Offset

The world of 3D printing continues to evolve and improve, making it easier and more efficient to create 3D pieces.

Auto levelling plays an important role in this regard, as it allows you to calculate the ideal distance between the nozzle and the print bed by using a simple adjustment called the offset. We explain what the offset is below.

What is offset?

The offset is the distance between the end of the nozzle and the print bed when detected by the levelling sensor. That is to say, the difference between the sensing distance and the distance between the nozzle and the sensor.

Sensing distance and offset distance.

Sensing distance and offset distance.

New BQ 3D printers include an inductive sensor which facilitates the use of the machine, and is located on the side of the extruder, near the nozzle.

This high-precision sensor is used both to detect the bed in the Z axis of the Witbox 2 and Hephestos 2 as well as to take measurements of certain points of the bed and thus calculate the inclination of the bed with respect to the axes of coordinates. It can even serve as a reference when levelling manually.

Measuring the offset from the installation wizard

ssh://git@stash.bq.com:7999/tresd/witbox-fw.git

During the initial setup wizard, in the case of the Hephestos 2, and after carrying out a hard reset, in the case of the Witbox 2, you will have to make an initial adjustment of the offset. With this first adjustment, the nozzle will stop at the height at which the inductive sensor detects the bed. This height will be saved as the offset distance, which will be specific to our extruder block and inductive sensor.

Adjusting the offset

Adjusting the offset

To adjust the offset distance, you should gradually lower the nozzle of the printer until it touches the base and then move it away slightly.

The LED of the sensors turns on when it detects metal.

The LED of the sensors turns on when it detects metal.

You should carry out this process the first time you set up your printer and whenever you replace any of the following components:

  • Print bed
  • Inductive sensor
  • Extruder/Hot-End

In this case, you can force the wizard to appear by selecting the hard reset option in the Settings menu.

Restore the printer’s settings by using the Hard Reset option

Restore the printer’s settings by using the Hard Reset option

Correcting the offset

Sometimes, the initial offset is not performed correctly, and the nozzle is either too close or too far from the bed. If this happens, you may experience the following problems:

  1. The print piece comes away from the bed.

If, despite using spray, the print piece comes away from the bed, this might be because the offset is too far from the bed.

Filament coming away from print bed

Filament coming away from print bed

  1. The first layers are too close together, the PLA spreads too far, and the filament may even be prevented from exiting the nozzle.

This might be because the nozzle is too close to the base. It should be moved away slightly, but not too far, as they should be lightly touching.

Filament stuck to print bed

Filament stuck to print bed

In both cases, you can go to Adjust offset in the Settings menu in order to make a finer adjustment.

Readjusting the offset in the settings menu

Readjusting the offset in the settings menu

When this option is selected, the nozzle will position itself over the bed. You can then adjust it by moving the wheel.

Why calculate the offset?

As the sensor is displaced with respect to the printing reference point (the centre of the nozzle) in the X, Y and Z coordinates, it is necessary to enter three new variables which will determine the exact position at which the sensor takes the measurements.

When printing, there is therefore a displacement which must be correctly calibrated so that the tip of the nozzle is correctly positioned during printing. There are thus three offset measurements: The X offset, the Y offset and the Z offset.

Offset measurements of the different axes

Offset measurements of the different axes

Of these three measurements, only the Z offset needs to be calculated manually, as the others are the same in each printer.

Why does the Z offset need to be measured manually?

The inductive sensor in the Witbox 2 and Hephestos 2 contains electronic elements whose measurements can differ in relation to other inductive sensors. Furthermore, the magnetic component of the print bed may vary in terms of sensitivity.

How is the offset calculated internally?

The basic way of calculating the offset of  BQ 3D printers is by moving the extruder in the Z axis from the position at which the sensor detects the bed to the point at which the nozzle touches the surface of the bed. This distance is stored in the memory and the sensing position is then rectified, for levelling as well as homing.

As the sensor is displaced with regard to X and Y, the offset should be calculated when the bed is completely horizontal, as any inclination will cause the calculation to be incorrect.

Calculating the offset

Calculating the offset

Manual levelling is a process which we have deemed to be completely optional, and it would be too difficult for users to have to level the base manually before calculating the offset. Because of this, a system for calculating the offset has been developed which takes the inclination of the bed into account. This system is explained below.

The first step is to calculate the inclination of the base. To do so, you should measure Z at three different points. These three points define the rotation matrix of the Mbase plane. This process does not require the offset value, which by default is 0.

Calculating the plane using three reference points.

Calculating the plane using three reference points.

Once the orientation of the bed is known, the distance the extruder must move in the Z axis is determined. This process is manual, whereby you must determine the correct position of the nozzle with regard to the upper surface of the bed. This distance may vary according the the existing inclination of the base, and is called Z offset’.

The offset allows to print with the bed inclined.

The offset allows to print with the bed inclined.

Next, once the Z offset’ value has been calculated, a vector is defined based which is defined as:

v=(X offset ,Y offset, 0)

This vector is part of a virtual horizontal plane.

As you now have the Z offset’ value and the inclination defined as Mbase, the next step is to reduce the v vector against the real (inclined) plane to obtain the Z defined in the previous example.

v’=v M base

v’=(X offset’ ,Y offset ‘ ,Z)

The last step in order to calculate the printer’s real offset is:

Z offset =Z offset’ -Z

Z may have a positive or a negative value.

It is important to remember that the value of the offset (Z offset) never depends on the inclination used to perform the measuring process. However, the value established by manually bringing the nozzle to the bed is a fictitious offset value (Z offset’) which depends entirely on the inclination of the bed during the measuring process, and may be smaller or greater than the Z offset.

As such, when the offset is calculated at different inclinations, you will get different Z offset’ values but only one Z offset value.

It can thus be concluded that the offset value is a single value for each printer, as long as the positions of the nozzle do not vary in relation to the sensor, the width of the base or its magnetic material.

We hope that these indications have helped you learn a little more about your 3D printer.