That Weird 3d Sound

Aside from the music 3d printers can make, mine was making a very strange sound which, up until this point, I couldn't nail down. It sounded like cutting glass using a scorer. I asked around on the IRC channels, but nobody had heard this sound from their printers before. The sound was coming from the extruder, so I thought the hobbed bolt was shearing off filament or slipping in some way. Looking down into the extruder, I couldn't see any filament shavings. Same thing when I occasionally took the extruder apart to clear jams (point a fan at your hotend to prevent jams!), no filament shavings. So for a long time, I ignored the sound thinking that if it was shearing or slipping it couldn't be that significant or there would be plastic everywhere.

Although I had assumed it was the hobbed bolt / filament, this past weekend someone suggested to me that it could be the gears making the sound. I'm not sure why this hadn't occurred to me. Looking at the gears, I did notice a small bit of plastic shards underneath. Using pronterface (because it keeps the e motor locked), I advanced the extruder a few mm at a time and then wiggled the large gear to see if there was any backlash. I repeated this procedure until I'd completed a full revolution of the large gear.

I did a similar test after first installing the herringbone gears and found that only one spot during the rotation had some backlash. At the time I chalked this up to the gears being printed poorly. After all, I had printed them as my 3rd or 4th print. They were still much better than the regular gears I'd started with, so I ran with them. Now, I was finding there was backlash around almost the complete revolution. Tightening up my e motor eliminated the backlash. It seems that the herringbone gears were just breaking themselves in. The teeth mesh very well now. There is no backlash that I can detect. In fact, I was able to reduce my retraction down to 0.6mm from 1.2mm. This are printing much more cleanly as a result.

This is my hollow pyramid print. I haven't cleaned it up at all.