Add fillets, chamfers, and shells to your 3D geometry to refine the design.
Once you have extruded your basic 3D shape, modifiers are the tools that transform a rough block into a production-ready part. They relieve stress concentrations, reduce weight, improve aesthetics, and prepare geometry for manufacturing processes like 3D printing, CNC machining, and injection molding. Mastering modifiers is what separates a beginner's boxy prototype from a professional-grade robotics component.
In this module, we cover the essential modifier and tool operations you will use in nearly every robotics CAD project: edge treatments (fillets and chamfers), shell and draft operations, pattern and mirror tools for repeated geometry, and Boolean operations for combining bodies.
These are the two fundamental edge treatments. Both modify sharp edges, but they serve different purposes and have different mechanical implications.
A fillet replaces a sharp edge with a smooth, tangent arc of a specified radius. The transition is continuous and curved, which distributes stress evenly across the surface.
Typical robotics radii: 0.5–1.0 mm for cosmetic edges, 1.5–3.0 mm for structural internal corners, 0.2–0.5 mm minimum for CNC-machined aluminum (limited by cutter radius).
A chamfer removes material at an angle (commonly 45 degrees) along an edge, creating a flat bevel. The transition is a straight cut rather than a curve.
Typical robotics sizes: 0.5 mm for general edge breaks, 1.0–2.0 mm for assembly lead-ins on bolt holes, 0.3 mm for cosmetic edge breaks on aluminum panels.
Select one or more edges, then specify a radius. The CAD tool replaces each sharp edge with a smooth arc. Start with structural internal corners first, then add cosmetic fillets to external edges. Most CAD tools let you select multiple edges and apply the same radius in one operation.
Select edges and specify either a single distance (equal chamfer) or two distances (asymmetric chamfer). A 1.0 mm x 45-degree chamfer is the most common edge break in robotics. Use asymmetric chamfers when you need a larger lead-in on one face than the other.
Select the face(s) to remove, then specify wall thickness. The tool hollows out the solid, leaving a uniform shell. Always shell before adding fillets to internal corners to avoid geometry failures.
Select vertical faces and specify a draft angle (typically 1–3 degrees). The faces tilt outward from the parting line, allowing the part to release from a mold. Even for 3D-printed parts, a slight draft can improve surface quality on vertical walls.
Instead of manually copying features, use pattern tools to repeat holes, bosses, or cutouts in rectangular or circular arrays. Use mirror to duplicate geometry across a symmetry plane, ensuring both halves remain identical when you edit the source.
The Shell operation converts a solid body into a thin-walled hollow part. You select one or more faces to remove (these become the "openings"), and the tool offsets all remaining faces inward by the specified thickness.
Uniform shell applies the same wall thickness to every remaining face. This is the default and works well for most enclosures. Variable-thickness shell lets you assign different thicknesses to specific faces — for example, making the bottom of a battery box thicker (2.5 mm) for rigidity while keeping the side walls thinner (1.5 mm) to save weight.
The faces you select for removal become open sides of the shell. For a simple electronics enclosure, you remove the top face so the lid can be attached later. For a battery box, you might remove one side face as the insertion opening. You can remove multiple faces in a single shell operation.
A draft angle is a slight taper applied to vertical faces of a part. Instead of walls being perfectly perpendicular to the base, they lean outward by a small angle (typically 1–3 degrees). This seemingly minor modification has a major impact on manufacturability.
When a plastic part is molded, it shrinks onto the mold core as it cools. Without draft, the part grips the core and is extremely difficult to eject — risking surface damage, warping, or broken ejector pins. Draft angles allow the part to release cleanly the moment the mold opens. Industry standard is a minimum of 1 degree per side, with 2–3 degrees preferred for textured surfaces.
While 3D printing does not use molds, draft angles still help. Vertical walls on FDM printers can show visible layer lines and slight ringing artifacts. A 2–5 degree draft on tall vertical faces reduces these artifacts and can eliminate the need for support material on near-vertical overhangs. For SLA/resin printing, draft helps the part peel away from the FEP film more easily during each layer.
