Prusa i3 MK2(s) & MK3 Spool Holder - Frame Mount - Top Loader
by MartinMajewski, published
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Update: As the Beta 5 of the Mark 3 arms show to be functional and easy to print they reached the final status. Therefore, when you have a Prusa i3 MK3, use the Mark 3 arms. All other parts can be reused from the Mark 2. Maybe I will rename them in the future to make the download section more homogeneous.
Right after l got the Prusa i3 MK2 up and running, I noticed one major flaw of that otherwise fabulous machine:
The spool holder!
Not only that it looks not as well engineered as the other parts of the machine, but it is also not very convenient regarding usage. Especially if you want to do layer based color printing you have to switch the filament quite often during one print. Bending the original spool holder to release the mounted spool, as well as adjusting it to hold the new one in place (mainly if the spools width differs from the first one) is time-consuming and tedious.
There are already a few alternatives here on Thingiverse, but after trying a few of them out, I found that they have some other issues.
- Their wheels are often mounted using printed hexagonal screws (bolts) and nuts. This results in the need for tools like a wrench to tighten them up.
- The mounting to the metal frame of the Prusa i3 MK2 is often not well designed. Most of the alternative spool holders rely on pure force to clip the holders onto the frame. This has two consequences:
- The required force is not good for the printer's construction, nor the printed parts, that can easily break (as happen to some of the spool holder arms I printed).
- It makes adjustment for other spool widths very hard.
- If the extruder reaches a certain hight on the z-axis it could crash into the overhanging construction.
- ... it is an all tool-less design. All parts are just slid into each other. The nuts can be tighten by hand.
- ... the arm's clamping design is optimized for an easy attaching onto and detaching from the printers metal frame, with as little strain on the parts as possible. This makes also the width adjustment very unproblematic and fast.
- ... due to the small footprint of the holder's arms, you can mount two spool holders side by side.
For arms that fit other frame sizes please take a look here:https://www.thingiverse.com/thing:2356183
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Prusa i3 MK2
10% for all small parts, 20% for the arm.
You'll have to print the following parts in the following quantities.
First off, choose an arm version:
- For the Prusa MK2(s) or MK3:
- For the Prusa MK2(s) only:
Then the rest of the parts:
- 4x Mark2_AxisScrew.stl
- 4x Mark2_AxisSpacer.stl
- 4x Mark2_AxisNut.stl
- 4x Mark2_SpoolHolder_HexStyle_Tire.stl
Optionally you may want to print the SpoolHolder_HeyStyle_PressingHelper.stl to press the bearings into the printed tires.
Note: users have reported that the bolts are breaking easily during the assembly. To avoid this from happening I highly suggest that you print them with 100% infill!
To make the spool holder work, you need four 608 bearings for one set, which you can get from a local skate shop or from Amazon (by using the provided links you can support me and my work):
- Amazon.de: 8x 608 ABEC 9 bearings as used on my prints
- Amazon.co.uk: 8x 608 ABEC 9 bearings, similar to the ones I used
You often find eight bearings inside a set, which is enough for two spool holders.
The spool holder is designed in a way, that you can use all parts of both sides of the holder and also switch the axis screws from back to front and vice versa. If you like to have the "smooth bed side" oriented to the outside, you have to mirror the right arm along the x-axis.
For the infill, I used a honeycomb pattern because it matches the hexagonal style of the holder and my filament is quite translucent. Honeycomb gives good mechanical support in general.
I printed three perimeters for all small parts. Four parameters were used for the arm and six perimeters for the wheels to avoid infill there. Each part has four top and bottom layers on my prints - except for the arms, where I printed without any top and bottom layer to achieve the honeycomb look.
Even though I printed my final design in 200µm (0.2 mm) layer resolution, I also tested 100um (0.1 mm) for the wheels. That gave them a smooth finish, but it had no effect on the mechanical properties.
Screws and nuts...
The screws and nuts should fit nicely with a small clearance. Nevertheless, it can be hard to get the nuts screwed onto the bolts the first time. So give them a gentle press and try to find the sweet spot where the threads match up.
For the Mark 1, I used a small amount of PTFE grease on the threads to make handling easier.
The Mark 2 and Mark 3 have more clearance between the threads of the bolts and nuts. Therefore, you should be fine even without PTFE grease.
However, if you still want to use grease, here is my recomendation:
Finish Line Fett Synthetik Casual Grease
This grease is also very useful for other mechanical components.
Print one single wheel/tire first! Then check, if you can press in the bearing. Bearings and wheels will always come with variances. What works for me, might not necessarily work for you. If you have a hard time to press fit the bearing (I mean a really, really hard time - I took the back of my screwdriver to press them in) into the tire, scale the tire up by a tiny amount (something of 0.01%) and try again.
I've printed the tires with 0.1 mm layer height for best results!
The wheels are modeled with a small clearance. Therefore it is not always easy to press the 608 bearings into them. But here is how you will succeed:
The wheel has a wider side. This side is oriented towards the holder's arm. Lay the wheel with the larger side flat on a table.
Place the bearing on the wheel. Try to push the bearing slightly into the wheel. It is enough if the bearing just barely sticks.
Flip the wheel with the bearing around and press with your hand hard on the wheel's wider side. The bearing should pop in and be level with the wheel"s narrower side.
- Place the pressing-helper tool onto the bearing and flip the wheel again, so that the pressing helper rests on the table. Now you can push again hard onto the wheel, and the bearing should pop completely into the wheel.
The pressing helper tool has a smaller diameter than the wheel"s inside diameter and should, therefore, be easy to remove.
Starting with the Mark2 the wheels become symmetrical. However, there is still an entrance side for the bearings, but it should not affect functionality if you put the wheels in reverse onto the axis.
Be aware that the wheels are asymmetrical. They have a wider side regarding diameter. This larger side should be oriented towards the arms.
How I Designed This
This spool holder was designed in Autodesk Fusion 360 and optimised to be printed on an Original Prusa i3 MK2s - because that is what I have. :-)
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Prusa i3 MK2(s) & MK3 Spool Holder - Frame Mount - Top Loader by MartinMajewski is licensed under the Creative Commons - Attribution - Non-Commercial - Share Alike license.
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