Reduce the Complexity in Controlled Surface Desgn
I have been working on a project that is enclosed within an organized, organic shape. I learned numerous things about surfaces during the process. While I cannot show the project details directly, I can discuss the workflows and key factors that made surfaces successful in this application.
Where Are Surfaces Useful?
- Managing Data
- Basic Surface Tools
- Working in Symmetrical Sections
- Using Derived Surfaces to Create Components
Anytime you need to control the limits of a complex shape, surfaces can play an important role. Furthermore, when that shape is also the control of other component features, especially in situations where concentricity and parallelism play a role, surfaces may be your only realistic option.
Figure 1: A tube-contained robotic concept—surfaces, work planes, and parameters drive the entire build.
I had an outer shape that was important to my client, which subsequently limited most of the design. The outer shell and inner shield needed to be held at a consistent offset; surfaces became the control for most of the design.
Another area where surfaces become very important is in sheet metal. There are some shapes that Autodesk® Inventor® Sheet Metal cannot create, and there is simply no alternative but to employ surfaces to create those components.
Managing Data
I am always looking to parameters to provide an easy method of managing the overall design and making the recall of feature-specific data easier.
I typically load a master skeleton part file with all the overall design parameters, typical tooling sizes, and sheet thicknesses. I then use these as a tool to develop the outer shape limits that the client desired. Work planes are used to transfer organized division throughout the design. Once completed, the skeleton usually contains two or three stitched surfaces, numerous work features and user-created parameters. Solids are rarely employed in my master skeleton files.
- Parameters pass numeric control to the surfaces (and work features).
- Surfaces pass geometric control to the component features.
Figure 2: The Master skeleton. Note the parameters, surfaces, and work features. The work planes make it seem busy, but the Model Browser shows that there are really only a few key features to control the main shape of the machine. The outer shape is automatically controlled by equations that rely on the length to OD ratio.
Once the skeleton is developed, it is passed onto the entire design in one manner or another. When the outer shape is changed, or a liner thickness needs to be altered, changes in the master skeleton are perpetuated throughout the design.
Sub-assembly and Construction Skeletons
In order to reduce complexity in the design, I will usually employ sub-assembly skeletons. This tactic allows the following:
- Keeps the master skeleton much less complex.
- Reduces the number of duplicated operations such as the same surface offset in adjoining components.
- Allows convenient creation of multi-body parts and feature-specific parametric controls.
In each sub-assembly skeleton, I would derive only the surfaces, work features, and parameters necessary to drive the individual components in the sub-assembly.
Figure 3: The derived outer casing skeleton. Note that only a few features were required to be derived from the main skeleton part file.
Note: These skeletons do not necessarily have to represent the actual sub-assembly to which the features will ultimately be assigned. In some cases you could call these construction skeletons.
The outer shell and inner liner represented components that were similar and concentric in nature. One option would be to develop the entire outer shell pair and possibly the inner liner in a single multi-body part file—all offset from a single derived surface.
Derive the Simplest Forms Possible
Too many parameters, sketches, and surfaces can convolute a skeleton or basic part file. A clean set of instructions is preferred. Go a step further and derive only the simplest surfaces required (see Figure 4). This can reduce needless downstream complexity.
Figure 4: Individual surface parts remain after stitching. It is possible to derive these instead of the entire surface. This option can be a valuable resource when editing derived components, so name these appropriately.
Work Directly on Surfaces
Not only can you benefit from the simpler partial surfaces in the master file (or conversely, trimming down the derived surfaces), but also there is value in applying as much work directly to the origin planes, derived work features, and surfaces.
You have likely been encouraged to stick close to your origin planes and work features, but what about when the solid bodies have been thickened? The tendency is to work off of the solid features.
Tip: If you have the opportunity, turn off the solid body and apply your construction geometry and references to the base surface. Then turn on the solid, and get back to work.
Why?
Because solids can get fouled up after a bad feature application/edit. The surfaces will often remain unharmed even when a solid disintegrates. If the surface survives, so do all your downstream construction features.
Basic Surface Tools
After the derived feature process is complete, I use the sketched geometry and the following surface tools to develop the components.
Figure 5: The main surface tools.
Surface Patch – creates a surface based on the shape of bounding features and sketch geometry.
Surface Trim – allows seamless, precise fitting of surface pieces, trimmed by surfaces, planes, and sketch geometry.
