CourseWiki/Engineering Graphics and CAD/CAD Modeling

Sweeps and Lofts

Use a sweep to carry a section along a path and a loft to transition through ordered sections, with references that control the result.

A sweep needs a profile and a path

A profile describes the cross-section; a path describes where it travels. A circular profile swept around a smooth bend makes a bent solid rod. An annular profile can make a tube. For a basic sweep, keep the section size constant and choose how it stays oriented along the path.

Profile + path → sweep → bent rod. The path is a reference curve, not another solid.

Place the profile on a plane normal (perpendicular) to the path at its start for an easy first construction. Constrain its position to the path. A shape that looks connected in one view can still be separated in three dimensions. Select the complete intended path, not a neighboring model edge.

A sweep can create material or remove it from an existing part. Advanced sweep options can control twist or scaling; a constant circular section is a simpler starting case. See Onshape: Sweep.

Sweep lab: how tight can a bend be before a thick profile folds?; Interactive

Sweep lab: how tight can a bend be before a thick profile folds?

Choose a profile and a path, then tighten the bend. Watch where the inside of the sweep turns red: there, the profile reaches the centre of the bend and the inner wall has to fold back through itself.

The rule this lab tests. Carry a profile along a path. At each point the path has a bend radius ρ: the radius of the circle that best fits the path there, measured to its centre of curvature. The profile also has a reach toward that centre: how far its edge extends from the path toward the inside of the bend. The inner wall folds through itself where

reach toward the centre of curvature ≥ local bend radius ρ

For a round rod centred on the path, the reach is its radius, so a rod of diameter D fails around a bend when D/2 ≥ ρ. Exactly equal is already a failure: the inner wall shrinks to an edge at the centre of curvature.

Reach 8 mm is less than the 12 mm bend radius, so the inner edge stays a smaller arc with 4 mm to spare.

Reach 15 mm passes the 12 mm bend radius. The inner edge (red) passes the centre of curvature and runs backwards on the far side, folding the wall through itself.

Profile on a 12 mm arc	Reach toward centre	Result
Ø16 mm rod	8 mm	Builds (8 < 12)
Ø24 mm rod	12 mm	Fails: inner wall collapses to an edge (12 = 12)
Ø30 mm rod	15 mm	Fails: folds (15 > 12)
30 × 15 mm bar, 30 mm side across the bend	15 mm	Fails: folds
Same bar turned, 15 mm side across the bend	7.5 mm	Builds

Only the local bend matters for this test. On an S-curve the bend radius changes along the path, so the failure appears only near the tightest bends. Here a Ø20 mm rod (reach 10 mm) follows an S-curve whose tightest bend radius is 8 mm: 6% of the path fails.

Where the reach line rises above the bend-radius line, that stretch of the sweep folds (red bands).

A passing local test is not a complete check. This lab calls a sweep “builds” only on the U-bend, where the geometry also rules out clashes; anywhere else a passing local test is reported as only that, with global clearance not established. A Ø20 mm wire wound with a 30 mm coil radius bends gently (bend radius 30.2 mm), but with a 16 mm pitch neighbouring turns are only 15.9 mm apart, so the wire hits the next coil. Coils collide: neighbouring turns are 15.94 mm apart along the chord between them, but the profile needs 20 mm. The local bend test cannot see this clash.

The local test passes everywhere, yet the turns overlap. A real CAD check also looks for clashes between distant parts of the same feature.

Teaching model: the profile is centred on the path and carried without avoidable twist (a rotation-minimising frame). The red regions come from the local test only. It is not a complete validity check. The helix chord check can prove that neighbouring coils collide, never that they clear. A given CAD product may report failures differently or offer other orientation options. Sources: Patrikalakis, Maekawa and Cho, Local self-intersection of pipe surfaces and Global self-intersection of pipe surfaces; Onshape: Sweep.

A loft passes through profiles selected in order

A loft transitions between two or more sections. Put sketches on suitable planes, then select the sections in the order the shape should pass through them. A broad oval transitioning to a smaller oval is a simple example; the section size changes along the length.

Ordered sections → transition → solid. A guide can control the side between the sections.

Use a guide curve when the profiles alone do not control the shape between them. For a straightforward boundary guide, constrain it to meet every profile at the intended boundary point. A Pierce relation can locate a sketch point where a curve crosses that sketch plane. Do not accept a connection solely because it looks right in an isometric view.

Match corresponding locations around successive profiles. If one section's corner connects to an unrelated corner on the next, the loft can twist. Inspect connection handles and a section view before adding more profiles.

