Prusa Mendel RepRap: Calibration Part One

After you finish getting the machine built, what's next? The good news is that you've finished the machine! Take a break. Congratulate yourself. Have a cocktail, or a rootbeer float. My wife and I had a toast to my success with an expensive single malt. Try to resist the urge to fire the machine up and try to print straight away, because the bad news is that now you're just half way to your ultimate goal of a working printer. It needs calibration for you to have any real success at printing.

Calibration, in my experience, takes as long as building the machine. In part that's because you're also learning some new skills and software, and in part it's because there isn't a comprehensive guide to Prusa calibration that I've found and so I had to fumble through it a great deal and get a lot of help from the IRC channel. So let me take a stab at it here. The directions below assume you have succeeded at getting the RAMPS board firmware loaded (sprinter), the print host (pronterface) installed and operational.

Step One: Physical Calibration

1. Level the Print Bed - You may have done this once as part of setting the Z axis end-stop, but you'll need to do it again.
   
   
1. Make sure you have covered the print bed with blue tape so you have an actual working height to level to. With needle nose pliers and hex key wrench, move the nozzle to its home position and adjust the tension screw in that corner to the nozzle height. Notice that even 1/8th of a turn of the screw makes a significant difference in the height of the bed relative to the nozzle.
   
2. It's very important to get this just right. If you're using a .5 or .35 nozzle, you will adjust the height of the bed so that two pieces of printer paper slide under the nozzle without a gap. For the .25 nozzle I used a single piece of newsprint paper (.07 thick) to gauge the height.
   
3. That done, move the nozzle across one axis, 10mm at a time, and adjust the screw across from the screw near home in that direction as you go to get the height just right. When all four corners are adjusted to the correct height, go around the horn again and also move the nozzle to the center of the print bed. Pay attention to the height of the nozzle in the midpoints. 
4. If your bed is bowed a little, like mine, you want the corners to be the same height (regardless of whether or not they're the gauge height and get the nozzle the right height in the center. The center is where the height is most important.
   
   
2. Tighten down the idler on the extruder. For my extruder I needed to swap the springs out for longer ones and tighten them down to near full compression for the extruder to work properly. So, maybe you almost can't get the idler too tight, as long as there is still a little spring compression left for the idler to work with as the width of the filament varies. Test the extruder and feel the filament as it goes through the extruder. Your fingers are very sensitive and you'll feel it if the filament slips in the extruder. If it slips, you either need a better hobbed bolt or to tighten the idler down more.
   
   
3. Tighten the belts. You will be plagued with backlash unless you get the belts tight enough, but not too tight. There is such a thing as too tight, and you'll know it when you see the motors and motor mounts warp significantly out of shape. I added a special Y tensioner feature to the Y belt so I could get it just right. Be sure your pulley gears are well seated on the flat of the motor shafts, and they are tight, but not so tight as to crack the pulley plastic.
   
   
4. Turn down the stepper driver pots -- there are little tiny screws next to the heat sinks on the stepper drivers.
   
   
1. Take a jeweler's screwdriver and carefully, with a light touch, turn the screws counter-clockwise until they stop. They are fragile and it's easy to break them if you try to turn too far. 
2. Turn the pots back up by one 1/4 turn.
3. Try to move the motors. Gradually, in 1/16th turn increments, turn the pots up until the motors move across the full range of movement without skipping. When you find that point, turn it up another 1/16th turn.
4. Include the extruder in this procedure. 
5. With the hot end off, clip any filament sticking out past the bottom of the extruder.
   
1. Measure the length of filament extruded when you set pronterface to extrude 50mm of thread (at a speed of 150). Is it close to right? Save this for firmware calibration in the next step. 
2. Clip off the extruded filament.
3. Hold up the clipped-off filament next to the filament feeding into the extruder and mark the feeding filament at the top of the sample piece. If you are using black filament I've found that red dry-erase marker works well.
4. Put the hot end back on and heat it up to a reasonable temp. I started at 185 deg.
5. Hit the button in pronterface to extrude the same amount of filament.
6. Did the mark made on the filament get close to the extruder or go in? If not, and your filament is not slipping (see slipping calibration above) then you can try to turn up the pot on the extruder's stepper driver a little bit. Try again and see if that makes a difference.
7. It's not crucial that the off/on extrusion measurements match exactly. You'll later adjust the e steps (the calibration in the firmware) to the practical results you get for extrusion. But this comparison process is useful to help negotiate the fine line you need to walk with the pot setting.
   
6. What you want to avoid is running the stepper drivers and/or motors at a high temperature. If you can stand to hold your finger on the heat sink for a couple of seconds without burning your finger, you are okay. If the motors are more than just a little warm to the touch, you have turned the pots up too high. If they wind up being warm at all you should still figure out a way to have a computer fan blowing on the electronics. It will extend the life and safety of the electronics.

In Calibration Step Two, I'll outline how to precisely calibrate the firmware.