Boss, Rib, Web and Rest

Overview#

Four features cover most internal structure in a molded part:

• Boss: a cylindrical post, typically for a screw or insert, that joins two solid bodies (for example lid and base) and reinforces thin areas. Autodesk describes it as a feature that fastens two solid bodies together; you select a sketch point between the bodies and set parameters per side and for the fastener. Available on the Plastic tab and requires the Design Extension.
• Rib: a thin feature made from an open sketch profile, extruded to the nearest faces on a solid body, with the extrusion parallel to the sketch plane. In Autodesk's help, Rib is a standard Solid > Create command.
• Web: like Rib, but the thin feature is extruded perpendicular to the sketch plane.
• Rest: a feature that intersects a solid body and forms a flat area for other features or component placement (for example a flat pad for a PCB standoff or a battery seat). You select a closed sketch profile and a body, then specify thickness. Plastic tab, Design Extension.

Autodesk's Design Extension notes say draft angle, draft pull direction and fillet radius can be added to Web and Rib, and that the Boss command gained ribs in a May 2022 update (a new tab for ribs; the dialog populates from material selection).

Where to find it#

• Design > Plastic > Create > Boss and Rest (verify, since panel layouts change).
• Design > Solid > Create > Rib and Web.
• All can read draft, thickness and radius from the assigned plastic rule.

Step by step#

Boss#

1. Assign a plastic rule to both components first.
2. Put the two parts in position (lid above base) and sketch a point in the plane between them where the screw axis should be.
3. Start Boss and click the sketch point. A preview appears.
4. Set the parameters for each side: outer diameter, hole diameter or insert size, height, draft, and base fillet. Set the fastener or screw dimensions if the dialog offers them.
5. Use the Ribs tab (added in 2022) to add supporting ribs around the boss.
6. Click OK.

Rib or Web#

1. Create a sketch with an open line or polyline where the stiffener should lie. For a Rib, sketch on a plane perpendicular to the wall. For a Web, the extrusion goes perpendicular to the sketch plane; sketch on the plane of the web's face.
2. Start Rib or Web and pick the profile.
3. Set the thickness (typically 50 to 60 percent of the wall), direction, and optional draft and fillet.
4. Click OK. The rib stops at the nearest faces automatically.

Rest#

1. Sketch a closed profile where the flat area should be.
2. Start Rest, pick the profile and the body.
3. Set thickness. Join or cut behaviour depends on the geometry; check the preview.

Options reference#

Tips and pitfalls#

Tip: Rib thickness at 40 to 60 percent of the wall and height under about 3 times the wall are standard. Draft ribs by at least 0.5 degree per side.

Tip: Boss outer diameter is typically 2 to 2.5 times the screw diameter, with a hole sized for the specific thread-forming screw or heat-set insert. Follow the screw maker's chart.

Warning: Never put a thick boss directly on a thin wall; sink marks show on the cosmetic side. Connect bosses to walls with ribs, and core out the boss.

Warning: Ribs that end at the same height as the wall's cosmetic face will print through. Keep them below the surface.

Tip: Rib and web patterns can be created with the Pattern command so the pattern follows parameters.

Rib, web and boss geometry in numbers#

All values below are general guidance for typical unfilled thermoplastics; the resin supplier's data sheet and your molder's advice take priority.

Why the percentages? A rib that is as thick as the wall makes a thick intersection at its root (the rib plus the wall plus the fillet), which cools last and sinks, leaving a dent on the cosmetic side. Keeping the rib thinner shifts the thickest point away from the surface.

Worked example: stiffening a 100 x 60 mm lid#

A 2.5 mm ABS lid deflects too much under 5 N at its centre. Add three ribs running along the 100 mm length, spaced 15 mm apart (6 times the wall, ample), each 1.4 mm thick (56 percent), 7 mm tall (2.8 times the wall), with 0.5 degree draft and a 0.8 mm root fillet. The second moment of area of the lid section increases substantially because the stiffness of a plate rises roughly with the cube of its depth, so a 7 mm rib is far more effective than thickening the wall to 3.5 mm. In Fusion, sketch one line per rib on a plane perpendicular to the lid, pick the sketch lines in Rib, set thickness and draft, and use a pattern for the rest. Check each rib stops at the nearest wall faces.

Rib versus web: choosing the right command#

Autodesk's help describes both as thin features created from an open profile: Rib extrudes parallel to the sketch plane (the profile is sketched on a plane perpendicular to the wall, in the plane of the rib), and Web extrudes perpendicular to the sketch plane (the sketch lies on the plane of the supporting face). A simple test: if the profile is a line you drew in front of you and you want the rib to stand out of the page, you want a Web; if you drew the rib as seen in its own plane, use a Rib. Both take thickness, direction, draft and fillet.

Boss planning#

A Boss spans two bodies, such as a lid and a base. Sketch a point in the plane between the bodies at the screw axis, then set the dimensions for each side separately. The practical sequence:

1. Fix the screw: for a thread-forming M3 screw into ABS, the pilot hole is about 2.5 mm diameter and the boss outer diameter about 7 mm.
2. On the lid side the boss holds a clearance hole (about 3.4 mm); on the base side it holds the pilot hole.
3. Set draft (about 0.5 to 1 degree) and base fillets on both sides.
4. Add supporting ribs from the Ribs tab, usually three or four at 90 to 120 degree spacing, with height about two thirds of the boss height.
5. Keep the boss away from the wall by at least one rib gap, or attach it to the wall with ribs so it does not make a thick pad.

Rest: flat seats for components#

A Rest creates a flat area that intersects a solid body and gives other features or components a seat, for example a flat pad for a PCB standoff or a battery. You choose a closed sketch profile and a body, then specify the thickness. Check the thickness against the wall: a 4 mm pad on a 2.5 mm wall is a thick section and may sink; hollow the pad underneath, or use a thinner rest with ribs. Confirm that the Rest command and its inputs match your version, as Autodesk's published Design Extension summary did not list it.

Common failures and fixes#

Practice exercise#

Design and size the internal structure of a small enclosure and check it numerically.

1. Create a plastic rule ABS_2.5mm (2.5 mm wall, 1 degree draft, 1.5 mm radius) and assign it to a base and a lid, each 80 x 50 mm.
2. Calculate boss dimensions for an M3 thread-forming screw: pilot 2.5 mm, outer diameter about 7 mm, wall about 2.25 mm. Write them down before you start.
3. Place a sketch point between the two bodies near each corner, 6 mm from the walls. Start Boss and enter your dimensions. Add three ribs on each boss from the Ribs tab.
4. Add two Ribs on the lid, running along its length, 1.4 mm thick and 7 mm tall with 0.5 degree draft, 15 mm apart.
5. Section the model through a boss and measure the outer diameter at the base, the rib thickness and the height. Compare with step 2.
6. Run Design Advice. Make one rib 2.5 mm thick, and note any warning.

Expected result: bosses with about 7 mm outer diameter and ribs reaching the walls, rib thickness within 40 to 60 percent of the wall, and no warnings until you thicken the test rib. Record the dimensions you measured and any differences from your calculation.

Related#

• Plastic rules, draft and wall thickness
• Lip, groove and snap fit
• Design for manufacture