Bug or Feature

jason_bennett190

I noticed an odd behavior when drafting bodies with round tops:

• If the neutral plane is under the top of the part, the height of the part shrinks.
• If the neutral plane is over the top of the part, the height of the part increases.

When drafting bodies with flat tops:

• If the neutral plane is under the top of the part, the height of the part stays the same.
• If the neutral plane is over the top of the part, the height of the part stays the same.

I created an example drawing to demonstrate:
https://cad.onshape.com/documents/a0d7730bebc80b06f9a58ad7/w/5a5c07c1f9d5fa539240d4b9/e/974d27fb624aed5b9eaa28c1

In other CAD software, my experience is that draft does not change the height of the part.

I shared this directly with Onshape support via the drawing and their response is that "This is working as designed."

Pretty much all of the work that I do in CAD involves altering models for molding which includes lots of drafting. I can't think of any situation where I would want draft to adjust the height. Is having different draft behavior between flat and round tops really working as designed or is it a bug?

NeilCooke

Hi @jason_bennett190 that is working as designed - if you were to draft the side faces and leave the top where it is your part would look like a lollipop so Onshape is adjusting the top to keep the faces tangent. If I remember correctly most CAD systems would turn it into a lollipop or fail. 

jason_bennett190

In spaceclaim:

• Green original
• Red drafted by plane below top
• Blue drafted by plane above top

For all of the height remains the same and the geometry of the rounded feature adjusts to maintain tangency.

You can look through the different versions in the link I shared above to see how draft application is inconsistent between a fully rounded top and any flat top (completely flat or flat with fillets).

lougallo

There is an option to reapply fillets. This is not currently working as designed when the top round is a full round like you example. When resolved using that option will react in the way you are expecting. 

NeilCooke

Interesting - the result is what Parasolid gives us, so it will be the same in SolidWorks, NX, Solid Edge, plus many others. SpaceClaim uses ACIS - it would be interesting to see if other ACIS modellers give the same results - anybody?