Manifold Geometry // Многообразная Геометрия

Sheet metal

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Another case with a broken sheet metal part

In this article, I'd like to give a couple of tips for fixing yet another "dirty design" problem. Look at the sheet metal part here:

This solid cannot be recognized (as a sheet metal) and unfolded because of a little flaw near one of the bend features. It feels like the two halves were modeled separately and then joined using a solid Boolean operation. Here is this slight (~0.24 mm) shift of one half with respect to another:

Surprisingly, such defects are quite tricky to fix, maybe because they are not always curable with fancy "push and pull" modeling operators, if you know what I mean. There is a little more to do with a part's geometry to make it clean and practical. We start with partitioning the part, i.e., separating its two halves onto individual solids. To do that, we first have to build a separation plane. In Analysis Situs, you can select a tiny "elevation" face and make a plane out of it using the following command:

> make-surf plane

This command extracts the host surface from the selected face and makes it an explicit variable in the project tree.

Once the plane is ready, you can use it to partition the model. The idea behind it is similar to that of a standard CAE-ish partitioning, where the goal is to generate non-manifold cells for subsequent FEA. Let's utilize the same apparatus to simply break down one solid into two pieces.

Partitioning turns the initial solid geometry into a couple of non-manifold cells as it can be seen from the following image:

To extract the solid bodies from the partition result, we go like this:

> explode -solid

What I did next is exported both solids to STEP files and opened them in Fusion360 where employed "Assemble / Joint" command to match the opposing faces. It remains then to fuse the parts back, e.g., like this (in Analysis Situs):

> bop-fuse res "SOLID 1" "SOLID 2"

And there we go. The part can now be recognized and unfolded:

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