Shell, Draft, and Scale

Overview#

Three Modify tools shape the body as a whole rather than adding individual features.

• Shell hollows a solid to a uniform wall thickness, optionally removing faces to open it up. It is how you make enclosures, cups, and covers from a simple outer shape.
• Draft applies a fixed-plane or parting-line draft angle to rotate planar faces of a solid body, which helps a part release from an injection mold or casting.
• Scale enlarges or reduces bodies, components, or sketches uniformly (same factor on all axes) or non-uniformly (separate X, Y, Z values).

Where to find it#

• Workspace: Design.
• Tab / panel: Solid tab > Modify > Shell, Modify > Draft, Modify > Scale.
• Shortcut: none listed in Autodesk's shortcut reference.
• Sketch scaling: the Sketch tab has its own Scale tool for sketch geometry (a separate tool).

Step by step#

Shell#

1. Model the outer shape first; apply fillets before shelling so the inside corners follow with offset radii.
2. Open Modify > Shell.
3. Select the faces to remove (for example the top of a box) in the canvas, or select a solid Body in the Browser to create a hollow body without removing faces.
4. Set Direction: Inside (offsets the faces toward the inside), Outside (toward the outside), or Both.
5. Use the manipulator handles or type the Inside Thickness and Outside Thickness.
6. Choose the Type: Sharp Offset (sharp offset corners) or Rounded Offset (rounded offset corners). Uncheck Tangent Chain if you do not want tangentially connected edges selected with the one you click.
7. Click OK.

Draft#

1. Open Modify > Draft.
2. Choose Type: Fixed Plane or Parting Line.
3. Select a plane or face to define the Pull Direction (click the pull direction manipulator, or use Flip Pull Direction, to reverse it), then select the Faces to draft. For Fixed Plane, also choose Draft Sides (One Side, Two Side, Symmetric) and the Angle Mode (Relative, the default, or Absolute). For Parting Line, pick a Parting Tool (plane, face, edge or sketch curve) and a Parting Line Type (Fix Parting Line or Move Parting Line).
4. Enter the Angle (as general guidance, plastics often use about 1 to 3 degrees) and flip the pull direction if the draft tilts the wrong way. With Two Side or Symmetric, Angle 1 and Angle 2 appear.
5. OK.

Scale#

1. Open Modify > Scale.
2. Select the body or bodies (or components or sketches) to scale.
3. Pick a fixed anchor Point for the scaling.
4. Choose Scale Type: Uniform or Non Uniform.
5. Enter the Scale Factor (Uniform), or X, Y and Z values (Non Uniform).
6. OK.

Worked example: a plastic tray#

1. Extrude a 120 x 80 x 30 mm box, fillet the vertical edges 8 mm.
2. Draft the four side faces 2 degrees with the base face as the fixed plane.
3. Shell with the top face removed, thickness 2 mm.

Draft before Shell so the outside and inside both have draft.

Options reference#

Tips and pitfalls#

Tip: For a lid that fits a box, shell both with the same thickness, then offset the lid lip by roughly 0.2 mm for clearance using Offset Face (general guidance).

Tip: If Shell fails, shell first, fillet later, or lower the thickness. Features like thin slots and small radii often prevent shelling.

Warning: A Shell thickness greater than an internal radius or about half the thinnest section tends to fail (general experience). Remove tiny fillets, try Sharp Offset, or reduce thickness.

Warning: Non-uniform Scale of a body makes cylinders elliptical, so use it for freeform shapes, not mechanical parts (general guidance).

Note: not confirmed in Autodesk's documentation: how Scale interacts with dimensions and parameters of features added before or after it. Scale early and check dimensions afterward.

• Autodesk describes Draft as rotating planar faces; check that the faces you select can be drafted.
• A draft on a pocket tapers the walls so the pocket is larger at the open end.
• When Shell fails on a complex body, split it and shell separately, then Combine.
• Rescale before assembling rather than after (general advice).
• Use Scale to convert units of an imported model scaled wrongly by 25.4 (inch to mm) or 1000.
• Extrude's Taper Angle is a quick draft for simple features, no Draft command needed.

Wall thickness and draft with real numbers#

Shell thickness (general guidance, not Autodesk data). For injection molding 1.5 to 3 mm is typical for ABS or PP; for FDM printing use multiples of the nozzle line width (0.4 mm nozzle: 1.2, 1.6 or 2.0 mm). A 1.2 mm wall prints as three perimeters.

