Tag: hobbed gear

Extruder Gear Misalignment on Prusa i3 MK2 Build

As I mentioned in the last post, after building up the extruder/hot end assembly I tried to feed some filament into it as you would in normal use. I noticed that this could only be done by opening the tensioner and having to use a screwdriver to push the filament so that it lined up with the teflon tube.

Shaded area represents the straight path

Even once you have done that, because of the forced curve in the filament around the gear, it is very tight to push the filament down through the hot end.

The basic issue is that hole in the extruder body which fits the teflon tube and the hole in the top where you insert filament are offset from the surface of the hobbed gear. I am going to guess the the MK8 hobbed gear is a slightly larger diameter than that the body part was designed for.

I have already filed the left side hole out in these photos

By drilling/filing out both holes I have reduced the problem considerably, moving the filament is a lot easier but it is still not great. I know that the extruder stepper and driver already are a bit limited on power so the additional friction cause by this situation isn’t good.

I will see how it works once I start printing. There are solutions, but nothing that simple. You quickly run out of material around both holes if you drill out anything more than 5mm. Also there is then nothing holding the teflon tube in place.

We shall see….

How to Build a Cheap Prusa I3 MK2 3D Printer – Building Z Axis & Extruder

Aside of being careful to keep everything lined up, parallel & square the build of the Z Axis, mounting the Y Axis and putting all the extruder and hotend parts on was pretty simple and in line with the official Prusa Manual.

Really the only thing I had to find a solution for was mounting my lead screw nuts. Toms version uses 5mm threaded rod and he provides STLs for parts to hold a M5 nut for that. The original MK2 looks like they use their own lead screw nuts. The ones that came with my lead screws had their mounting holes too close together and there was no way to centralise the nut in the Z carriers. I customised Toms Z nut holdahs to do the job – which it seems to do fine.

I will say this though, after building it all I examined the extruder to “sanity check” it and it turns out that it was very tricky to feed filament through the extruder and into the telfon tube running to the hotend. I will do an extra post discussing that as I think it is a mis-match between the extruder body design and the commonly available MK8 hobbed gear. I think if this was left unchanged, apart from having to use tools to get it to feed in, it would put a lot of load on the extruder stepper just to overcome this tightness.

I would also recommend that if you can squeeze an extra £20/$30 to get a Z axis frame laser cut – do that. It will save you a lot of time and insecurities about the Z axis being parallel and square. My wood will probably work fine, but it’s just a lot harder to make sure it is correct.

© 2023 Jules Gilson


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