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Question about gear library

kdtop
I am trying to model a gear for an Atlas metal lathe.  I have some original (real-life) gears, but want to print one with a different number of teeth.

I have found this gear library by GregFrost:  https://www.thingiverse.com/thing:3575/files
I know that this list is not responsible for the library, but it seems to be a good one, and I am hoping someone here can help.

My question is about the units of the input parameters.  For example, in the file parametric_involute_gear_v5.0.scad there are some example gears.  I modified one slightly as follows:

gear (number_of_teeth=64,
      circular_pitch=285.0,
      circles=0);

I chose the value for circular_pitch experimentally.  First I put an Echo() statement in the gear_shape module such that it reports the minor diameter, pitch diameter, and major diameter (in mm).  I then slowly modified the circular_pitch value until it matched the measurements of my actual real-life gear, taken with my micrometer.

I am confused about what circular_pitch is supposed to represent.  I reviewed standard gear terms from here:  https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch
Circular pitch should be the distance (in mm) from tooth to tooth.  In my actual real-life gear, the value is about 5 mm.

On a slightly different tact, I am also confused about gear terminology used for my lathe.  The gears are typically described using inches and diametral pitch (which is supposed to be 1/circular_pitch).  According to this site: https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ (which references here: http://www.lathes.co.uk/atlas/), Atlas gears use a 16DP, which I think means 16 diametral pitch or 16 teeth per inch.  I'm not sure I believe this number, because my real-life gear with 20 teeth has a diameter of 1.475 inches, circumference of ~4.3 inches, and thus 20/4.3 = ~4.6 teeth/inch.

Summary:
  1. Can anyone help me understand the units for the circular_pitch parameter in the gear() module as described above?
  2. Can anyone see the source of my confusion about the 16DP

Thanks in advance,
Kevin T

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Re: Question about gear library

nophead
I am not familiar with Greg's library but DP is not about circumference. It is the number of teeth per inch of its pitch diameter, an odd measure that doesn't really have a physical meaning because the teeth go around the circumference, not across the diameter.

Also the pitch diameter is not the outside diameter, it is the diameter where the teeth mesh, sort of half way down the tooth. So you can mesh two gears if they have the same DP and you space them apart by half the sum of the pitch diameters.

I have a vague memory that Greg's library has a bug in the way circular_pitch is used.

On Sat, 22 Dec 2018 at 13:16, Kevin Toppenberg <[hidden email]> wrote:
I am trying to model a gear for an Atlas metal lathe.  I have some original (real-life) gears, but want to print one with a different number of teeth.

I have found this gear library by GregFrost:  https://www.thingiverse.com/thing:3575/files
I know that this list is not responsible for the library, but it seems to be a good one, and I am hoping someone here can help.

My question is about the units of the input parameters.  For example, in the file parametric_involute_gear_v5.0.scad there are some example gears.  I modified one slightly as follows:

gear (number_of_teeth=64,
      circular_pitch=285.0,
      circles=0);

I chose the value for circular_pitch experimentally.  First I put an Echo() statement in the gear_shape module such that it reports the minor diameter, pitch diameter, and major diameter (in mm).  I then slowly modified the circular_pitch value until it matched the measurements of my actual real-life gear, taken with my micrometer.

I am confused about what circular_pitch is supposed to represent.  I reviewed standard gear terms from here:  https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch
Circular pitch should be the distance (in mm) from tooth to tooth.  In my actual real-life gear, the value is about 5 mm.

On a slightly different tact, I am also confused about gear terminology used for my lathe.  The gears are typically described using inches and diametral pitch (which is supposed to be 1/circular_pitch).  According to this site: https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ (which references here: http://www.lathes.co.uk/atlas/), Atlas gears use a 16DP, which I think means 16 diametral pitch or 16 teeth per inch.  I'm not sure I believe this number, because my real-life gear with 20 teeth has a diameter of 1.475 inches, circumference of ~4.3 inches, and thus 20/4.3 = ~4.6 teeth/inch.

Summary:
  1. Can anyone help me understand the units for the circular_pitch parameter in the gear() module as described above?
  2. Can anyone see the source of my confusion about the 16DP

Thanks in advance,
Kevin T
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Re: Question about gear library

alexgibson

Hi, I’ve used Greg’s excellent library a lot without ever getting into the fine detail of this – I’m wondering why you actually need to?

 

My assumption is that you might be trying to change only one gear of a pair?

