I'm printing some change gears to check the fitment of the gear train on a banjo. I have been fighting with this particular brand of PETG trying to get a clean and accurate print since last week.
Notice on the 2 20T gears the poor bores and under extruded 5 layer top surface. They were printed with a hardened .40mm nozzle on a Bambu A1 Mini. They do NOT fit the spindles on the banjo at all.
Now look at the larger 80T gear. Clean teeth and bore with proper flow rate on the top surface. (Yes the infill is showing a bit due to the mere 3 top and bottom layers I printed it with to save on filament). This gear fits as it should right off the plate.
Same exact filament, same printer, same exact slicer setting. Just a hardened .60mm nozzle swapped for the hardened .40mm nozzle. And dried to 14%Rh for days and I printed directly for the dryer.
Soooo much time and filament wasted calibrating and recalibrating for that .40mm nozzle and it still failed. All I needed was the .60mm nozzle. But, still a new and interesting phenomenon to learn from for me.
If it's just for fitment then all you need at the critical dimensions. You could have just printed the ID of the gear centers to see if they fit on the shift or added a ton of open space on the big gear like how a spoked wheel looks to save lots of filament and print time.
I did say it was a lazy design choice. And change gears a mostly made as solid gears. You very seldom see spoke designs. It has to do with casting/machine time costs. So, solid gears are what I think about with change gears. The gears are still cheap to print and they only took a good afternoon to print all 4 gears and the job is done. And the post is about the weird crappy finish I got with GeeeTech PETG. And the simple nozzle swap solving the problem. And at this point, I just want to burn up this spool to just get rid of it. I won't buy this brand again.
Turned out the real design sticker was that I needed a 16mm keyed center hole to fit the banjo shafts. And while 16mm hole just barely fits on a mod 1 20T gear, the 4mm key needed does not. I ended up modeling the 20T and 80T gears as one piece and the single 20T gear needed to be designed with a solid 30mm round attached to it to have some place for the drive key to go.
The upside to doing it how I did things meant that yesterday I used those 3D printed gears as made to do the lathe job I needed to do, burnishing a section of shaft in my lathe. Sometimes you just gotta go for it and hope it works out. I might even reprint them with a better PETG brand like DeepLee or better yet, MatterHackers and make them with 4 or 5 perimeters and 6 top and bottom layers with 80% gyroid infill. I have both on hand. For the few times I might need a .0025" per rev feed, they might well last for years. If I find a #2 and #6 pair of used gear cutters for cheap, I might buy them and machine a set from steel. But even decent gear cutters are stupid expensive. And if I don't have a customer to bill the expense out to, it's probably not worth thinking about.