Interesting Sketch/Beam Interaction

matthew_stacy

Today I learned an interesting characteristic of multi-element Beam features.  Each Beam feature includes arrows that you can drag to control which node of the profile sketch anchors to the guide sketch.  When multiple beam segments are created within a single feature, I found that this guide/profile realignment very much depends on the direction in which the guide segments were sketched.  Consider the following example:

• https://cad.onshape.com/documents/3058b3efbf72b81d54cc64cc/w/1a2ff82917001b6029536c56/e/6bf2104af33ba6b75ee1865d

Sketch 1 is comprised of 3 horizontal line segments:

1. upper-most line segment sketched from left-to-right
2. middle line segment sketched from right-to-left
   
3. bottom line segment sketched from left-to-right (parallel to 1, anti-parallel to 2)

It should be noted that there are no clues that I am aware of, by which a user can distinguish the origin of a line segment from it's terminus (so I cheated by hanging a triangle, that represents an arrow head at the terminus).  All three of these segments are utilized as guides in a single beam feature, Beam 1. 

Edit the beam feature by clicking and dragging the alignment arrow:

Notice that the beam elements associated with parallel line segments translate up/down in lockstep with each other.  However, beam elements associated with anti-parallel line segments move in opposite directions, toward or away from each other.  This is not a bug per se, but rather a software idiosyncrasy that you can either struggle with or take advantage of.

When you can't get multiple beam elements to behave the way you want.  You can either go back to the guide sketch and redraw specific line segments to reverse direction, or apply additional independent Beam Features.

matthew_stacy

Here is another example:  https://cad.onshape.com/documents/3b6ce2285c88bb7d0920155a/w/02441452ffd686e9f2e91944/e/e972483a5447bfa9c9b86122

Both Part Studios show essentially the same weldment (beam) structure modeled in almost exactly the same way.  But note that the diagonal brace is INSIDE the sketch line in Part Studio 1 and OUTSIDE the line in Part Studio 2.

The difference is that in Sketch 1 the direction of the diagonal line segment was reversed.  That change is not visible at the sketch level, but manifests in the Beam 3 feature.

EvanReese

yep! every curve, even 3D ones, have an explicit "start" and "end" point, which in Beam is used to orient the beam direction. It's also relevant, I think, to how chamfer decides which way is which (this matters with non-45° chamfers). Sometimes I have to use two chamfer features where I'd like to have just one, because the orientations don't match and there's no way to flip it on a per-edge basis.

matthew_stacy

@Evan_Reese, can you envision a way to identify start/end points on a sketch entity or 3d curve?  Right now it seems to be a swing, miss, try again proposition to swap start/end points.  One might hope to guess correctly 50% of the time ... but my luck has been running below expectation.

Alternatively would it be possible to create a Swap-Start-End-Point feature script? 

EvanReese

I think right now you have to just guess and check or remember which way you drew your lines. I'm sure a custom feature that lets you see them is possible, and probably pretty simple. I don't know about flipping the direction of an edge, but just making a new curve on the selection that's flipped wouldn't be too hard.