 

If you can change both gears, then you don’t need to worry much about the pitches – simply specify the distance between shafts and the gear ratio, and Greg’s library will take care of it all very nicely.

 

If you’re hoping to keep using one of the gears, you will need to much more deeply understand the pitches, or if your main motivation is to understand how Greg’s library is doing its job, please ignore this useless intervention!

 

Cheers,

 

Alex Gibson

 

admg consulting

 

edumaker limited

 

·         Project management

·         Operations & Process improvement

·         3D Printing

 

From: Discuss [mailto:[hidden email]] On Behalf Of nop head
Sent: 22 December 2018 13:52
To: OpenSCAD general discussion
Subject: Re: [OpenSCAD] Question about gear library

 

I am not familiar with Greg's library but DP is not about circumference. It is the number of teeth per inch of its pitch diameter, an odd measure that doesn't really have a physical meaning because the teeth go around the circumference, not across the diameter.

 

Also the pitch diameter is not the outside diameter, it is the diameter where the teeth mesh, sort of half way down the tooth. So you can mesh two gears if they have the same DP and you space them apart by half the sum of the pitch diameters.

 

I have a vague memory that Greg's library has a bug in the way circular_pitch is used.

 

On Sat, 22 Dec 2018 at 13:16, Kevin Toppenberg <[hidden email]> wrote:

I am trying to model a gear for an Atlas metal lathe.  I have some original (real-life) gears, but want to print one with a different number of teeth.

 

I have found this gear library by GregFrost:  https://www.thingiverse.com/thing:3575/files

I know that this list is not responsible for the library, but it seems to be a good one, and I am hoping someone here can help.

 

My question is about the units of the input parameters.  For example, in the file parametric_involute_gear_v5.0.scad there are some example gears.  I modified one slightly as follows:

 

gear (number_of_teeth=64,

      circular_pitch=285.0,

      circles=0);

 

I chose the value for circular_pitch experimentally.  First I put an Echo() statement in the gear_shape module such that it reports the minor diameter, pitch diameter, and major diameter (in mm).  I then slowly modified the circular_pitch value until it matched the measurements of my actual real-life gear, taken with my micrometer.

 

I am confused about what circular_pitch is supposed to represent.  I reviewed standard gear terms from here:  https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch

Circular pitch should be the distance (in mm) from tooth to tooth.  In my actual real-life gear, the value is about 5 mm.

 

On a slightly different tact, I am also confused about gear terminology used for my lathe.  The gears are typically described using inches and diametral pitch (which is supposed to be 1/circular_pitch).  According to this site: https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ (which references here: http://www.lathes.co.uk/atlas/), Atlas gears use a 16DP, which I think means 16 diametral pitch or 16 teeth per inch.  I'm not sure I believe this number, because my real-life gear with 20 teeth has a diameter of 1.475 inches, circumference of ~4.3 inches, and thus 20/4.3 = ~4.6 teeth/inch.

 

Summary:

  1. Can anyone help me understand the units for the circular_pitch parameter in the gear() module as described above?
  2. Can anyone see the source of my confusion about the 16DP


Thanks in advance,

Kevin T

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Re: Question about gear library

boxcarmib
In reply to this post by nophead
If you know the diametral pitch of a gear train you should be in pretty good shape as you can work backwards to determining the pitch diameter and other necessary dimensions.
As others have noted, diametral pitch is the distance between gear teeth, but this is not measured at the outer diameter of the gear but at a calculated point between the outer diameter of the gear teeth and the diameter of the base of the teeth, and this is know as the pitch circle or reference diameter.

pitchDiameter = numberOfTeeth / diametralPitch;

So if you want to calculate the pitch diameter for Greg’s routines you can just count the teeth on the gear you are trying to copy and plug that into the formula above. If Atlas gears do have a diametral pitch of 16 and you count the number of teeth accurately :) then you’re gonna end up with a gear the correct size.
If you want to check that against what you’ve measured as the outside diameter of your gear you can do the following, noting that the dimension between the pitchDiameter and the outside diameter is called the addendum

addendum = 1 / diametralPitch;

outsideDiameter = pitchDiameter + (2 * addendum);

Looking at the link you posted (https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ ) it mentions that atlas gears use a 14.5 degree pressure angle so in addition to the routine in Greg’s library you’re already calling you’ll want to include the pressure_angle parameter as well, as the default of 20 is not what you want., e.g.

gear (number_of_teeth=64,
//      circular_pitch=285.0, // call greg’s routine with the diametral pitch parameter.. not circular pitch
 diametral_pitch = 16,
    pressure_angle = 14.5,
      circles=0);


Hope this is helpful.



