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Create a ionternal gear correct workflow


Alibre Super User
Hi Group,

Not quite sure if the methode is correct, but I want to create a internal (ring) gear).
To create this I follow these steps:
1. Create a spur-geare using the alibre sample script and save it as a part
2. Create a round part that is slightly larger than the gear
3. Put boht parts in a assembly and align them ( see the assembly in the package.
4. Then do a bool subtract and create the desired Ring-gear.
Is this procedure correct or are there other way's to make a Ring-gear??

Thanx Berdien


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Alibre Super User
I think the concept is correct I'd change the order though.
Start with the round part open then start the Boolean Subtract function and select the spur gear. Once the spur gear is aligned and the Boolean is accepted you have the ring gear.
Once the ring gear is created then you can put it into an assembly.

This doesn't account for clearances though, I'll let those users more experienced in gear modeling speak to that.


Alibre Super User
I wouldn't rely on the alibrescript gear shapes. They are not true involute curves, just approximations for virtual models.
Also, a boolean subtract will have no clearance between the teeth or have the right rolling shape.

Because you are going to 3d print them, the clearance you will need will all depend on the precision to which the gears can be printed.
External gears are normally measured to a calculated value over so many gear teeth minus the amount of clearance (backlash) you want in the gear.
Internal gears are measured between determined pins plus an amount for clearance.

If someone who has printed gears before can give the amount of backlash that they would use. I can give the relevant measurements.

Actually, if you tell me what gears you are using, maybe I can generate proper involute dxfs and create models for you that will include the clearance that you want.

This will most likely be a trial and error thing to get a good rolling gearset, but I will help where I can.

Have a look at this page: https://www.instructables.com/A-Practical-Guide-to-FDM-3D-Printing-Gears/ he suggests a .4mm clearance and 25°PA teeth. Hmm, my program won't let me put that much clearance on, but I still may be able to tweak it manually. I have another program that will tell me the correct sizes but won't create an output file.
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Alibre Super User
You will have to do try and error, it depends on the characteristics of your printer.
That 0.4mm tolerance recommendation is way too much if your gear forms are anywhere near good.
I have tried both epicycloid and involute gear forms and find the epicycloid to work better.
Epicycliod are used in clocks and watches as they allow smaller pinions.
The only down side of epicycloid is that they are less tolerant of axle distance error.
At least on my printer, it seems to print the epicycliod better.
Another thing I found that makes a big difference is using a backing to strengthen the teeth.
This I would highly recommend if you are transmitting any power.


Alibre Super User
That 0.4mm tolerance recommendation is way too much if your gear forms are anywhere near good.
You're right, .4 is way too much for gears that size. I can do .1mm backlash (.05mm per gear) models though.