The Problem

Printing a large model can be difficult.  Particularly one with large parts than need to fit together.

I printed this Hex Nut Sorter and it printed perfectly.  The lid was a perfect fit to the base.

When I printed a large round box at the maximum of the print bed, the results were not so good.  I have good bed adhesion, no warping, but the lid was not a perfect fit.  Clearly the center part of the bed was OK, but the outer parts not so much.

For a big model the bottom of the model will match the shape of the bed.  If the model has to be smooth and flat, the degree to which bed leveling is needed may be a problem.

I printed this Z-Axis Pointer for the Z-Axis lead screw and was not happy with the result. The indication was not bad, but not good enough for the large models I wanted to print.  The pointer was moving a lot on the outer edges of the plate.

The Solution

I set about the task of checking that my MK3S+ print bed was as level as I could make it - and then leave only the last little bit to the Prusa bed leveling.

I created a Z-Axis Scale that you will find in the files section.  It has 1mm divisions.  The Z-Axis pointer was moving backwards and forwards several divisions across the plate extremities.  

When I first started printing about 12 months ago, I watched a lot of videos on YouTube and had seen Chris Riley's “nylock nut” leveling video.  I installed the nylock nuts and used OctoPrint “plugins” called Bed Visualizer and Prusa Leveling Guide to do the bed leveling.  It all worked very well.  As Chris points out in his video, one can chase one's tail around and around trying to get it perfect.

I heated the bed and extruder appropriately. But, m,any times I ran the OctoPrint Bed Leveling plugins, the “screw turning instructions” changed, sometimes by large amounts even though I had not adjusted the screws.  I achieved a good result, by averaging repeated bed leveling runs, but how could I verify the result more directly?

Bed Verification

The Z-Axis pointer fits onto the Z-Axis lead screw and the scale fits over the Z-Axis stepper motor.  One can see at a glance how much the software needs to move the extruder to match the bed.  The scale brings the objectivity needed to measure and make adjustments effectively.

Also in the files section are models for the first layer that one can print and watch the indicator to see where the software moves the extruder up or down to match the plate.

When you print these models, depending on what you observe, you may be persuaded to implement the “nylock nut” hack for the Prusa print bed.

If you have the “nylock nuts” in place …

When the pointer moves LEFT - the software is raising the extruder to match the bed - the screws for that location need to be “screwed in” to lower the plate.  When the pointer moves RIGHT the software is lowering the extruder to match the bed - the screws need to be “screwed out” to raise the bed.

Print one of the models - observe the pointer - adjust one or two of the screws - print the same model again and get direct feedback of your efforts.   The models use CENTS of filament and take only a few minutes.  The longest is only about 10 minutes.

Be aware - we are dealing with THOUSANDTHS of a MILLIMETER - only very small turns of the screws are needed.  You will get direct visual feedback of the effect of adjusting the screw.  Move only ONE or TWO screws at a time.  The bed will react to your adjustments.  Make very small adjustments.

Be Cautious

If you install the “nylock nuts” DO NOT use the tools included here to level the bed.   Follow the procedure set out by Chris Riley.  Once you have a level bed from that procedure, use the tools included here to refine the result.

You may not even need to adjust anything, after using Chris Riley's procedure.  But you will know what his process actually achieved. With apologies to Chico Marx in the movie “Duck Soup”:  Who are you going to believe, OctoPrint, or your own eyes?

The First Layer Models

All this work was done with Fusion360, PrusaSlicer, and a Prusa I3 MK3S+.  There are no special GCodes.  The only setting needed is “Perimeters = 6".  The included project is using a .4 mm nozzle and 6 perimeters and includes all the STL files needed.  Select a model, rearrange the plate and print.

The Fusion 360 model is included and has full parameterization - adjust for your bed size etc.  It is already configured for a Prusa MK3S+ print bed.

As far as possible, the models have been designed to move the extruder around the plate continuously by printing perimeters and avoiding any infill.  Prusa Slicer has its own algorithms as to how to print what when.  Adjust the Perimeter count to remove as much infill as possible.

I did all my adjustments with a .6mm nozzle and using the LCD panel “printer knob”, adjusted the speed up to 300%.  One is not interested in quality, only extruder movement. 

What can be achieved

When I started the pointer moved one division left and right.  After the “nylock nuts” were installed, it moved about half that.   Using these tools, I was able to get it within the 1mm center division.   There is some variation with temperature and the accuracy within the Prusa printer's ability to measure the bed distance before every print.

It is above my pay grade to calculate the variation from the pitch of the screw, the change in angle of the pointer and the distance from the lead screw to the end of the pointer.

My recommendations

1. Preheat the bed and the extruder for at least 20 minutes before you start printing any large model.  Normally small models print around the center bed screw where there is little variation so not waiting has less affect.
2. Keep the bed at a constant temperature.   In PrusaSlicer change the End G-code to set the bed and extruder to a temperature that keeps them warm until the next print. 
3. Install the nylock nuts to the screws on the print bed.  The process is cheap, simple and easy to do and results in a marked improvement in the first layer and the overall quality of larger models.
4. Make a first adjustment, then do a lot of printing.  Repeat the adjustment.
5. Check and adjust the bed level at the end of a long print.
   • Do not check the bed ten minutes after you turned the printer on.
   • The stresses and strains in the bed will tend to even out over time.  Take advantage of the heat soak from a six-hour print.   Longer is better.
6. Do not expect a perfect result up front.  Heat the bed and extruder and retest after many hours of printing.   Expect that you may have to repeat the adjustment a few times.

It worked for me.  Your mileage may vary.