UMMD: A Better Way to Set Up the Origin and RepRapFirmware Manual Bed Leveling Assist

Setting Up the Printer's Origin

In a previous post I explained how to set up the endstops and origin of a 3D printer.  In the method I outlined, slic3r is easy to set up, but Cura required some custom gcode to get prints dropped on the center of the bed.  It turn out that it's easier to set up the slicers to drop prints on the center of the bed if the printer's origin is at the printable center of the bed.

This post describes how I put UMMD's origin at the printable center of the bed with the new Duet Ethernet controller board, and how you can do the same for your printer.

First, you have to know the dimensions of the printable area of your printer's bed.  It may sound strange, but some machines can't print on the entire bed surface.  So, move your printer through it's motion limits and watch the nozzle relative to the bed.  If it is unable to print on part of the bed, mark a line (or lines) on the bed where the nozzle can't go any further.  Now mark the center point of the printable area of the bed (you can find the center by drawing diagonals between opposite corners of the printable area).

Next, move the extruder to the "home" position (where the X and Y end stop switches are both triggered). Use a ruler to measure the distance from the printable center of the bed to the nozzle in X and Y and write the numbers down.  Now move the extruder carriage to the diagonally opposite corner of the motion limits and measure again, and write down the numbers.

Make a sketch of the top view of the printer, showing the limits of nozzle travel and the outline of the printable area within those limits, like this one that I made for UMMD:

Top view of UMMD's XY stage.  The outer rectangle represents the limits of XY motion of the extruder nozzle.  The  printable area of the bed is a 300x300 mm square that fits within those limits.  The leveling screws are shown for reference (we'll use those later).  The home position is in the right rear corner of the machine because that's where the end stop switches are located.

The origin is set to the dead center of the bed's printable area.  Notice that the bed is not centered within the range of motion.  That's OK.

In the Duet config.g file, the following statements define the origin as the center of the bed's printable area:

M208 X-151 Y-185 Z0 S1 ;  sets the minimum values for all axes
M208 X150 Y153 Z680 S0  ; sets the maximum values for all axes

With the Duet (RepRapFirmware), the fact that the upper right corner is the home position is a function of where the endstop switches are positioned on the X and Y axes and the motor rotation directions.  In SmoothieWare, there are explicit statements that the X and Y axis home to max or min, and then the ordinate values to assign to each.

Now mark the coordinates of the corners of the printable area of the bed:

When you set up Cura you tell it the dimensions of the printable area of the bed (in this case, 300x300 mm), and check the "origin at center" box:

Cura custom machine setup.  There's no need to make changes to the start gcode to position the origin.

Plater view in Cura, origin at center of bed.

When you set up Slic3r, you enter the dimensions of the printable area of the bed and then enter offsets that put the origin at the center:

Slic3r bed set-up.  You enter dimensions of the printable bed area and offset values that put the origin at the center of that printable area.

And this is what you see in the Plater view- origin at center- it matches the diagram perfectly.

Why is this better?  Besides the easier setup in the slicers, it makes the gcode a little more portable between different printers, assuming they use origin at center.  Of course, you still need other things to be right for gcode to be moved from one machine to another.  You won't be able to use gcode for a 300x400x200mm print in a machine with print capacity that's 200x200x200, for example.

Manual Bed Leveling Assistant

The Duet has been working fine for a few weeks now and I am still exploring some of the options in the firmware.  One of the really great ones for people with printers like UMMD that have flat, stable beds that don't require frequent releveling, is called the "manual bed leveling assistant".  The assistant "probes" (actually, you do the "probing" with a piece of paper placed under the extruder nozzle) the bed at a few locations then does a least-squares fit and tells you how much to adjust each leveling screw up or down to minimize leveling error .  It's a quick process that works extremely well.  In order to use it, you'll need to add the coordinates and pitch of the leveling screws in a config file statement, so start by adding the coordinates to the diagram we drew above by measuring the distance from the bed center to each of the screws:

Leveling screw coordinates added.  These coordinates will be used in the M671 statement in the config.g file.