On Dec 22, 2018, at 1:52 PM, nop head <[hidden email]> wrote:

I am not familiar with Greg's library but DP is not about circumference. It is the number of teeth per inch of its pitch diameter, an odd measure that doesn't really have a physical meaning because the teeth go around the circumference, not across the diameter.

Also the pitch diameter is not the outside diameter, it is the diameter where the teeth mesh, sort of half way down the tooth. So you can mesh two gears if they have the same DP and you space them apart by half the sum of the pitch diameters.

I have a vague memory that Greg's library has a bug in the way circular_pitch is used.

On Sat, 22 Dec 2018 at 13:16, Kevin Toppenberg <[hidden email]> wrote:
I am trying to model a gear for an Atlas metal lathe.  I have some original (real-life) gears, but want to print one with a different number of teeth.

I have found this gear library by GregFrost:  https://www.thingiverse.com/thing:3575/files
I know that this list is not responsible for the library, but it seems to be a good one, and I am hoping someone here can help.

My question is about the units of the input parameters.  For example, in the file parametric_involute_gear_v5.0.scad there are some example gears.  I modified one slightly as follows:

gear (number_of_teeth=64,
      circular_pitch=285.0,
      circles=0);

I chose the value for circular_pitch experimentally.  First I put an Echo() statement in the gear_shape module such that it reports the minor diameter, pitch diameter, and major diameter (in mm).  I then slowly modified the circular_pitch value until it matched the measurements of my actual real-life gear, taken with my micrometer.

I am confused about what circular_pitch is supposed to represent.  I reviewed standard gear terms from here:  https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch
Circular pitch should be the distance (in mm) from tooth to tooth.  In my actual real-life gear, the value is about 5 mm.

On a slightly different tact, I am also confused about gear terminology used for my lathe.  The gears are typically described using inches and diametral pitch (which is supposed to be 1/circular_pitch).  According to this site: https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ (which references here: http://www.lathes.co.uk/atlas/), Atlas gears use a 16DP, which I think means 16 diametral pitch or 16 teeth per inch.  I'm not sure I believe this number, because my real-life gear with 20 teeth has a diameter of 1.475 inches, circumference of ~4.3 inches, and thus 20/4.3 = ~4.6 teeth/inch.

Summary:
  1. Can anyone help me understand the units for the circular_pitch parameter in the gear() module as described above?
  2. Can anyone see the source of my confusion about the 16DP

Thanks in advance,
Kevin T
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Re: Question about gear library

Neon22
In reply to this post by kdtop

Hi Kevin,
As others have pointed out Circular pitch means something slightly different and direct measurement is difficult (also pressure angle)..

I suggest looking at the links in this reference:
- https://github.com/jnweiger/inkscape-gears-dev

This plugin will draw gears in Inkscape ( a free OS vector drawing program).
This might help you to match the gear you have by direct comparison with a 2D drawing.
You can also export to openSCAD directly but this will only make simple gears with no 3D profiling but you can use it to learn about these parameters and make direct 2D comparisons. Then create gears that match in Greg's library.

Cheers, Mark...

On 12/23/2018 2:14 AM, Kevin Toppenberg wrote:
I am trying to model a gear for an Atlas metal lathe.  I have some original (real-life) gears, but want to print one with a different number of teeth.

I have found this gear library by GregFrost:  https://www.thingiverse.com/thing:3575/files
I know that this list is not responsible for the library, but it seems to be a good one, and I am hoping someone here can help.

My question is about the units of the input parameters.  For example, in the file parametric_involute_gear_v5.0.scad there are some example gears.  I modified one slightly as follows:

gear (number_of_teeth=64,
      circular_pitch=285.0,
      circles=0);

I chose the value for circular_pitch experimentally.  First I put an Echo() statement in the gear_shape module such that it reports the minor diameter, pitch diameter, and major diameter (in mm).  I then slowly modified the circular_pitch value until it matched the measurements of my actual real-life gear, taken with my micrometer.

I am confused about what circular_pitch is supposed to represent.  I reviewed standard gear terms from here:  https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch
Circular pitch should be the distance (in mm) from tooth to tooth.  In my actual real-life gear, the value is about 5 mm.

