I am a complete beginner at this 24-48 hours in, and am trying to work out how to use FreeCAD. Have been through the Youtube videos which were really useful as I have no mechanical design / drafting experience.
I have a question:
What I have picked up so far is that the basic process is to make 2D drawings (sketches) which one then extrudes (pads) into 3D objects, which can then be added to or cut away by other 3D objects to make the desired final model. To do this, one picks as a starting point a 2D plane on a body to make the sketch on (XY, XZ, YZ), so the other coordinate position (Z in the case of an XY plane) is zero. Later one can create a new sketching plane based on a surface of your model, making another extrudable drawing on that.
But, as far as I can see, in all cases the sketch is nailed to the origin (0) on the 3rd axis, being the one you are not drawing. Is that right?
Consider this: you have an extruded block 10mm deep (drawn on X/Y extruded on Z), but you want to put a circular cutout inside it, say 20mm diameter, 3mm deep, 5mm from the bottom surface, so buried 2mm below the top surface. Then in the centre of the cutout, a 3mm diameter hole to the top surface. Never mind why one might want to do this - it is just an example.
If one places a sketch on - say - the top surface of the extruded block, it seems nailed to the surface, therefore one can create a 20mm circle on it and extrude it to make a cut, but it will always start cutting at the surface it is on. So is there a way of offsetting the sketch from the surface so it starts - in this case - 2mm lower?
I can see other ways of making it, build up the 10mm block with a 5mm block, then a 3mm block with a 20mm cutout in it on top, then a 2mm block with a 3mm cutout in it on top of that. I assume that would work.
However, this isn’t a real design, I am just trying to find out what you can do, so the question in summary is:
“Once one has created a sketch either on a surface, or - say - has selected XY as the 2D drawing plane of a body, can one offset that sketch along the Z axis?”