Geometry Scaling

Geometry Scaling#

Scale geometry using the scale_factor parameter. Useful when converting between units (e.g., mm to cm for neutronics).

Basic Usage#

import cadquery as cq
from cad_to_dagmc import CadToDagmc

sphere = cq.Workplane().sphere(5)
small_sphere = cq.Workplane().moveTo(10, 0).sphere(2)

assembly = cq.Assembly()
assembly.add(sphere)
assembly.add(small_sphere)

model = CadToDagmc()
model.add_cadquery_object(cadquery_object=assembly, material_tags=["mat1", "mat2"])

# Scale geometry 10x (e.g., mm to cm)
model.export_dagmc_h5m_file(
    min_mesh_size=0.5,
    max_mesh_size=1.0e6,
    scale_factor=10.0,
)

Scaling at Different Stages#

When Adding Geometry#

Scale when adding CadQuery objects or STEP files:

# Scale CadQuery object
model.add_cadquery_object(
    cadquery_object=assembly,
    material_tags=["mat1"],
    scale_factor=0.1,  # Scale down (mm to cm)
)

# Scale STEP file
model.add_stp_file(
    filename="geometry_in_mm.step",
    material_tags=["mat1"],
    scale_factor=0.1,  # Scale down (mm to cm)
)

When Exporting#

Scale at export time:

# Scale at export
model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    scale_factor=0.1,
)

Common Scale Factors#

From

To

Scale Factor

mm

cm

0.1

mm

m

0.001

cm

mm

10.0

m

cm

100.0

inch

cm

2.54

foot

cm

30.48

Different Scales for Different Parts#

When combining geometry from different sources with different units:

from cad_to_dagmc import CadToDagmc
import cadquery as cq

model = CadToDagmc()

# STEP file in mm
model.add_stp_file(
    filename="component_in_mm.step",
    material_tags=["steel"],
    scale_factor=0.1,  # Convert mm to cm
)

# CadQuery object in cm (no scaling needed)
sphere = cq.Workplane().sphere(10)  # 10 cm radius
model.add_cadquery_object(
    cadquery_object=sphere,
    material_tags=["tungsten"],
    scale_factor=1.0,  # No scaling
)

model.export_dagmc_h5m_file(filename="combined.h5m")

Neutronics Unit Conventions#

Most neutronics codes (OpenMC, MCNP, etc.) work in centimeters. If your CAD software uses millimeters (common in mechanical engineering), apply a scale factor of 0.1:

# CAD created in mm, simulation needs cm
model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    scale_factor=0.1,  # mm to cm
)

Mesh Size and Scaling#

Warning

Mesh size parameters (min_mesh_size, max_mesh_size, set_size) are applied after scaling. Set them in the output units, not the input units.

Example:

# Geometry is 100mm sphere, scaling to cm
# Want 0.5 cm mesh size (not 5 mm)
model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    scale_factor=0.1,      # mm to cm
    min_mesh_size=0.5,     # 0.5 cm (in output units)
    max_mesh_size=2.0,     # 2.0 cm (in output units)
)

See Also#

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