Friday, July 26, 2013

Z-gap consistency or inconsistency

The CubeX uses reed switches / sensors for the xyz homing. The switch activates thru a magnetic field (from a magnet). The z-sensor / switch is a black ~ 4mm diameter, 25mm long cylinder that sticks down vertical close to the jets back by the large single fan on the carriage.
The magnet to trigger the sensor is glued on the bottom of the print platform. The z-sensor points towards the magnet. Once the magnet with its magnetic field gets close enough to the sensor when the print platform moves up, the platform stops moving (-/+ the offset that you calibrate when setting the z-gap).

The z-sensor is positioned between the print jets and fans.
The temperature in that area can vary with the jets on or off and the fans on or off,
additionally the 6 fans (if you have a trio) can produce a different magnet field when they are on or off.
So all this "noise" could have an effect on the repeatability of the z-sensor.

Since I repositioned the z-sensor and magnet to a less "noisy" location, the z-gap has been very consistent.
The sensor repositioning kit came with the heated printbed, but you can get the z-sensor kit by itself from GRM products.
It installs easily without modifications to the current z-sensor setup. The current sensor just gets unplugged and the new sensor from the kit plugs into the connector instead.

This little modification is well worth it.

Also, make sure to check the "Print bed leveling aid" post, for easier print bed leveling.

Just as a reference, here are pictures of the location for the X and Y homing sensors:

In general, this is what I do before printing:
  1. Select the "Home" button. This will move the print jet behind the print bed so you can do step three.
  2. Make sure the jets don't have any (not even a tiny piece at the tip) of filament left stuck on them otherwise it can affect your z-gap measurement.
  3. Hit the "z-gap" menu button and check or adjust the z-gap with the up/down button.
  4. Then print. 
Note: Always "Home" before setting or checking the z-gap. This has been most consistent.

(Note: I am not affiliated with GRM products and I have to buy my stuff like everybody else, but I gladly promote companies with great products and services that make my life easier).


1 comment:

  1. thanks, I ordered it.

    unfortunately, the Z-gap is more complicated than the magnet and reed switch.

    the reed switch is a switch. its either off or on, and there is no other functionality. its not a sensor in the sense of a capability of sensing a variable or continuum of magnetic field strength.

    Furthermore, the Z reed switch goes from its "OFF" to "ON" state at a jet-bed separation that is a comparatively loooonnngggg distance when compared to the ideal Z-gap setting of near ~0.1 mm.

    You can watch this happen when you home, start a print job, press "z-gap", etc. The print bed raises and at a certain point it stops, then reverses direction momentarily.

    that is the point where the reed switch turns on. the reversal of direction is a way for the system to get a 2-point measurement of the point at which the reed switch turns on. You will notice that this occurs at a separation that is long before the very small gap that we ideally would like to program into the device. that separation... which is, IIRC, at least several milimiters between the print jet tips and the print bed.... represents what we might call a "starting point" or "Zero point" or perhaps the "origin" for the print bed.

    after (above) that point, the system is recording the duration of time that the stepper motor is turned on, most likely as a function of current input. Similarly, when the print bed is lowered below that point, the system is still recording the stepper motor duration.

    in general, thats OK and is not necessarily a bad or poor design concept because these types of stepper motors are extremely precise, and the electronics should be very capable of extremely precise measurement of the current that drives the motors during the adjustment or print process

    My concern however is that, during the fine tuning process, repeatedly pressing the up/down button.... especially in extremely small increments like 0.02 milimeters, will result in errors that compound.

    I have tested it already, so I definitely know that there is a correlation.

    If you set the z-gap (stock or presumably with the relocation kit) by pressing the "UP" button continuously up to .... whatever your particular unit represents as very close (mine reads in the 28.xx range, with 28.9x being the closest I have set it, and 28.4x being the largest gap I have set it).....

    and compare the resultant behavior to what occurs if you repeatedly press the up button in small bursts or small/brief intervals of time, so that you adjust it in increments of 20-60 microns (0.02-0.04 mm) at a time, possibly including pressing the "DOWN" button several times, you will notice that adjustment involving these distinct behaviors will produce drastically different results when you compare the consistency of the machine's Z-gap setting by successively checking its own ability to reproduce the z-gap that you had previously set(by setting it, then saving it, then pressing the Z-gap again to see if the machine can reproduce the position that you had set it)

    Additionally, it should be noted that Cubify/3D systems themselves actually admit in one of their documents that the CubeX is subject to ~0.05 mm of error in resolution. I cant remember off the top of my head which document it was in.... hell it might even be in the CubeX guide... but I'll look for it.

    Sorry for the long comment, lol


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