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why Modeling Strategy Matters in Procedural CAD Modeling why Modeling Strategy Matters in Procedural CAD Modeling

Why does modeling strategy matter so much in procedural CAD? Two scripts can produce the exact same geometry while showing very different generation times.

Using a concrete build123d example, this article explains why certain operations, such as fillets, become expensive when repeated at scale, and how a simple change in approach can drastically reduce computation time.

A clear overview of the key challenges in procedural modeling, highlighting the direct impact of design choices on CAD engine performance.

A selection of custom 3D models

This STL file provides 2 compartments measuring 80×26×10mm each. The total dimensions are 84×58×12mm. The cavities do not feature any fillet.
Parameter Value Unit
number of rows 2
number of columns 1
compartment length 80 mm
compartment width 26 mm
compartment height 10 mm
wall thickness 2 mm
compartment fillet (radius) 0 mm

📦 Model #4476

1 object(s)
- format STL
Tubing adapter STL 3D file ⌀ 40–37 mm (Length: 115 mm)
Tube adapter ⌀40 mm to ⌀37 mm in STL 3D format. Length of this connector is 115 mm. The thickness of the tubes is identical: 5 mm. The larger-diameter tube has a sleeve length of 35 mm, the smaller one of 40 mm. The ends are rounded to facilitate the connection of the two tubes.
Parameter Value Unit
side A length 35 mm
side A outer diameter 40 mm
side A thickness 5 mm
side B length 40 mm
side B outer diameter 37 mm
side B thickness 5 mm
transition length 40 mm
axis offset 0 mm
ends fillet fillet on bo...
Download this STL model of a simple U-shaped drawer handle. The overall dimensions are 50×100×20 mm. The handle has a circular cross-section combined with a softened transition. The mounting holes are 5 mm diameter with a center-to-center distance of 80 mm.
Parameter Value Unit
width 50 mm
length 100 mm
thickness 20 mm
shape (0:square,1:circle) 1
transition (0:right,1:rounded) 1
fillet radius 0 mm
hole diameter 5 mm

📦 Model #1995

1 object(s)
- format STL
O-ring STL file ID 67 × CS 7 mm
File of O-ring (torus-shaped seal) with dimensions Inner diameter 67 mm by CS (cross section) 7  mm. Outer diameter (OD) resulting corresponds to 81 mm.
Parameter Value Unit
inner diameter (ID) 67 mm
cross section (CS) 7 mm
3D model in STL format of a round-to-rectangular adapter with an external diameter of ⌀128 mm and a rectangular section with internal dimensions 114×58 mm. The shell thickness is 3 mm and the total length is 220 mm. Chamfers are applied on the outside of the cylindrical end and on the inside of the rectangular end to ease insertion.
Parameter Value Unit
cylinder outer diameter 128 mm
cylinder inlet length 50 mm
rectangle internal length 114 mm
rectangle internal height 58 mm
rectangle inlet length 99 mm
offset Z 0 mm
offset Y 0 mm
total length 220 mm
thickness 3 mm
chamfer chamfers on ...
3D file in STL format of a round-to-rectangular adapter with an external diameter of ⌀58 mm and a rectangular section with inner dimensions 62×82 mm. The wall thickness is 2 mm and the total length is 100 mm. A chamfer is present on the inside of the rectangular end.
Parameter Value Unit
cylinder outer diameter 58 mm
cylinder inlet length 30 mm
rectangle internal length 62 mm
rectangle internal height 82 mm
rectangle inlet length 50 mm
offset Z 0 mm
offset Y 0 mm
total length 100 mm
thickness 2 mm
chamfer chamfer on t...

