9 Replies Latest reply on Apr 16, 2018 11:26 AM by Dan Pihlaja

# Offsetting 2 Tangent Arcs - Works for unequal radii - fails when equal

Hi All

I'm surprised I haven't run into this before.  Here is a simplified version of my sketch - the left half of a 4 center arch.

For model reusability I was hoping to be able to make the radii equal (a 2 center arch) but SW refuses to play ball

Here I tried to make the 100R = 50

It will accept slightly above or below 50 but not equal.  Anyone any ideas what the problem is?

Thanks for looking

2016 attached

• ###### Re: Offsetting 2 Tangent Arcs - Works for unequal radii - fails when equal

Rob,

Hopefully one of the folks smarter than I am can figure it out, but I suspect it's just some kind of bug.  I got the same results you did when trying to change the 60 to 50, and also when trying to link them with an equation; however, when I deleted the 60 dimension and added an equal relation between the two arcs it worked fine.

• ###### Re: Offsetting 2 Tangent Arcs - Works for unequal radii - fails when equal

Glenn Schroeder wrote:

Rob,

Hopefully one of the folks smarter than I am can figure it out, but I suspect it's just some kind of bug. I got the same results you did when trying to change the 60 to 50, and also when trying to link them with an equation; however, when I deleted the 60 dimension and added an equal relation between the two arcs it worked fine.

Thanks Glenn

You replied while I was writing my second post, there's not many smarter than you!

• ###### Re: Offsetting 2 Tangent Arcs - Works for unequal radii - fails when equal

Im guessing that it may be something to do with the arc centers

From this sketch both types of equal relations on the arcs are suppressed.  The top radius is set to driven, as is the height, yet the top arc is black, maybe it's just my monday brain, but surely this sketch should have more blue

• ###### Re: Offsetting 2 Tangent Arcs - Works for unequal radii - fails when equal

Interesting.  If you don't chain the offsets and create three separate offsets, it can solve the sketch:

But it does not behave well when changes are made after you have made them equal:

The problem seems to be the point where the two arcs meet.  It is not fully defined so SW doesn't know how to move it when changes are made.  If you add an angle, to locate it, the sketch behaves:

• ###### Re: Offsetting 2 Tangent Arcs - Works for unequal radii - fails when equal

Thanks Jim and Dan , both great suggestions that helped....

I've managed to get a sketch that works...  although not very well.

I didn't apply an offset to either of the arcs only to the vertical line below them.  I manually drew the the two offset arcs and merged their centre points with the cp of the corresponding other arc (two less points for the solver)

To get the tangent relation - I used a horizontal relation, and a coincident relation on a construction line, joining the shared arc vertex and the center points.

To get the arcs equal, I set the radii to driven and unsuppressed the equal relations.

Going this way is easy, the difficulty is returning to an unequal state.

I had to change something else to get the arcs to solve independently.  Nudging the height (1730) while one of the dims was still driven worked - once the radii were slightly different the sketch could solve again.

This seems like a bit of a future nightmare to be honest, but I'm not sure of a good alternative.

Thanks again, this will have to do for now

2016 part and video attached

• ###### Re: Offsetting 2 Tangent Arcs - Works for unequal radii - fails when equal

Well, can you make them 2 separate sketches?  1 that ends at the end point of the first radius, and another that begins at the same point.   Then you can control the end point of the first sketch and just make the 2nd sketch whatever you want it to be (you can control these then with Global coordinates (or equations)).  Then, if you still need it to be all 1 sketch, then just create a 3rd sketch and convert entities to get it.

This seems like it would work in my head.

• ###### Re: Offsetting 2 Tangent Arcs - Works for unequal radii - fails when equal

I was thinking along similar lines, maybe something like this?

but this is my skeleton for a reasonably complex model, I'm nervous about starting out only to hit complications later

• ###### Re: Offsetting 2 Tangent Arcs - Works for unequal radii - fails when equal

Rob Edwards wrote:

I was thinking along similar lines, maybe something like this?

but this is my skeleton for a reasonably complex model, I'm nervous about starting out only to hit complications later

Seems to work pretty well.  See attached video.