Drawing a screw thread - problem getting top and bottom faces

[john_mcclenahan]

I’ve followed @DaveR’s video for drawing screw threads, working at 1000x full scale.

Live Model Along! Assembly... with screws!

[Screenshot - 4_15_2020 , 2_33_53 PM] [Screw Thread Tut]

I’m using a somewhat unusual thread - one used since the late 19th century for securing porcelain telegraph and telephone pole insulators onto their metal supports, patented by J. H. Cordeaux.

But as the topic title says, I can’t get the flat face at top and bottom of the thread to separate from the rectangle used when I Intersect faces with the thread to cut it to length.

After much googling, I found a specification for this thread. Here’s what I found.

Cordeaux Thread for Telegraph Insulators

Asymmetric thread. Upper flank: F = 29 deg.; Rc = Rr = 0.015 inches. Lower flank: F = 35 deg.; Rc = Rr = 0.022 inches.
Sc = 0.0166 inches; Sr = 0.0331 inches; h = 0.0642 inches (7TPI) or 0.0832 inches (6 TPI).

And I was able to draw the profile at my second attempt. On the first attempt, I couldn’t work out how the radii fitted, and just drew a flat on the inside and outside of the thread.

I thought I’d managed to create one with flats instead of rounded edges on the threads, but then found it wasn’t solid.
Looks like this:

Cordeaux thread 3d.skp (258.9 KB)

So I tried again, and DID manage to make one solid, but then found I’d cut it off (and then bevelled it) at the wrong height - short by a fraction of a turn of the thread, but the correct pitch (6tpi).

So I thought I’d draw it again with rounded tops and bottoms to the thread.

Here’s an image of the revised profile, and the SketchUp drawing of it.

Cordeaux thread profile rounded.skp (52.2 KB)

And here’s where I’ve got to, redrawing the thread, before cutting to length.

Cordeaux thread rounded 3d.skp (1.7 MB)
But when I use Interesect faces with selection, and trim off the surplus, on the bottom it looks like this:

Cordeaux thread profile rounded.skp (52.2 KB) (saved back to v2017 but drawn in SU 2020).

The problem I have is that the bottom face of the screw doesn’t separate from the whole rectangle, and the same happens on the top. I’ve tried redrawing the lines in the plane, but can’t get the face to separate. I’ve zoomed right in on every vertex in the plane, and can’t see any gaps or anything out of plane.

I don’t know why not, and wonder if anyone can see why not, and help me fix it?

The overall thread portion is six turns at 6tpi, so 1" high - scaled up to 1000" between the planes.

Any help to create a clean solid would be helpful.

[TDahl]

I see similar results occasionally when cutting helix shapes (simple saw-tooth screw threads in my case). Sometimes tracing an edge of the intersection will allow SketchUp to discover that there is a closed loop that defines a face (screw cross-section). Sometimes there is a tiny gap in the intersection result, for some reason. Close visual inspection of all the apparent intersection vertices will sometimes allow a gap to be found, which can then be manually healed one way or another.

I work at a 100X scale, for what that’s worth. That is a compromise between not large enough to defeat SketchUp’s close-endpoint tolerance in some cases, vs. too large to make editing the normal-scale version of the model (in the same file) difficult due to the irritating front-clipping issue.

[john_mcclenahan]

Thank you for looking, and for your suggestions.

In addition to looking very closely zoomed WAY in at each vertex, I’ve also tried drawing a diameter, and various radial lines, to see if I can isolate where there are any gaps.

I’ve turned on Endpoints in Styles to see more easily where there are multiple lines nearly collinear.

I can get half of the bottom to separate easily, with one line drawn across the middle, but not the rest.

[LATER]
Working my way round the vertices with Endpoints turned on, using more-or-less radial lines, I can get a face where only three radial lines remain, and although they are not showing as profile edges, deleting any one of them removes a segment (or more) of the face.

I can ‘cheat’ and hide them, I suppose, and no one will ever see them in normal viewing.

I can examine the Z-value of each of the outer endpoints of those three lines, and try to move them into the same Z-plane.

But I want to put a plain cylindrical (unthreaded) portion below the thread, and merge it with the thread. If I do, I suspect that Solid Inspector2 will complain about an open or overlapping border on the bottom of the thread.

And I also want to bevel the top, as Dave shows in his video.

But why does SketchUp, even at this much enlarged scale, not put all the intersected edges in the plane that created them?

[mihai.s]

solid-group.skp (1.9 MB)

[DaveR]

I got pinged on this topic but was busy doing an online session so I’m late to the party. Is it correct to say that you have this sorted now?

[john_mcclenahan]

MANY thanks @mihai.s.

How did you do it? Given the high edge count, I think you used something like a 72 segment circle for the helix - is that right?

I have been trying to balance poly count with ‘apparent roundness’. The item will be only 1" high when scaled down, and this one is I think over detailed - it will be seen only as the threaded part of a metal shaft for mounting insulators on a telephone pole arm.

If you could either re-do it with at most 24 segments in plan view, I’d be very grateful. Or better, tell me how you managed it at only 1000 times scale.

I had one more go at 10,000x full size, and finally got something that did create top and bottom faces.

But then I found that there’s a Surface Boundary problem between successive threads - I think probably because I didn’t get the height of the profile exactly the same as the thread pitch. Maybe off by one in the thousandth of an inch place of decimals.

@DaveR

Is it correct to say that you have this sorted now?

More or less, but even my latest effort isn’t a solid. (See above.)

Do you understand why the face isn’t created when I Intersect faces/with selection? It worked better drawn at 10,000 times full size, but still not perfectly, though I was able to create the faces without too much fiddling at that scale, but gave up at 1000x.

PS.
Bit of a frustrating day - I spent several hours doing and redoing this. It’s probably gilding the lily but I DO like to get clean models, made up of solid components for the most part.

[mihai.s]

You’re welcome!

I created it based on the existing one (it’s in layer 1), with 24 segments for the circle/helix. The problem was in 2D profile, which had a very small deviation.

[john_mcclenahan]

@mihai.s

The problem was in profile, which had a very small deviation.

What was the nature of the deviation? As you’ll see from my most recent post, I suspect I had the height VERY slightly different from the helix pitch - but was either top or bottom not quite horizontal?

[mihai.s]

[john_mcclenahan]

Ah, slightly too tall, not as I had thought, too short.

I’ll correct that and try again tomorrow - had enough of it for today/tonight.

[DaveR]

Looks like mihai has you sorted.

The profile needs to be the same as the pitch exactly.

[Shep]

Here’s something I had stuffed under a lathe. It really doesn’t have that much to do with the more modern objects above but some may find it interesting. This thing looks as if the threads were more of a suggestion than anything.
Shep

[john_mcclenahan]

Well, I’ve tried several times more, taking great care to make the profile height EXACTLY the same height as the helix pitch.

There’s still something slightly wrong in the result, and I still get a Surface Border error between successive threads. I’ve given up, and used @mihai.s thread to start with, added a bottom unthreaded portion, and bevelled the top.

Thank you all who have looked, and @mihai.s in particular for drawing something I can use.

Something still isn’t quite lining up for me, and I still can’t see what.

[DaveR]

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.

[john_mcclenahan]

Thank you, @DaveR.

Busy at the moment for a couple of hours.

[DaveR]

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.

[john_mcclenahan]

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.

[DaveR]

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.

[john_mcclenahan]

@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.

[DaveR]

@john_mcclenahan

But Upright Extruder couldn’t get the first two segments to line up, creating a surface border issue between first and second turn.

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.