Tracing from a sketch on one plane onto another

[Rayzorman]

Hi,

I'll try and explain what I'm trying to do as it will probably make more sense this way.

Imagine a round plastic cup with 4mm thick base.

The the underside of the base has had a number of rectangles in a circular pattern around the base that have been cut away.

Now as this is a plastic enclosure I need to do a reverse/negative cut away on the inside base surface.

So what I am trying to do is create a sketch on the inside base surface that effectively encloses the plastic that has not yet been cut away with an offset of 2mm from the base cut-outs so that I end up with the base being effectively 2mm thick over the entire area.  Hope that makes sense!

So I select the internal base as the sketch plane and try to follow the sketch on the base with a view to offset this 2mm later. Problem the sketch line tool keeps dropping out so I cannot get a contiguous enclosed area and there is no way to join the bits of line up so I can't end up with the correct shape to extrude/cut away. 

Projecting doesn't appear to be the answer as I'm not trying to get the bottom sketch onto the top sketch as such I just want to use the edges to describe a different area.   I've done this using Onshape as that software has a function called 'use' that allows you to select features from other planes and project them individually onto your drawing plane. You can then use constraints like offset to reference these features without them actually being in the sketch. This seems to be pretty fundamental requirement of a CAD package so I'm sure I must be missing something. Many Thanks in advance for any advice.

[TheCADWhisperer]

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@Rayzorman wrote:

I'm sure I must be missing something. 

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Can you File>Export your *.f3d file to your local drive and then Attach it here to a Reply?

Make up a dummy file if your design is proprietary.

[Rayzorman]

I've knocked together a much simpler drawing which basically demos the issue. The jpg shows the contiguous area I'm trying to form that can be cut away.  I just can't get this area to form properly. Just end up with bits or I can only select the whole internal surface of the base.  If I could get the ends of the various bits of line to join this would be simple. Surely there is an opposite action to split... turn these bits of line into one contiguous line with joined up vertices. Can't understand why I can't find it!

[jeff_strater]

perhaps I do not completely understand your final goal, but something like this seems to be in the general direction (I had to turn opacity on - too hard for me to visualize where the faces are with the model translucent)

[Rayzorman]

Ok but that is not what we need to do. You are starting on the wrong side!  You need to select the inside base of the cup. You need to create the sketch there. We are cutting down towards the base not up from the base. Note you cannot go to the outside edge as this will just make holes in the bottom of the vessel. Thus you start with a blank canvas. Thus you need a bit of translucency so you can see areas you need to avoid etc. 

[jeff_strater]

OK, thanks, that helps.  As I said, I was not clear on the intention.  Same basic idea inside of the cup.  Project that face from the bottom onto the sketch inside, then offset it.  I could not get the offset to avoid the inside walls without it decomposing into islands, so I've left it interfering with the side walls.  You'll have to decide what your design intent is here: