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Drawing sheet for a POM gear

Question asked by Eg Gloo on Mar 27, 2017
Latest reply on Mar 28, 2017 by Dennis Dohogne

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Hi all,

I'm trying to design a 28 teeth POM gear for an education project. We'll need about 3,000 units, so we decided injection moulding is probably the best way to go. I have several questions:

1) The injection moulder's draft angle is 4 degrees - is it too big for POM gears?

2) I have produced a drawing sheet, but I can't seem to specify the draft angle. Is it typically acceptable to specify the draft angle as a note?

3) Usually, what tolerances are acceptable for POM gears? Is 0.05mm (linear? angular?) too tight?

4) Are tolerances usually specified for each dimension or stated as a note on the sheet?

5) Are fillets needed around the teeth?

Here's the sheet so far:

Does it look adequate for manufacturing?

Any help appreciated.

**EDIT:**

A 4 degree draft angle is too big, so I'll find a manufacturer that can do 0.5 degree.

Dennis Dohogne Mar 27, 2017 10:01 AM

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Eg Gloo wrote:

> At 4 degrees of draft this part might be self-ejecting. That is great for the molder

What you said is probably true. I got quotes from two shops. The tooling cost with 4 degrees is somewhat cheaper than the one with 0.5 degree.

You make part of my point regarding the draft.

Toolbox gears are only crude representations and are not at all suitable for actual use unless your gears are going to be made from the specifications (which you don't have on your drawing) instead of the geometry (as the molder would want to use to start with). Toolbox gears only have arcs to represent the tooth profile instead of actual involute curves.

Here is a link to a parametric file for a true involute gear profile that you can use: Does Anyone have a Parametric Bevel Gear Model to Share? Just go to the Design Table and set the module, pressure angle, and number of teeth.

0.7 module is a non-standard module. How did you arrive at this? Modules of 0.8 and 0.6 are standard.

You still have not mentioned the pressure angle.

What about the mating gear? Model the mating part and add your draft to see what effect it has on the mesh of the teeth.

You really must do more research on specifying gears or else you will end up buying 3k worth of parts and a mold that do not serve your purpose.

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14 Replies

Wojciech Paterski Mar 27, 2017 7:30 AM
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so how is this gear going to mesh with something else? as at the moment it isn't a flat gear
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- Eg Gloo @ Wojciech Paterski on Mar 27, 2017 9:22 AM
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  > as at the moment it isn't a flat gear
  
  It is meant to be flat. I used the SolidWorks Toolbox to generate it. Is it because of the large draft angle? I've updated my post with smaller draft angles.
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Peter Brinkhuis Mar 27, 2017 7:44 AM
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Those teeth don't look right. Have you done gear designs before? I really suggest you get a gear manufacturer involved so the tooth and gear design is proper. The 4 degree angle seems too large, I'd rather mill those gears

The drawing doesn't seem complete yet and there are multiple dimensions that are unclear to me. A drawing has to make the design perfectly clear. The function of the diameters on the right view is unclear to me for example.

To add the draft angle, you could add a horizontal sketch line and use that to add the dimension.
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- Eg Gloo @ Peter Brinkhuis on Mar 27, 2017 9:13 AM
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  >The 4 degree angle seems too large, I'd rather mill those gears
  
  That's what I thought. I probably should find another manufacturer.
  
  > Those teeth don't look right.
  
  The gear was originally generated using SW's Toolbox (Power Transmission). I just set the module value, teeth number, etc. Is the Toolbox reliable?
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- Eg Gloo @ Peter Brinkhuis on Mar 28, 2017 2:40 AM
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  To add the draft angle, you could add a horizontal sketch line and use that to add the dimension.
  
  Do you mean adding a horizontal line in the drawing sheet? I've made some changes and used the design table provided by Dennis Dohogne. Are draft angles meant to be specified for each drafted face? That'd seem to be a bit pedantic - right? Here's what my sheet looks like now:
  
  I haven't specified the drafts for all faces though - but should I?
  
  Thanks
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Dennis Dohogne Mar 27, 2017 8:24 AM
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Gears are very special shapes (involute profile). The important parameters are module (for metric gears), pressure angle (which is not even called out on your drawing), and number of teeth. The precision (or tolerance) is specified by AGMA for different classes of gears.

An injection molder will tell you what he wants if he doesn't care about it affects you. Draft is a function of a lot of things including the material type, any texturing, depth, and size of the part, etc.

I do not understand what you are referring to with POM. Is that the material, an application, or what?

From my experience with injection molding 4 degrees is a lot of draft. If your gear has this much draft then you had better model the mating gear and see just what that will do to your mesh. With this much draft your face width becomes irrelevant because you won't have proper load sharing unless the mating gear has a similar draft and is specifically mounted with opposite facing. This is not a good way to go at all.

At 4 degrees of draft this part might be self-ejecting. That is great for the molder, but if you agree to this (with your drawing) you might be paying for 3000 parts you cannot use. It would make the mold only slightly more expensive to incorporate injection pins, but you'd be able to then get this molded with minimal draft, on the order of .5 degree or less.
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- Eg Gloo @ Dennis Dohogne on Mar 27, 2017 9:32 AM
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  > Gears are very special shapes (involute profile). The important parameters are module (for metric gears), pressure angle (which is not even called out on your drawing), and number of teeth.
  
