Tentative basic documentation for gear()
On Tue, Jul 14, 2015 at 10:44:36AM -0700, TimM wrote:
a couple of the descriptions are a bit off <<
I'm not surprised, as I'm not an expert, although I did research these
things a lot. So that I can update my own documentation, as well as for the
sake of others, can you say which descriptions are off, and how? Thanks!
This seems a bit more complete:
http://forum.openscad.org/file/n13121/InvoluteGearsDoc.txt
The parts you got incomplete in
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/MCAD are the parts relating
to pitch, and twist.
Twist is to make helical and herringbone gears (which have advantages entirely
unrelated to resisting delamination, heck they probably delaminate easier than
normal spur gears), but practically nobody specifies helical gears in terms of
the amount of twist (you can't even tell whether they mesh without calculating
the helix angle). In the dev branch, I added helix_angle to replace that and
plan to drop the "twist" option in the new API.
I have worked out a formula to convert circular pitch from length units
to whatever gear()requires. I've provided a function for this called
convertcp(). <<
You don't need a function. Just multiply the circular pitch you want by
57.29578, which is 180/pi.
Now try doing that for every time you make a gear. I can never remember whether
it's 180/pi or pi/180, which was why I made the function in the first place.
Doug suggested using deg(x) which is easier to recall, but is pretty much a
lucky coincidence -- deg(x) is a conversion from radians to degrees, but
circular pitch is a measure of length, not angle.
In fact, circular pitch should have nothing to do with angles whatsoever. The
circular pitch of a gear that meshes with a rack is equal to the pitch of the
rack, and the pitch of a rack.
--
Kind regards,
Loong Jin
Loong Jin - Thanks for the reply. Aha... I see the source of confusion. The document that you cited as being more complete is the one that I submitted and that is under discussion! The UserManual in MCAD was written by someone else, no idea who.
The points that you made about twist for helical gears sound interesting and valuable. I hope that you or someone else will expand on them at some point. From your comment, it sounds like you are improving the code. Great! I have put my InvoluteGearsDoc.txt file into the able hands of the members of this forum, in the hope that it will be tweaked and expanded for the benefit of all. The only parameter that is totally mysterious to me is involute_facets. I'd love it if someone could document this parameter. Thanks!
I've subscribed to this thread, and I'll modify my own documentation with any useful updates, and submit a revised version whenever the document reaches a new level of authenticity.
TimM
-----Original Message-----
From: Chow Loong Jin [via OpenSCAD] ml-node+s1091067n13155h96@n5.nabble.com
To: TimM TimothyMasters@compuserve.com
Sent: Wed, Jul 15, 2015 3:21 am
Subject: Re: Tentative basic documentation for gear()
On Tue, Jul 14, 2015 at 10:44:36AM -0700, TimM wrote:
a couple of the descriptions are a bit off <<
I'm not surprised, as I'm not an expert, although I did research these
things a lot. So that I can update my own documentation, as well as for the
sake of others, can you say which descriptions are off, and how? Thanks!
This seems a bit more complete:
http://forum.openscad.org/file/n13121/InvoluteGearsDoc.txt
The parts you got incomplete in
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/MCAD are the parts relating
to pitch, and twist.
Twist is to make helical and herringbone gears (which have advantages entirely
unrelated to resisting delamination, heck they probably delaminate easier than
normal spur gears), but practically nobody specifies helical gears in terms of
the amount of twist (you can't even tell whether they mesh without calculating
the helix angle). In the dev branch, I added helix_angle to replace that and
plan to drop the "twist" option in the new API.
I have worked out a formula to convert circular pitch from length units
to whatever gear()requires. I've provided a function for this called
convertcp(). <<
You don't need a function. Just multiply the circular pitch you want by
57.29578, which is 180/pi.
Now try doing that for every time you make a gear. I can never remember whether
it's 180/pi or pi/180, which was why I made the function in the first place.
Doug suggested using deg(x) which is easier to recall, but is pretty much a
lucky coincidence -- deg(x) is a conversion from radians to degrees, but
circular pitch is a measure of length, not angle.
In fact, circular pitch should have nothing to do with angles whatsoever. The
circular pitch of a gear that meshes with a rack is equal to the pitch of the
rack, and the pitch of a rack.
--
Kind regards,
Loong Jin
OpenSCAD mailing list
[hidden email]
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
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On Wed, Jul 15, 2015 at 03:27:26AM -0700, TimM wrote:
Loong Jin - Thanks for the reply. Aha... I see the source of confusion. The
document that you cited as being more complete is the one that I submitted and
that is under discussion! The UserManual in MCAD was written by someone else,
no idea who.
The points that you made about twist for helical gears sound interesting and
valuable. I hope that you or someone else will expand on them at some point.
From your comment, it sounds like you are improving the code. Great! I have
put my InvoluteGearsDoc.txt file into the able hands of the members of this
forum, in the hope that it will be tweaked and expanded for the benefit of
all. The only parameter that is totally mysterious to me is involute_facets.
I'd love it if someone could document this parameter. Thanks!
Oh, involute_facets is simply $fn for the involute gear tooth shape. I think
it's a separate variable from the standard $fs, $fa and $fn set so that you can
treat gear teeth faceting differently from the bore and material-saving circles.
I've subscribed to this thread, and I'll modify my own documentation with any
useful updates, and submit a revised version whenever the document reaches a
new level of authenticity.