Draft arithmetic. A face of height h drafted at angle a moves outward by h x tan(a). For 2 degrees on a 40 mm tall wall, the shift at the top is 40 x tan(2 degrees) = 1.4 mm; a 1 degree draft on a 100 mm deep cavity adds 1.75 mm of taper. As general molding guidance (not Autodesk data), plastics need roughly 1 to 3 degrees on smooth surfaces, about 1 additional degree per 0.025 mm of texture depth, and deep draws often use 3 to 5 degrees.

Scale arithmetic. Scale by a factor f multiplies lengths by f, areas by f^2, and volumes by f^3. A 0.5 scale gives an eighth of the volume. Shrink compensation of 0.5 percent is a factor of 1.005 (shrink rates vary by material; use your resin's data sheet). Inches to mm is 25.4; a model that arrives 25.4 times too small was exported in inches.

Order of operations#

1. Main shape (extrudes, lofts, revolves).
2. Fillets on the outside edges you want rounded.
3. Draft on the walls.
4. Shell.
5. Internal features (ribs, bosses) and holes.
6. Small cosmetic fillets and chamfers.

Fillets before shell give rounded inside corners automatically: an outside fillet of R3 with a 2 mm wall gives an inside corner of R1. Draft before shell gives parallel walls with draft on both inside and outside. If you shell first and draft later, the inner and outer walls tilt together only if you select both sets of faces.

Shell direction worked example#

A 50 x 50 x 30 mm box, 3 mm shell:

• Inside keeps the box at 50 x 50 x 30 mm outside, with a 44 x 44 cavity.
• Outside makes the cavity 50 x 50 and the outer size 56 x 56 (one wall added outward). Use it to build a cover that fits over a part of known size.
• Both with 2 mm inside and 1 mm outside yields an outer size of 52 x 52 and a cavity of 46 x 46.

Removing the top face leaves an open tray; selecting no faces creates a hollow closed body. A closed hollow body is useful for volume or mass studies but cannot be printed without an escape hole.

Draft types#

• Fixed Plane: drafts from a fixed plane you select, with a pull direction defined by a plane or face. Use the parting plane of the mold or the flat base.
• Parting Line: drafts along a parting line defined by a Parting Tool (plane, face, edge or sketch curve). Fix Parting Line keeps the line in place; Move Parting Line moves it to apply the draft, with Fixed Edges to prevent deformation. Split Face and Silhouette Split results can serve as parting lines (see the Combine, Split page).
• Draft Sides: Two Side drafts both sides of the plane or line at different angles; Symmetric uses one angle for both; One Side drafts only one.

A draft can fail when neighbors cannot adapt (general experience). Faces roughly parallel to the pull direction are the ones that benefit.

Non-uniform scale#

Non Uniform scale lets you scale separately along X, Y and Z (for example 1.2, 1.0 and 0.8). A cylinder becomes an elliptic cylinder and holes lose their round shape, so use it for organic shapes only. To compensate a printer's axis shrinkage, apply it to a copy of the final body, never to the working model.

Failure modes and fixes#

Practice exercise#

Design a drafted, shelled cover and a shrink-compensated copy, and verify numerically.

1. Model a 60 x 40 x 30 mm box with 5 mm vertical fillets and 3 mm top fillets. Place the base on the XY plane.
2. Draft the four side faces 3 degrees with the bottom face as the fixed plane and the pull direction up. Measure the top width: it should be 60 - 2 x 30 x tan(3) = 56.9 mm if the base stays 60 mm. Check this with Measure.
3. Shell inward 2 mm, removing the bottom face. Measure the wall thickness (Inspect > Section Analysis helps). Try 5 mm and note the first failure; then reduce to the largest value that works.
4. Repeat the shell using Direction Both (1.5 mm inside, 0.5 mm outside) on a copy and measure the outside size change (the outside grows by 0.5 mm per side).
5. Copy the body again, Scale uniformly by 1.005 about the base center, and compare the volume change (expected +1.5 percent, since 1.005^3 = 1.015).
6. Scale a copy by 0.5 and verify the volume is one eighth.
7. Apply Non Uniform scale X 1.2, Y 1.0, Z 0.8 to a copy and observe that the fillet circles become ellipses.

Expected result: a drafted hollow cover with measured dimensions, a shrink-compensated copy at +1.5 percent volume, and a miniature at one eighth of the volume.

Related#

• Fillet and chamfer
• Rib, Web, and Emboss (stiffening the shelled part)
• Loft (organic shapes to shell)
• Move/Copy