Arc / Circle / Curve Segmentation in Sketches

larry_hawes

Thought I added this comment earlier but why not a sketch tessellation setting; coarse, medium, fine, extra fine etc. with performance hits as required?

kvdmolen

I have the same issue: trying to fit a circle onto a curve:



Zoom out:



But I get this and the connection of the circle and curve (after cutting the down-part of the circle):





@NeilCooke Has this bug been looked at since June 2019?

john_mcclary

can you share your model above? 

NeilCooke

@Klaas - I don't know, probably not since it is something we've always known about. The unfortunate issue with computer graphics tesselation is that there is a finite number of segments that an arc can be split up into and that limit has to be set somewhere as a trade-off between graphics and performance. If you zoom in close enough, there will always be a graphical mismatch when arcs are involved even if we increased the tesselation. If the constraint is set, the geometry is accurate, despite how it looks in the sketch. For small fillets like these, it would be better to add them to the model rather than the sketch. Vote on an improvement request to get this changed (I should not have classed this as a bug originally).

kvdmolen

@john_mcclary dit is the (cleaned) document:

https://cad.onshape.com/documents/1dc31ec0617759f1ad29162f/w/8efa318f66b229012d883ec4/e/59f3871d8c7cb810700cb4eb

@NeilCooke Thanks for the answer.

More detailed, my situation: I have a lever (drawn inc fillets). The lever sinks into a part, and needs to rotate about 50 degrees.
The base has a 0.2mm clearance wrt to the lever. 

Creating the fillets in parts is a good option, but because of the offset & clearance I prefer using fillets in the sketch.

In the document you can see the double points as described above after cutting the lower part of the circle in sketch 2.
Now I can't create the 0.2mm offset....

PS. Perhaps there are other solutions for this particular situation, however, I came across the limited "precision".
As you mentioned, of course there is a maximum of points to render.
However, when making cuts or edits in drawings, then I would expect onshape to use parametric data?

Thanks!

john_mcclary

The overlap you are seeing in your sketch is not a glitch or bug.

Your white piece is a squared shape inside of the arc.
see this exaggerated image:

The fillet's endpoint will not touch the arc. 
You "use" the fillet of the white arm's sketch, so the fillet is still full length in your new sketch. You will need to trim there as well to remove that artifact.

If you follow through with your trimming it would look like this:

kvdmolen

@john_mcclary you are absolutely right. Thank you for the clarification. The "rough" tessellation put me on the wrong foot.

I definitely vote for tessellation-setting in sketches. We are only talking 1/10ths of millimeters here..
It will avoid these kind of mistakes.

For example, the piece in between, rendered at a different location than the actual curve:

john_mcclary

Rough tessellation is in every CAD I have ever used, it's just something you get used to. At least in AutoCAD they let you use the "regen" command on the fly which would scale the tessellation for the level of zoom you are at

Solidworks will improve tessellation as you zoom in with drawings, but that ends up being a huge lag monster when zooming in and out.
Of course you can't just increase the tessellation all of the time, because even the dumbest models could kill your speed.

Its a fine balance between pretty and functional. So is the limit of digital technology.

I actually get better tessellation and performance in Onshape than i do in Solidworks, so they are doing something better than the competition in this regard. Not bad for something that can run on a cell phone.

kvdmolen

Ok, I see your point, let me get used to this

Though I do like the AutoCAD option: regen on request.
Something for Onshape then as well?

john_mcclary

Klaas said:

Ok, I see your point, let me get used to this

Though I do like the AutoCAD option: regen on request.
Something for Onshape then as well?

I would vote for that 

michael_kruger

Something related to this issue that I just discovered is that the resolution of exported STL models seems to be derived from the model tessellation quality, so if the model tessellation is not set fine enough, the STL model will also not have a smooth surface. This can be over-ridden by manually specifying the STL tessellation parameters when exporting, but I think it would be better if the default parameters were set to some generally good values, rather than being derived from the model tessellation.

eric_pesty

michael_kruger said:

Something related to this issue that I just discovered is that the resolution of exported STL models seems to be derived from the model tessellation quality, so if the model tessellation is not set fine enough, the STL model will also not have a smooth surface. This can be over-ridden by manually specifying the STL tessellation parameters when exporting, but I think it would be better if the default parameters were set to some generally good values, rather than being derived from the model tessellation.

I like the "default" behavior to export what you are looking at so there are no surprises. Not sure what "generally good" values should be (especially since they could be lower than what you were displaying at the time of exporting). Solidworks also exports based on the currently selected display quality so it's not too surprising. Having the option to override when exporting provides pretty much all the control needed.