The story of the SOLIDWORKS Boundary surface cannot be told without referring to the Loft feature. Loft was in the original 1995 release of SOLIDWORKS, and was only a solid feature at that time. Surfacing functions were not added to the software until 1998 onwards. The boundary solid and surface features were added to SOLIDWORKS in 2008. Since most or all of the available applications of Boundary involve surfacing, that is the part of the Boundary feature that this article addresses. Remember that a solid boundary feature already exists, but if you are making something sophisticated enough to need a boundary feature, you are probably already working using surfaces.

When Boundary was first released, it looked like a mostly redundant Loft feature. However, as the details of the new feature slowly came into focus through a lot of trial and error, comparison and research, Boundary quickly gained traction—and eventually won out over the Loft feature among those who work with surfaces daily.

Boundary has several advantages and differences when compared to the other interpolating features Loft, Sweep and Fill. Spoiler alert: Boundary is the first feature that many—maybe even most—SOLIDWORKS surface modelers use when called on to make a complex shape.

Accuracy

One of the advantages of the Boundary feature that the developers explained during the initial release was that the accuracy of the Boundary surface compared to the sketched curves is better for Boundary than for other features (except Fill). Before Boundary, many surfacers would complain that sometimes a surface resulting from multiple profiles didn’t actually touch the initial profiles. These profiles could be sketches, edges or curve features.

Loft and Sweep approximate profiles, especially non-spline shapes, which get converted to splines. Boundary may still do this, but it does a better job and approximates closer to the input geometry. This was a big deal, and helped propel Boundary over Loft.

Equality Between Direction 1 and Direction 2

Another factor that distinguishes the Boundary feature is that it treats Direction 1 the same as Direction 2. We all know that most surfaces in CAD are based on NURBS, and NURBS depends on a 2-directional U-V mesh of.

On a Sweep, one direction is called the path and the other direction is called the profile. On a Loft, one direction is called the profile and the other is called the guide curve. But in the Boundary surface, you just have Direction 1 and Direction 2, and they both have exactly the same controls. The Loft surface only allows you to apply Curvature Continuity (C2) to the beginning and the end of the loft, not to the sides.

The importance of the equality of the directions is shown in this example. We have the following shape with a hole in it that needs to be completed (without using the Delete Hole or Fill features). Let’s use Loft, and then Boundary.

Figure 1: Original Surface.

Figure 2: Loft patch, not as nice as the Boundary patch.

From these images, you can see from the smoother zebra stripe display that the Boundary surface patch (on the top) is able to get good results with Curvature to Face in both directions. Loft can only manage to use Tangency in both directions, and the zebra stripe display is not very good. Even though Curvature is available in the Profile constraints, Loft cannot make it happen when guide curves are also selected, and tangency is the only option available for the guide curve constraints.

A sample file for this is available in SOLIDWORKS 2020 format at this link.

Arrangements of Curves

The Boundary surface allows a bigger range of arrangements of curves compared to the Loft. Loft requires at least two profiles, but it can work with or without guide curves. Boundary requires at least two curves, and doesn’t care about which directions or combinations. It can use one Direction 1 and one Direction 2, or two Direction 1, or two Direction 2.

Loft also ends the surface at the profiles, and it won’t give you any extra surface if your guide curve extends past the profile. So, the curves might look like the lines in the letter T, L or E, but you could use an arrangement of curves that looks like an X or #.

For example, using the mesh of intersecting 3D splines shown below, with the purple being guide curves (Direction 2), and the blue being profiles (Direction 1), the resulting Boundary surface uses the complete length of curves in both directions, while Loft only uses the entire profile, but not the guide curve.

Figure 3: Original mesh of curves.

Figure 4: Loft doesn’t make use of entire length of guide curves.

Figure 5: Boundary makes use of entire length of all curves unless the Trim By Direction options are used.

Also notice the difference in the callout flags; Boundary allows you to use them to set tangency conditions, while Loft does not.

Boundary also has the often-overlooked option called Trim By Direction. So, you could have a ‘#’ arrangement, and then trim it back so that it only gives the center square as a result, or it might also give an ‘H’ type result. Trim By Direction can be useful, especially when you are using existing edges as profiles. You can download this mesh example from this link.

Figure 6: Trim By Direction allows you to decide how much of which curves to use.

