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Problems with Thread
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March 15, 2019, 01:53:00 AM
I want to create a cap that screws into the main body with a short throw screw action (a couple turns), so I tried creating male and female threads.

In the attached image and drawing, I created the wider blue Cylinder and the thinner green Cylinder concentrically before 3D Subtracting them (keeping the subtrahend), so I have a hollow, blue Cylinder and a solid green Cylinder.

I then created a Thread (Genuine option selected) with a 3mm pitch on the inner surface of the (now hollow) blue Cylinder and another Thread on the outer surface of the green Cylinder.

The resulting Threads have two different diameters, making them unusable in my application.  I know that I could create the male thread and subtract it to create the female thread, but that would require very tight tolerances.

I can't find any formula for calculating the thread size from the diameter of the base cylinder, so I don't know if it's even possible to do what I'm trying.

The only thing that occurs to me is to create a male thread on one cylinder and subtract it from a wider cylinder, then scale it slightly in X & Y to create a gap to allow for manufacturing tolerances and to reduce friction, but that would change the angles of the threads.

So... any suggestions on how do to this properly?

Jeff

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* March 15, 2019, 02:40:51 AM
#1
Jeff Id do what you suggest by creating a male thread then subtracting and then scale just a smidgin larger.

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Daz
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March 15, 2019, 03:28:21 AM
#2
Actually, I created the two as before, then scaled the male thread to fit (also had to be adjusted vertically).


Jeff

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* March 15, 2019, 04:12:33 AM
#3
Rather than scaling the thread, which as you pointed out, will change the thread angles (although possibly only by a tiny amount).  can you not create the external thread from the correct diameter cylinder,  that way the threads should match.

For example. from this internet page.  I selected 2" x 4.5tpi,  and left the tolerances as default.  After clicking calculate,  for the internal thread I subtracted a cylinder 1.795 diam, (max minor diam), and for the external I used a cylinder of 1.9971 (Max major diam.  Then I used the same  0.2222 pitch on both parts.      ### Edit ### Actually, just checked the image again on the site (lower down the web page), and I did use the possibly the wrong figure,  should have probably been between min / max, which is why my thread is a slack fit as I just used max..   but you get the idea.  ### End Edit ###

seems to fit ok, though maybe a touch slack... but it avoids any problems of scaling any threads.
« Last Edit: March 15, 2019, 04:19:48 AM by Andy H »

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* March 15, 2019, 08:24:44 AM
#4
Jeff,
  The first set didn't work because threads are cut into the blue cylinder, not added to the cylinder. So for example for a 1/2"-13 thread, the inner cylinder (green) would have a diameter of 0.5 in. The inner diameter of the blue cylinder (actually a hole) would need to be 0.406 in. That is the diameter of a hole
drilled into a plate when tapping for 1/2"-13.
  I tried this, but the problem becomes lining up the threads so they mesh. And I can't confirm if the tolerances and actual profiles of the inner and outer threads are correct.

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* March 15, 2019, 08:54:12 AM
#5
If you scale by the same factor in all three axes the angles won't change.

I'd thought of Shelling a copy of the male thread, using a Thickness equal to whatever gap you'd like between male and female threads (maybe .05mm), and then subtracting that to create the female thread -- but Genie sez she can't shell that object.

Henry H

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* March 15, 2019, 09:43:08 AM
#6
In my replay above, I gave the incorrect diameter of a hole to be drill for 1/2"-13. It should be 0.422 in.
If I use the Thread tool on a cylinder of 0.5 in and a hole of 0.422 in, the threads seem to work, although I still don't know if it is true to life.
I did this using the Thread tool, NOT the SDK Add On of Insert Thread3D which creates a screw. The threads on it don't seem to come out right.

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March 15, 2019, 05:20:36 PM
#7
What I ended up doing was duplicating the cylinder before creating the male thread.  I then measured the thread depth (which varies with the cylinder diameter) and determined the desired separation for the female thread.

Going to the duplicate cylinder, I used Extract Entity / Extract Face, with Offset Distance set to the male's thread depth (minus the 0.25mm buffer I decided to use) and Thickness set to the thread depth plus the desired wall thickness (1.6379 + 2.0 mm), then used the Thread operation (remembering to set the Genuine option) and created a full-height thread.

I then had to rotate the male thread to align with the female thread (I used Dynamic Cut Plane in RedSDK).  The necessary amount was something close to 220°.  This should be built-in.


Jeff

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