  I'm using the Toolbox to generate the gear (Power Transmission -> Gears). I've set the module, pressure angle, and other parameters there.
  
  > At 4 degrees of draft this part might be self-ejecting. That is great for the molder
  
  What you said is probably true. I got quotes from two shops. The tooling cost with 4 degrees is somewhat cheaper than the one with 0.5 degree.
  
  > I do not understand what you are referring to with POM. Is that the material, an application, or what?
  
  By POM, I mean polyoxymethylene. It seems to be a very popular material for plastic gears.
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  - Dennis Dohogne @ Eg Gloo on Mar 27, 2017 10:01 AM
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    Eg Gloo wrote:
    
    > At 4 degrees of draft this part might be self-ejecting. That is great for the molder
    
    What you said is probably true. I got quotes from two shops. The tooling cost with 4 degrees is somewhat cheaper than the one with 0.5 degree.
    
    You make part of my point regarding the draft.
    
    Toolbox gears are only crude representations and are not at all suitable for actual use unless your gears are going to be made from the specifications (which you don't have on your drawing) instead of the geometry (as the molder would want to use to start with). Toolbox gears only have arcs to represent the tooth profile instead of actual involute curves.
    
    Here is a link to a parametric file for a true involute gear profile that you can use: Does Anyone have a Parametric Bevel Gear Model to Share? Just go to the Design Table and set the module, pressure angle, and number of teeth.
    
    0.7 module is a non-standard module. How did you arrive at this? Modules of 0.8 and 0.6 are standard.
    
    You still have not mentioned the pressure angle.
    
    What about the mating gear? Model the mating part and add your draft to see what effect it has on the mesh of the teeth.
    
    You really must do more research on specifying gears or else you will end up buying 3k worth of parts and a mold that do not serve your purpose.
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    - Eg Gloo @ Dennis Dohogne on Mar 28, 2017 4:43 AM
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      Thanks for the pointer to your design table.
      0.7 module is a non-standard module. How did you arrive at this? Modules of 0.8 and 0.6 are standard.
      It was mostly derived from the diameter of the gear and the number of teeth. But module 0.6 works will fit just as fine.
      
      You still have not mentioned the pressure angle.
      
      What about the mating gear? Model the mating part and add your draft to see what effect it has on the mesh of the teeth.
      
      The pressure angle will be 20 degrees. The mating gear will be 16T and of the same module and pressure angle.
      
      As for plastic gears, what tolerances are reasonable? Is 0.1mm too much?
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Roland Schwarz Mar 27, 2017 10:39 AM
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There are vendors that specialize in molding POM gears with near-zero draft on the teeth. You get what you pay for. You also usually don't get what you don't pay for.

The 4° draft can be dealt with if you flip-flop the draft on mating gears. Not the best solution, but better than not.

Involute
If you don't understand what that word means and how it applies to gears, just STOP!
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- Dennis Dohogne @ Roland Schwarz on Mar 27, 2017 10:43 AM
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  Roland Schwarz wrote:
  Involute
  If you don't understand what that word means and how it applies to gears, just STOP!
  AMEN to that, Roland!!!!
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- Eg Gloo @ Roland Schwarz on Mar 28, 2017 2:42 AM
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  Involute
  If you don't understand what that word means and how it applies to gears, just STOP!
  
  Yes, I know about the importance of involute. I just thought the gears generated by the toolbox were already accurate enough.
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  - Roland Schwarz @ Eg Gloo on Mar 28, 2017 9:49 AM
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    I've seen more than one warranty disaster caused by "good enough" gears.
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  - Dennis Dohogne @ Eg Gloo on Mar 28, 2017 10:21 AM
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    Eg Gloo wrote:
    
    Involute
    If you don't understand what that word means and how it applies to gears, just STOP!
    
    Yes, I know about the importance of involute. I just thought the gears generated by the toolbox were already accurate enough.
    Toolbox gears are only accurate enough to simply represent a toothed member. They are not good for any kind of actual function. Period. They are quick and dirty representations of gears and nothing more. Why did they do it this way? Probably because at the time they made the gears in TB nobody went to the trouble to work out all the parameters for a true involute. Besides, it is a situation similar to threads. You typically just show a representation (of threads or gear teeth) and use appropriate standards and notes to specify the feature including its precision. Sure, things change and more and more folks need the actual and accurate geometry. SWX has listened and is finally providing a tool for generating physical threads (caveat emptor = let the buyer beware = You had better check these threads instead of just assuming you get what you intend). I've generated my own physical threads when I needed them and even true involute gear models (that I've shared). The person using these files still has the responsibility to verify the files give you what you want instead of just accepting them on blind faith.
    
    As far as us advising you on what is accurate enough for your application. . . Well, you have a lot more faith in our clairvoyance than is warranted!
    
    By definition, if you are using a true involute geometry in SolidWorks then it is "perfect" from a modeling standpoint. The accuracy of the physical parts comes from the tolerances you allow on the drawing. Those tolerances will dictate what processes (and materials) can be used to make your parts. AGMA classifies gears based on tolerances, or rather, they have established gear classifications which specify the accompanying tolerances.
    
    YOU have to decide what you can live with in the way of tolerances and costs. Nobody on this forum can do that for you.
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Drawing sheet for a POM gear

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