📦 Model #2390

1 object(s)
- format STL
Round air vent STL 3D file ∅ 115 mm, slat angle: 45°
3D file of a round grille for air circulation in STL format. Its male diameter is 115 mm. The slats have an angle of 45° and a high thickness of 2 mm. This ventilation grille has a collar of 10 mm. The full diameter of this model is 135 mm.
Parameter Value Unit
male diameter 115 mm
slat angle 45 °
slat thickness 2 mm
flange width 10 mm
central reinforcement no

📦 Model #4503

1 object(s)
- format STL
Tube adapter STL 3D file ⌀ 125–80 mm (Length: 105 mm)
Straight tube coupler ⌀125 mm to ⌀80 mm in STL format. Total length of this junction is 105 mm. The thickness of the tubes is identical: 5 mm. The larger-diameter tube has a sleeve length of 35 mm, the smaller one of 40 mm. The ends are raw.
Parameter Value Unit
side A length 35 mm
side A outer diameter 125 mm
side A thickness 5 mm
side B length 40 mm
side B outer diameter 80 mm
side B thickness 5 mm
transition length 30 mm
axis offset 0 mm
ends fillet no fillet

📦 Model #1260

1 object(s)
- format STL
Tube adapter STL 3D file ⌀ 45–10 mm (Length: 90 mm)
Inline sleeve ⌀45 mm to ⌀10 mm in STL format. Total length of this connector is 90 mm. The larger-diameter tube has a thickness of 2.5 mm and a length of 30 mm. The smaller-diameter tube has a thickness of 2 mm and a length of 30 mm as well. The ends have no fillet.
Parameter Value Unit
side A length 30 mm
side A outer diameter 45 mm
side A thickness 2.5 mm
side B length 30 mm
side B outer diameter 10 mm
side B thickness 2 mm
transition length 30 mm
axis offset 0 mm
ends fillet no fillet

📦 Model #1458

1 object(s)
- format STL
Tube adapter STL 3D file ⌀ 104–50 mm (Length: 85 mm)
Inline tube reducer ⌀104 mm to ⌀50 mm in STL 3D format. Total length of this adapter is 85 mm. The thickness of the tubes is identical: 2 mm. The larger-diameter tube has a sleeve length of 30 mm, the smaller one of 25 mm. The two axes of the tubes are off-center by 26.9 mm. The ends are rounded on the inside to facilitate the connection of the two tubes.
Parameter Value Unit
side A length 30 mm
side A outer diameter 104 mm
side A thickness 2 mm
side B length 25 mm
side B outer diameter 50 mm
side B thickness 2 mm
transition length 30 mm
axis offset 26.9 mm
ends fillet fillet on th...
3D square protective honeycomb grid model as a STL file. Mounting holes are placed at the four corners (center-to-center 107x107 mm), each with a diameter of Ø5 mm. The overall dimensions reach 117x117 mm, with very large 10 mm cell size for maximum airflow. This grid serves as both a protective guard and a ventilation panel.
Parameter Value Unit
length or center-to-center 107 mm
width or center-to-center 107 mm
mesh size 10 mm
dual color no
holes yes
hole diameter 5 mm
Download this STL model of a simple U-shaped furniture handle. The overall dimensions are 35×140×14 mm. The handle has a square cross-section combined with a smooth transition. A fillet along the edges allows for a more comfortable grip. The mounting holes are 4 mm diameter and have a 126 mm center-to-center distance.
Parameter Value Unit
width 35 mm
length 140 mm
thickness 14 mm
shape (0:square,1:circle) 0
transition (0:right,1:rounded) 1
fillet radius 1 mm
hole diameter 4 mm
Download this rectangular protective honeycomb grid model in STL format. Mounting holes are placed at the four corners (center-to-center 210x110 mm), each with a diameter of Ø5 mm. The overall dimensions reach 220x120 mm, with very large 10 mm cell size for peak ventilation. This grid serves as both a protective guard and a ventilation panel.
Parameter Value Unit
length or center-to-center 210 mm
width or center-to-center 110 mm
mesh size 10 mm
dual color no
holes yes
hole diameter 5 mm
File of a pipe elbow with a 15° angle in STL format. This elbow has an outer diameter of 77 mm and an inner diameter of 75 mm. The tube thickness is therefore 1 mm. End fillets make it easier to insert.
Parameter Value Unit
outer diameter 77 mm
inner diameter 75 mm
angle 15 °
end fillets yes
Model of a pipe elbow with a 60° angle in STL format. This pipe elbow has an outer diameter of 32 mm and an inner diameter of 28 mm. The tube thickness is therefore 2 mm. The ends feature fillets to make it easier to assemble.
Parameter Value Unit
outer diameter 32 mm
inner diameter 28 mm
angle 60 °
end fillets yes
Download this model in STL format of a round-to-rectangular adapter with an external diameter of ⌀100 mm and a rectangular section with inner dimensions 135×75 mm. The shell thickness is 2 mm and the overall length is 160 mm. The adapter has an offset of 30 mm along the Z axis.
Parameter Value Unit
cylinder outer diameter 100 mm
cylinder inlet length 35 mm
rectangle internal length 135 mm
rectangle internal height 75 mm
rectangle inlet length 60 mm
offset Z 30 mm
offset Y 0 mm
total length 160 mm
thickness 2 mm
chamfer no chamfer

