Tentative basic documentation for gear()

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Tentative basic documentation for gear()

TimM
I am a hobbyist wanting to print gears for gizmos.  The involute_gears library is fantastic, but for someone with no engineering background the calling parameters are cryptic.  In searching forums for hints I found MANY people in the same boat, and even a few people using it professionally without understanding what they were specifying.  So I put what I've learned into a short text document.  My hope is that an expert can review what I wrote for correctness and then, after any needed edits, a moderator can post it somewhere for use by other hapless newbies.  I think I uploaded the text file correctly, but this forum format is new to me, so please let me know if I have to do something differently.
------------------------------------

InvoluteGearsDoc.txt
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Re: Tentative basic documentation for gear()

doug.moen
Thanks for writing this documentation, it was educational for me.

One comment so far:
	   Standard practice is circular_pitch = pi / diametral_pitch (See the next parameter).
	   But for murky historical reasons,  the gear() code defines circular_pitch = 180 / diametral_pitch.
Note that pi radians == 180 degrees, and that OpenSCAD uses degrees, instead of radians, in its trigonometric functions.

I think this documentation should be placed in involute_gears.scad itself.

On 12 July 2015 at 07:22, TimM <[hidden email]> wrote:
I am a hobbyist wanting to print gears for gizmos.  The involute_gears
library is fantastic, but for someone with no engineering background the
calling parameters are cryptic.  In searching forums for hints I found MANY
people in the same boat, and even a few people using it professionally
without understanding what they were specifying.  So I put what I've learned
into a short text document.  My hope is that an expert can review what I
wrote for correctness and then, after any needed edits, a moderator can post
it somewhere for use by other hapless newbies.  I think I uploaded the text
file correctly, but this forum format is new to me, so please let me know if
I have to do something differently.
------------------------------------

InvoluteGearsDoc.txt
<http://forum.openscad.org/file/n13121/InvoluteGearsDoc.txt>



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Re: Tentative basic documentation for gear()

jdawgaz
great doc. I wonder if it could be put in a libre office document, with pictures to show what the various parts are; visuals are so much more effective.

thanks.
jerry


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On Sun, Jul 12, 2015 at 7:54 AM, doug moen <[hidden email]> wrote:
Thanks for writing this documentation, it was educational for me.

One comment so far:
	   Standard practice is circular_pitch = pi / diametral_pitch (See the next parameter).
	   But for murky historical reasons,  the gear() code defines circular_pitch = 180 / diametral_pitch.
Note that pi radians == 180 degrees, and that OpenSCAD uses degrees, instead of radians, in its trigonometric functions.

I think this documentation should be placed in involute_gears.scad itself.

On 12 July 2015 at 07:22, TimM <[hidden email]> wrote:
I am a hobbyist wanting to print gears for gizmos.  The involute_gears
library is fantastic, but for someone with no engineering background the
calling parameters are cryptic.  In searching forums for hints I found MANY
people in the same boat, and even a few people using it professionally
without understanding what they were specifying.  So I put what I've learned
into a short text document.  My hope is that an expert can review what I
wrote for correctness and then, after any needed edits, a moderator can post
it somewhere for use by other hapless newbies.  I think I uploaded the text
file correctly, but this forum format is new to me, so please let me know if
I have to do something differently.
------------------------------------

InvoluteGearsDoc.txt
<http://forum.openscad.org/file/n13121/InvoluteGearsDoc.txt>



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Re: Tentative basic documentation for gear()

tp3
I'd like to ask if it could be added to the MCAD page on the Wiki.
This page is really in need of some love and it looks like the
gear scripts are likely the most used part of MCAD.

https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/MCAD

Or, maybe even better as Doug suggested it could be added to the
script itself.

Loong Jin, what do you think?

ciao,
   Torsten.


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Re: Tentative basic documentation for gear()

Peter Falke
Lets first put it on the wiki, so we all can make miner correction easily, and expand it a little.

2015-07-12 15:16 GMT+02:00 Torsten Paul <[hidden email]>:
I'd like to ask if it could be added to the MCAD page on the Wiki.
This page is really in need of some love and it looks like the
gear scripts are likely the most used part of MCAD.

https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/MCAD

Or, maybe even better as Doug suggested it could be added to the
script itself.

Loong Jin, what do you think?

ciao,
  Torsten.



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Re: Tentative basic documentation for gear()

Chow Loong Jin
In reply to this post by tp3
On Sun, Jul 12, 2015 at 03:16:22PM +0200, Torsten Paul wrote:

> I'd like to ask if it could be added to the MCAD page on the Wiki.
> This page is really in need of some love and it looks like the
> gear scripts are likely the most used part of MCAD.
>
> https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/MCAD
>
> Or, maybe even better as Doug suggested it could be added to the
> script itself.
>
> Loong Jin, what do you think?
Hmm, a couple of the descriptions are a bit off, and I'm hoping to eventually
get Doxygen documentation on the entire library, but first comes tidying up the
messy interfaces.