Connectors

Boundary also allows you to use Connectors in both directions, whereas Loft only allows this in one direction. Connectors are like ad hoc guide curves, and they are the reason why you don’t hear the old lie about how “your loft profiles need to all have the same number of segments” anymore. Connectors are very powerful. Sometimes they can be the difference between a Boundary working and not working, and sometimes they can allow you to change the flow of a feature subtly, to make it really the right shape.

In the image below, the connectors are the purple dots that connect between the purple curves. In this case, the connectors help keeping the U-V sections lined up as the surface transitions around the bend. The alternative to using connectors is drawing a lot of profile curves. The connectors can be edited on the fly while the Boundary command is active.

Figure 7: Connectors help you control the flow of surfaces.

Internal Profiles

It is common to control the tangency of the first and last profiles on a loft or boundary. Sometimes you need to control the tangency direction of an internal profile. So, maybe the second of three profiles needs you to control the direction. Usually, you would do this with Direction 2 curves, but it never hurts to have more control. Try to place internal profiles to match the flow of the feature as best you can, but there are times where you need a little extra help.

Special Functionality

Degenerate Surfaces / Loft To Point

Degenerate surfaces are surfaces where all of the lines in one direction, U or V, come together in a single point—like at the North Pole on the globe. This is a situation you find in CAD models from time to time, but sometimes it causes problems. It can cause problems with offsets, shells, fillets and other types of features.

Both Loft and Boundary enable you to create this type of geometry, where the profile on one end is just a point. The typical example of this is a surfboard. When shelling out a model like this causes a problem, the typical solution is to cut off the degenerate end, and replace it with a Fill surface, which will be the topic of a future article. You can download this sample file at this link.

Figure 8: Degenerate features can be useful, but can also cause issues with other downstream features such as shell, offset and fillet. Boundary and Loft can both create features that terminate in a tangency to a point.

Drag Sketch

Drag Sketch is an option that is only available when all of the curves in the active Boundary (or Loft) feature have been created in a single sketch. This is usually 2D or 3D splines in a 3D sketch. It lets you dynamically drag sketch points as the surface updates. The option is found in the PropertyManager. This is very helpful for visualization while creating or editing surfaces. Download this sample file at this link.

Tangency Weighting / Apply To All

Tangency weighting allows you to control the strength or stiffness of tangency end conditions. It works similar to the tangency arrows at the ends of splines. The image shown below only uses two profiles, but the tangency weighting arrows are controlling the shape of the feature.

Figure 9: Tangency weighting can help you control the shape of a feature without adding curves.

Figure 10: The weighting strength directions can interact and give interesting shapes.

The Apply To All option is on by default. This means that any tangency weighting you apply to a curve applies to all nodes of that curve. If that option is turned off, then you can change the weighting for each individual node of a curve, as shown below. The nodes at the close end of the curves are set to heavy weighting, while nodes at the far end are set to weak weighting. This allows you a lot of control in certain situations where you might need extra flexibility without adding additional curves to your feature. Download this sample part at this link.

Curves Influence

The Curves Influence options depend on the type of profiles you have. If you have profiles with sharp corners, or multiple segments, it can give you options that other profiles don’t allow (such as a single smooth spline).

The options are:

• Global
• To Next Curve
• To Next Sharp
• To Next Edge
• Linear

These options mean that the curves in the given direction will affect the overall shape of the feature “globally” or “to the next curve” and so on. If you have a profile such as a rectangle with sharp corners, then the curves will only influence the shape up to the next sharp corner of the curve in the other direction.

Not all of the options apply to every situation. The solutions depend on how many curves are selected in each direction, and if your curves have multiple segments, non-tangent segments or other factors. In most cases, there are only a couple of solutions for all of the options. You might find it best to just cycle through the options and see which one is best.

Disadvantages

It’s not all peaches and cream, however. There are several functions that you can’t get with the Boundary that you can get with Loft.

• Centerline Loft (works like multi-section sweep).
• Closed Loop (you can make a closed loop with Boundary, but you have to use one of the curve directions to create an actual closed loop curve).
• Add Loft Section.
• Separated tangency/directional controls.
• Micro Tolerance .

Summary

The Boundary surface feature in SOLIDWORKS can be as simple or as complex as you want to make it. It is the basic surface type that most users go to when they need to make a complex shape. There are many options you can use like the Drag Sketch to make this feature easier to edit in real time. Many more examples can be found on the author’s subscription site, SOLIDWORKS Surfacing Episodes.

To learn more about SOLIDWORKS, check out the whitepaper Developing Better Products in the Cloud.