📦 Model #3502

1 object(s)
- format STL
Protective grid STL 3D file, 60x60mm, mesh: 8mm
Download this 3D square honeycomb grid model in STL format. The overall size is 60x60 mm, with wide 8 mm cell size for optimal air circulation. This type of grid serves both protection and ventilation roles.
Parameter Value Unit
length or center-to-center 60 mm
width or center-to-center 60 mm
mesh size 8 mm
dual color no
holes no

📦 Model #1321

1 object(s)
- format STL
Tube adapter STL file ⌀ 85–70 mm (Length: 80 mm)
Straight tube fitting ⌀85 mm to ⌀70 mm in STL 3D format. Length of this sleeve is 80 mm. The thickness of the tubes is identical: 3 mm. The larger-diameter tube has a length of 30 mm, the smaller one of 30 mm as well. The two axes of the tubes are off-center by 7.4 mm. The ends are raw.
Parameter Value Unit
side A length 30 mm
side A outer diameter 85 mm
side A thickness 3 mm
side B length 30 mm
side B outer diameter 70 mm
side B thickness 3 mm
transition length 20 mm
axis offset 7.4 mm
ends fillet no fillet
3D file of a mounting bracket with a central reinforcement in STL format. The dimensions are 150 mm in length, 150 mm in height, 14 mm in width, and 6 mm in thickness. The screw holes are designed with a diameter of 6 mm. Chamfers are applied to the holes to improve the seating of the heads. The central reinforcement bar reduces bending and provides two clearances for screwdriver access. No support is needed to print this bracket, printed flat on the build plate.
Parameter Value Unit
length 150 mm
height 150 mm
width 14 mm
thickness 6 mm
hole diameter 6 mm
chamfer on the holes yes
Rectangular duct transition downloadable as STL 3D file. The internal cross-section on side A is 96x51.2 mm (rectangular), while side B is 40x26.2 mm (rectangular). Both side A and side B sleeves share the same insertion length of 30 mm. Between the two sleeves, the transition section adds 55 mm, giving an overall length of 115 mm. Inner and outer edges at both ends are left unchamfered. Relative to side A, side B is shifted by 25 mm along the Z axis.
Parameter Value Unit
side A internal width 96 mm
side A internal height 51.2 mm
sleeve length on side A 30 mm
side B internal width 40 mm
side B internal height 26.2 mm
sleeve length on side B 30 mm
thickness 2 mm
transition length 55 mm
Y offset 0 mm
Z offset 25 mm
end chamfers none
3D model of washer / gasket in STL 3D file format. This part features an inner diameter of ⌀3.9 mm and an outer diameter of ⌀6.9 mm. The total thickness is 4.3 mm. This part has no finish applied.
Parameter Value Unit
inner diameter 3.9 mm
outer diameter 6.9 mm
thickness 4.3 mm
finish none

📦 Model #4475

1 object(s)
- format STL
Tube adapter STL 3D file ⌀ 40–37 mm (Length: 118 mm)
Straight tube fitting ⌀40 mm to ⌀37 mm in STL format. Total length of this connector is 118 mm. The thickness of the tubes is identical: 5 mm. The larger-diameter tube has a sleeve length of 38 mm, the smaller one of 40 mm. The ends have a fillet to make tube connection easier.
Parameter Value Unit
side A length 38 mm
side A outer diameter 40 mm
side A thickness 5 mm
side B length 40 mm
side B outer diameter 37 mm
side B thickness 5 mm
transition length 40 mm
axis offset 0 mm
ends fillet fillet on bo...

