---
jupyter:
  description: 'Preparing a part for laser cutting in Python with planex2d: read a planar STEP file, check the thinnest
    web and the smallest opening against the sheet thickness, offset the outline by half the kerf, count cut length
    and pierces, and write the cutting DXF in millimeters.'
---

# Preparing parts for laser cutting

A part to be cut by a laser, a waterjet or a plasma cutter needs a few checks and one change before it goes to the
machine. Its webs and openings must not be narrower than the sheet is thick, the cutting path runs half the kerf
outside the part, and cut length and pierces give the cutting time. planex2d does all of it on the exact geometry and
writes the path as DXF. The part can come from a planar STEP or BREP file, a DXF or SVG drawing, a sketch from a
.CATPart file or a Shapely polygon. The first cell is the complete workflow, ready to copy and run, and the section
Step by step runs it in parts, with the plots one looks at along the way.

The example is a sensor bracket cut from steel sheet 2 mm thick, 90 × 40 mm with corners of radius 3, two adjustment
slots 6.6 wide, a hole of diameter 10 and a window of 30 × 10 near its top edge. The file
[sensor-bracket.step](sensor-bracket.step) was exported from a 3D CAD program as a planar STEP file in millimeters.

```python nbsphinx="hidden"
from docs_figures import Row
```

```python
import planex as px
import planex.measurements as pm

bracket = px.FromStep("sensor-bracket.step")
print("thinnest web", bracket.Measure(pm.MinThickness()), "| sheet", 0.002)
print("smallest opening", min(min(w.bounding_box[1][i] - w.bounding_box[0][i] for i in (0, 1)) for w in bracket.inner))

cut = bracket.Offset(0.0001, inplace=False)
print("cut length", sum(w.mass for w in cut.wires), "| pierces", len(cut.wires))
px.WriteDXF(cut.wires, "sensor-bracket-cut.dxf", unit=px.mm)
cut
```

## Step by step

### Reading the part

`px.FromStep` reads the planar face of the STEP file, with its holes, and converts the file's millimeters into
meters, as every reader of planex2d does (see [STEP and BREP](../core/files/step-brep.md) and
[Units](../core/basics/units.md)). The bracket is one face with four openings.

```python
import planex as px
import planex.measurements as pm

bracket = px.FromStep("sensor-bracket.step")
print(type(bracket).__name__, "with", len(bracket.inner), "openings | size", bracket.bounding_box)
# px.Plot(bracket)
```

### Checking webs and openings

A common rule for laser-cut steel is that no web and no opening is narrower than the sheet is thick, here 2 mm.
`pm.MinThickness()` rolls the largest disc that fits along the whole outline and returns the thinnest wall, 2.5 mm
above the window. The wall-thickness map shows where it is (Figure 1). The narrowest opening is the smaller side of
each opening's bounding box, which is the width of a slot and the diameter of a hole. The narrowest are the slots,
6.6 mm wide. See
[Part analysis](../core/measuring/analysis.md) for wall thickness, clearances and the other measurements.

```python
print("thinnest web", bracket.Measure(pm.MinThickness()), "| sheet", 0.002)
print("smallest opening", min(min(w.bounding_box[1][i] - w.bounding_box[0][i] for i in (0, 1)) for w in bracket.inner))
```

```python nbsphinx="hide-input"
Row((bracket, "The wall thickness, thinnest above the window",
     {"field": pm.MinThickness(), "samples": 900, "bar": "right"}),
    caption="The sensor bracket's wall thickness")
```

### The cutting path

The laser cuts a slot as wide as its kerf, 0.2 mm here, so its path runs 0.1 mm outside the part, outward around the
outline and inward into every opening. `Offset` of the face by 0.1 mm does both at once, since a positive offset grows
the material, and `inplace=False` keeps the part as it is (see [Offsets](../core/sketch/offsets.md)). Cut length is
the length of all loops of the path, and every loop starts with one pierce of the sheet, five in all (Figure 2).

```python
cut = bracket.Offset(0.0001, inplace=False)
print("cut length", sum(w.mass for w in cut.wires), "| pierces", len(cut.wires))
```

```python nbsphinx="hide-input"
pierces = [tuple(w.edges[0].start)[:2] for w in cut.wires]
Row((cut, "The cutting path, a pierce at the start of each loop", {"fill": False, "marks": pierces}),
    caption="The cutting path, 0.1 mm outside the part")
```

### Writing the cutting file

`px.WriteDXF` of the path's loops writes them alone, without a fill, and `unit=px.mm` writes millimeters, converted
from the meters of the geometry. Read back, the path is 0.1 mm larger on every side than the part. The file is
[sensor-bracket-cut.dxf](sensor-bracket-cut.dxf). See [Files](../core/files.md) for the other writers.

```python
px.WriteDXF(cut.wires, "sensor-bracket-cut.dxf", unit=px.mm)
print(px.Compound(px.FromDXF("sensor-bracket-cut.dxf")).bounding_box)
```

## Other formats

| Format | Reader | See |
|---|---|---|
| STEP, BREP (planar) | `px.FromStep(path)`, `px.FromBrep(path)` | [STEP and BREP](../core/files/step-brep.md) |
| DXF | `px.FromDXF(path, layers=..., unit=..., tol=...)` | [Files](../core/files.md) |
| SVG | `px.FromSVG(path)` | [SVG files](../core/files/svg.md) |
| .CATPart sketches | `px.FromCATPart(path)` | [CATIA V5](../integrations/catia.md) |
| Shapely | `px.FromShapely(geometry)` | [Shapely](../integrations/shapely.md) |

## Pitfalls

- **Compensating twice.** Many cutting controllers compensate the kerf themselves. For those, the part's own outline
  goes into the file, without the offset.
- **Units.** The geometry is in meters, so a kerf of 0.2 mm is an offset of 0.0001. The writer converts to the unit
  given, and `scale=1` would write the meters' numbers unchanged.
- **Open contours.** A drawing with gaps gives no closed loops to cut. See
  [Cleaning up imported drawings](cleaning-up-a-drawing.md).
- **The rule of thumb.** Web and opening at least the sheet thickness is a common guide for steel. The actual limits
  depend on the material, the machine and the cutting shop.

## In the browser

The [contour checker](https://planex2d.org/contour-checker/) checks a DXF, SVG or STEP file for open contours, gaps,
doubled lines and small edges and gives cut length and pierces, and the [nesting tool](https://planex2d.org/nesting/)
places many parts on a sheet or a roll.