More profiles do not automatically make a better surface

Extra sections and guides can introduce unwanted waves or conflicting conditions. Start with the fewest that describe the intended shape, then add a control for a specific problem. Profile and guide rules vary with solid, surface, and thin options. See Onshape: Loft and SOLIDWORKS: Description of Sketch Relations.

Loft lab: what moving a start point or reordering profiles does; Interactive

Loft lab: what moving a start point or reordering profiles does

Three profiles stay in place: a square, a circle and a slot. Change only which points the loft pairs and the order you select the profiles, then watch the connector lines twist, pinch or double back.

What decides a loft’s shape. Each profile is divided into the same number of points, counted from its start point (the connector) in the same direction. Point 1 of the first profile joins point 1 of the next, and so on; the loft passes through the profiles in the order selected. Three profiles sit on parallel planes: a 60 mm square at the bottom, a Ø44 mm circle 45 mm up, and a 56 × 24 mm slot at 90 mm.

Matched start points: each connector runs straight from square to circle to slot.

Circle start point moved 90°: the connectors spiral. No checked section crosses itself, but it is twisted.

Circle start point moved 180°: paired vertices sit on opposite sides, so sections pass through the middle and cross themselves (red).

Selected square, slot, circle: the loft climbs to the slot, then comes back down to the circle through heights it already occupies (red).

Change	Result	Fix
Start points do not correspond	Twist; a large mismatch pinches or crosses itself	Line up the connectors or start points
Profiles selected out of their order along the part	Loft doubles back across heights already occupied	Select profiles in sequence
Whole selection reversed	Same surface, built from the other end	Nothing to fix

Teaching model: every profile runs counter-clockwise, and corresponding points are joined by a smooth curve through all three profiles. A profile drawn in the opposite direction is a separate problem, not modelled here. Twist and crossing come from the actual sections. Source: Onshape: Loft (sequential profile order; connections control twist).

Use the controlling geometry to choose sweep or loft; Practice

Use the controlling geometry to choose sweep or loft

What you know	First feature to consider	What to inspect
One section following a bent centerline	Sweep	Path continuity, section position, and orientation.
Different sections at several locations	Loft	Profile order, connections, and shape between sections.
One section along a straight direction	Extrude	Depth or end condition.
A section repeated around an axis	Revolve	Axis position and rotation angle.

These are starting choices, not exclusive capabilities. For example, a loft can follow a centerline, and a sweep can vary with advanced controls. Prefer the feature that states the intended change clearly.

Practice profile and path Practice changing sections

Challenge: choose the feature, then the one input that fixes it; Interactive

Challenge: choose the feature, then the one input that fixes it

Each target below comes with a failing attempt. Name the feature that states the shape, then pick the change that makes the attempt match the target.

Choose the feature, then the input to change. For each target, decide whether a sweep, loft or extrude states it, then which change fixes the failing attempt.

U-shaped handrail. One Ø20 mm round section follows a U-shaped centreline with a 30 mm bend radius. Current attempt: fails: the inner wall folds through itself.
Answer
Sweep. Yes: one constant section follows a bent centreline, which is what a sweep states. Change: Open the bend radius to 30 mm.
Square-to-round duct transition. A duct changes from a 60 mm square, through a Ø44 mm circle, to a 56 × 24 mm slot, with straight, untwisted sides. Current attempt: twisted: no crossing section found.
Answer
Loft. Yes: the section changes shape at known positions, which is what a loft states. Change: Move the circle’s start point back to 0° so it lines up with the square and slot.
Three-section transition built out of order. The same square → circle → slot duct, built bottom to top. Current attempt: wrong shape: the loft doubles back.
Answer
Loft. Yes: three different sections at fixed heights. Change: Select square, circle, slot: the order the profiles sit along the part.
Compression spring. Ø16 mm round wire wound with a 24 mm coil radius and a 28 mm pitch. Current attempt: fails: neighbouring coils collide.
Answer
Sweep. Yes: one wire section follows a helical path. Change: Increase the pitch to 28 mm.

Troubleshoot the inputs before changing every option; Checklist

Troubleshoot the inputs before changing every option

Check that the selected profile regions match the intended solid, surface, or thin result.
For a sweep, check path continuity and the profile's location and orientation.
For a loft, check profile order and each intended guide intersection.
Look for self-intersection: a thick swept section may overlap itself around a tight bend.
Inspect sections and confirm the expected part count; repair the first failed reference.

Shelling a complicated loft is a separate geometry problem: offset walls can collide where the shape becomes tight. Build and inspect the outer shape first, then check the wall thickness. See shell and other applied features.

Appears in

DET 102 DET Part Features