicate and Overlapping Geometry
A line drawn twice. A polyline traced over the top of existing lines. A block inserted twice at the same point. On screen it’s invisible; on the machine the laser cuts the same edge twice, dumping heat into an already-cut kerf and leaving dross — and your quote just doubled the cut length for that edge.
Spot it: total cut length is noticeably longer than the perimeter you’d estimate by eye, or you see a pierce landing on an edge that’s already been cut. Fix it: a duplicate-removal pass (AutoCAD calls it OVERKILL) before you nest. Then compare cut length against a rough perimeter as a sanity check.
3. Splines and Over-Segmented Curves
CAD tools export curves two bad ways: as true splines, which many controllers and nesters won’t take, or exploded into hundreds of straight segments a few thousandths long. The first gets rejected. The second nests, but the machine decelerates at every vertex, the cut looks faceted, the program is enormous, and the nester crawls because it’s doing collision math on a thousand tiny edges.
Spot it: an entity count wildly out of proportion to the part’s complexity; a “curve” that shows as a polyline with hundreds of vertices. Fix it: convert splines to arcs and lines with a tolerance, and arc-fit segmented polylines back into real arcs. Native arcs also post cleanly as G02/G03 instead of a wall of G01 moves.
4. Text and Dimensions Still in the File
Text entities aren’t cut geometry, and dimensions are annotation. Left in the file, one of two things goes wrong: the nester ignores the text and the customer who wanted their part number engraved gets a blank plate, or the nester treats dimension lines as geometry and tries to cut them.
Spot it: stray geometry outside the part outline, arrows, leader lines, or characters in the preview. Fix it: strip the annotation layers. If text genuinely needs to be cut or etched, explode it to outlines deliberately and put it on an etch layer so the post-processor treats it correctly.
5. Wrong Units or Scale
An inch drawing exported with the millimeter flag set, or the reverse, and the part comes in 25.4 times too big or too small. Less dramatic but just as costly: a drawing exported at 1:2 or 2:1 sheet scale. The DXF header has a units field, but exports set it incorrectly often enough that you can’t trust it.
Spot it: a bracket that measures four feet wide, or a bolt hole that comes in at 0.02". Fix it: always check one known dimension — a hole diameter, an overall length — against the print before nesting. It takes ten seconds and catches every unit and scale error at once.
6. Construction Geometry and Junk Layers
Centerlines, bend lines, hidden lines, the drawing border, the title block. Flat pattern exports drag all of it along, and a nester that doesn’t know your layer conventions will happily nest the title block as a part or cut a bend line straight through the blank.
Spot it: a rectangle much larger than the part, dashed geometry, lines that cross the interior of an otherwise closed outline. Fix it: layer discipline. One cut layer, one etch layer if you need it, and everything else deleted before the file goes anywhere near a nest. You can ask customers for cut-only flat patterns, and you should — but you’ll still get everything, so the cleanup has to be routine.
7. Not Actually Flat
A 3D model saved as DXF, geometry sitting in a plane other than XY, entities with non-zero Z values, or a sheet metal part exported folded instead of as a flat pattern. Depending on the viewer, the part shows up edge-on as a single line, or as a perfectly convincing side view of something that can’t be cut from flat stock.
Spot it: the part renders as a line or a sliver; dimensions don’t match the flat pattern on the print; the outline has features that only make sense on a formed part. Fix it: flatten all Z to zero and project to XY. If the part was exported folded, go back to the source — an unfolded flat pattern from the native CAD, or unfold it yourself from the STEP file.
The Ten-Second Check Before You Nest
- Closed contour count matches the part (one outside, one per hole).
- Total cut length is in the neighborhood of the perimeter you’d expect.
- One known dimension verified against the print.
- No text, dimensions, or borders left in the file.
- Cut and etch geometry on their own layers, everything else gone.
- Every entity at Z = 0, in the XY plane.
Fixing It Without Leaving the Program
None of this is hard. What makes it expensive is the round trip — opening a second program, fixing the file, saving it, re-importing, discovering the next problem, and doing it again. PolygonLogix imports DXF, DWG, IGES, STEP, and SVG with full geometry extraction, unfolds SolidWorks and Inventor sheet metal, and includes an embedded CAD editor with an AutoCAD-style command line and snap engine, so closing contours, cleaning layers, and removing junk happens in the same window as the nest. And when the “drawing” is a phone photo instead of a file, the free Image to DXF tool gets you to real cut geometry in about a minute.