wrap misalignment

damir_kucherenko

Hi everyone,

I faced a behavior of wrap feature that confused me.
I'm not sure if it's a bug so I decided to ask a question first instead of reporting it.
I've created a cylinder and a plane aligned with it's surface
then I created 3 separate sketches with rectangular shapes and all of them positioned on the same distance from the center line but with different size.
I expected that all of them will be aligned after applying wrap function, but by some reason they are not

document:
https://cad.onshape.com/documents/ffbc031eafa46a162e4e271d/w/799e7c087acda3adf22b1623/e/3567f769253baf463a591274

in the "problem" part you can see the issue that I faced
in the "expected" part you can see how I think it should work by placing all rectangles in the same sketch

Am I doing something wrong?
Maybe I got a wrong idea how it should work?

emmett_weeks

In the absence of manually entered anchor points, wrap generates automatic anchor points by taking the bounding box of all of the tools, getting the center, and then projecting it on the target. If more precise alignment is needed, then manual anchor points need to be provided. https://cad.onshape.com/documents/bb7c6da59b9813d4f730c842/w/070df00caf7da8977cfd3c99/e/046428fd1dad0f3b895b487c is not great from a design perspective, but it does illustrate a way to align the individual wraps.

ben_bouws

Also, I think I figured out what is actually happening that causes them to be offset, because each sketch does not directly intersect the cylinder, when Onshape calculates the wrap it rotates each rectangle about it's center point to make it tangent to the cylinder, and then it is able to wrap each rectangle. Because each rectangle is a different width they each have different center points that they are being rotated about which is what causes them to be misaligned.

emmett_weeks

In the absence of manually entered anchor points, wrap generates automatic anchor points by taking the bounding box of all of the tools, getting the center, and then projecting it on the target. If more precise alignment is needed, then manual anchor points need to be provided. https://cad.onshape.com/documents/bb7c6da59b9813d4f730c842/w/070df00caf7da8977cfd3c99/e/046428fd1dad0f3b895b487c is not great from a design perspective, but it does illustrate a way to align the individual wraps.

ben_bouws

I don't quite understand why you can just use all one sketch like you did in the expected tab, however, if you want to use separate sketches I think the problem is that Onshapeis getting confused about where to place each wrap because your tools never directly intersect the cylinder you are attempting to wrap around. There are a few ways you can work around this, one is is to create a separate sketch and wrap surface that you can use as anchor points for the rest of your wraps to line them all up (solution 1). Another solution would be to not offset each sketch from the center line but instead have it coincide with the center line, this would make the sketch and the cylinder intersect giving Onshape a better reference for were to place each wrap. (solution 2)

ben_bouws

Also, I think I figured out what is actually happening that causes them to be offset, because each sketch does not directly intersect the cylinder, when Onshape calculates the wrap it rotates each rectangle about it's center point to make it tangent to the cylinder, and then it is able to wrap each rectangle. Because each rectangle is a different width they each have different center points that they are being rotated about which is what causes them to be misaligned.