On a slightly different tact, I am also confused about gear terminology used for my lathe.  The gears are typically described using inches and diametral pitch (which is supposed to be 1/circular_pitch).  According to this site: https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ (which references here: http://www.lathes.co.uk/atlas/), Atlas gears use a 16DP, which I think means 16 diametral pitch or 16 teeth per inch.  I'm not sure I believe this number, because my real-life gear with 20 teeth has a diameter of 1.475 inches, circumference of ~4.3 inches, and thus 20/4.3 = ~4.6 teeth/inch.

Summary:
  1. Can anyone help me understand the units for the circular_pitch parameter in the gear() module as described above?
  2. Can anyone see the source of my confusion about the 16DP

Thanks in advance,
Kevin T

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Re: Question about gear library

Parkinbot
In reply to this post by kdtop
Kevin,

having programmed, designed and printed a lot of gears and gear types, the
last I would recommend you is to use a 3D-printed gear in a metal lathe.
Even if you have a SLM printer at hand, the result won't be usable on the
long run in a production machine. You risk a lot when putting a minor
quality gear into such a device.
Gears used in lathes are usually of standard pitch und can be bought from
the manufacturer even for old machines. Did you do an Ebay search?

But to answer your question and to complete your information about gears. DP
is a kind of a crook to be able to measure gears somehow in inch. Using it
is somehow magic and not very intuitive.

The metric system is more clear. You have a modulus m and a number z of
theeth. With this your pitch diameter d=m*z. The outer diameter of a gear is
usually od=d+addendum, with addendum < 2*m.

So, starting with addendum = 2*m, od=1.475 inches = 37.47 mm and z=20 will
give you a modulus of 1.70xx, which is uncommon. A common module is m=1.75.
Why? because you can buy cutting tools for this module, and many lathes use
it. This will result in a pitch diameter of 35mm or 1.378 inch. Further you
must know the pressure angle w, which you can't measure. 20° is normal, but
there are other systems. Maybe it its 14.5°, as was mentioned before. And
last but not least many gears have a profile shift to further adapt the
pitch diameter.

If you want to check all this with your given gear set, you can see my post:
https://www.thingiverse.com/thing:636119 and download my library gears.scad.
It will prompt you the pitch and the outer diameter and some other useful
information to the console. However it uses millimeters. It also provides a
help function. This is your gear:

use<gears.scad>
help(); // see what the lib contains and the prototypes of the modules
gear(m=1.75, z=20, w=14.5);

and your new gear will simply be:

gear(m=1.75, z=64, w=14.5);









--
Sent from: http://forum.openscad.org/

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Re: Question about gear library

kdtop
In reply to this post by nophead
Nop head,

Thank you for your reply.  The reason I was talking about circumference is because I was calculating the actual number of teeth per inch on my actual gear.  To calculate that, I had to know the distance along the surface to divide the 20 teeth between.  That involves the circumference.

I'll keep my eyes out for a bug in the library.

Thanks. 

On Sat, Dec 22, 2018 at 8:53 AM nop head <[hidden email]> wrote:
I am not familiar with Greg's library but DP is not about circumference. It is the number of teeth per inch of its pitch diameter, an odd measure that doesn't really have a physical meaning because the teeth go around the circumference, not across the diameter.

Also the pitch diameter is not the outside diameter, it is the diameter where the teeth mesh, sort of half way down the tooth. So you can mesh two gears if they have the same DP and you space them apart by half the sum of the pitch diameters.

I have a vague memory that Greg's library has a bug in the way circular_pitch is used.

On Sat, 22 Dec 2018 at 13:16, Kevin Toppenberg <[hidden email]> wrote:
I am trying to model a gear for an Atlas metal lathe.  I have some original (real-life) gears, but want to print one with a different number of teeth.

I have found this gear library by GregFrost:  https://www.thingiverse.com/thing:3575/files
I know that this list is not responsible for the library, but it seems to be a good one, and I am hoping someone here can help.

My question is about the units of the input parameters.  For example, in the file parametric_involute_gear_v5.0.scad there are some example gears.  I modified one slightly as follows:

gear (number_of_teeth=64,
      circular_pitch=285.0,
      circles=0);

I chose the value for circular_pitch experimentally.  First I put an Echo() statement in the gear_shape module such that it reports the minor diameter, pitch diameter, and major diameter (in mm).  I then slowly modified the circular_pitch value until it matched the measurements of my actual real-life gear, taken with my micrometer.

