6.3 Loft Boss/Base & Loft Cut
Key Takeaways
- A loft blends two or more profiles on different planes, so it is the tool for transitions where the cross-section changes—round-to-square adapters, tapered bosses, and blended handles.
- Loft connects profiles in the order you click them, and it pairs vertices with green connectors; clicking the wrong corner or the wrong order produces a twisted solid.
- A sketch point or vertex may serve as a loft profile only at the first or last station—never in the middle.
- Start/End Constraints (None, Normal To Profile, Direction Vector, Tangency To Face) control how the loft leaves each end profile and change the solid's volume.
- Loft volume is not the average of the two end areas; use the prismatoid estimate h/6 × (A1 + 4Am + A2) only as a sanity band and read Mass Properties for the graded value.
6.3 Loft Boss/Base & Loft Cut
Quick Answer: A loft blends between two or more profiles that sit on different planes. Extrude keeps one section, revolve spins one section, sweep drives one section along a path—only loft lets the section change. SOLIDWORKS names lofts in the official CSWA hands-on skill list, so a transition shape on an advanced-part item is fair game.
When the drawing is telling you to loft
Read the section views. If the shape has different cross-sections at different heights and the material between them changes smoothly, that is a loft:
- A round-to-square duct or hose adapter
- A tapered boss whose top is smaller than its base and whose sides are not a simple draft angle
- A blended handle that starts as an ellipse and ends as a rectangle
- A funnel or transition collar
Contrast this with a plain taper. If the drawing shows a rectangular base and a geometrically similar smaller rectangle on top, an Extrude with a draft angle may be simpler and faster. Loft earns its cost when the section changes shape, not merely size.
| Cross-section behavior | Best feature |
|---|---|
| Identical at both ends | Extrude |
| Same shape, uniformly smaller | Extrude with draft |
| Same shape, follows a curve | Sweep (6.2) |
| Different shapes at two or more stations | Loft |
Building a loft: the plane-first workflow
Lofts are the feature that makes Chapter 11's reference planes non-optional. The usual recipe:
- Create the offset planes. Insert → Reference Geometry → Plane, first reference = an existing plane, Offset Distance = the height from the drawing. Repeat for each station.
- Sketch one profile per plane. Each profile lives in its own sketch. Fully define every one.
- Insert → Boss/Base → Loft.
- Click each profile in order, near corresponding points.
- Check the preview, set constraints, click OK.
Selection order is a real setting
Loft joins the profiles in the order you select them. Select bottom → middle → top and you get the intended stack. Select bottom → top → middle and SOLIDWORKS tries to run the solid up, back down, and up again. If the preview looks like an hourglass or a knot, the order is wrong. Use the up/down arrows in the Profiles box to reorder without restarting.
Connectors: the fix for a twisted loft
When you select profiles, SOLIDWORKS shows small green connector dots that pair a vertex on one profile with a vertex on the next. Those pairings define how the surface runs between stations.
- Click near the same relative corner on every profile (for example, always the front-left corner) and the connectors line up.
- Click a square's front-left corner and then the next square's back-right corner, and the solid twists 180° between them.
- To repair without restarting: drag a green connector dot to the vertex it should pair with, or right-click a connector and choose Reset Connectors.
Trap: A twisted loft often still builds successfully and still reports a mass. It does not error out. You must look at the preview—an item graded on center of mass will punish a twist that mass alone would hide.
Points as end profiles
A sketch point or a model vertex can act as a loft profile, which is how you build a cone, a pyramid, or a tapered spike. The rule: a point may be the first or last profile only, never an intermediate one. A loft from a Ø40 mm circle up to a single point 50 mm above it is a cone; that is a legitimate, fast construction for a conical feature.
Start and End Constraints
These control how the solid leaves the first profile and arrives at the last one. They change the surface shape—and therefore the volume and the mass.
| Constraint | Behavior | When the drawing wants it |
|---|---|---|
| None | Straight ruled surface between profiles | Default; flat-sided transitions |
| Normal To Profile | Surface leaves perpendicular to the end profile's plane | Bosses that must start square to their base face |
| Direction Vector | Surface leaves along a selected edge, axis, or plane | Controlled lead-in at a specified angle |
| Tangency To Face | Surface runs tangent to the adjacent existing face | Blending a loft onto geometry already modeled |
Each constraint also exposes a Tangent Length (start/end tangency) slider that pushes the surface further before it curves away. Increasing tangent length makes the shape fuller and increases mass. If the drawing does not ask for tangency, leave the constraint on None—adding it invents volume the answer key does not have.
