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

[solidad]

I have been trying to get what I think is a simple shape concept built for weeks and I keep running into one small problem after another.

The gist is that I want to carve a double helix pattern into a cylinder.

Using curve maker and upright extruder I get the following shape with one of the helix right and it carved into the cylinder on the left (using subtract).

The problem is that the bottom of the helix starting face doesn’t match the top. So when I go to carve the second helix both edges don’t butt up to eachother. I wish I could choose a begining extrusion point and an end extrusion point so they would both match.

OR

I wish I could find an easier method to do this double helix…

temp-test.skp (217.8 KB)

[DaveR]

Maybe I don’t understand what you are trying to accomplish. Can’t you just rotate the helix 180°?

@solidad

So when I go to carve the second helix both edges don’t butt up to eachother.

What is it you are trying to do?

You need to be using a second helical shape if you want the bottoms of the helices to meet.

Are you trying to do something like this?

[solidad]

If I make another helix shape (rotating it and puting the 2 shapes together I get the following:

The bottom of the helix is butted together but the top isn’t…

And yes, I am trying to do what you have in the pic.

[DaveR]

So you want like this?

I split the first and last segments of the helix at their midpoints and extruded the profile along the remaining bits. I used two full turns instead of your 2-1/2 turns.

[solidad]

Yes that is what I want.

I don’t understand what you mean by “split the first and last segments”.

[DaveR]

I drew short line segments from the midpoints of the edge segments and then erased beyond that point. Sort of like this.

That results in the ends of the extrusions lining up as you can see if this plan view of them.

[solidad]

Ok. I think I understand. I didn’t realize you could do something like that. So Those small “guide lines” basically cut the path into sections that are more controllable?

[DaveR]

@solidad

So Those small “guide lines” basically cut the path into sections that are more controllable?

Well, any time you draw an intersecting edge segment, it will split the segment. I don’t know that it makes it more controllable. It just makes the end segments align the way you need to do.

[solidad]

ok. So did you just duplicate the extruded sections and just stick them together?

[DaveR]

I made a copy of the first one and flipped it to mirror it. I left them as separate groups, though.

[solidad]

OK. I think I got it. Thank you.

[solidad]

Actually one more thing. How did you stich the sections together and remove the lines? Every time I try it removes part of the face of the section.

[DaveR]

I didn’t stitch anything together. I use Trim from the Solid Tools to trim the cylinder with the helices. Afterward there were a few edges to soften which I did by selecting all of the cylinder’s geometry, right clicking on it and selecting Soften/Smooth.

[solidad]

I mean when building the helix from the sections.

[DaveR]

There wasn’t any need to stitch anything together for the helices, either. I Just drew the helix, trimmed it as I described before, added a rectangle at the bottom for the profile and used Upright Extruder to extrude it around the helix.

[solidad]

Now I am extremely confused. That is exactly what I have been doing all along but the top part doesn’t match the bottom… You said you drew short segments at the midpoint of each section and “erased beyond that point”. I feel like I am missing something else.

[DaveR]

I started a 3 turn helix but cut it down to two turns dividing edge segments at their midpoints.

[solidad]

I don’t know what I am doing wrong but I don’t want to waste any more of your time. I tried dividing the midpoints of the helix. The first one worked. The second one didn’t. I don’t understand what is different between the 2 times I have done this. I am getting more and more frustrated. I wish I could do this in a different way because this seems so counter-intuitive.

[DaveR]

I’m not sure why you are finding it so difficult.

3-turn helix.
Divide bottom segment at midpoint and segment two full turns up.
Erase the unneeded parts of the helix.
Add profile for the extrusion.
Run Upright extruder.

[solidad]

That helped more in understanding what you are doing. One thing that is frustrating me is finding the midpoint. How do you do that easily?