I am confused about what circular_pitch is supposed to represent.  I reviewed standard gear terms from here:  https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch
Circular pitch should be the distance (in mm) from tooth to tooth.  In my actual real-life gear, the value is about 5 mm.

On a slightly different tact, I am also confused about gear terminology used for my lathe.  The gears are typically described using inches and diametral pitch (which is supposed to be 1/circular_pitch).  According to this site: https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ (which references here: http://www.lathes.co.uk/atlas/), Atlas gears use a 16DP, which I think means 16 diametral pitch or 16 teeth per inch.  I'm not sure I believe this number, because my real-life gear with 20 teeth has a diameter of 1.475 inches, circumference of ~4.3 inches, and thus 20/4.3 = ~4.6 teeth/inch.

Summary:
  1. Can anyone help me understand the units for the circular_pitch parameter in the gear() module as described above?
  2. Can anyone see the source of my confusion about the 16DP

Thanks in advance,
Kevin T
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Re: Question about gear library

kdtop
In reply to this post by alexgibson
Alex,

Yes, I am trying to add a gear into an existing gear train.  So I have to exactly match the pitch of the others.  

Thanks!

Kevin

On Sat, Dec 22, 2018 at 9:36 AM Alex Gibson <[hidden email]> wrote:

Hi, I’ve used Greg’s excellent library a lot without ever getting into the fine detail of this – I’m wondering why you actually need to?

 

My assumption is that you might be trying to change only one gear of a pair?

 

If you can change both gears, then you don’t need to worry much about the pitches – simply specify the distance between shafts and the gear ratio, and Greg’s library will take care of it all very nicely.

 

If you’re hoping to keep using one of the gears, you will need to much more deeply understand the pitches, or if your main motivation is to understand how Greg’s library is doing its job, please ignore this useless intervention!

 

Cheers,

 

Alex Gibson

 

admg consulting

 

edumaker limited

 

·         Project management

·         Operations & Process improvement

·         3D Printing

 

From: Discuss [mailto:[hidden email]] On Behalf Of nop head
Sent: 22 December 2018 13:52
To: OpenSCAD general discussion
Subject: Re: [OpenSCAD] Question about gear library

 

I am not familiar with Greg's library but DP is not about circumference. It is the number of teeth per inch of its pitch diameter, an odd measure that doesn't really have a physical meaning because the teeth go around the circumference, not across the diameter.

 

Also the pitch diameter is not the outside diameter, it is the diameter where the teeth mesh, sort of half way down the tooth. So you can mesh two gears if they have the same DP and you space them apart by half the sum of the pitch diameters.

 

I have a vague memory that Greg's library has a bug in the way circular_pitch is used.

 

On Sat, 22 Dec 2018 at 13:16, Kevin Toppenberg <[hidden email]> wrote:

I am trying to model a gear for an Atlas metal lathe.  I have some original (real-life) gears, but want to print one with a different number of teeth.

 

I have found this gear library by GregFrost:  https://www.thingiverse.com/thing:3575/files

I know that this list is not responsible for the library, but it seems to be a good one, and I am hoping someone here can help.

 

My question is about the units of the input parameters.  For example, in the file parametric_involute_gear_v5.0.scad there are some example gears.  I modified one slightly as follows:

 

gear (number_of_teeth=64,

      circular_pitch=285.0,

      circles=0);

 

I chose the value for circular_pitch experimentally.  First I put an Echo() statement in the gear_shape module such that it reports the minor diameter, pitch diameter, and major diameter (in mm).  I then slowly modified the circular_pitch value until it matched the measurements of my actual real-life gear, taken with my micrometer.

 

I am confused about what circular_pitch is supposed to represent.  I reviewed standard gear terms from here:  https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch

Circular pitch should be the distance (in mm) from tooth to tooth.  In my actual real-life gear, the value is about 5 mm.

 

On a slightly different tact, I am also confused about gear terminology used for my lathe.  The gears are typically described using inches and diametral pitch (which is supposed to be 1/circular_pitch).  According to this site: https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ (which references here: http://www.lathes.co.uk/atlas/), Atlas gears use a 16DP, which I think means 16 diametral pitch or 16 teeth per inch.  I'm not sure I believe this number, because my real-life gear with 20 teeth has a diameter of 1.475 inches, circumference of ~4.3 inches, and thus 20/4.3 = ~4.6 teeth/inch.