Like some have noticed, circular_pitch in the gear module uses broken units
(something to do with radians that I don't quite understand, but 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 use it like
this for a 2D gear with a circular pitch (distance along the pitch circle's
perimeter between teeth) of 5 and 10 teeth:

  gear (number_of_teeth = 10, circular_pitch = convertcp (5), flat = true);

I'd like to change this so that it no longer needs convertcp() which I see as a
kludge, but this requires breaking the MCAD API and, as a result, older designs
using this library.

As such, there is an ongoing effort to unify the naming conventions of all MCAD
modules and functions into mcad_foo_bar_baz() format (we currently have
lowerCamelCase, UpperCamelCase and bare_underscores all in the same tree and
it's quite a clusterfuck), while providing compat shims with the API of the
current master branch so that older designs will work. The doxygen comments will
also be added along the way.

--
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Loong Jin

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Re: Tentative basic documentation for gear()

Neon22
Good references on what these gear calcs mean and a good ref for them on this page(at bottom):
    - https://github.com/jnweiger/inkscape-gears-dev
    - http://www.gizmology.net/gears.htm
    - http://www.micro-machine-shop.com/gear_theory.pdf
along with a python module that performs same - if you want to check anything....


On 7/14/2015 6:23 PM, Chow Loong Jin wrote:
On Sun, Jul 12, 2015 at 03:16:22PM +0200, Torsten Paul wrote:
I'd like to ask if it could be added to the MCAD page on the Wiki.
This page is really in need of some love and it looks like the
gear scripts are likely the most used part of MCAD.

https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/MCAD

Or, maybe even better as Doug suggested it could be added to the
script itself.

Loong Jin, what do you think?
Hmm, a couple of the descriptions are a bit off, and I'm hoping to eventually
get Doxygen documentation on the entire library, but first comes tidying up the
messy interfaces.

Like some have noticed, circular_pitch in the gear module uses broken units
(something to do with radians that I don't quite understand, but 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 use it like
this for a 2D gear with a circular pitch (distance along the pitch circle's
perimeter between teeth) of 5 and 10 teeth:

  gear (number_of_teeth = 10, circular_pitch = convertcp (5), flat = true);

I'd like to change this so that it no longer needs convertcp() which I see as a
kludge, but this requires breaking the MCAD API and, as a result, older designs
using this library.

As such, there is an ongoing effort to unify the naming conventions of all MCAD
modules and functions into mcad_foo_bar_baz() format (we currently have
lowerCamelCase, UpperCamelCase and bare_underscores all in the same tree and
it's quite a clusterfuck), while providing compat shims with the API of the
current master branch so that older designs will work. The doxygen comments will
also be added along the way.



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Re: Tentative basic documentation for gear()

Chow Loong Jin
On Tue, Jul 14, 2015 at 07:37:21PM +1200, Mark Schafer wrote:
> Good references on what these gear calcs mean and a good ref for them on this
> page(at bottom):
>     - https://github.com/jnweiger/inkscape-gears-dev
>     - http://www.gizmology.net/gears.htm
>     - http://www.micro-machine-shop.com/gear_theory.pdf
> along with a python module that performs same - if you want to check
> anything....

Yeah, I looked through all of that while dissecting the gear module, and
everywhere I looked, circular pitch is defined as distance between the same spot
of adjacent teeth along the circumference of the pitch circle.

Distance *should* be measured in mm or inches (let's just designate this as
$length_unit for the time being), but for whatever reason, the original gear()
module used circular pitch values measured in units of
"(PI / 180) $length_unit".

In order to preserve backward compatibility with old designs that use this
module, we have to accept values specified in terms of that odd unit.

--
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Loong Jin

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Re: Tentative basic documentation for gear()

TimM
In reply to this post by Chow Loong Jin
>> 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!


>> 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.

TimM
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Re: Tentative basic documentation for gear()

doug.moen
>> 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.

You can also use the deg(x) function from <MCAD/math.scad>, which performs a radians to degrees conversion.
It's the same function.

OpenSCAD is different from every other programming language I know, in that the trig functions take arguments measured in degrees instead of radians. If you are porting software from another language into OpenSCAD, you should take this into account. I suspect that gear() was ported from another language, but without converting radians to degrees in the calls to sin(), cos() etc.

On 14 July 2015 at 13:44, TimM <[hidden email]> 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!


>> 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.

TimM



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Re: Tentative basic documentation for gear()

Chow Loong Jin
In reply to this post by TimM
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

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Re: Tentative basic documentation for gear()

TimM
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] <[hidden email]>
To: TimM <[hidden email]>
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

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Re: Tentative basic documentation for gear()

Chow Loong Jin
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

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Re: Tentative basic documentation for gear()

TimM
>> 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] <[hidden email]>
To: TimM <[hidden email]>
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

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Re: Tentative basic documentation for gear()

Chow Loong Jin
In reply to this post by TimM
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

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Re: Tentative basic documentation for gear()

Chow Loong Jin
In reply to this post by TimM
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

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Re: Tentative basic documentation for gear()

TimM
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] <[hidden email]>
To: TimM <[hidden email]>
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

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Re: Tentative basic documentation for gear()

Chow Loong Jin
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

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