There's going to be some gcode presented below.  You can find definitions of all the gcode supported by RepRapFirmware at this site.

You'll also need to select probing points, at least one for each leveling screw.  If you have 3 leveling screws, you might choose to use just 3 probing points.  You must use at least as many probing points as there are leveling screws, so if you have 4 screws, you need at least 4 probing points.  I chose to use five points, one near each corner of the bed and one at the center:

Probing point coordinates added.  P0-P4 designators are used in G30 statements in the bed.g file.

The config.g file has to contain a few specific lines to enable use of the manual bed leveling assistant.  First, there's and M667 statement that tells the firmware the architecture of the printer you're setting up (coreXY, delta, etc.).  Then you need a couple statements that set up the origin of the printer because everything to come will depend on the coordinates.  You need an M558 statement to tell the assistant how the probing is to be done, and an M671 statement to tell the assistant where the leveling screws are located.  In the M671 statement, list the screw coordinates reference first, then pitch, then roll.  UMMD's config.g file will contain:

M667  S1  ;  set coreXY architecture
.
.
.
M208 X-151 Y-185 Z0 S1 ;  set minimum travel limits (front left corner) for X, Y, and Z
M208 X150 Y153 Z680 S0;  set maximum travel limits for X, Y, and Z
.
.
.
M558 P0 F180 H5 T6000  ; no probe, probe at 180mm/min, start 5 mm above the bed, travel between probing points at 6000 mm/min
.
.
.
M671 X-161:161:0 Y0:0:-161 P0.7  ; defines leveling screw locations and thread pitch

Finally, you need to have a bed.g file that specifies the coordinates of the probing points.:

bed.g file:

G28 ;  home
G30 P0 X-140 Y-140 Z-99999  ; first probe point coordinates
G30 P1 X140 Y-140 Z-99999  ; second probe point coordinates
G30 P2 X140 Y140 Z-99999  ; third probe point coordinates
G30 P3 X-140 Y140 Z-99999  ; fourth probe point coordinates
G30 P4 X0 Y0 Z-99999 S3  ; fifth probe point coordinates, 3 leveling screws

Once all this stuff is in place, you can start the manual bed leveling assistant from the Panel Due by first preheating the bed and nozzle to print temperatures, homing all the axes, then touching the wavy looking icon under "P0" on the right side of the control screen.

Heat up the bed and nozzle, home all axes, then touch the sine wave looking icon on the right side to start the manual bed leveling assistant. Note: I did not heat the bed and nozzle for this photo...

Then you'll see a screen like this for each of the probing points:

The manual leveling assistant at work.  The nozzle will start at the height set by the H parameter in the M558 statement in the config.g file, in UMMD, that will be 5 mm above the bed.

Put a piece of paper between the bed and the nozzle and lower the nozzle using the buttons on the screen until the nozzle just grabs the paper.  After the last point has been probed, the assistant stops. and you go back to the ordinary control screen.  What happened?!!

Fear not!  Switch to the console screen and you will see a message telling you how far off the leveling is at each leveling screw, and how much to rotate it to correct the error:

The message at the bottom tells you the result of the manual leveling assist process.  The first leveling screw is considered the reference and the error and correction are always zero there.  

The example above shows that there is no error or adjustment required at the reference screw (it will always show that, and that's why you put the reference screw coordinates first in the M671 statement on config.g), the bed is low by 20 um at the pitch adjust screw, and the bed is low by 60 um at the roll adjust screw.  Since I told it the pitch of the screws are 0.7mm (the P parameter in the M671 statement in config.g), the bed Pitch adjust screw needs to be turned 0.03 of one rotation (that's not much!) in the direction that raises the bed to correct the leveling error, and the bed Roll adjust screw needs to be turned 0.08 of one turn in the direction that raises the bed to correct the leveling error.

You twist the leveling screws by the stated amounts to bring the bed into "level" (true meaning is parallel to the XY plane of the printer defined by the X and Y guide rails).  If you are full-on OCD or just borderline like me, you repeat the process as many times as it takes to satisfy you that the bed is as level as it can possibly be.