 

Summary:

  1. Can anyone help me understand the units for the circular_pitch parameter in the gear() module as described above?
  2. Can anyone see the source of my confusion about the 16DP


Thanks in advance,

Kevin T

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Re: Question about gear library

kdtop
In reply to this post by boxcarmib
Hugo, 

Thank you for this helpful reply.  See comments below

On Sat, Dec 22, 2018 at 10:52 AM Hugo Jackson <[hidden email]> wrote:
If you know the diametral pitch of a gear train you should be in pretty good shape as you can work backwards to determining the pitch diameter and other necessary dimensions.
As others have noted, diametral pitch is the distance between gear teeth,

I think your definition is not quite right.  The units for diametral pitch are teeth/distance, not distance/tooth.   But I get what you mean.
 
but this is not measured at the outer diameter of the gear but at a calculated point between the outer diameter of the gear teeth and the diameter of the base of the teeth, and this is know as the pitch circle or reference diameter.

pitchDiameter = numberOfTeeth / diametralPitch;

This formula helped me quite a bit.  I think I might have been conflating the two different terms: diametral pitch and pitch diameter.  But I see now that they are clearly different.  
 
So if you want to calculate the pitch diameter for Greg’s routines you can just count the teeth on the gear you are trying to copy and plug that into the formula above. If Atlas gears do have a diametral pitch of 16 and you count the number of teeth accurately :) then you’re gonna end up with a gear the correct size.

So this worked:

pitch_diameter (inches) = 20 teeth / 16 teeth/inch   = 1.25 inches  <--- matches my actual gear.

So the gear() module accepts either circular_pitch OR diametral_pitch as inputs.  In a bit of a gotcha, however, the units for the diametral_pitch module have to be in mm, not inches -- even though diametral pitch is typically only makes sense in inches.  E.g. 16 teeth/inch makes sense to talk about.  The equivalent value, 0.63 teeth/mm seems crazy. 

So I mentioned before that I carefully adjusted the circular_pitch until I got values that matched my real gear.   Well, I then made another gear using the diametral_pitch, and they exactly matched.  Hurrah!

I STILL would like to know the units for the circular_pitch Greg is using.  Wikipedia (https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch) says circular pitch is the inverse of diametral_pitch, e.g. 

circular_pitch = 1/ diametral_pitch.

But in the gear() module, this conversion is made

circular_pitch = 180/diametral_pitch.   <--- what's that 180??
 
If you want to check that against what you’ve measured as the outside diameter of your gear you can do the following, noting that the dimension between the pitchDiameter and the outside diameter is called the addendum

addendum = 1 / diametralPitch;

outsideDiameter = pitchDiameter + (2 * addendum);

Looking at the link you posted (https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ ) it mentions that atlas gears use a 14.5 degree pressure angle so in addition to the routine in Greg’s library you’re already calling you’ll want to include the pressure_angle parameter as well, as the default of 20 is not what you want., e.g.

gear (number_of_teeth=64,
//      circular_pitch=285.0, // call greg’s routine with the diametral pitch parameter.. not circular pitch
 diametral_pitch = 16,
    pressure_angle = 14.5,
      circles=0);


Hope this is helpful.


This was very helpful. I put in the correct pressure_angle and it now looks just like my actual gear.

Thanks so much!!

Kevin
 



On Dec 22, 2018, at 1:52 PM, nop head <[hidden email]> wrote:

I am not familiar with Greg's library but DP is not about circumference. It is the number of teeth per inch of its pitch diameter, an odd measure that doesn't really have a physical meaning because the teeth go around the circumference, not across the diameter.

Also the pitch diameter is not the outside diameter, it is the diameter where the teeth mesh, sort of half way down the tooth. So you can mesh two gears if they have the same DP and you space them apart by half the sum of the pitch diameters.

I have a vague memory that Greg's library has a bug in the way circular_pitch is used.

On Sat, 22 Dec 2018 at 13:16, Kevin Toppenberg <[hidden email]> wrote:
I am trying to model a gear for an Atlas metal lathe.  I have some original (real-life) gears, but want to print one with a different number of teeth.

I have found this gear library by GregFrost:  https://www.thingiverse.com/thing:3575/files
I know that this list is not responsible for the library, but it seems to be a good one, and I am hoping someone here can help.

