Gears in FreeCAD: FC Gear

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ramblur
Posts: 15
Joined: Thu Nov 02, 2017 10:34 pm

Re: Gears in FreeCAD: FC Gear

Post by ramblur » Sun Dec 29, 2019 6:25 pm

Hello all,

I recently started playing around with FC Gear and I'm very impressed with it. Thanks for all your work on this looo! I've run in to a difficulty with making an internal gear for a planetary gear system, however (see screenshot). The steps I take to get this far are:
  1. create Body (Part Design WB)
  2. make sketch (circle) and pad (Part Design WB)
  3. make involute gear with beta = 30 degree, and double helix = true (Gear WB)
  4. select gear and use Boolean Cut operation (Part Design WB)
If I pad the circle to the same length as the gear, I get the spurious faces on the both ends of the cylinder. If I pad the circle to less than the gear length, then I only have the spurious face on the x-y plane.

Lastly, I don't have this problem for straight, non-helical gears. Using a single helix (i.e. not a herringbone), I get the spurious face on just one side, away from the x-y plane.

I'm using the FreeCAD AppImage on an up-to-date Arch system (installing FreeCAD is presently difficult to impossible on Arch).

Thanks in advance for your help.

Cheers,
-R
Screenshot_2019-12-29_10-23-34.png
Screenshot_2019-12-29_10-23-34.png (144.08 KiB) Viewed 300 times
chrisb
Posts: 21445
Joined: Tue Mar 17, 2015 9:14 am

Re: Gears in FreeCAD: FC Gear

Post by chrisb » Sun Dec 29, 2019 6:32 pm

Looks like a rounding issue. In FreeCAD it is sensible to avoid coplanar faces for cuts anyway so you should make the object to be cut a bit larger than necessary anyway.
ramblur
Posts: 15
Joined: Thu Nov 02, 2017 10:34 pm

Re: Gears in FreeCAD: FC Gear

Post by ramblur » Mon Dec 30, 2019 1:15 am

chrisb wrote:
Sun Dec 29, 2019 6:32 pm
Looks like a rounding issue. In FreeCAD it is sensible to avoid coplanar faces for cuts anyway so you should make the object to be cut a bit larger than necessary anyway.
Thanks for the information; I didn't know that there were issues with co-planar surfaces and boolean operations. I never had this problem with Involute Gear in Part Design, but you make a good point and it's easy enough to design around the potential problem.

Cheers,
-R
ramblur
Posts: 15
Joined: Thu Nov 02, 2017 10:34 pm

Re: Gears in FreeCAD: FC Gear

Post by ramblur » Mon Dec 30, 2019 4:31 am

Another question:
When I enter a beta angle to make an involute gear a helical gear, it seems that the whole gear gets bigger. This is reflected in an increased pitch diameter (computed value 'dw' in the Data tab). In the attached screenshot, both gears are the default involute gear generated by clicking the 'Involute gear' button in the Gear workbench, except that the one on the right has been beta = 45 degrees. dw for the default gear is 15 (I assume mm), and is 21.21 for the helical version (screenshots of the Data tabs for each also attached).

Why does this happen? I thought a helical gear was just an involute profile swept along a helix, so the increased size of the gear is very surprising. Is this expected behaviour, or a bug? If it's a known bug, is there a work around? I'm designing some herringbone gears and I could use the Part Design + Parts workbenches work flow of Involute Gear -> Helix -> Sweep -> Mirror, but it would be nice to do everything in the Gears workbench if possible.

(I'm using the FreeCAD AppImage in Arch linux, in case that makes a difference)

Thanks,
-R
Screenshot_2019-12-29_20-26-06.png
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Data tab for the default gear:
Screenshot_2019-12-29_20-28-34.png
Screenshot_2019-12-29_20-28-34.png (35.6 KiB) Viewed 236 times
Data tab for the helical gear:
Screenshot_2019-12-29_20-29-06.png
Screenshot_2019-12-29_20-29-06.png (36.2 KiB) Viewed 236 times
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looo
Posts: 3113
Joined: Mon Nov 11, 2013 5:29 pm

Re: Gears in FreeCAD: FC Gear

Post by looo » Mon Dec 30, 2019 4:50 am

ramblur wrote:
Mon Dec 30, 2019 4:31 am
Why does this happen? I thought a helical gear was just an involute profile swept along a helix, so the increased size of the gear is very surprising. Is this expected behaviour, or a bug?
It's by design. The used module is the module of the tool to produce the involute which is the same rack-profile for straight and helical gear. The only difference is that the tool of the helical gear is rotated by beta.
Defining gears this way makes sense for gears produced by cutting with racks or involute profiles. It doesn't make too much sense for printed gears.
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ramblur
Posts: 15
Joined: Thu Nov 02, 2017 10:34 pm

Re: Gears in FreeCAD: FC Gear

Post by ramblur » Tue Dec 31, 2019 5:01 am

Thank you for the information, looo, and also for all your work on the Gear workbench.

Cheers,
-R
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