does anybody know, what criteria is relevant for a line to be centerline of a symmetric constraint?

Ulrich_Hanisch

i've created some sketch with just 3 lines in it. all 3 lines are the same, none of them is from type "construction" with dash-dot style. the orientation of all 3 lines is more or less arbitrary. they even cross each other. now i select them by rectangle selection and click "symmetric" ... by doing so 3 times (and do a redo between) i got 3 different results ... somehow quirky. it would be very very nice to be able to foresee a reproducable result for a constraint type which is that simple !!

here's my starting point of the tests:

https://cad.onshape.com/documents/3282f67f328e68e36f02a626/w/7d90ff8c606025b970e9ba14/e/f89e6309230563660cd3cf53?renderMode=0&uiState=62a87599a2481d64883fcb86

tim_hess427

I think the order of selection matters. So, perhaps, when you select by rectangle, the lines are being added to the "selection set" in different orders in the background. 

If you're super careful in making sure your box highlights the three lines in the same order each time, do you get the same result?

Ulrich_Hanisch

during my box selection, i get no highlighting. so i can not check this in detail and i would say: box selection plus symmetrical constraint is not usable.

i just can confirm: when i select the 3 lines click by click, it "seems" that the first line i select, is used as centerline.

"seems" because: in real life, i have some part, where this logic (first wins the center) does not work ... hm.

john_mcclary

the order of selection is important.

in general, you will want to separate your selections between usage of the tool.

so, either pre-select the geometry, click the symmetry tool, then select the line that would be the mirror line.
or select the symmetry tool, click the mirror line, then window select the rest of the geometry (but not the mirror line again)

if you select the symmetry tool, then just window select everything willy Nilly, then how would the computer know what line is intended to be the mirror line? So it just chooses the first instance it calculated from the query.
It isn't just finding the center most line. 

robert_scott_jr_

Ah! Thanks for that explanation. Now that I know it's protocol of use I now have a another tool at my disposal. - Scotty

Ulrich_Hanisch

ah, ok. i've tested some more, and yes, it's about the order of selecting the elements. doesn't matter if using ...

- element1 >> symm command >> element2 >> element3
- symm command >> element1 >> element2 >> element3
- selecting by single click or rectangle select
- selecting lines, edges, construction lines

... the first element1 is the centerline.

the only exception of all things is just the case, i had some days ago, which led me to my testings:

if you sketch three lines, all parallel to each other, and try to make the right one the centerline (by selecting the right one first) this doesn't work. maybe a bug or a limitation of the constraint solver ... i would expect here, that one of the other lines (element2 or 3) should flip over to the other side of the "centerline".