Finally, it's a good idea to readjust the Z=0 position after you're satisfied that the bed is level.

You can find info on using the manual bed leveling assistant here, and definitions of all the gcode that RepRapFirmware supports here.

Setting Up a CoreXY Printer's Origin and EndStops

In this explanation, I'm going to use SmoothieWare as an example for the config file entries, but there are similar entries for whatever firmware you are using. Look them up!

If your printer is not a coreXY type, please refer to this post for setting up its origin and end stops.

I will refer to the motor that connects to the X or alpha output on the controller as the alpha motor, and the motor connecting to the Y or beta output as the beta motor.  The Z axis motor connects to the Z or gamma output. I will largely ignore the Z axis because it's pretty straightforward- you're going to have an endstop at the Z=0 position at the top of the Z axis (where the bed touches the nozzle).

CoreXY motion can be a little confusing when trying to set up endstops and motor direction in firmware. The printer's firmware needs to know:

1. That a corexy mechanism is being used
2. The locations of the printer's endstop switches and origin
3. The length of each axis
4. The direction to spin each motor

Step by step CoreXY firmware setup

1. Build your printer, and mount the motors and limit switches.
2. Choose origin location
3. Set home_to direction for each axis, plug in the endstops
4. Assign appropriate ordinate values for each axis
5. Set motor rotation directions for all three motors.

Mounting Motors and Switches

You can mount the motors either pulley-up or pulley-down or one up and one down - it doesn't matter.  You can put limit switches at either end of either axis, but you have to make appropriate assignments in the firmware and plug the switches into the appropriate inputs on the controller board. We'll get to that in a minute.

Example corexy layout, viewed from the top of the printer, that will be used to illustrate firmware configuration.  Motors are at the front of the machine, origin is at the left-front (L-F) corner, X axis endstop is at the right (bright green box), Y axis endstop is at the rear (red box).

First things first: you have to tell the controller that your printer uses a corexy mechanism.  You do that in SmoothieWare by using this line in the config file:

arm_solution corexy

In RepRap Firmware (Duet board):

         M667   S1 ;  set up corexy kinematics

Origin Location

The printer's origin (0,0,0) can be located literally anywhere but the directions of increasing ordinate values must be in the proper orientation for right-hand-rule coordinates or your prints will come out mirrored because the CAD software that designed the print used right-hand-rule coordinates. People typically set the origin up at the left-front (L-F) corner (X increases as the extruder moves to the right and Y increases as the extruder moves to the rear) or the right-rear (R-R) corner of the bed or printer (where X increases as the extruder moves left and Y increases as the extruder moves toward the front of the machine), but there are some significant advantages to setting it up at the center of the bed. Continue reading this post, and once you understand it, see the newer post, here, about setting the origin at the center of the bed.

Slicers commonly default to showing the origin at the left-front, and the jog controls in Pronterface assume a left-front origin, so you can save yourself some mental gymnastics by doing the same. There is no explicit statement in the config file that tells the controller where the origin is- its location is implied by the homing directions and endstops used.

The Z=0 position is at the level of the bed surface. Z increases as the nozzle goes up relative to the bed (the bed moves down relative to the nozzle). 

Setting "home_to" Direction for Each Axis

If the switch is at the origin end of its axis, you'll set that axis for home_to_min.  If it's at the far end, set that axis for home_to_max.  

Printer Origin	X axis		Y axis
	endstop location	alpha home-to	endstop location	beta home-to
L-F	left	min	front	min
L-F	right	max	front	min
L-F	left	min	rear	max
L-F	right	max	rear	max
R-R	left	max	front	max
R-R	right	min	front	max
R-R	left	max	rear	min
R-R	right	min	rear	min

Let's say that the origin is at the L-F and the switches are located at the right and rear.  In SmoothieWare, you'll have entries like this:

corexy_homing true
alpha_homing_direction home_to_max
beta_homing_direction home_to_max
gamma_homing_direction home_to_min

In the example above, the alpha and beta endstop switches are both located at the maximum ends of the X and Y axes, so you have to plug the endstop switches into the Xmax and Ymax endstop inputs on the controller board.  The Z endstop should plug into the Zmin endstop input, assuming you have positioned the Z limit switch at the physical top of the Z axis, at the level of the extruder nozzle.

