coincident vs. pierce

rune_thorsen229

john_mcclary I found use for pierce. Still I do not fully understand why there have been implemented two types of constraints: coincident and pierce that fundamentally seems to do the same?.
Why is coincident not simply implemented as a pierce when the planes differs?

NeilCooke

Pierce is used when you want to make a sketch entity coincident to the point where external geometry intersects the current sketch plane. Coincident is for "projected" geometry only. We could solve behind the scenes and call it all coincident, but the two types are there to make it clear which type of geometry you are referencing.

rune_thorsen229

Found this old one: January 2015brucebartlett .  I tried to make some experiment that illustrates the difference. The only difference I can find is that coincident does not always work and then I resort to pierce (as john_mcclary) taught me. 

@NeilCooke sooner or later a student or collegue will ask me so could you please provide an example of the functional difference?
Thanks in advance

NeilCooke

rune_thorsen229

NeilCooke. It moves the line so it touches the wave. How would a coincident behave differently?
What I mean exactly is that the Pierce does what the word 'Coincident' says, whereas Coincident behaves more like a projection.

Dashed line: coincident (with the plane)
Solid: pierce

NeilCooke

That point is now fixed - looking at it from the front it looks like it is coincident to a vertical line and therefore should be able to slide up and down. 

rune_thorsen229

NeilCooke , I don't fully understand. Could you provide the animation to confront with?

NeilCooke

https://cad.onshape.com/documents/36758a4ca8d3bbab9cd1474a/w/7b2a62ef43f244c973242d55/e/a6f69d292ce778aae9188c75

rune_thorsen229

Hmm, maybe I should have cited January 2015 brucebartlett  "Most people don't understand the difference between pierce and coincident in a sketch and I don't see the need to separate these into 2 different constraints."

john_mcclary

You need them to be separate functions. There are times where both will actually constrain.

Pierce will actually touch the line/curve where it intersects the sketch plane.
Coincident will project the constrained point along the entire line/curve as if it were projected onto your sketch as a vertical line (in the example I have shown)


https://www.youtube.com/watch?v=YbN9Oilv1AU