funny behavior using helix

[idslk]

I've tried to make a "real" thread using Helix.
To start with something the sketch has not all details, but the main dimensions are M6 x 1.
For this sample i have used Helical Boss with no "core" for first test...
First attempt has 1mm pitch and a sketch with a triangle with 1mm heigth. It "failed" with no warning...
Second attempt has 1mm pitch and a sketch with a triangle with 0.999mm heigth. This worked as expected.
Part with 0.999 heigth is appended. (AD V2018.2)
For metrical threads no real problem because the profile triangle is lower than the pitch...
(Haven't tried it with e.g. Whitworth Tube thread)
Tought maybe the helix should work also with a pitch with same heigth?
What do you think?
Regards
Stefan

Attempt with 1mm:


Attempt with 0.999mm:

 

[Ralf]

Hi Stefan,

 

[idslk]

Hi Ralf,

of course (i'm always using fine) ;-)
Have you changed the sketch dim from 0.999 (which is set in the sketch of the file) to 1?

Best Greetings from here
Stefan

 

[oldfox]

Hi Stefan, my thinking has always been that external threads are "cut" (subtractive) and not "applied" (additive). Therefore I use "Helix Cut".
The pitch is still 1.000mm but the "length" of the thread is 0.999. I have found the same problem you have been experiencing. The answer is to
use a length of the thread, slightly less than the pitch. (i.e. 0.999 vs 1.000) and it works every time. For my "almost" real threads I use a chamfer
on the end of the rod and an "outfeed" at the finishing end of the cut. For "eye candy" it all comes out pretty nice I think.

 

[idslk]

Hello oldfox,

of course threads are normaly cut, no matter external or internal (except you are using a 3D printer...).
And yes, if it is for "eye candy" everything is perfect. Also normaly it won't cause any real problems in the real world.
Only the behavior of AD was unexpected...(does not generate sleepless nights ;-)...some kind of funny as headline says.

Regards
Stefan

 

[oldfox]

Only the behavior of AD was unexpected

I think AD is seeing the situation as "intersecting wires" and therefore just says... (no, no)

 

[bigseb]

I've tried to make a "real" thread using Helix.
To start with something the sketch has not all details, but the main dimensions are M6 x 1.
For this sample i have used Helical Boss with no "core" for first test...
First attempt has 1mm pitch and a sketch with a triangle with 1mm heigth. It "failed" with no warning...
Second attempt has 1mm pitch and a sketch with a triangle with 0.999mm heigth. This worked as expected.
Part with 0.999 heigth is appended. (AD V2018.2)
For metrical threads no real problem because the profile triangle is lower than the pitch...
(Haven't tried it with e.g. Whitworth Tube thread)
Tought maybe the helix should work also with a pitch with same heigth?
What do you think?
Regards
Stefan

Attempt with 1mm:
View attachment 26621 View attachment 26622 View attachment 26623 View attachment 26624

Attempt with 0.999mm:
View attachment 26625 View attachment 26626 View attachment 26627 View attachment 26628

This is normal behavior for Alibre. Has been around for years. The solution is exactly you already figured out i.e. change one dimension by a micron.

 

[JST]

Agree. It seems that Alibre does not like overlaps. Happens in a related way with helical cut, which I use for this same purpose when needed.

 

[neilr]

just a thought as I haven't had a chance to try it yet but what if you did 1mmx1mm thread but only one revolution, so there would be no intersecting wires. then did a linear pattern up the rod?

 

[oldfox]

Neil, Linear Pattern... Boo, hiss.
Anything with a pattern can NOT be made flexible. And it is so much easier to enter a "short" dimension in the first place.

 

[bigseb]

Agree. It seems that Alibre does not like overlaps. Happens in a related way with helical cut, which I use for this same purpose when needed.

This.

I think it has something to do with trying to calculate a solid with zero thickness (the point where two peaks meet). I think adding a solid core might sort it out.

 

[NateLiquidGravity]

It doesn't like the zero thickness but I had no trouble with overlaps. It also worked with both added lines constrained horizontal or colinear.

 

[JST]

it fails reliably doing a cut, unless tiny dimension changes are made. I do not do thread models enough to recall just what dimension has to be changed, but it has to do with an overlap of some sort, unquestionably,

The exact match and zero thickness is quite familiar as I try to recall the way it is solved, that has to be part of it.

But on a helical cut, there is a solid core, so that is not the issue itself. It has to do with the cutting shape. Possibly if the cutting shape cuts a previously cut volume there is an error.

 

[bigseb]

Even with a cut there is a zero thickness cut. Decreasing the width of the cut by 1 micron is enough to avoid the mathematical error.

 

[JST]

Yes, that does fix it. Not sure it is always a zero that causes the issue, but may be.