Loft Cut
Insert → Cut → Loft removes the lofted volume instead of adding it. Same ingredients, same connector rules. Exam-shaped uses include a tapered pocket that is rectangular at the opening and circular at the bottom, or a blended relief cut into a boss. As with every cut, the volume you remove lands directly in the graded mass number.
Guide curves and centerlines (know the names)
At associate level you rarely need these, but you should recognize them:
- Guide curves steer the surface between profiles, so the transition bulges or pinches along a defined edge. Guide curves require Pierce relations to each profile, exactly as in 6.2.
- Centerline Parameters loft along a centerline so the sections stay perpendicular to it—useful for curved transitions.
If a drawing gives only end sections and a height, you need neither. Adding a guide curve you were not asked for changes the shape and the mass.
Why lofts fail
| Symptom | Root cause | Fix |
|---|---|---|
| Solid is twisted or pinched | Connectors paired to the wrong vertices | Drag connectors, or reselect near matching corners |
| Preview loops back on itself | Profiles selected out of order | Reorder with the arrows in the Profiles box |
| Result is a surface, not a solid | One or more profiles are open | Close every profile |
| Cannot select a point in the middle | Points are valid only as first or last profile | Move the point to an end station |
| Self-intersection error | Profiles too dissimilar or too close together | Increase the station spacing, or add an intermediate profile |
| Mass is too high | An unintended tangency constraint added fullness | Set start/end constraints back to None |
Mass properties on a loft
A loft between two parallel planar profiles is a prismatoid, and the prismatoid formula gives a good estimate:
V ≈ (h ÷ 6) × (A₁ + 4Aₘ + A₂)
where h is the distance between end planes, A₁ and A₂ are the end areas, and Aₘ is the area of the section halfway up.
Work one: a loft 50 mm tall from an 80 × 80 mm square (A₁ = 6,400 mm²) to a 40 × 40 mm square (A₂ = 1,600 mm²), with a midsection of roughly 60 × 60 mm (Aₘ = 3,600 mm²):
V ≈ (50 ÷ 6) × (6,400 + 14,400 + 1,600) = 8.333 × 22,400 ≈ 186,667 mm³
Notice what the naive average would have given: (6,400 + 1,600) ÷ 2 × 50 = 200,000 mm³—about 7% high. On a numeric-entry item, 7% is a zero.
Use the prismatoid number only as a sanity band. Guide curves, tangency constraints, and non-similar profiles all break the assumption. Apply the specified material and read Mass Properties for the answer you type in.
Worked exam-style scenario
Drawing: a transition block, 60 mm tall. Bottom section is a 70 × 70 mm square on the Top plane. Top section is a Ø40 mm circle. Sides blend straight between them. Material Alloy Steel. Report mass in grams and center of mass relative to the default origin.
- Units → MMGS.
- Sketch the 70 × 70 square on the Top plane, centered on the origin. Fully define with a Midpoint or symmetric relations so it is centered—this matters for the CoM answer.
- Reference Geometry → Plane, first reference Top plane, Offset 60 mm.
- Sketch the Ø40 circle on that new plane, concentric with the origin.
- Insert → Boss/Base → Loft. Click the square, then the circle, near the same side of each.
- Inspect the preview for twist. Leave Start/End Constraints on None—the drawing says sides blend straight.
- Apply Alloy Steel, then Mass Properties.
Because both profiles are centered on the origin, CoM X and Y should read ≈ 0. If either is non-zero, a profile is off-center—fix the sketch, do not fudge the typed answer.
Section checklist
- One profile per sketch, each on its own plane, all fully defined.
- Select profiles in order, clicking corresponding corners.
- Inspect the preview for twist before clicking OK.
- Points allowed only as the first or last profile.
- Leave Start/End Constraints on None unless the drawing asks for tangency.
- Sanity-check with the prismatoid estimate; type the Mass Properties value.
A loft between two square profiles builds successfully but the solid is visibly twisted 180 degrees. What is the most likely cause?
Where may a sketch point be used as a profile in a loft?
A drawing shows a boss whose base and top are the same rectangular shape, with the top uniformly smaller. Which feature is the faster correct choice?
For a 50 mm tall loft from an 80 x 80 mm square to a 40 x 40 mm square, why is averaging the two end areas and multiplying by the height an unsafe way to get the answer?