In RepRap Firmware (Duet board) it will look like this:

          M574 X2 Y2 Z1 S1              ; Set active high endstops

X2 and Y2 define the positions of the endstops at the maximum end of the X and Y axes.  Z1 means the Z axis endstop is at the minimum end of the Z axis.  S1 sets them as active high.  The endstop switches will be plugged into the X, Y, and Z endstop inputs (there are no min and max inputs, just a single input for each axis).

Set Ordinate Values for Each Axis

Measure the length of the X axis by manually moving the extruder carriage from the far left to the far right.  Do the same for the Y axis by measuring the distance the extruder nozzle moves from the front to the back of the machine.  And, of course, measure the usable Z range of motion.

Let's say the X axis range of motion is 380 mm, the Y axis is 340 mm, and the Z axis is 400 mm.  In SmoothieWare you'll have entries like this:

alpha_min 0
alpha_max 380
beta_min 0
beta_max 340
gamma_min 0
gamma_max 400

In RepRap Firmware (Duet board) the assignments will look like this:

          M208 X0:380 Y0:340 Z0:400          ; Set axis minima:maxima

Setting Direction of Rotation

Setting direction of rotation is done either by reversing the connectors at the motors or controller board (only with power off or you may destroy the motor driver chip!), or by inverting the direction logic via the firmware.  

Here is how the mechanism works, ignoring any of the electrical stuff (rotation of the drive pulleys, viewed from above, motors at the front of the mechanism):

Left Motor (alpha)	Right Motor (beta)	Extruder Motion Toward:
CW	CW	left
CW	CCW	front
CCW	CW	rear
CCW	CCW	right
CW	off	left-front
CCW	off	right-rear
off	CW	left-rear
off	CCW	right-front

Remember, when homing the mechanism, the location of the switches are important, not the location of the origin.  Homing should always send the extruder carriage toward the switches. Using the table above, just the top four entries, notice that, when homing, if the switches are at the

• left and front, the alpha motor must turn CW.  
• left and rear, the beta motor must turn CW.
• right and front, the beta motor turns CCW
• right and rear, the alpha motor turns CCW

In Smoothieware, the motor rotation direction is set by these lines in the config file:

alpha_dir_pin 0.5 

beta_dir_pin 0.11

gamma_dir_pin 0.20

We can use the table to easily set the motor rotation directions.  For example, if the switches are located at the right and rear, manually push the extruder carriage to the center of the build area, tell the controller to home all axes (G28), and watch the rotation of the alpha motor.  It should turn CCW.  If it doesn't, reverse its direction either by shutting off power and reversing the cable connection to the motor, or by appending a "!" in the config file, like this:

alpha_dir_pin 0.5!

Once the alpha motor is turning the right way, push the extruder carriage to the center of the XY space, send another home-all-axes command (G28), and watch the mechanism.  If it moves toward both switches, both motors are turning in the right directions.  If not, reverse the beta motor direction like this:

beta_dir_pin 0.11!

The Z motor should likewise move the bed toward the limit switch, normally at the level of the extruder nozzle.  If you have positioned the switch at that location, Z is homing to minimum (Z=0) and the G28 command should drive the bed upward, toward the switch.

In RepRap Firmware (Duet board), P selects the driver and S sets the rotation direction:

          M569 P0 S1          ; motor A goes forward
          M569 P1 S0          ; motor B goes backward
          M569 P2 S1          ; Z motor goes forward

Endstop wiring

If you are using simple, reliable, snap-action switches for the endstops, they can be wired either normally open (NO) or normally closed (NC).  For safety, it is best to wire them NC. That way, if a wire breaks or becomes disconnected the controller will interpret that as a switch closure and it will quickly become apparent that something is wrong.

The SmoothieBoard config file defaults to NC. If you wire any of the switch(es) NO, you have to invert their inputs in the config file.  Refer to the SmoothieWare endstop configuration documentation here.

