CSWA: The Groove That Pinches at the Corners

The Groove That Pinches at the Corners

An O-ring groove must be cut into the face of a 150 × 100 × 12 mm cover plate. The path is a closed rectangle inset 15 mm from the plate outline, with 1.5 mm corner radii added so the path is continuous. The cutting profile is a 3 mm wide × 2 mm deep rectangle, centred on the path and drawn on a plane normal to the path at its start point. The first sweep fails outright. Raising every path corner radius to 3 mm lets the sweep build, but the resulting groove has sharp, pinched inner walls at all four corners and the seal vendor rejects the plate because a 3 mm cord will not seat. A suggestion on the floor is to 'just set the sweep to thin feature and it'll ignore the corners'.

1. Derive the minimum path corner radius for this groove from the profile dimensions, and explain why 1.5 mm failed while 3.0 mm built.

Read the worked answer

The limit is set by the offset of the profile's inner edge from the path, not by the profile size. The path runs down the middle of a 3 mm wide groove, so the groove's inner wall sits 1.5 mm inside the path (half the width). Sweeping turns the path into two offset surfaces: outer wall at path + 1.5, inner wall at path − 1.5.

At a corner of path radius R, the inner wall radius is R − 1.5:

R = 1.5 → inner wall radius 0. The inner offset collapses to a point and the swept solid self-intersects at the corner. The kernel refuses.
R = 3.0 → inner wall radius 1.5 mm. Legal, so it builds - but 1.5 mm is a very tight inside corner for a 3 mm cord to bend around, which is why the vendor rejects it even though the geometry is valid.

General rule: R_path > half the profile width, and in practice the inside wall radius should be several times the cord diameter for the seal to conform. The 3 mm build is a geometry pass, not a design pass - the vendor requirement, not the kernel, is what sets the final number.

2. The corners cannot be enlarged much because the groove must register against a mating part. What are your options, and which do you recommend?

Read the worked answer

The variables you actually own are the groove cross-section, the path, and the corner treatment.

Narrow the groove. Reducing width from 3.0 to 2.4 mm drops the half-width to 1.2 mm, so a 2 mm path radius yields a 0.8 mm inner wall radius - buildable, but check the squeezed cord percentage against the vendor's spec before committing.
Enlarge only the corner radius, not the corner position. A larger fillet radius at the corner can be swept while keeping the corner's nominal location; verify the groove centreline near the corner still clears the mating part's sealing land.
Negotiate a molded corner seal so the seal itself carries the corner geometry, and let the plate keep a generous corner radius.
Split the cut into straight sweeps plus a corner feature, which changes nothing about the offset geometry - the corner still needs R > 1.5. This is not a fix; do not spend time on it.

Recommendation: option 1 combined with option 2 - narrow the groove to the vendor's minimum accepted width and take the largest corner radius the mating part allows, then confirm the corner by sectioning the model. 'Thin feature' is not an option: it changes how the profile is interpreted along the path, it does not guarantee the corners seal, and it will not satisfy an inspection drawing.

3. Where should the profile sketch be placed, and what would you check in the model before releasing the plate to the vendor?

Read the worked answer

Placement. The profile must be on a plane normal to the path at the start point, and the start point should be in the middle of a straight segment, not on an arc. Sketching on a plane that is merely convenient - the plate's top face, for instance - leaves the profile at an angle to the path, and the resulting groove is skewed and deeper on one side than the other. The symptom is a groove that looks right in isometric and wrong in section.

Release checks:

Section through each corner, perpendicular to the path, measuring the remaining wall between the groove's inner wall and the plate edge - that is the failure you cannot see in a top view.
Measure the groove width and depth at a straight segment against the cord spec, then at a corner to confirm the corner did not narrow it.
Check the plate's remaining thickness under the groove: 12 mm plate minus a 2 mm groove leaves 10 mm, which is fine, but confirm nothing else (a counterbore, a boss) reduces it locally.
Confirm the sketch is fully defined so the groove is reproducible if the plate family is resized - an under-defined profile will drift on the next configuration.