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  • Simple FreeCAD design question regarding axes

    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?"

  • #2
    Originally posted by nmw01223 View Post
    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?"
    I guess what you want is to set a Z offset for the sketch. If you click on the sketch (it must be closed) there should be a section with "attribute" and "value" below. There is also a line named "attachment offset" with several coordinate next to it in the value column (you may need to drag the vertical bar to see the "offset" text), click at the end of these coords and a button with "..." appears, which opens a window, where you can move and rotate your sketch. It is kind of hidden.

    I personally use a datum plane for this. The advantage is that you create a real plane like XY, XZ and YZ, where you can anchor multiple sketches to. So If you want to change the lets say height of your model, you would need to search and fix all the offsets in all sketches, while using a datum plane just needs one tweak. Datum planes also support parametric values, so you can insert the width of your model minus 2 e.g. "Sketch.contrain_width-2mm" and the datum plane will just adapt every time you change the models height. Also with the datum plane all sketches change position.

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    • #3
      I highly recommend you view the videos by Joko Engineeringhelp on YouTube. I would watch the very beginning one and then the one about creating an engine manifold. In the manifold video he really shows how datum planes can be used to create things inside a solid. Geit is the one who put me onto these videos, yesterday I think.

      Comment


      • #4
        Very helpful, thank you.

        attachment offset
        and
        datum plane
        .

        I will look into both of the above.

        Joko Engineeringhelp on YouTube
        I did look at a Joko video but didn't really understand it. Now I understand a bit more about the concepts, I think it would be more help.

        Comment


        • #5
          I know what you mean about Joko. I'm working on a conceptual idea of the philosophy of FCAD. I'm going to do a couple of tests. If they work the way I suspect they might, I will report back, because the concept might help others get a grip on FCAD.

          Comment


          • #6
            Originally posted by nmw01223 View Post

            I did look at a Joko video but didn't really understand it. Now I understand a bit more about the concepts, I think it would be more help.
            It is not really a tutorial video. You need to know a lot already.

            This is more useful:

            https://www.youtube.com/playlist?lis...8kUgQCNEiDUwSH

            The variate of the videos is quite high and he covers stuff you rarely find good videos, like "FEM workbench" and "Curved Shapes workbench".
            Last edited by Geit; 10-09-2020, 06:22 PM.

            Comment


            • #7
              Yes, that's the "very beginning" video I was referring to earlier.

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