My question is about the units of the input parameters.  For example, in the file parametric_involute_gear_v5.0.scad there are some example gears.  I modified one slightly as follows:

gear (number_of_teeth=64,
      circular_pitch=285.0,
      circles=0);

I chose the value for circular_pitch experimentally.  First I put an Echo() statement in the gear_shape module such that it reports the minor diameter, pitch diameter, and major diameter (in mm).  I then slowly modified the circular_pitch value until it matched the measurements of my actual real-life gear, taken with my micrometer.

I am confused about what circular_pitch is supposed to represent.  I reviewed standard gear terms from here:  https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch
Circular pitch should be the distance (in mm) from tooth to tooth.  In my actual real-life gear, the value is about 5 mm.

On a slightly different tact, I am also confused about gear terminology used for my lathe.  The gears are typically described using inches and diametral pitch (which is supposed to be 1/circular_pitch).  According to this site: https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ (which references here: http://www.lathes.co.uk/atlas/), Atlas gears use a 16DP, which I think means 16 diametral pitch or 16 teeth per inch.  I'm not sure I believe this number, because my real-life gear with 20 teeth has a diameter of 1.475 inches, circumference of ~4.3 inches, and thus 20/4.3 = ~4.6 teeth/inch.

Summary:
  1. Can anyone help me understand the units for the circular_pitch parameter in the gear() module as described above?
  2. Can anyone see the source of my confusion about the 16DP

Thanks in advance,
Kevin T
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Re: Question about gear library

kdtop
In reply to this post by Parkinbot
Parkinbot,

Thanks for the replay.  See comments below. 

On Sat, Dec 22, 2018 at 5:44 PM Parkinbot <[hidden email]> wrote:
Kevin,

having programmed, designed and printed a lot of gears and gear types, the
last I would recommend you is to use a 3D-printed gear in a metal lathe.
Even if you have a SLM printer at hand, the result won't be usable on the
long run in a production machine. You risk a lot when putting a minor
quality gear into such a device.
Gears used in lathes are usually of standard pitch und can be bought from
the manufacturer even for old machines. Did you do an Ebay search?

There are different types of gears used on metal lathes.  You are probably thinking about gears that are directly connected to the motor and turn the spindle.  That is not what I am doing.  Instead, lathes have a secondary drive rod that moves the carriage along slowly, and these can be plastic.  In fact many of the smaller bench-top lathes use plastic gears for this.  The gears are combined in various combinations to get the drive gear to move at different speeds compared to the spindle, and this allows creating threads when making screws etc.   These are called "change gears."  There is a great Youtube creator named TubalCain (https://youtu.be/V6Aq0d1070I) that did a great video on this topic showing that the plastic change gears work fine.  Furthermore, I have printed out a set already (https://www.thingiverse.com/thing:143893) and they seem to be working OK.  But I just need a size not provided there. 

I have looked on Ebay for the gear sets, but they are typically expensive, i.e. about $150 for a set.  Also, I want to make a larger gear than is usually provided, so that I can slow down my carriage travel even slower.  That gives a better surface finish.  
 

But to answer your question and to complete your information about gears. DP
is a kind of a crook to be able to measure gears somehow in inch. Using it
is somehow magic and not very intuitive.

The metric system is more clear. 

Yes, I agree that the metric system is better.  Ten thousand times better.  But I have to work with the information I have.  And the gear specifications for this lathe use the imperial system.  :-(
 
You have a modulus m and a number z of
theeth. With this your pitch diameter d=m*z. The outer diameter of a gear is
usually od=d+addendum, with addendum < 2*m.

So, starting with addendum = 2*m, od=1.475 inches = 37.47 mm and z=20 will
give you a modulus of 1.70xx, which is uncommon. A common module is m=1.75.
Why? because you can buy cutting tools for this module, and many lathes use
it. This will result in a pitch diameter of 35mm or 1.378 inch. Further you
must know the pressure angle w, which you can't measure. 20° is normal, but
there are other systems. Maybe it its 14.5°, as was mentioned before. And
last but not least many gears have a profile shift to further adapt the
pitch diameter.

If you want to check all this with your given gear set, you can see my post:
https://www.thingiverse.com/thing:636119 and download my library gears.scad.
It will prompt you the pitch and the outer diameter and some other useful
information to the console. However it uses millimeters. It also provides a
help function. This is your gear:

use<gears.scad>
help(); // see what the lib contains and the prototypes of the modules
gear(m=1.75, z=20, w=14.5);

and your new gear will simply be:

gear(m=1.75, z=64, w=14.5);


This library looks great.  I'll check it out.

