Leveling the Print Bed

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Introduction

There are many variable inputs to your RepRap prints. The key to great results is to go after the low-hanging fruit, and nail down as many of those variables as you can. One of the simplest things you can do, which many of us avoid, is to level your print bed. These instructions are based on my experience leveling a Prusa. I'm sure other machines will use similar, if not identical steps. There are a few prerequisites that we must cover before moving on to leveling, but theses too are low hanging.

Prerequisites

0. The print bed material should be absolutely flat. 1. The Y-Axis smooth rods should be leveled to each other. 2. The Y-Axis base plate should make firm contact against the smooth rods on all four contact points (PLA bushings in the case of Prusa). There should be no rocking when pressing the corners. 3. The X-Axis smooth rods should be level across the Y-Axis rods. 4. The Z-Axis Opto/Mechanical switch and flag should be located on the motor side of the X-Axis carriage.

If you can meet at least some of these prerequisites, it will be better than nothing. Each will contribute significantly to improving the quality of your prints.


Leveling the Print Bed

The process of leveling is fairly straightforward, and does not require any fancy tools. It involves only six (6) easy steps: 1. Stiffen the print bed support screws. 2. Set adequate tension in three (3) springs. 3. Adjust the Z-Axis homing flag. 4. Level first diagonals. 5. Level second diagonals. 6. Secure hardware.


Stiffen the Print Bed

To stiffen the print bed, tighten the nuts under the bed so that the support screws are absolutely perpendicular to the print bed surface. Nylon-backed nuts insure that the screw will remain set, but if those are not available, Krazy glue, LockTite, paint, or nail polish will get the job done. If using these, allow time to set before moving on, as vibrations tend to loosen things up.


Set Spring Tension in Three Springs

The springs most commonly used on Prusa Mendels are the type you find in many ball point pens. While more than adequate for the job, they provide little resistance when fully extended, so they need to be compressed enough so that they provide sufficient support for the weight of the bed, while still allowing for slight compression during the occasional head crash, or low to the surface print (more on this below). We ultimately want to set the tension in all the springs, but for now, we only want to set three (3), leaving the last spring relatively loose. We will leave the front-right spring loose. To set the tension, tighten the nut under the base plate of each support screw, beginning with the two in the back, until the front and back of the print bed are about 2mm above the top of the fender washers. It is not important that they be level at this point, only that they be within this range, as this is where we will establish our Z-Axis home position.


Adjusting the Z-Axis Home Position

As mentioned in the prerequisites, the Z-Axis min limit switch should be located on the same side as the X-Axis motor. The reason for this is that this side of the X-Axis weighs more, and is less likely to lag behind during downward movements like homing. Through repetition, bring the Z-Axis nozzle down to the surface of the print bed at the near corner on this side of the print bed. In my case, this is the front-left corner. It does not have to be perfect, but should be within +/- 0.5 mm, so that we don't have to make huge adjustments to the tensional nuts. I prefer to be a little above the surface, so as not to reduce the clearance to the fender washer further, but either way will work.


Leveling Diagonal 1

The leveling order is Front-Left, Back-Right, Back-Left, Front-Right. We refer to the first pair as Diagonal 1. With the Z-Axis homed, and positioned over the Front-Left corner of the print bed, use the tension nut to adjust the height of the print bed until it just touches the tip of the nozzle. Re-home the Z-Axis and observe that the tip touches, but does not compress the print bed. Adjust the nut as needed until you can achieve touch without movement of the print bed.

Once you have set the front-left corner, repeat the process with the opposite diagonal corner in the back-right. Return to the front-left corner and verify that nothing has changed. On or two repetitions may be required.


Leveling Diagonal 2

The second diagonal is leveled in the same manner as the first, but starting in the back-left corner, and ending with the front-right. It may be that after completing the procedure to this point, you find that Diagonal 1 is no longer set. It will most likely be a small adjustment, and a second repetition of the leveling process will correct this in most cases.


Securing the hardware

One thing I cannot recommend enough, is that you secure the hardware. The RepRap is subject to high and low frequency vibrations, which will loosen any part, given sufficient time. A quick dab of hobby paint at the base of each nut will insure that your leveling effort pays off for the duration. On my machine, I use a second nut to lock each of the tensioning nuts, and then add a drop of red paint. It gets the job done, and is easy enough to remove with a hobby knife, if I need to take something apart.