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Hello,
First let me say I know nothing about '3d printing'
That being said, Could a small nylon gear be copied / produced ??
How cost effective would it be?
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Is printing cost effective when there is one on the shelf to purchase? Not usually when you count the reat time involved. If you have a good 3D model to begin with it is cheaper.
Getting/making the model is part of the cost. I have uploaded a few projects. Even those that seem simple have taken many hours designing, adapting and test fitting. Many small holes do not print "as designed". Long explanation I will avoid. Getting the best press fit takes some iterations.
The main advantage of 3D printing is to overcome long shipping times, for functional parts that don't exist or your own customization.
What he said. 
I've only recently arrived on the 3D printing scene, but even designing surprisingly simple parts can be time consuming. I haven't attempted a gear, but since I'm pretty hard-over on metal gears anyway, I have no real desire to spend time designing a crappy plastic gear. 
One of my customers provides 3D printed gears for the Industrial Rail trolleys. The problem with them is finding the right material that is hard enough to prevent excessive wear, but ductile enough that they don’t split when pressing on the shaft. They work, but I don’t think they will last as long as the originals, the changing direction when banging into the bumpers doesn’t help with longevity, so he uses the trolley sparingly.
Check out these two options.
https://www.amazon.com/s?k=sma...f=pd_sl_938caigabt_e
https://www.rushgears.com/tech...pe/replacement_parts
Jan
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Thanks for the comments.
The gear in question is part of a worm drive set, used in the 6-12782 Lionel Lift Bridge.
Where the worm pinion is made of brass while the worm gear is made of nylon as designed to wear to protect other components.
This worm gear is press fit on a 3 mm shaft. It has a diameter of 10 mm. and is 13mm long. The teeth have a concave face with a depth of 1.5 mm. There are 20 teeth total.
AFAIK, These worm gear are obsolete, even though there appears to be a demand for them. Finding the right one maybe like finding the correct needle in a stack of needles! Which is why I thought of replicating them.
Making one with CA glue and baking soda may be an option, but I would be concerned about the integrity of the gear.
