How to concave a cylinder with Onshape?

Question

If I have a 3D cylindrical extrusion (in Onshape), how can I scoop, carve, or indent a concave/parabolic curve in one of the ends?

The yellow surface is the target. Here is the basic shape I'm trying to bore out (like a satellite dish, parabolic antenna, or even a contact lens.. You get the idea).

It doesn't necessarily have to be with Onshape. That's just what I'm using currently. If you can show how to do it with another CAD software, I can probably adapt the technique for my purpose. — voices, Nov 10 '17 at 13:07

score 3 · Answer 1 · answered Nov 09 '17 at 17:54

3

As is typical with many CAD type programs, a feature in Onshape known as revolve may be your solution. As you have a clear formula for the cross-section, half of the work is complete.

You would generate a sketch representing the curve, then use the Revolve feature with the axis oriented to vertical. According to the Onshape video, you can generate a solid or a surface from the options that appear when selecting that feature.

As you can see in the images above, the axis selected in the tutorial video is horizontal. Other features of revolve are covered in the video. Your post suggests it will also be necessary to perform a subtraction action on the assembly in order to get the scoop/concave result.

answered Nov 09 '17 at 17:54

fred_dot_u

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Yes, that's right. Nice example by the way. The problem in this case is that I already have a solid object; I'll sketch a 3 point arc that starts at the edges of, and descends below the surface. -[-)-]- Set the revolve tool to cut, specify the arc, with the horizontal axis running through the middle. But it doesn't seem to work. – voices Nov 10 '17 at 13:01
I also tried using different software, like Fusion 360. But I must be missing a crucial concept. – voices Nov 10 '17 at 13:02
Just to clarify: I used this technique to redraw the object from scratch, but would prefer to simply modify the original 3D object. – voices Nov 10 '17 at 13:13

score 2 · Answer 2 · answered Dec 11 '17 at 23:18

There is no tool to create a parabola in a sketch in Onshape but you could use a 3 or 5 point spline to create a similar curve. If you knew exactly the curve you wanted you could create it with another cylinder but let's skip that for now and use a spline.

Once you have the curve, add a line to close it until the geometry is shaded in light gray. I would try using the project command to select the smaller circular edge of your part.

Now draw another line perpendicular to the first line, from the midpoint of the first line up to the curve. If you used a spline you can now use this second line to make the points of your spline symmetrical across that line so you can control your parabola-like curve. Once you're happy w/ your curve, select the split/trim command and delete the half of the curve and half of the first line until you have half the shape you want to cut out. Now complete the sketch.

Now use the revolve tool, select the sketch, select the second line you drew as the axis, and make sure that "remove" is selected and that the original body is selected for merging.

This will create the desired effect.

score 0 · Answer 3 · answered Dec 13 '17 at 00:56

Create a midplane going through it, and then sketch half if a profile for what you want yo carve out on it that plane, and in the spot where you need it to carve out. Now revolve that profile around it's Central axis and this should cut wlthe piece you want out.

How to concave a cylinder with Onshape?

3 Answers3