Be sure to plug the endstop switches into the appropriate inputs on the controller board.  For example, if the Y axis endstop is at the maximum end of the Y axis, plug that endstop switch into the Ymax endstop input.

The Duet controller boards have only one endstop input per axis, and it is defined as a minimum or maximum using the M574 command as above.

Setting Up a 3D Printer's Origins in Firmware and Slicers

Update:  This post explains, among other thing, how to set up a printer with the origin at one corner of the bed.  I have since decided that placing the origin at the center of the bed is a better idea.  I wrote another blog post on how to do that.  I suggest you read through both posts and decide for yourself which way you prefer.



If you build your own 3D printer design, one of the confusing things about configuring its firmware is the limit switches on each axis, homing the printer, and setting up the printer description in slicing software.

One of the first things you need to do when you finish building your printer is measure its limits of motion.  Move each axis as far as it can go and physically measure how far it went.  Write down the numbers.

Three main considerations for homing and slicing:

1. The printer's origin and limits
2. The bed's dimensions
3. The bed's origin

Note: if your printer is a coreXY type, I wrote a separate blog post on setting up its origin and endstops, here.

The Printer's Origin:

First principle: the home position - i.e. the place the extruder ends up when you (or the gcode) issues a home-all-axes command - is not necessarily the printer's origin.  If you read through the material below this will make sense.

Simplifying assumptions:

• CAD software and slicing software use right-hand-rule coordinate space.  Your printer should, too, or your prints will come out mirrored. For FDM printers, that means that the printer's origin, the (0,0,0) point, must be located at the left-front or right-rear corner of the printer with the extruder nozzle at bed level.
• The Z axis limit switch is almost always placed at the Z=0 position at the bottom of the Z axis if the extruder moves up, like in a Prusa i3, or at the top of the Z axis if the bed moves down.  For the discussion below I'll assume that the Z axis switch is located at Z=0 and we'll simply ignore the Z axis.

Right hand rule says origin must be at left-front or right-rear corner of printer.  Note- they are equivalent- one is just a rotation of the other.

Slicer's and host software have default views of the print bed that they present to the user.  Slic3r, Cura, and Pronterface all default to show the printer's origin at the left-front corner of the bed.  If you set your printer up that way, the view presented in the slicer and host will match what you see when you look at your printer.  If the printer's origin is at the right-rear of the printer, the slicer view will show the view of the bed from the back of the printer.

Setting up the printer's origin involves multiple settings in the controller's firmware.  You can control the motor rotation direction for each axis, whether the motor is to move toward maximum or minimum when executing a home instruction, and finally, the ordinate value to set for each axis after a home instruction has been executed.

3D printers typically have one limit switch on each of the 3 axes, though you can have two on each, one for minimum and one for maximum.  The Z axis limit switch is almost always set at Z=0, so we'll ignore it for now.  Everything that applies to the X and Y axes also applies to the Z axis.  For simplicity, we'll assume there's one limit switch for each axis.

Basic rules and sequence for establishing printer origin in the controller's firmware:

1. Mount your limit switches at whichever end of each axis is most convenient
2. Set the motor rotation direction to drive the mechanism toward the limit switches when a home command is executed.
3. Set home to min or max depending on where you put the switches and where you want the printer's origin to be (see the table)
4. Assign the ordinate values for X and Y after a homing instruction depending on where you want the printer's origin to be.
5. Make sure you plug the limit switches into the appropriate inputs on the controller board. 

When the printer is ordered to home the extruder via the gcode or via a command from a host computer, the motors should drive the mechanism toward the limit switches in each axis.  If the mechanism moves away from the switches in any axis, you have to reverse that motor's direction of rotation either by changing a firmware definition or by powering down the printer and physically reversing the connector on the motor or the controller board.

Once the motors are all turning in the right directions, you can assign ordinate values.  Assuming the most common configuration in which there is one limit switch per axis, you may put the switches at either end of each axis, depending on where it is most convenient.  You might want to keep cables short, or have other specific reasons for placing switches at one end or the other.  It doesn't matter.  The Z axis limit switch is normally put at the Z=0 position and we'll assume that for the examples, below.