📦 Model #4396

1 object(s)
- format STL
Honeycomb grid STL file, 250x250mm, mesh: 10mm
Square grid model as a STL file. The overall size is 250x250 mm, with very large 10 mm cell size for maximum airflow. This grid serves as both a protective guard and a ventilation panel.
Parameter Value Unit
length or center-to-center 250 mm
width or center-to-center 250 mm
mesh size 10 mm
dual color no
holes no

📦 Model #1483

1 object(s)
- format STL
Tube adapter STL file ⌀ 200–50 mm (Length: 80 mm)
Tube reducer ⌀200 mm to ⌀50 mm in STL 3D format. Total length of this junction is 80 mm. The thickness of the tubes is identical: 3 mm. The larger-diameter tube has a sleeve length of 30 mm, the smaller one of 30 mm as well. The ends have no fillet.
Parameter Value Unit
side A length 30 mm
side A outer diameter 200 mm
side A thickness 3 mm
side B length 30 mm
side B outer diameter 50 mm
side B thickness 3 mm
transition length 20 mm
axis offset 0 mm
ends fillet no fillet

📦 Model #4497

1 object(s)
- format STL
Tubing adapter STL 3D file ⌀ 32–19 mm (Length: 65 mm)
Tube fitting ⌀32 mm to ⌀19 mm in STL 3D format. Length of this junction is 65 mm. The thickness of the tubes is identical: 2 mm. The larger-diameter tube has a sleeve length of 35 mm, the smaller one of 20 mm. The ends are not rounded.
Parameter Value Unit
side A length 35 mm
side A outer diameter 32 mm
side A thickness 2 mm
side B length 20 mm
side B outer diameter 19 mm
side B thickness 2 mm
transition length 10 mm
axis offset 0 mm
ends fillet no fillet
Download this model in STL format of a round-to-rectangular tube adapter with an external diameter of ⌀100 mm and a rectangular section with internal dimensions 180×100.2 mm. The wall thickness is 3 mm and the overall length is 190 mm. The adapter has an offset of 45 mm along the Z axis.
Parameter Value Unit
cylinder outer diameter 100 mm
cylinder inlet length 50 mm
rectangle internal length 180 mm
rectangle internal height 100.2 mm
rectangle inlet length 50 mm
offset Z 45 mm
offset Y 0 mm
total length 190 mm
thickness 3 mm
chamfer no chamfer
Enclosure with screw-mounted lid STL 3D file: 120×60×75 mm (with ventilation on the lid)
View of object #0
Enclosure with a screwed lid in 3D STL format. The dimensions are 120 mm long by 60 mm wide and 75 mm high. The cover only measures 10 mm high. Side thickness is 3 mm. This enclosure features significant cooling area on the lid.
Parameter Value Unit
length 120 mm
width 60 mm
total height 75 mm
lid height 10 mm
wall thickness 3 mm
screw margin 0 mm
fit clearance 0.1 mm
cooling level 8
cooling zone(s) cutouts on t...

