1. Printed gears usually require a little post processing prior to use. Expect to have to bore holes to the right size & clean up teeth with a blade.

Center hole shrinkage is a very common issue that occurs even on expensive printers. This is the result of multiple factors. Some to thermal contraction of cooling plastic and some is because the holes are modeled as polygons that have lots of segments cutting short around the perimeter of the hole. (Always export gear STLs with high segment counts.)

Slicing software can also have an effect as different programs may choose different spots for the hole to actually start. If you consider the absolute innermost edge of the extruded plastic to be the inside edges of the hole and shoot to make that the desired hole size, then the hole diameter is easy to stretch out of tolerance by a tiny bit when you press something inside. So a slicer designer may choose to intentionally make holes tighter.

Also, any misalignment between layers and any discrepancy between the actual and intended extrusion width can have a measurable hole tightening effect. To combat this I usually oversize holes in the model by ~.005” across the diameter. For similar reasons and to make sure gears have enough space to function when printed next to each other, I recommend leaving a .4mm clearance between meshing gear teeth in your model. This creates a bit of backlash but will prevent printed gears from locking together in use.

2. Another common challenge is getting a solid infill can be difficult on small gear features. Gaps inside tiny teeth are common even when the slicer is set to 100% infill.

Some software is better than others at automatically fixing this, but one way to manually solve is to increase the layer overlap. RichRap did a great job documenting the problem and various solutions on his 3d printing blog.

3. Standard FDM 3D printing challenges: Thin walled parts are weak, overhanging parts need breakaway scaffolding, part strength is significantly weaker in the Z-axis. All the same, my recommended print settings for gears are no different than for anything else 3d printed. Based on testing I did a while back I recommend that you use the rectilinear infill with a minimum of 3 perimeters. I would recommend as fine of a layer height as you have the ability & patience to print, so as to create smoother teeth.

4. But then again, plastic is cheap and your time isn't. If the application is critical or just cumbersome to replace a broken gear on then you may as well print the gear mostly solid to avoid the chance of any non-wear related failure. The most common failure modes of printed gears are:

*Tooth wear until slipping. (A long term failure mode, see Step 10 on lubrication.)

*Tooth breakage. (If overloaded.)

*Connection to shaft failure. (See Step 7 on attaching to shafts)

*Hub or spoke breakage. (This is a rare failure mode that only occurs if the gear was printed poorly, with not enough infill, or was designed with too thin of support spokes.)