Thanks so much for your help.

Kevin T

 







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Re: Question about gear library

kdtop
In reply to this post by Neon22
Mark,

Thanks for the reply.  As per the other posts, I think I will be able to do this all in OpenSCAD, and not have to use multiple applications.  But it is good to know of those resources.  

Thanks!

Kevin

On Sat, Dec 22, 2018 at 1:48 PM Mark Schafer <[hidden email]> wrote:

Hi Kevin,
As others have pointed out Circular pitch means something slightly different and direct measurement is difficult (also pressure angle)..

I suggest looking at the links in this reference:
- https://github.com/jnweiger/inkscape-gears-dev

This plugin will draw gears in Inkscape ( a free OS vector drawing program).
This might help you to match the gear you have by direct comparison with a 2D drawing.
You can also export to openSCAD directly but this will only make simple gears with no 3D profiling but you can use it to learn about these parameters and make direct 2D comparisons. Then create gears that match in Greg's library.

Cheers, Mark...

On 12/23/2018 2:14 AM, Kevin Toppenberg wrote:
I am trying to model a gear for an Atlas metal lathe.  I have some original (real-life) gears, but want to print one with a different number of teeth.

I have found this gear library by GregFrost:  https://www.thingiverse.com/thing:3575/files
I know that this list is not responsible for the library, but it seems to be a good one, and I am hoping someone here can help.

My question is about the units of the input parameters.  For example, in the file parametric_involute_gear_v5.0.scad there are some example gears.  I modified one slightly as follows:

gear (number_of_teeth=64,
      circular_pitch=285.0,
      circles=0);

I chose the value for circular_pitch experimentally.  First I put an Echo() statement in the gear_shape module such that it reports the minor diameter, pitch diameter, and major diameter (in mm).  I then slowly modified the circular_pitch value until it matched the measurements of my actual real-life gear, taken with my micrometer.

I am confused about what circular_pitch is supposed to represent.  I reviewed standard gear terms from here:  https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Circular_Pitch
Circular pitch should be the distance (in mm) from tooth to tooth.  In my actual real-life gear, the value is about 5 mm.

On a slightly different tact, I am also confused about gear terminology used for my lathe.  The gears are typically described using inches and diametral pitch (which is supposed to be 1/circular_pitch).  According to this site: https://www.practicalmachinist.com/vb/general/need-atlas-lathe-gear-208658/ (which references here: http://www.lathes.co.uk/atlas/), Atlas gears use a 16DP, which I think means 16 diametral pitch or 16 teeth per inch.  I'm not sure I believe this number, because my real-life gear with 20 teeth has a diameter of 1.475 inches, circumference of ~4.3 inches, and thus 20/4.3 = ~4.6 teeth/inch.

Summary:
  1. Can anyone help me understand the units for the circular_pitch parameter in the gear() module as described above?
  2. Can anyone see the source of my confusion about the 16DP

Thanks in advance,
Kevin T

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Re: Question about gear library

Parkinbot
In reply to this post by kdtop
kdtop wrote
> Instead, lathes have a secondary
> drive rod that moves the carriage along slowly, and these can be plastic.
> In fact many of the smaller bench-top lathes use plastic gears for this.

I have a metal lathe on my own and know exactly what you want ... and what
forces are involved. However, plastic gears and 3D printed gears are not the
same. At least you should use PETG or Nylon. ABS and PLA will not do the job
(for long) as a 1.75 module is not very strong. You can't use normal grease
lube for plastic gears. Rather use silicone grease.
But try, and have fun.






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Re: Question about gear library

kdtop
Parkinbot,

Fingers crossed that they don't break soon!  I have nylon (Taulman 680, I think)  and can print them in that if they do.

Thanks again
Kevin

On Sun, Dec 23, 2018 at 7:03 AM Parkinbot <[hidden email]> wrote:
kdtop wrote
> Instead, lathes have a secondary
> drive rod that moves the carriage along slowly, and these can be plastic.
> In fact many of the smaller bench-top lathes use plastic gears for this.

I have a metal lathe on my own and know exactly what you want ... and what
forces are involved. However, plastic gears and 3D printed gears are not the
same. At least you should use PETG or Nylon. ABS and PLA will not do the job
(for long) as a 1.75 module is not very strong. You can't use normal grease
lube for plastic gears. Rather use silicone grease.
But try, and have fun.






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