Lip, Groove and Snap Fit

Overview#

An enclosure normally has two halves that must align and then be held shut. Lip and Groove aligns them and seals the seam against dust. Snap Fit holds them together without screws. Fusion provides dedicated commands for both on the Plastic tab (Design Extension).

Where to find it#

• Design > Plastic > Create > Lip (Autodesk's help: Plastic > Create > Lip).
• Design > Plastic > Create > Snap Fit.
• Both read dimensions from the plastic rule assigned to the components; assign the rule first.

Step by step#

Lip and Groove#

1. Assign a Plastic Rule to both bodies.
2. Start Lip. Select one or more tangent edges on a solid body to define the path. Select the inside edge to place the reveal on the outside of the design.
3. Select the Type: Lip (raised), Groove (recessed) or Lip And Groove (both, across a pair of bodies).
4. Optionally adjust Side 1 / Side 2 Transparency so you can see through.
5. Choose a Guide Type:
   • Face Guide: select a face on either side of the edge.
   • Pull Direction: select a plane or face; the edges must be tangent continuous.
6. Use Flip to move the feature to the other body.
7. Optionally set one or two Extents by selecting points, vertices, faces or planes, then an Extent Result.
8. In the Advanced tab, adjust the lip dimensions. For Lip And Groove, the groove dimensions follow automatically from the lip dimensions and clearance.
9. Click OK.

For Lip And Groove, the two bodies must touch, their edges line up, and the faces are normal to each other.

Snap Fit#

1. Place sketch points on the parting plane between the two bodies where each snap fit should go. Leave them unconstrained for a quick layout, or dimension them for precise location.
2. Start Snap Fit and choose Selection Mode: Automatic (all visible sketch points) or Manual.
3. Choose the type:
   • Parallel Hook and Groove: hook and groove parallel to a face.
   • Perpendicular Hook and Groove: perpendicular to a face; set the base fillet radius and the hook undercut.
   • Hook and Loop: a loop on one body and a hook on the other.
4. Pick the hook body, set its dimensions, then tick Groove (or Loop) and set those dimensions.
5. Choose a rotation: Uniform, Independent or Aligned, and flip as needed.
6. For the parallel and hook and loop types, choose the extent: To Next (with a base fillet radius) or Distance (with a depth).
7. Use Section Analysis and the Side 1/Side 2 Transparency to check the engagement.
8. Click OK.

Options reference#

Tips and pitfalls#

Tip: Lip height of about the wall thickness, lip width about half the wall, and a clearance of 0.1 to 0.15 mm per side are common starting points; confirm with your molder.

Tip: Design snap hook deflection with strain in mind. The permissible strain for ABS is a few percent; for brittle or filled resins it is much less. Calculate from the beam formula or simulate; the Fusion command only sets geometry.

Warning: A hook that is too thin or too short will break or yield on the first assembly. Base fillet radius at the root of the cantilever should be at least 0.5 times the thickness to reduce stress concentration.

Warning: Undercut hooks need side-action cores or a through-window in the other wall to mold. Check Design for manufacture before committing.

Tip: Place at least three snap fits around a perimeter, and put them near corners and spaced evenly so the lid does not bow.

Lip and groove: sizing and tolerances#

A lip and groove joint aligns two halves and keeps dust out. The lip is a raised rim on one half, the groove a matching channel on the other. The usual rules of thumb for a general enclosure:

With a 1.25 mm lip and 0.12 mm clearance per side, the nominal groove is 1.49 mm wide. Injection moulding tolerances of about plus or minus 0.1 mm mean the actual clearance may range from nearly zero to about 0.3 mm, which is why a larger clearance (0.15 mm) is used for a loose fit, and a smaller one (0.05 to 0.1 mm) for a tight seal. A groove is cut into the wall, so ensure the wall stays thicker than the groove depth plus a margin; a 2.7 mm deep groove cannot fit in a 2.5 mm wall, so on thin walls either put the lip on an inner step, thicken the wall locally with a rim, or reduce the lip height to about 1.5 mm.

In Fusion, the Lip command uses a path of tangent edges, a type (Lip, Groove or Lip And Groove), a Guide Type (Face Guide or Pull Direction), optional extents and Advanced dimensions. For Lip And Groove, the two bodies must touch, their edges line up and their faces are normal to each other, and the groove dimensions follow from the lip and clearance.

Worked example: a 100 x 60 mm enclosure seam#

Select the inner rim edge loop on the base, use Pull Direction with the Z axis (the edges are tangent continuous, as required), choose Lip And Groove, and set the lip to 1.5 mm high and 1.2 mm wide with 0.12 mm clearance. Switch Side 1 and Side 2 Transparency on to see both bodies, and check the section for a visible gap of about 0.12 mm per side. If Pull Direction fails, look for a corner that is not tangent continuous: add small corner radii (at least 1 mm) to the rim and retry, or switch to Face Guide.

Snap fits: beam mechanics you can check by hand#

Fusion's Snap Fit command lays out geometry, but it does not calculate stress. The cantilever hook is a beam of length L, thickness t and width b, deflected by a distance y at its tip during assembly. For a constant-section cantilever, the maximum strain at the root is

strain = 1.5 x t x y / L squared

Example: an ABS hook with t = 1.5 mm, L = 10 mm and an undercut (deflection) y = 0.8 mm gives strain = 1.5 x 1.5 x 0.8 / 100 = 0.018, or 1.8 percent. ABS usually permits a few percent for a single assembly, but only roughly 1 to 2 percent for repeated use, so this design is acceptable for a service panel that is opened rarely and marginal for a battery door. Increasing L to 14 mm reduces strain to 0.9 percent (since (10/14) squared is 0.51). Tapering the beam from t at the root to 0.5 t at the tip also lowers strain at the root. A root fillet of at least 0.5 times the thickness (here at least 0.75 mm) reduces stress concentration.

Assembly force is roughly proportional to the stiffness b x t cubed / L cubed times the deflection, so doubling the thickness multiplies the force by eight. Keep the lead-in angle about 25 to 35 degrees for easy assembly and 60 to 90 degrees on the retention face for a hold that resists pulling, or a shallower angle if you want it to release.

Choosing a snap-fit type#

Autodesk's help lists these three types, with rotation options Uniform, Independent and Aligned, and extents To Next or Distance. Place hooks near corners, and use at least three around a perimeter so the lid does not bow.

Common failures and fixes#

Practice exercise#

Design a two-part enclosure seam and check it by calculation.

1. Create a 2.5 mm plastic rule and assign it to a base and a lid, each 100 x 60 mm, 20 mm deep.
2. Use Lip And Groove with a 1.5 mm high, 1.2 mm wide lip and 0.12 mm clearance per side. Add 1 mm corner radii if the edge path is not tangent continuous. Use Section Analysis to measure the gap on each side; it should be about 0.12 mm.
3. Add four Parallel Hook and Groove snap fits, one near each corner, 10 mm from the corner. Set the hook thickness to 1.5 mm and a deflection (undercut) of 0.8 mm.
4. Calculate the root strain with the formula above for your hook length. If it exceeds 2 percent for ABS, lengthen the hook or reduce the undercut, and recheck.
5. Add a root fillet of at least 0.75 mm.
6. Use Section Analysis through a hook and confirm the hook sits behind the groove.

Expected result: halves that register with a visible clearance of about 0.12 mm per side, four snap fits with calculated strain of about 1 to 2 percent, and a section showing hook engagement. Record the strain value for each design change.

Related#

• Boss, rib, web and rest
• Plastic rules, draft and wall thickness
• Design for manufacture