Sunday, November 24, 2013

Oversized Prusa I3 finished and then some

It has been so long since my last post. I have been very busy with work and play that I never got around to updating the blog. I have made it the home page on my browser to remind me. So here is my oversized I3 story in one brief summary.

My I3 has been finished for months and has been well used. As a matter of fact its my preferred printer now. It has an oversized print base. Currently configured with a 300x300 build plate. It can actually accommodate a 400x300 build plate if necessary. However to make this oversize version needed some modifications to the original build.

When I first built it I used the standard (at the time) 8mm steel smooth rods. It was quickly discovered that with the extra lengths in my build, this allowed to much flexure, which affected print quality. The quality was poor. So... back to the drawing board.

I managed to source very high quality (strong) 12mm smooth rods and bearings from a local CNC parts shop. So I reprinted the parts to handle those. These rods are very rigid, very sturdy and very heavy hehe, the two rods on the X carriage weigh over 1Kg.
Anyway this rebuild with all of the upgraded steel rods and threaded rods work a treat. It is extremely ridged.
This thing is big, it is hard to tell from the photo, but I had a friend's standard I3 here for a while and it is only slightly wider than the carriage and can almost fit through the gap  :-) It looked so cute!.

In the original build I also discovered two other issues which I resolved when I rebuilt it:
  • Moving the much heavier build plate around (at speed). There is always some print job that has fast back and forth fills that just find the right frequency to want to shake the printer apart or at least make it skip a step in the Y direction ruining the print. I adjusted the Acceleration and XY Jerk settings in the firmware to reduce the intensity of the direction changes to help resolve this. This increased the print times a little but the top speed is still very fast so on big objects t flies. Another option would be to upgrade the Y stepper to a nema 23 but I have not got around to that.
  • In the original build, looking at the original prints under a microscope (as you do), it appeared that I was getting some slight movement of the x carriage that matched the thread of the z threaded rods. Basically as the rods turn, the thread can put pressure on the X carriage and case a very slight shift affecting the print quality up the Z.  I was not sure if this would carry across to the new build but to make sure I redesigned the mounting of the x carriage to decouple it from the threaded rod.
    This was done by adding a separate lifter that holds the bolts that ride up and down the Z threaded rod. The X carriage just sits on top of these and rides up and down. Any lateral movement of lifters does not translate to the X carriage as they are not hard coupled in any way to the carriage. If there is any minor X and Y movement caused by the threads, it does not get translated by pushing or pulling the X carriage.
    This also has the advantage of limiting the damage that can be done in the dreaded event of the Z end stop being missed. If that happened, the hot end can be pulled into the Print Bed with some substantial torque causing major damage if you are not quick enough to stop it. However with this design, the X carriage is simply resting on the Z lifters so in this case they can ride further down the threaded rod and the X carriage will simply rest on the print bed and not be pulled down onto it. This also means in an emergency, I can simply lift the X carriage by hand if necessary as it just rids the smooth rods.      
The image below shows the Z lifter on the threaded rod. The carriage simply sits on the lifter and is not attached at all. I have also included a couple of prints at .2mm layer height. They have not been cleaned up.