Making a claw clutch using an additive helix

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Simonski
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Joined: Wed Nov 24, 2021 11:31 am

Making a claw clutch using an additive helix

Post by Simonski »

I am trying to model a simple three claw-clutch in the Part Design WB.

I am forming a claw by sweeping a rectangular profile (ClawSketch) 120 degrees along a helix. I then intend to use a polar array to duplicate this for the other two claws.

The problem I’m having is that the helix pitch needs to be 30mm but I can’t set it more than about 17.5mm without it failing.

As a test, I’ve created another body (shown in blue) consisting solely of the claw and here I can set a pitch of 30mm without problem.

So, what am I doing wrong - why can’t I create a 30mm pitch helix in the main body?


OS: Ubuntu 20.04.3 LTS (KDE/plasma)
Word size of FreeCAD: 64-bit
Version: 0.20.26498 (Git) AppImage
Build type: Release
Branch: (HEAD detached at 1895593)
Hash: 18955931c7f0926a4cd9d2719be5a433b49ae56e
Python version: 3.9.7
Qt version: 5.12.9
Coin version: 4.0.0
OCC version: 7.5.3
Locale: English/United Kingdom (en_GB)
Attachments
ClawClutchHelixPitch.png
ClawClutchHelixPitch.png (210.35 KiB) Viewed 1204 times
ClawClutch.FCStd
(43.51 KiB) Downloaded 23 times
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Willem
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Re: Making a claw clutch using an additive helix

Post by Willem »

I made a try but I also could not get a pitch of 30 with the part design helix function. My solution is to use the Part Helix insterad and make a facebinder in the body. A sweep will do it. Finally you have to fill the open space
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ClawClutch-Willem.FCStd
(122.03 KiB) Downloaded 18 times
drmacro
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Re: Making a claw clutch using an additive helix

Post by drmacro »

if you rotate the sketch for the claw away from the seam line:
Snip macro screenshot-fb6d96.png
Snip macro screenshot-fb6d96.png (170.69 KiB) Viewed 1132 times
(note: if you aren't using OCCT 7.6 this file will likely show up blank... You can see though in the Attachment dialog on the left I changed the Roll by 4 degrees.)
Attachments
ClawClutch-mac.FCStd
(50.66 KiB) Downloaded 16 times
Star Trek II: The Wrath of Khan: Spock: "...His pattern indicates two-dimensional thinking."
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papyblaise
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Re: Making a claw clutch using an additive helix

Post by papyblaise »

I can pitch = 30mm, height 9mm or 11, but not 10, no idea why
Attachments
claw clotch.PNG
claw clotch.PNG (25.97 KiB) Viewed 1096 times
ClawClutch_1.FCStd
(54.17 KiB) Downloaded 14 times
Simonski
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Re: Making a claw clutch using an additive helix

Post by Simonski »

drmacro wrote: Tue Nov 30, 2021 3:38 pm if you rotate the sketch for the claw away from the seam line:
Thanks, that worked.

However, I then tried setting the roll to 90 deg, which moved the sketch as expected (background of screenshot), but when I clicked Apply, the sketch suddenly jumped to near the X axis instead (screenshot inset) and generated an incorrect claw. What is causing this? It’s very confusing to a new user.
Attachments
ClawClutch90Deg.png
ClawClutch90Deg.png (193.76 KiB) Viewed 1064 times
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papyblaise
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Re: Making a claw clutch using an additive helix

Post by papyblaise »

I see it's a coplanar problem : I turn 10° ClawSketch > now it's OK
Attachments
claw clotch1.PNG
claw clotch1.PNG (13.15 KiB) Viewed 1048 times
Simonski
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Re: Making a claw clutch using an additive helix

Post by Simonski »

papyblaise wrote: Tue Nov 30, 2021 6:12 pm I see it's a coplanar problem : I turn 10° ClawSketch > now it's OK
OK, I can see the problem is that I created both sketches are on the same plane (XZ). But why did the helix work when the pitch was less than 17.5mm?
drmacro
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Re: Making a claw clutch using an additive helix

Post by drmacro »

Simonski wrote: Tue Nov 30, 2021 6:49 pm
papyblaise wrote: Tue Nov 30, 2021 6:12 pm I see it's a coplanar problem : I turn 10° ClawSketch > now it's OK
OK, I can see the problem is that I created both sketches are on the same plane (XZ). But why did the helix work when the pitch was less than 17.5mm?
Same solution I showed.

It happens because the OCCT engine is just finicky about the placement of the seam lines. It is especially so, when the seams are on cylindrical objects that share their axis of rotation.

It is common for it to work with one dimension and not some other. In this case, I'd guess the longer helix was deformed enough by the helix angle that it avoided the coplaner tolerance.

When the seams aren't on cylinders, for example where two flat surfaces meet, in many cases, setting Refine=true in the properties of the last object before an operation fixes the problem...in some cases it makes it worse. In the case of a cylinder, the seam is always there (i.e. Refine doesn't remove it) and two cylindrical objects always have the seam in a coplaner situation and the only way to make it work is to rotate one cylinder so the seams don't create a coplaner condition.

This is a foible of the OpenCasade 3D engine and all we can do is learn the tricks to fool it into working. :mrgreen:
Star Trek II: The Wrath of Khan: Spock: "...His pattern indicates two-dimensional thinking."
Simonski
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Re: Making a claw clutch using an additive helix

Post by Simonski »

drmacro wrote: Tue Nov 30, 2021 7:10 pm It happens because...
...don't create a coplaner condition.
Thank you for that explanation; it sounds reasonable.
drmacro wrote: Tue Nov 30, 2021 7:10 pm This is a foible of the OpenCasade 3D engine and all we can do is learn the tricks to fool it into working.
Is there a good ‘tips and tricks’ guide anywhere that explains these foibles? Without the help of people on this forum, I don’t think I would have worked out what the problem was by myself.
drmacro
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Re: Making a claw clutch using an additive helix

Post by drmacro »

Simonski wrote: Tue Nov 30, 2021 9:48 pm

Is there a good ‘tips and tricks’ guide anywhere that explains these foibles? Without the help of people on this forum, I don’t think I would have worked out what the problem was by myself.
Experience and the forum are pretty much it. Some day I may publish my personal notes... 8-)

The Sketcher Lecture is over 70 pages and is under continuous review and update. (thanks! @chrisb)
The problem of writing the "t&t" guide is, it would change daily, rival or exceed the Lecture in length, and be rather confounding to those without the experience to write. At least, IMO. ;)
Star Trek II: The Wrath of Khan: Spock: "...His pattern indicates two-dimensional thinking."
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