Here are a few examples of different set-ups to illustrate how to configure the firmware.  In all examples, we'll assume that the printer's limits of motion are 310 mm x 248 mm x 215 mm measured by jogging or manually pushing the extruder carriage, Y, and Z axis as far as they will go and measuring the distances traveled.

Consult your firmware documentation for the exact syntax required to set motor rotation direction, home to min/max, and ordinate values.

Example:  bed moves in Y, origin is at left-front of printer

If you put a limit switch at the left end of the X axis, you must plug the limit switch into the Xmin limit switch input, set the motor to move the extruder carriage toward the switch, and specify that the X axis homes to minimum in the firmware configuration, and assign an ordinate value of 0 to X after homing.

If you put a limit switch at the right end of the X axis, you must plug the switch into the Xmax limit switch input, tell the firmware to rotate the motor in the right direction- i.e. the extruder carriage should move toward the switch, and tell it to "home to max" in X, then once it has done so, assign an ordinate value of 310 mm to X.

If you put the Y axis limit switch at the back of the printer, you plug the limit switch into the Ymin input, tell the firmware to spin the motor to move the bed toward the switch, and set the Y axis as "home to min" in the configuration file, (because the switch will be activated when the extruder is near the front edge of the bed), and set Y=0 when the bed bumps the limit switch.

Example 2: bed moves in Z (extruder moves in X and Y), origin is at left-front of printer

Using the same limits of motion for the printer, and the left-front of the printer as the origin, placing the switches at the left end of the X axis and at the back of the machine for Y, will require setting firmware to "home to min" in X and "home to max" in Y, then assign ordinate values of 0 for X and 248 for Y.  The limit switches will plug into the Xmin and Ymax inputs on the controller board.

Example 3: bed moves in Z, origin is at right-rear of printer

Placing the X axis limit switch on the left side of the X axis and the Y limit switch at the front of the printer will require that both axes "home to max" and you'll assign ordinate values of 310 to X and 248 to Y.  You will plug the switches into the Xmax and Y max inputs on the controller board.

This table summarizes all the possibilities for FDM printers (using the 310 x 248 mm limits from the examples, above):

Bed		Homing	Switch			Controller
Motion	Printer’s	Switch	Location	Home	Ordinate	Limit Switch
Axis	Origin	Axis	on Axis	to:	Value	Input
Y	Left-Front	X	Left	min	0	Xmin
Y	Left-Front	X	Right	max	310	Xmax
Y	Left-Front	Y	Front	max	248	Ymax
Y	Left-Front	Y	Rear	min	0	Ymin
Y	Right-Rear	X	Left	max	310	Xmax
Y	Right-Rear	X	Right	min	0	Xmin
Y	Right-Rear	Y	Front	min	0	Ymin
Y	Right-Rear	Y	Rear	max	248	Ymax
Z	Left-Front	X	Left	min	0	Xmin
Z	Left-Front	X	Right	max	310	Xmax
Z	Left-Front	Y	Front	min	0	Ymin
Z	Left-Front	Y	Rear	max	248	Ymax
Z	Right-Rear	X	Left	max	310	Xmax
Z	Right-Rear	X	Right	min	0	Xmin
Z	Right-Rear	Y	Front	max	248	Ymax
Z	Right-Rear	Y	Rear	min	0	Ymin

Slicer setup - the print bed's dimensions

Your printer's bed is all that matters to the slicer.  It doesn't know or care about the limits of the printer's motion, except as they may limit the printable area of the bed.  The slicer needs to know two things: the printable size of the bed and the bed's offset from the printer's origin.

When you enter the print bed size in the slicer, you want to enter the printable dimensions which are not necessarily the same as the physical dimensions of the bed.  If the nozzle can't go there, it can't print there, so you don't want to tell the slicer it can.  By entering the printable dimensions and the offset from the printer's origin, the slicer will be able to set prints at the center of the printable area.

