Setting Z axis height

I have had great problems using the IR sensor to set the bed height as described in the instructions.
I find it is much more reliable to set the Z axis height manually. Now I have done this am able to do succesful prints most of the time.

These are the instructions I have written for myself:

Setting Z Axis height

1.Ignore the IR sensor and set height by hand each time the Duet is powered up.

2. Type in M561 THIS IS IMPORTANT AS IT CANCELS THE Z HOMING ON THE SD CARD

3.Then set the Z height as you want it i.e close to the bed.

4.When it is correct height type G92 Z0.

5.Lift Z probe a couple of mm and then type G1 Z0 to check it returns to the position set

6.Then open SD card on Pronterface and select setbed.g

You are now ready to select file from SD card and print

I also set the bed parameters by hand each time rather than using a fixed setbed.g file, because I have found that the slope of the bed changes every time I take off and replace the glass (though it has become considerably better after I made some alterations to the bed, so I may be able to dispense with that now).

Dave
(#106)

I agree. I am less than a week into have a built machine but have found sometimes too high, sometimes too low (on the bed) when setting up.
Unless I missed something somewhere, the 561 / 92 commands are a solid fix.

The original IR sensor only works well when there is no significant sunlight or incandescent light falling on the white tapes. The modulated IR sensor fixes that. RRP will hopefully have them available within the next few weeks, or if you can do SMD soldering then you can modify the existing board as described in this thread [forums.reprap.org]. Before I had the modulated sensor, I was doing much the same thing as you i.e. manually levelling the bed and then setting the z-height at the centre of the bed.

Edited 1 time(s). Last edit at 01/30/2014 12:11PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

my ormerod is located in a room whitout windows, the only light is a fluorescent tube, from which the ormerod is shielded (as it sits in a shelf).
Still I can not get the combination of z homing and bed compensation to work correctly.
What I do:
- Home all axis
- Run G32 (automatic bed compensation)
- Move to x100 y100
- manually set Z to be "a paper thick" above the bed
- G92 Z0 -> This will often set effective reportet Z to someting like 0.07 or -0.03

This has worked "ok" in that I can print even relative large objects, but
it "feels wrong" and I do not understand why I have to "re-zero" after bed compensation. any ideas?
thanks a lot for all input!

Here are some other possible reasons for Z homing and bed compensation not working well:

1. Running Z homing and bed compensation when cold, then heating the bed. Best to heat the bed, then home Z and run compensation.

2. Using the original alu tape. Tiny differences in the y-position when the sensor probes can make a big difference to the IR. Use white tape or white paper instead of alu tape.

3. Head sagging by a variable amount. Head sag affects the relative heights of the nozzle and the IR sensor. So if the head sag is not the same as when you did the G32 calibration, then z-homing will be wrong. [I'm going to try to come up with a modification that puts the IR sensor in line with the nozzle, to eliminate this.] I found that the original nozzle mount allowed a variable amount of head sag, so I'm using a modified design. Another possible cause of head sag is the X-plate not being flat.

4. Bed support rib not fitting well on the front y-bearing. Some people have tie-wrapped the rub to the bearing.

Even with a working IR sensor, I still check the nozzle height at X=100 Y=100 for at least the first print of the day. I've added bed-levelling screws so that I have the bed as nearly level as possible in the first place.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].