Still it is not clear to me either, it looks as if you start moving the head the axes are parellel no ?
The idea is that you have the smaller X/Y rods exactly parallel with the thicker X/Y rods in the frame. There is a little room for skew there, and you don't want that as it puts extra stress on your machine and makes your printed models skewed.
- 2 months later...
I don't get this either. My axes still have quite a bit of friction (granted, they are not greased yet). I understand what the instructions are asking me to do, what I fail to understand is how it is helpful. The openings in the sides of the frame are radiused and so is the end of part 3B. However, the only way to make contact between the wooden block and the frame is to use part 3B such that the radiuses don't fit together (not that they're the same anyway). But that leads to the part wanting to push away from the side of the frame as it moves along the radius of the corner. Clearly this is not a repeatable setup. Wouldn't a better solution be to have a pair of gauges in the shape of a Z (but with a straight center section, if that makes sense). You could then gauge the distance between the cross rod (the ones through the extruder carriage) and the edge of the frame.
I have my partially completed ultimaker with me today at work so I will probably bring it in during lunch and measure the perpendicularity using the portable CMM on my desk I just need to prepare myself for the shock of how bad it's going to be. I'm used to instruments that are repeatable down to about 5-10 microns so bearings pressed into wood will not quite measure up (so to speak). I have some other questions/concerns about the assembly but I'll save those for another thread.
You use the 3B part to put between the slider blocks and the edge of the frame, this will give you the same distance at both ends of the X/Y rods. It's just a simple measuring tool in this case.
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