OnShape and etched fold lines

Due to flooding, progress on real modelling has halted, and I’ve had to move into the virtual realm.  This is okay, as there is loads of drawing work yet to do on 622.

I speculated a couple of months ago that some careful manipulation of OnShape’s new Sheet Metal Model feature might yield a good pattern for etching.  Tonight, armed with an excellent resource on design for etching from Geoff Baxter at Hollywood Foundry, I decided to test this theory.

My first task was to figure out how to make accurately-sized fold lines.  According to Geoff, these should be the same thickness as the etched material.  The resulting fold has two edges meeting on the inside of the fold, trapping an elliptical void.  The inside radius of such a fold is 0.

Unfortunately, the inside radius of a fold can’t be zero in OnShape, probably due to some nasty divide by zero error if you try.  Setting the bend radius to .0004″ is about as small as you can go.  This rounds to zero for most dimensions, unless there are two folds side by side, when it rounds to .001″.

It’s easy to get excited about a thousandth of an inch, for some reason.  But let’s put that in perspective: it is unlikely that the folds will be perfect anyway!  In practice, when you fold on an etched line, one side overlaps the other by a thousandth or two.  So, let’s not worry too much about this inaccuracy, but be aware of it and the possibility of compounding it.

To make the fold line in the flat object equal to the material thickness, when the radius is .0004″, set the K-Factor of the model to 0.625.  Using this factor, OnShape will automatically lengthen a .020″ thick sheet by .020″ to accommodate each 90 degree fold.

Sadly, 180 degree folds are not possible in OnShape, probably because the resulting geometry behaves poorly.  The best you can do is 179 degrees, and this results in a fold line that is twice the material thickness.  To make a 180 degree fold, you could chain two 90 degree folds with a .0004″ flat between.  The fold line will then be more than double the material thickness, however.

Naturally, because OnShape doesn’t know about etched sheet metal models, it doesn’t distinguish between regular folds and under folds.  It also doesn’t know about half-etching the part before bending it up.  These nuances will need to be resolved in editing after the fact.

Finally, I need to figure out the workflow for turning the flat models into something that I can place in an etch.  Exporting from OnShape yields an error when importing directly into InkScape.  QCADcam can convert the OnShape DXF into a file that InkScape can read, but along the way, I am losing the scale.  More experimentation to come!

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