How to change a sketch offset?

josh_levine445

Here the inner pac man is created by offseting the outer one...



...and then that inner pac man is extruded to make a solid...


How can I go back and change the amount of the original offset so that the solid extruded from it adjusts?

(This is a test case, in my actual problem the shape that was offset is very complicated and there is lots and lots of stuff built on top of that extrusion). 

Thanks!

john_mcclary

The offset is just a dimension in the original sketch. Edit the sketch and change the dimension

If it is not updating as you expect, try fully constraining the "blue" circles in your sketch (Which is good practice anyway)

john_mcclary

The offset is just a dimension in the original sketch. Edit the sketch and change the dimension

If it is not updating as you expect, try fully constraining the "blue" circles in your sketch (Which is good practice anyway)

brucebartlett

Find the sketch in the tree and edit it,


Another tip, if you click "final "while editing you will get the finished part studio to show

josh_levine445

I guess I do not understand how offset works in onshape. Is that dimension special, or is is just a normal dimension that fixes the distance between the two referenced entities and the offset constraints drive the behavior? What does the offset constraint mean? It seams to relate two sets of entities to each other, but I can't figure out exactly what the semantics are. Is there any documentation for it? It does not even seem to be listed on the "Working with Constraints" page. Thanks so much!

john_mcclary

@josh_levine445

I made a short video hopefully it answers your questions about offset.
https://youtu.be/p2eo610PNdw

I forgot to mention in the video, The reason it won't appear on the "Working with Constraints" page is because, it is not a standard mate (coincident, tangent, ect.) The only way to add an offset is to use the offset command and select all of the entities you need. It is done in this way so many entities can all share the same offset dimension. Rather than individually selecting 2 objects and offsetting them, then adding a 3rd reference to make them equal.

Here is where the documentation is found
https://cad.onshape.com/help/index.htm#sketch-tools-offset.htm?Highlight=offset

john_mcclary

Really? A down vote? Did I offend someone? Sorry if I did.

josh_levine445

Wasn't me!

Thanks for the video. I'd still love to better understand the semantics of the offset constraint between entities. Does it map endpoints to be coincident to a line that passes though the original two endpoints or something like that? 

Thanks!

Jason_S

Sketch offset works by projecting entities normal to their seed(s), not a 'copy and transform ' in a certain direction. Each offset entity doesn't know about another entity until you add it to the selection of the offset.

Line: Self explanatory

Arc: The arc length is increased because you are offsetting the radius of the arc. The angle that the arc travels stays the same. Remember back to Θ=s/r from geometry class

Circle: Self explanatory

Hexagon: As I add input lines, the latest offset line has a longer length than its other entities, because that latest line has no idea about the next entity in the hexagon. As I complete my selection, the offset closes the shape because of the known coincident constraints on the seed entity.

Spline: The spline succeeds in one direction, but not the other. This is because the curvature is too sharp at the point I pointed out for our sketcher to see it as a valid spline (for now).




Feel free to check out out our Video Library and our new Learning Center!

Hope this helps.

Jason