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Z Height is Suddenly off 5mm

Posted by Garry Bartsch 
Z Height is Suddenly off 5mm
July 09, 2016 03:14PM
After printing happily for months upon months tonight something weird pops up. I had been printing small parts without problems (qualifications below) when after homing and lowering for a new print the machine stops with the nozzle roughly 5mm above the glass and begins printing from that height. Indeed, lowering the extruder as low as possible (after homing) with the Z control in Proterface and sending M114 shows the machine believes the nozzle is at 0 when it's actually about 5mm above the glass.

Trouble-shooting Steps:

1. Confirmed the Z height in firmware is: #define Z_HOME_POS (198.12)
2. After homing using the HOME button M114 returns: Z:198.02
3. After homing using the Z button M114 returns: Z:198.12
4. After homing using the HOME button M119 returns: x_min:L y_min:L z_max:H (the plastic part holds the switch open)


1. The last prints that worked showed a progressive increase between the nozzle and the glass on the first layer. That is, The bottom of older parts shows the nozzle being a bit too high and subsequent prints show the first layer further from the glass. Finally the plastic would not even adhere to the glass because the nozzle was much to far from the glass.

So suddenly the Z_HOME_POS is off by 5mm. I didn't even open the firmware file let alone upload any changes. The discrepancy in Z can be seen when controlling the machine using Pronterface so that would rule out the slicer having anything influence.

Has the Z limit switch gone bad?

Or can someone suggest an alternative problem?

Thank you in advance for any replies.

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Re: Z Height is Suddenly off 5mm
July 09, 2016 04:48PM
Odd because Z_HOME_POS is normally around 203. Not sure why it would be as low as 198 unless you are using a different hot end?

If you are using a longer hot end 5mm longer and it was working with 198 then I think something must have broken to make 5mm difference. Is the x-carriage and the Wades block still bolted together properly and neither of them broken?

Re: Z Height is Suddenly off 5mm
July 09, 2016 09:02PM
Thank you Chris.

Sorry for the confusion. Way back I made a Z smooth rod clamp that sets the switch 5mm lower so it would trigger long before the X end on the motor end could hit the right Z clamp. That is responsible for the different Z_HOME_POS.

The Wade's block and X-carriage appear intact and fixed together properly. A wiggle test doesn't uncover any movement in the assembly.

I was able to do an imprecise measurement of the gap - rather than a guess - and it's closer to 2mm than 5mm.

When someone can help me (I'm the cripple) I will check the switch to see when it triggers.

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Re: Z Height is Suddenly off 5mm
July 09, 2016 11:17PM
The switch does not click till the switch button is pushed completely into the switch body (top of the button is completely flush with the face of the switch housing). The other switches click far earlier. I will change the switch and expect the problem to be solved.

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