Lathe 1/4 round on end of 1/2 round rib--not quite what I expect

advice
modeling

#1

Jim here–little snow in Chi-town but sun is out this morning.

I always seem to have little weird issues with lathing ends.

I want to install a ‘rib’ in the plane and then ‘cap’ the ends. Again a 3-D printing of a part for five platonic bodies I’m working on. note: I did scale up 1000 Xs.

Here is my work.

thanks in advance.

Jimlathend01.pdf (457.8 KB)


#2

Follow Me requires that the profile be perpendicular to the first segment in the path and it’ll end with the face of the extrusion perpendicular to the last segment in the path. Make that happen in your set up and it’ll work out fine. I’ll make an example to show how I do it.

Since your profile wasn’t placed perpendicular to the first segment, SketchUp projects the profile to be perpendicular. This is important to understand because it results in the shape of the profile being modified. Here’s an example of that.


#3

Here’s an example.


Notice the top segment of the path is perpendicular to the vertical edge. The segment at the opposite end is perpendicular to its neighbor, too.

I started by drawing a circle. I rotated it half the angle between vertices and then drew lines between the midpoints on the edges at 90° to each other.
lathe.skp (51.7 KB)


#4

Because a half-circular path raises these problems with Follow Me, sometimes it’s faster and easier just to use a circular path and remove half the resulting shape.

-Gully


#5

@DaveR and @Gully_Foyle:

I’ve been pondering your tips for a day.

Testing reveals that both of your samples work fine.

Gully’s type, I was able to reproduce, though I’m not using a full 1/2 circle profile.

Dave, still a koan? I see the right-angles at the ends of the ‘path’.
Tried to rotate and mess with the new shape, but alas, no go.

I do see the problem though (it took all day)!

Finally, I broke down and copied Dave’s path into my drawing: Thanks to you Dave again.

Here is the Octahedron- next 3-d printout this week.

wei-Octo02.pdf (473.6 KB)


#6

In order to get a true 90 degree wedge, the ends of the arc need to be tangent at the end. It’s not quite clear in @DaveR’s example that he has rotated the original circle to get a 90 degree segment that has “straight” ends (as opposed to “angled”). An extreme example might illustrate this better by using an 8-sided circle (octagon) as the path:


#7

I don’t believe you mentioned that before. How’s this?

-Gully


#8

That’s what I’m going for!


#9

Thanks for further enumeration.


#10

Thanks for further enumeration.


#11

I thought I did make that clear. Evidently not.