Trying to get the beginning face of an extrusion to match the end

That’s a fundamental SketchUp thing. Inferencing will tell you when you’re on the midpoint.

I was a afraid you were going to say that. I keep over-shooting or under-shooting the midpoint so when I make the helix shape and it’s mirror either the top or bottom are too short or too long. I got the idea of what you are doing. I think I just have to fiddle around with things like sides of the helix. I think that’s part of the problem for me.

It can be tricky to get some snaps when they are close together. Zoom in close enough that you can clearly see the segment you want to split, move the mouse around near the center and you should readily see the midpoint inference as in @DaveR’s image.

The problem is that DaveR’s version may have sides that are different than mine (more or less sides). So finding the midpoint may not be the thing that I need to do. That’s the frustrating part. So when I cut off the section of the helix that I don’t need, I am either cutting off too much or too little. I don’t know why this is so infuriating but it is.

I used the extension’s default 36 sides but it doesn’t matter. The segments that need to get trimmed must be at some multiple of 360 from each other. Basic geometry.

I am at my whit’s end…

All I know is that the shape that gets extruded in the image below is wrong.

Left shows where I put a line to cut the helix. Center shows what I cut away and right shows the finished extrusion. When I go to make a mirrored version of this, they don’t fit. What am I doing wrong?

test-temp2.skp (103.1 KB)

Your helix has an extra half turn.

I don’t understand why that’s a problem. I need this to have 2.5 turns.

I’m at a loss to understand why you aren’t getting it.

Here it is with 2-1/2 turns.

I just wanted you to do it with 2 turns to get the idea. Maybe someone else will be able to help you sort it out.

You aren’t drawing your stop lines perpendicular to the first or last segment of the helix:



Hi Dave, before you split the helix at the base midpoint do you rotate the helix so that the midpoint is on the red axis, thus making the face perpendicular to the first segment?

Perhaps this illustration will help you understand how the orientation of path to shape affects the extrusion.


I do but it isn’t required.

I do usually rotate the helix, but in this case especially as the helix is split on the opposite side at the top I just thought it would be easier to split it in the right place using the red axis as a guide that’s all.

to identify the segments, you can use guidelines and rotate them half a segment:


Start a line at the midpoint of the segment and hoover over it to activate the inferencing machine. Press the Down key twice to draw perpendicular to the segment:

Copy, move, flip along and rotate:

Or Copy, flip the original, paste in place. No need to rotate.

I always add a little extra work !:joy:

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