Drawing a screw thread - problem getting top and bottom faces

If you want me to, I’ll look at it as soon as I get a chance and see what I can do for you.

Thank you, @DaveR.

Busy at the moment for a couple of hours.

I’ll just see if I can model it based on the info you’ve given.

I was figuring you were all set after mihai.s’s tuition.

Well, I have given up trying to draw this thread with rounded edges. I simply can’t get it to draw as a solid. That means I can’t bevel the top using Eneroth Solid Tools, and Intersect faces is too tedious to trim.

I’ve PM’d @DaveR who may have time to try to find out why it doesn’t work. But I HAVE made one solid with simplified straight edges to the thread, omitting the rounded corners.


Cordeaux Insulator spindle thread x1000 3D straight edges thread.skp (142.0 KB)
I’ll bevel the top tomorrow.
This one is a solid.

First try but I don’t think it’s exactly right. This is 6 tpi. I may have misinterpreted your drawing earlier . You indicated the diameter as 5/16 in. but based on the other dimensions and the way you indicated it, I assumed it should be 5/8 in. diameter instead. I expect it’s way more detailed than you need and I just stuck a 30° chamfer on each end which is probably wrong. At least it does make a solid and printable.

FWIW, it isn’t terribly critical where you start along the thread profile and where you end but the top and bottom of the profile edge need to be aligned in the blue direction.

@DaveR, thank you so much.

You are right, the nominal diameter should be 5/8" but the thread stops slightly short by (at normal size) 0.0166" in radius, and my 5/16" (0.3125") was for the radius.

So the actual radius of the outside of the thread is (at 1000x scale) 312.5 - 16.6, or 295.9.

I still don’t understand why I can’t get it to come out right. And I have really tried, as I described in my PM to you.

I fixed slight misalignments in the profile, and got both top and bottom outer corners exactly the same height as the pitch, and exactly at the same radius. But Upright Extruder couldn’t get the first two segments to line up, creating a surface border issue between first and second turn.

However, thanks to you and @mihai.s I have workable solutions for the rounded profile, and I have managed to draw one myself with a simpler straight edge profile.

Still puzzled why my drawing just doesn’t work exactly, and the Intersect faces won’t provide a top and bottom.

I’ll have one more go later today, starting from scratch, and being EXTREMEMLY careful to get everything exactly aligned.

1 Like

I get that same thing both at the start and at the end. I expect that to happen there and haven’t agonized why it happens. I just plan ahead for that, make the screw a few turns longer than needed and cut the ends off.

Thank you again - I thought it was something I was doing wrong. That’s reassuring.

1 Like

So, it’s not just me…
Parallel viewing of the basic helices helps show the distortion that unfailingly occurs when attempting to splice two “strands” together.
As seen here, the junction seems shifted slightly to the left, breaking the usually straight line one would have expected.


A longitudinal view of the same shows the cut-off point as an off-center circle,Screen Shot 2021-02-26 at 3.45.57 PM
whereas a successful intersection with a plane always features an indentation, with its attendant and opposed protrusion being hard to see…

What I see in your side view of the threads indicates that angles on the top and bottom of the thread are different.

When I model threads the angles are the same and there’s no problem stacking sections to make a longer screw.

Here’s an example of that with perfect alignment of two sections.

So how do I avoid the problem?
In other words, what should I do differently? The method I use is as follows: Form a helix and a cylinder of the same dimensions, alt-rotate the helix and scale it down in the x and y axes. That a distortion occurs at both ends of the result is unexpected and puzzling to many people, as this string attests.

Don’t use Scale. Use a properly drawn profile to sweep round a helical path. I’ve shown this before. John posted a link to it in the first post in this thread.

The method you are using is OK for approximating threads but it’s not accurate

Read the post before my OP, and again after your latest. Still don’t see the link you’re referring to, or at least fail to recognize it as such…

Really? You don’t see the video tutorial in my post there?

I do now…
Thanks.

There’s always the option of replacing the dirty “cycle” with a clean one, as shown here…

You could do that. Now you’ve got reversed faces in that area and the threads are still only a rough approximation or screw thread. If threads like that are acceptable for your modeling. It probably doesn’t make any difference if they line up properly so you might as well save yourself the trouble and let them be.

A little something I found on the internet and unabashedly stole the content and paraphrased a bit:

So you want to print 3d fasteners.pdf (56.5 KB)

1 Like

I use CAD files from McMaster-Carr quite frequently although for use in SketchUp the 3-D Step files can be pretty huge. If nothing else they are excellent as guides.

Those faces were reversed only for illustration purposes. Of course, they’d be right-side out if I intended to use that shape in a model.
Although my attempt at using it was a failure, your proposed solution is obviously the way to go, I’m only trying to understand the odd outcomes of some operations, such as the valentine-shaped intersection that’s a constant result of this way of creating workable threads.