In the examples below, we'll use the X and Y travel limits above (310 x 248 mm) with a bed plate that is 200 x 200 mm and is shown with the printer in the home position.

Just 4 of infinite possible variations, example A being the most common.

Example A is the most common situation where the entire bed surface is within the printer's limits of motion.  In this example, you would tell the slicer that the bed size is 200 x 200 mm.

Example B would only be printable over 180 x 175 mm, so those are the dimensions you set in the slicer.

Example C is printable over 165 x 170 mm, so those are the dimensions you set in the slicer.

Example D is printable over 200 x 180 mm, so those are the dimensions you set in the slicer.

If the bed is larger than the printer's limits of motion, you simply tell the slicer the bed dimensions are the same as the printer's limits.

Slicer setup - the print bed's origin

Slicers default to dropping the your print at the center of the print bed.  This is a good thing for several reasons.  Even if you have an unflat, unlevel, unevenly heated bed, the center is where prints are most likely to stick.  If you're printing multiple parts that are going to use a large portion of the printer's bed, having them centered makes it less likely that any of them are going to end up outside the printable area.

The slicer uses XY coordinates on the bed, and the bed's origin is normally the closest point on the bed to the printer's origin.  The bed occupies the first quadrant of the coordinate space, so the printer's origin is in the 3rd quadrant of the bed's coordinate space and the offsets are 0 or negative.  So when you enter the offset in the slicer, the values entered are normally zero or negative.

The slicer's view of the bed's origin.  The origin is left-front, but it could just as well be right-rear.

In example A, above, the printer's origin is at bed coordinates (-50, -30), so that's the offset you enter in the slicer.

Example B shows a situation with no offset, so you enter (0,0).

Example C shows an offset of (-145,-78).

Example D shows an offset in only the X axis, so the offset is (-50,0).

If you are trying to use the center of the bed for the origin, just use the appropriate offsets, and be sure to tell the slicer that the origin is at the center of the bed.  Of course, you want to specify the center of the printable area, which may not be the same as the dimensions of the bed plate.

What about Cura?

Cura is a little different.  It assumes that the home position puts the nozzle on the bed.  If that isn't true in your printer you have to play some tricks with custom gcode when you set up the printer in Cura.

When you set up the custom printer description in Cura, you tell it the size of the printable area of the bed.  In my printer, UMMD, I tell Cura that the bed is 298 x 300 x 695 mm.  The machine homes to max in both X and Y, landing at extruder nozzle at coordinates (300,330,0) as set by the controller's firmware.  The bed's origin is at (2, -27, 0).  I get Cura to place prints on the center of the bed by using custom gcode.  After homing the printer, which takes it to (300, 330, 0), I drop the bed 15 mm, then use a G92 command to set the coordinates at (300,330,15), then send the extruder to (2,27,15), then use the G92 command to reset the printer's coordinate system to (0,0,15) at that point.  Now the print will be placed in the same position that Cura set it when it was sliced.  The first G92 command ensures that the printer coordinates are correct if I run a second print after the first without resetting the controller.

Here's the start gcode for UMMD:

G28    ;Home
G0 Z15 F1200   ; drop bed 15 mm
G92 X300 Y330 Z15  ; set printer's coordinates to (300, 330, 15)
G0 X2 Y27 F12000; go to left-front corner of bed
G92 X0 Y0 Z15 ; set printer's coordinates to (0,0,15)
G92 E0   ; set extruder to 0

Summary

• The home position is not necessarily the printer's origin.
• Limit switches can be placed at either end of the X and Y axes.
• The firmware needs to turn the motors to move the mechanism toward the limit switch in each axis.
• The firmware needs to know where the origin of the printer is (left-front or right-rear corner).
• The firmware needs to know the machine's limits of motion in each axis.
• The slicer needs to know the printable dimensions of the bed, which are not necessarily the physical dimensions of the bed plate.
• The slicer needs to know the offset of the printer's origin from the bed plate's origin.
• With proper setup, the slicer will arrange prints around the center of the printable area of the bed plate.