Tangent arcs flip directions

christoph_schmitz

Hi,

in one sketch I use a couple of arcs that flow into straight lines. Now an arc can be tangent to a line by continuing in the same direction (0 degrees), or go right back in the direction the line came from (180 degrees). In my model, some of the tangent arcs flip back and forth between the two depending on other geometry.

Is there a way to constrain the arc and the line in a way that prevents the 180 degree case? I'd rather have my sketch break than turn into some contorted shapes that are hard to move back. I have tried to add an angle constraintgeometry at the joint between the arc and the line, but that doesn't seem to work.

Thanks

Christoph

NeilCooke

I just think it needs a little help - this seems to work:

1. Create 2 construction lines tangent to the ends of the other curves and dimension the length equal to your tangency value:



2. Create a spline coincident with the start of those curves and snap the spline tangency handles to the ends of those curves:

NeilCooke

Hi @christoph_schmitz - short answer is no. How severe are the topological changes to your sketch? You may want to consider either not adding fillets to your sketches and add them as fillet features to the model or make the sketches less complicated. If you share the doc URL I can take a look at why it might be happening.

christoph_schmitz

Thanks Neil,

I think in part I was just being stupid :-), see below.

Here's a document that illustrates what I observed:

https://cad.onshape.com/documents/f4fc462cbce27d8bfdbc5d95/w/089a8c7db69016406335e7b5/e/8046bbfbfe9d50ebd24f9401

... in the Misc/Tangent Problems part studio. I was trying to create a boomerang shape from six arcs and two straight lines by first drawing the shapes (Sketch 1), then connecting those with coincident constraints (Sketch 2), then adding tangent constraints (Sketch 3 is what I want). Sketch 3a is what I sometimes end up with when the tangent constraints being added on the arcs throw things around quite a bit and then I try to drag all the arcs and lines into their approximate places before dimensioning them and tying them down, so to speak.

The problem, I guess, is that I should not try to add the tangent constraints last and wait for things to fly all over the place.

Christoph

christoph_schmitz

There is a slightly less contrived example in the Sweep Half Airfoils/Configurations folder. In one configuration called "Traditional", the splines  that define the wingtips flipped outside in, while in other configurations it is ok. In this case it's a spline, though, not a tangent arc.

NeilCooke

@christoph_schmitz I see it. When you are creating these splines are they close to their final shape before you add dimensions? If the change (size/location) caused by the configuration is big, this could be the cause.

NeilCooke

I just think it needs a little help - this seems to work:

1. Create 2 construction lines tangent to the ends of the other curves and dimension the length equal to your tangency value:



2. Create a spline coincident with the start of those curves and snap the spline tangency handles to the ends of those curves:

christoph_schmitz

That's brilliant, thanks!