📦 Model #4447

1 object(s)
- format STL
Tube adapter STL 3D file ⌀ 100–79 mm (Length: 240 mm)
Straight tube fitting ⌀100 mm to ⌀79 mm in STL 3D format. Total length of this sleeve is 240 mm. The larger-diameter tube has a thickness of 4 mm and a sleeve length of 100 mm. The smaller-diameter tube has a thickness of 5 mm and a length of 100 mm as well. The axes of the tubes are offset by 11.4 mm. The ends feature an external fillet.
Parameter Value Unit
side A length 100 mm
side A outer diameter 100 mm
side A thickness 4 mm
side B length 100 mm
side B outer diameter 79 mm
side B thickness 5 mm
transition length 40 mm
axis offset 11.4 mm
ends fillet fillet on th...
Round box with lid STL 3D file ⌀ 134 mm - Height: 20 mm, Shell: 2 mm
View of object #0
Get this circular box model in 3D STL format. Its diameter is 134 mm and its total height is 20 mm. The wall thickness is 2 mm. A fillet located at the bottom of the box makes it easier to grip objects.
Parameter Value Unit
external diameter 134 mm
total height 20 mm
wall thickness 2 mm
fit clearance 0.2 mm
inner bottom fillet 1 mm
Download this model of an angle bracket with reinforcement in STL 3D format. The dimensions are 250 mm in length, 150 mm in height, 20 mm in width, and 5 mm in thickness. The screw holes have a diameter of 5 mm. Chamfers are applied to the holes to seat the screw heads. The reinforcing bar enhances mechanical strength and provides two passages for screwdriver access. No support is needed to print this bracket, placed flat directly on the build plate.
Parameter Value Unit
length 250 mm
height 150 mm
width 20 mm
thickness 5 mm
hole diameter 5 mm
chamfer on the holes yes

STL: Advantages and Disadvantages for 3D Printing

The STL format is, without question, a cornerstone of 3D printing. This exchange format has established itself as the universal standard for representing 3D models ever since the early days of stereolithography. Its main strength lies in its simplicity: it describes the surface of an object using countless small triangles that form a mesh. This approach, known as tessellation, makes STL 3D files universally compatible with nearly all CAD software and slicers. If you’d like to learn more about this format, check out our article STL: What Is This 3D File Format?.

One of the major advantages of the format lies in this universality: whether you’re using a complex modeling program or a simpler design tool, you can export your 3D models in STL 3D format with near certainty that they’ll be interpreted correctly by your 3D printer. This ease of exchange has played a key role in the widespread adoption of 3D printing, allowing anyone to share and print objects without worrying about software compatibility. Once again, simplicity is its greatest strength.

However, that same simplicity also brings certain limitations. The triangle mesh, while effective for describing geometry, contains no information about colors, textures, or materials. For more advanced projects requiring these details, the STL format starts to show its weaknesses. Additionally, print quality depends directly on the fineness of the tessellation: too few triangles can lead to rough or faceted surfaces, while an overly dense mesh can make the file unnecessarily heavy.

Another notable drawback is the lack of unit management. An STL file doesn’t specify whether dimensions are in millimeters, centimeters, or inches, which can sometimes cause scaling errors when importing into a slicer. Despite these limitations, the STL format remains the go-to standard for converting your 3D models into G-code — the language your printer understands. It continues to be the preferred choice for its robustness and broad compatibility, even as newer formats like 3MF emerge for more specialized needs.

What is parametric modeling?

Parametric modeling is a fundamental approach in computer-aided design (CAD) that reshapes how 3D models are created and managed. Far from being a simple drawing technique, it represents a genuine design philosophy where objects are defined not by fixed shapes, but by variables and intelligent relationships.

This method makes it possible to modify the length, width, or diameter of a part and have the entire design adapt automatically, without the need to redraw everything. At the core of the process are parameters—numerical values (length, angle, thickness, etc.)—linked together through constraints and formulas. For instance, the diameter of a hole can be defined as half the width of a plate; if the width changes, the hole’s diameter instantly adjusts, ensuring the consistency of the design. This interdependence makes 3D models flexible and responsive to changes. One of the main advantages of parametric modeling lies in its ability to simplify customization and enable rapid iteration of designs.

Whether through modeling software such as Fusion 360 or FreeCAD, or through code-based libraries like build123d, this approach allows effortless exploration of a wide range of variations. Such flexibility is especially valuable across multiple fields—from mechanical engineering and architecture to consumer product design. It saves considerable time, reduces errors, and improves the performance of parts.

By defining design intent from the start through these parameters and constraints, the model preserves its integrity and functionality even after numerous modifications. It is a powerful way to transform an idea into a tangible object, ready to adapt to new situations.