Point concentric on another layer OR extrude in two directions?

gary_boone

Here's my setup: 1) make a shape on a sketch (A). 2) Extrude it up. 3) Make another sketch (B) on top of the extruded shape. 4) Extrude that shape up.

Question: How do I make a hole through both shapes that is concentric on a point in sketch B?

The problem is that the point in sketch B is between the two shapes. If I make a hole concentric on a point in B, it can either go up through the top shape or down through the bottom shape. But not both.

I thought I could just make a point on the lower sketch A that is concentric on the point in sketch B, then make the hole go up from there through both layers. But I can't seem to figure out how to make such a point. Other forum messages show that points can be constrained by points on other layers. I can select both points, but that's when showing the 3D tools, not the sketch tools with the concentric constraint. If instead I select the sketch layer, I'm in one sketch and can't select the other point. Perhaps someone can suggest the exact steps or a better way to achieve the desired result.

andrew_troup

Try not to use the terminology of "Layers" for sketches; there are no layers in Onshape.

That mis-characterisation might be part of your problem, because layers (in the CAD packages which use them) are not time dependent. They exist "outside of time".

Sketches, OTOH, are time dependent. In fact, everything in the feature list is. This means that you can only create relations to items further up the list, or in other words, earlier in time. Parents are always above children, chronologically. 

It follows from this that you cannot create a relation in sketch (A) to a point in sketch (B) further down the feature list, because sketch B does not exist when you are creating (and hence, when in future you are editing) sketch A

andrew_troup

To achieve your result, you should be able to create the hole by creating a new feature, further down the tree from sketch B, and relating the centre point of the hole to the desired point in sketch B

gary_boone

Ah, now that makes sense. With a little rearranging of the sketches in the feature list, it worked. I moved sketch B above sketch A in the feature list, making it possible for the point in A to be constrained by a point in B.

Thanks, Andrew, for the insight!

shanshan

gary_boone,you moved sketch B above sketch A?but you created ketch B  on the top surface of extrude which was creat  by sketch A , no errors appeared? I suggest that you can upload some screenshots of your model here when asking some questions, it is helpful to understand your question well !

gary_boone

shanshan said:

gary_boone,you moved sketch B above sketch A?but you created ketch B  on the top surface of extrude which was creat  by sketch A , no errors appeared?

Yes, an error appeared for the reason you said. So I just removed the erroneous surface and added in 'top.' That worked in my case because my surfaces were parallel to top. Then I used Transform to move it back into position.