Select a pull direction (usually perpendicular to the base), select the faces to draft, and enter the angle. Most CAD tools show a preview so you can verify the taper direction. Always apply draft before adding fillets to the drafted faces — the fillet will adapt to the tapered geometry automatically.
Pattern and mirror operations let you replicate features without modeling them individually. They are parametric — editing the original feature automatically updates every copy. This keeps your design intent clear and your feature tree manageable.
Copies a feature in a grid along one or two linear directions. Specify count and spacing for each axis.
Use case: Rows of mounting holes on a chassis plate, ventilation slot arrays on an enclosure panel.
Copies a feature around a central axis at equal angular spacing. Specify count and total angle (360 degrees for a full circle).
Use case: Bolt hole circles on motor mounts and wheel hubs, gear teeth, spoke patterns on wheels.
Reflects features or entire bodies across a plane of symmetry. The mirrored copy is a linked duplicate of the original.
Use case: Symmetrical chassis halves, left/right brackets, mirrored mounting lugs on a center-split enclosure.
Boolean operations combine two or more solid bodies into a single result. They are the fundamental way to add material, remove material, or find the shared volume between bodies. In robotics CAD, you use Booleans constantly — every hole, pocket, and joined structure relies on them.
Merges two bodies into one, combining all material from both. Overlapping volume becomes a single solid. Use this to fuse separate modeled sections into a unified part — for example, joining a motor mount boss onto a chassis plate.
Removes the volume of one body (the "tool") from another (the "target"). The tool body is consumed. This is how you create holes, pockets, channels, and any negative space — for example, cutting a shaft bore through a bearing block.
Keeps only the volume shared by both bodies, discarding everything else. Less commonly used, but valuable for finding interference between parts or creating complex curved surfaces where two shapes overlap.
The order in which you apply modifiers matters. Applying them out of sequence is one of the most common causes of CAD geometry failures (rebuild errors). Follow this recommended order:
In particular: always shell before filleting internal corners. If you fillet an internal corner and then try to shell, the shell operation often fails because it cannot offset the complex fillet geometry at the corner. Shell first, then fillet the resulting internal edges.
The following table provides starting-point values for typical robotics applications. Always verify values against your specific material, manufacturing process, and load requirements.
| Modifier | Application | Typical Value | Notes |
|---|---|---|---|
| Fillet Radius | 3D-printed internal corners | 1.5 – 3.0 mm | Larger radii reduce stress risers; match nozzle diameter as minimum |
| Fillet Radius | CNC aluminum internal corners | 0.5 – 1.5 mm | Limited by endmill radius; specify slightly larger than cutter radius |
| Fillet Radius | Cosmetic external edges | 0.3 – 1.0 mm | Improves feel and appearance; does not affect strength significantly |
| Chamfer | Assembly lead-in on bolt holes | 1.0 – 2.0 mm x 45° | Guides bolts into position; size based on bolt diameter |
| Chamfer | General edge break / deburr | 0.3 – 0.5 mm x 45° | Removes sharp edges for safe handling |
| Chamfer | 3D-print bottom edge (elephant's foot) | 0.3 – 0.5 mm x 45° | Compensates for first-layer squish on FDM prints |
| Shell Thickness | 3D-printed enclosure (PLA/PETG) | 1.5 – 2.5 mm | Minimum 3–4 wall lines; thicker for load-bearing faces |
| Shell Thickness | Injection-molded ABS enclosure | 1.0 – 2.0 mm | Uniform thickness prevents sink marks and warping |
| Shell Thickness | CNC-machined aluminum housing | 1.5 – 3.0 mm | Depends on span and load; use FEA to verify for structural parts |
| Draft Angle | Injection molding (smooth surface) | 1 – 2° | Minimum for clean ejection; more for deep draws |
| Draft Angle | Injection molding (textured surface) | 3 – 5° | Texture requires additional draft to prevent drag marks |
| Draft Angle | 3D printing (tall vertical walls) | 2 – 5° | Optional; reduces layer-line visibility and may eliminate supports |