Surface Stitch – is much like stitching a quilt together out of pre-trimmed surface parts.
Almighty Thicken/Offset – allows surfaces to be either offset from another surface in a concentric manner, or a solid to be built by adding a thickness to a surface.
Specific surface features can be applied to the sections using sketch geometry and the Extrude, Revolve, and Sweep tools. In these cases, employing Project Cut Geometry in the sketch is a design necessity in order to reference the section surfaces.
Figure 6: Using derived parameters and surfaces as well as the Project Cut Edges tool to guide the lip’s sketched revolve geometry.
Working in Symmetrical Sections
Once the basic surfaces for your part are derived and offset, etc., if your part is symmetrical it is wise to apply any and all features that appear in that symmetry. The strategy is to:
- Minimize the complexity of the reference surface.
- Reduce the number of features applied to the surfaces.
We can do this by trimming down the overall surface into the appropriate single section of the symmetry/pattern and developing all basic features at the surface level. When parametric changes are passed down from the main skeleton files, only a fraction of the design is actually being adjusted. The rest of the design is simply a copy.
My Design Example
My project design was orthogonally developed about a centerline axis, so many components were easily built in a single quarter, and then mirrored about the origin planes of the hemisphere. In order to do this, the main-derived surface was cut into a hemisphere by a named work plane, and then quartered by the part origin planes using the Surface Trim tool.
Figure 7: The thickened liner example. Note the outline of the derived outer surface (blue), and the resulting surface build (red). The preliminary surface was derived from the overall, trimmed into a 1/6 section, and trimmed again by two controlled “division” work planes. A surface patch was applied to close up the angled face, and all were subsequently stitched together. The result was then duplicated six times in a circular pattern, and then stitched together again.
Note: A clean work plane trim is required for the final parts to be mirrored back into place properly.
To Fillet or Not to Fillet
Specific radii can be applied directly to the surfaces, such as roll formed and stamped sheet bends, using the Fillet tool. This allows consistent thickness offsets on the sheet when the solid is created by the Thicken tool, and only one fillet operation is needed.
Tip: If you are working in sections, you may want to hold your feature fillet operations until the entire surface (mirrored, patterned, etc.) has been stitched together as a complete unit. Quite often, additional fillet operations may still remain when the entire surface is stitched.
A single fillet was applied to the liner’s final surface shown in Figure 7. I used the “Feature” option, and picked the final surface object. Inventor then added the radii to all edges in one operation.
Note: In my project design, I was able to perform most of my fillet operations in the section prior to patterning. A somewhat rare opportunity, but this made the design quite simple.
In some cases it can be quite convenient to thicken the surface section and join patterned solids onto the final shape. Beware of your seams—convex thicknesses often leave gaps.
Other Areas of Concern
Another area that presented a specific problem was where a component was to be molded, and an odd rib had to be formed along a centerline seam for support. This had to be added as a sketched feature, and Swept into place after the solid was built.
Using Derived Surfaces to Create Components
Observe how surface features behave when thickened. The question that should be asked is, “Should the feature be applied to the sheet before or after it is formed?”
Holes applied to the surface and then offset will be stretched differently on the upper and lower sheet edges. If the hole needs to be consistently the same parallel diameter throughout, then the hole needs to be applied to the solid after the surface is thickened as if it will be drilled and reamed after the sheet part is formed. Sometimes however, this stretch is advantageous.
Using Sheet Metal Tools
Whenever possible, I used Inventor Sheet Metal to take care of the component design. The derived surfaces and work features were used to develop the base sketch geometry, and then the remaining faces and flanges were handled with sheet metal workflows.
Closing thoughts
It will take significant practice to determine which application of these ideals will fit your needs. I tried to stress the benefit of reducing the complexity by:
- Referencing surfaces
- Deriving the simplest information (including partial surfaces) necessary
- Leveraging as much symmetry as possible
- Understanding when to move ahead with features and solids
These will quickly enhance your update resilience.
Remember, if you are employing intersecting surface trims, etc, any sketches that reference drastically changing projected cut edges will go rogue. Another reason to derive the simplest surface necessary, and not the entire master shape.
If your surface trims become insane and cumbersome, you will lose the robustness of the design. Often, a little reflection and a return to the master skeleton file can help approach your automation from a different direction, and bring your seamless updates back to life.
It may sound as if I am trying to discourage you from using surfaces. Quite the contrary—I hope these tips and notes will help you make this powerful control option a success.
