Configuring Marlin for Unified Bed Leveling

Many of the common questions seen on the Marlin Discord involve setting up the firmware for for UBL (Unified Bed Leveling). Users find that Marlin tries to probe outside the bed or doesn’t probe the whole bed. This is usually due to a misunderstanding about how to configure the leveling process, and the interactions between X_MIN_POS, X_MAX_POS, Y_MIN_POS, Y_MAX_POS, X_BED_SIZE, Y_BED_SIZE, MESH_INSET, NOZZLE_TO_PROBE_OFFSET, and PROBING_MARGIN, as well as not understanding the philosophy behind UBL.

The purpose of this guide is to try to explain how UBL works with respect to determining the area to probe, and how to set up Marlin for the best results when using UBL.

Note that this guide only explains how to set up UBL on cartesian printers (motion systems such as CoreXY are included in the definition of “cartesian”). It does not include information on setting up UBL for deltas or SCARA printers.

When is the bed size not the size of the bed?

There’s a dirty secret that need to be understood before configuring your printer:

“Bed Size” might not be the physical size of the printer bed.

In Marlin’s configuration, “bed size” is the usable print area of your printer. It will never be bigger than your physical bed, but if the nozzle is unable to reach all parts of the bed, or there are physical obstructions such as clips, the print area will be smaller than the actual size of the print bed. So, when we set the X_BED_SIZE and Y_BED_SIZE parameters, we’re actually setting the size of the usable print area.

The world of UBL - Machine limits and bed position

The most important part of the configuration is ensuring that Marlin has a proper understanding of where the nozzle can safely move and where the usable print area is in relation to those limits.

As an example, let’s take a look at my printer. It has a physical bed size of 220mmx220mm and the nozzle can not only reach all of the bed with no obstructions (so in this case the “bed size” is actually the physical size of the bed) but the nozzle can also safely move to positions outside the bed on all four sides. When X is homed, the nozzle is 20mm to the left of the left edge of the bed, when Y is homed the nozzle is 21mm in front of the front edge of the bed. The nozzle can safely move 15mm past the right edge of the bed, and 5mm past the back of the bed.

How do we tell Marlin these dimensions? The important concept is that everything is relative to the front left corner of the bed. That is coordinate 0,0. We need to define the machine limits with respect to that position. In the case of my printer, I use the following configuration:

define X_BED_SIZE 220
define Y_BED_SIZE 220
...
define X_MIN_POS -20
define Y_MIN_POS -21
define X_MAX_POS (X_BED_SIZE + 15)
define Y_MAX_POS (Y_BED_SIZE + 5)

I could also have used

define X_MAX_POS 235
define Y_MAX_POS 225

but decided to take advantage of the preprocessor available when compiling Marlin to make it easier to see how the limits relate to the bed size.

This results in the following layout:

Machine Limits Example

The light blue area shows the machine limits, i.e. where the nozzle can reach. The pink area is the actual bed. The most important thing to understand is that in this case (and indeed this is the case for many printers) the minimum X and Y positions are negative, because they are relative to the 0,0 position which is defined as being the front left corner of the bed.

Determining machine limits on a printer that homes to Xmin/Ymin

To use the following procedure you need:

  • An LCD display on the printer that can be used to move the nozzle in a controlled fashion.
  • A connection to a terminal through software such as PronterFace, Repetier Host or OctoPrint.

Determining X_MIN_POS and Y_MIN_POS

  1. Build Marlin with X_MIN_POS and Y_MIN_POS both set to 0, and X_BED_SIZE/Y_BED_SIZE set correctly. Flash this new Marlin to your printer and reboot.
  2. Connect your terminal and home just X and Y by sending G28 X Y. Do not use G28 on its own - we do not want to home Z.
  3. Make sure the nozzle is at the end stops by sending G0 X0 Y0.
  4. The LCD display should show X=0,Y=0 and the nozzle should be as far left and to the front of the bed as it can move, and touching the end stop.
  5. Check out where the nozzle is with respect to the front left corner of the bed.
  6. If the nozzle is exactly over the front left corner of the bed, congratulations! Your X_MIN_POS and Y_MIN_POS values are correct, and you may jump to section “Determining X_MAX_POS and Y_MAX_POS”.
  7. If the nozzle is to the right of the left edge of the bed, or behind the front edge of the bed, or both, jump to the section “The Xmin/Ymin position is over the bed”.
  8. Using the LCD, move the nozzle carefully until it’s exactly over the front left corner of the bed. Read the X and Y values on the LCD. Take those values, make them negative, and use them as new values for X_MIN_POS and Y_MIN_POS. For example, if when the nozzle is over the front left corner of the bed you read X=20 and Y=21 on the LCD, then use
    define X_MIN_POS -20
    define Y_MIN_POS -21
    
  9. Build a new version of Marlin with the new X_MIN_POS and Y_MIN_POS settings and upload to your printer.
  10. Reboot, send G28 X Y followed by G0 X0 Y0. The nozzle should end up right over the front left corner of the bed.
  11. Go to section “Determining X_MAX_POS and Y_MAX_POS”.

The Xmin/Ymin position is over the bed.

If either the X minimum position or the Y minimum position is not on the edge or outside the bed, it means the nozzle cannot reach the whole bed. The safest solution in this case is to redefine the size of the bed to only include the area the nozzle can reach.

For example, let’s assume that the physical X dimension of the bed is 235mm, but when the printer homes and the nozzle is at the X minimum end stop it is over the bed, 8mm in from the edge. In this case, you would configure X_MIN_POS as 0, and reduce X_BED_SIZE by 8:

#define X_BED_SIZE 227
...
#define X_MIN_POS 0

The same applies to the Y axis.

Note that if the nozzle can also not reach the right and/or back edge of the bed, it may be necessary to reduce the bed size even further - see the next section about setting the maximum X and Y positions.

Determining X_MAX_POS and Y_MAX_POS

Once you have determined and configured the minimum positions following the method above, it is a simpler proposition to determine and configure the maximum positions.

  1. Home X and Y with G28 X Y.
  2. Disable software end stops with M211 S0.
  3. With your LCD, carefully move the nozzle to the right back corner of the bed. Watch carefully for any obstructions to motion (on some printers the nozzle is unable to reach the back corner of the bed).
  4. If possible, carefully move the nozzle to the right and behind the bed as far as it can safely go.
  5. The position shown on the display will be your X_MAX_POS and Y_MAX_POS values. It is a good idea to lower these by a few millimeters for safety.
  6. If X_BED_SIZE is more than the resulting X_MAX_POS (or Y_BED_SIZE is more than Y_MAX_POS), set X_BED_SIZE to the same value as X_MAX_POS (or Y_BED_SIZE to Y_MAX_POS).