First post about my MM2 build, and here is one of the first things I am going to make different.
I am upgrading my MM1.5 to an MM2, so I have the plate set only, some of this will not be the same on a full kit, so some things might need changing because of that, other things I just want to change because that is what I do…
The first thing I am going to change is the Z lead screws, I got some with lower friction between the nut and screw and at the same time more straight than the ones I could salvage from the MM1.5 too. There are however also a downside to this, the downside is that the flanges on the ones I am going to use instead isn’t the exact same type, so they will not fit directly onto the ends of the X axis, but more about this in another post when I get to that.
But here is the problem, the MM2 got a bearing at the bottom to guide the lead screws (notice, if your lead screws aren’t straight, or if you can’t align them properly, then constraining them in both ends is a bad idea), and also to help lift the X assembly so the motor doesn’t do all the lifting. Another advantage of this is also the coupling connecting the lead screw to the motor can be stretched so Z could vary a bit depending on how much they stretch, this bearing will eliminate that potential stretching.
My problem was now the lead screws I got, they are turned down to a diameter of 7.9mm 10mm in, on the opposite end than the motor, where a 608 bearing (which is the one the MM2 use) is only 7mm wide. This makes the end of the lead screw rest on the extrusion instead of the bearing. This might work just fine, but I don’t like it.
Instead I am going to make this little device to replace the 608 bearing at the bottom of the Z lead screw.
This contraption consist of a top and bottom part, with a thrust bearing (51101) in between. The thrust bearing is made to handle this kind of pressure, and at the same time this also lifts the end of the lead screw up so it will rest on the top piece instead of on the extrusion.
If you have no idea what a thrust bearing is, here is a little description.
A thrust bearing is pretty much like a normal ball bearing, just instead with the sides cut open. This allows the top and bottom half to rotate freely even with a lot of pressure on them. If a normal ball bearing was used for this the balls would press against one side in the inner and outer ring instead of the grove it was designed to ride in. But even when normal ball bearings are not made to be used as they are in the MM2 it is no problem because the forces involved are so small compared to the load they can handle.
And here we are, ready to assemble!
The bottom part fits tight into the hole in the black aluminium part.
The thrust bearing then fits lose into the bottom part, and same goes for the top part.
When the bearing is put in and the top is put on, there will be a 0.5mm gap between the two parts with the thrust bearing lifting the load from the X assembly.