--
Kind regards,
Loong Jin
Oh, involute_facets is simply $fn for the involute gear tooth shape. <<
Thank you! That makes perfect sense, as it allows one to reduce friction where it matters without introducing a huge number of polygons where they are not so important. I've added that to my documentation.
Tim
-----Original Message-----
From: Chow Loong Jin [via OpenSCAD] ml-node+s1091067n13169h30@n5.nabble.com
To: TimM TimothyMasters@compuserve.com
Sent: Wed, Jul 15, 2015 10:29 am
Subject: Re: Tentative basic documentation for gear()
On Wed, Jul 15, 2015 at 03:27:26AM -0700, TimM wrote:
Loong Jin - Thanks for the reply. Aha... I see the source of confusion. The
document that you cited as being more complete is the one that I submitted and
that is under discussion! The UserManual in MCAD was written by someone else,
no idea who.
The points that you made about twist for helical gears sound interesting and
valuable. I hope that you or someone else will expand on them at some point.
From your comment, it sounds like you are improving the code. Great! I have
put my InvoluteGearsDoc.txt file into the able hands of the members of this
forum, in the hope that it will be tweaked and expanded for the benefit of
all. The only parameter that is totally mysterious to me is involute_facets.
I'd love it if someone could document this parameter. Thanks!
Oh, involute_facets is simply $fn for the involute gear tooth shape. I think
it's a separate variable from the standard $fs, $fa and $fn set so that you can
treat gear teeth faceting differently from the bore and material-saving circles.
I've subscribed to this thread, and I'll modify my own documentation with any
useful updates, and submit a revised version whenever the document reaches a
new level of authenticity.
--
Kind regards,
Loong Jin
OpenSCAD mailing list
[hidden email]
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
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On Wed, Jul 15, 2015 at 03:27:26AM -0700, TimM wrote:
Loong Jin - Thanks for the reply. Aha... I see the source of confusion. The
document that you cited as being more complete is the one that I submitted and
that is under discussion! The UserManual in MCAD was written by someone else,
no idea who.
The points that you made about twist for helical gears sound interesting and
valuable. I hope that you or someone else will expand on them at some point.
Ah, right, I forgot to expand on this earlier. Helical and herring bone gears
are at https://en.wikipedia.org/wiki/Gear#Helical and
https://en.wikipedia.org/wiki/Herringbone_gear respectively.
Helical and herringbone gears are nice because you get a gear that's at a
different phase at different heights, so your gear teeth don't engage all at
once giving you that clack-clack-clack sound, but gradually, resulting in very
quiet operation (which is why so many people use herringbone Gregstruder gears).
The reason I say that they're more likely to delaminate is that they have less
surface area in common between one layer and the next (the teeth are slightly
out of phase resulting in a slight overhang). Additionally, the angle of the
helical gear tends to result in axial thrust parallel to the rotational axis of
the gear, which further pushes the layers apart.
Herringbone gears are basically mirrored helical gears stuck together so that
the axial thrust forces balance themselves out, but this also means that during
operation, 3d printed herringbone gears are always trying to pry the layers
apart. In practice, layer adhesion's usually good enough that it doesn't matter.
And finally, for theoretically perfect gear meshing, you need:
- same circular pitch
- same pressure angle
- opposite helix angles (or you could just flip the gear)
In practice, you can probably get a pretty wide range of circular pitches to
mesh with each other, and the same goes for pressure angle, but it probably
won't obey the Fundamental Law of Gearing:
The angular velocity ratio between two gears of a gearset must remain constant
throughout the mesh.
--
Kind regards,
Loong Jin
On Wed, Jul 15, 2015 at 07:52:49AM -0700, TimM wrote:
Oh, involute_facets is simply $fn for the involute gear tooth shape. <<
Thank you! That makes perfect sense, as it allows one to reduce friction where
it matters without introducing a huge number of polygons where they are not so
important. I've added that to my documentation.
Well, actually I was thinking of completely ignoring $fn and just using
mcad_polyhole() for the bore.
--
Kind regards,
Loong Jin
It sounds as if you have some excellent ideas for improving that wonderful gear library. I know that someone else (I can track down his name if you aren't aware) has made a modification to gear() that produces teardrop holes instead of round holes. Do you have a rough timeline for when you might be posting your new, improved gear code?
-----Original Message-----
From: Chow Loong Jin [via OpenSCAD] ml-node+s1091067n13172h23@n5.nabble.com
To: TimM TimothyMasters@compuserve.com
Sent: Wed, Jul 15, 2015 11:02 am
Subject: Re: Tentative basic documentation for gear()
On Wed, Jul 15, 2015 at 07:52:49AM -0700, TimM wrote:
Oh, involute_facets is simply $fn for the involute gear tooth shape. <<
Thank you! That makes perfect sense, as it allows one to reduce friction where
it matters without introducing a huge number of polygons where they are not so
important. I've added that to my documentation.
Well, actually I was thinking of completely ignoring $fn and just using
mcad_polyhole() for the bore.
--
Kind regards,
Loong Jin
OpenSCAD mailing list
[hidden email]
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
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NAML
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On Wed, Jul 15, 2015 at 10:21:39AM -0700, TimM wrote:
It sounds as if you have some excellent ideas for improving that wonderful
gear library. I know that someone else (I can track down his name if you
aren't aware) has made a modification to gear() that produces teardrop holes
instead of round holes. Do you have a rough timeline for when you might be
posting your new, improved gear code?
No timeline, but some of the improvements are already available in the dev
branch of MCAD. helix_angle, herringbone, and convertcp() are already there.
--
Kind regards,
Loong Jin