auto bed leveling not accurate

I have recently built a prusa i3 clone (basically followed the youtube videos by Tom Sanladerer) and it has been working fine for a while.

I just had a nozzle blockage, and after clearing the blockage (it is now extruding OK) the auto bed leveling is not working properly.

I have an inductive sensor (NPN NO LJ8A3-2-Z/BX-5V), and when I request a G29 , the nozzle is a good distance on the left side of the bed, however on the right side the nozzle hits the bed and it's more sever from front to back.

What could be causing this? I would have thought that the sensor should trigger at the same height regardless of the position on the bed.

2mm switching distance is very limited especially with a aluminium bed, this distance will be reduced, (2mm is with a ferrous material).

If you have the bed heater enabled while probing, the current going thru the heater can also interfere with the probe.

Quote
obewan
2mm switching distance is very limited especially with a aluminium bed, this distance will be reduced, (2mm is with a ferrous material).

If you have the bed heater enabled while probing, the current going thru the heater can also interfere with the probe.

I have an issue with the bed heater at the moment so it is always off, however I have tested it with the hotend heater off and that seems to fix the issue. Does this sound right, as I have only tried it a couple of times so far.

Just completed another g29 and all points are being sensed ok apart from the far right back point - as follows:

Bilinear Leveling Grid:
0 1 2 3
0 -0.34 -0.04 +0.18 +0.35
1 -0.32 -0.06 +0.12 +0.26
2 -0.44 -0.20 -0.05 -0.09
3 -0.62 -0.38 -0.27 -2.62

Edited 1 time(s). Last edit at 09/01/2017 10:16AM by vfwfw.

Also, have just run into another problem where the thermistor is reading the hotend temp as 362 degree celcius as soon as I turn it on. Any idea what could cause that?

From my own thermister open circuit and displaying -14C, would say you have an shorted thermister this should read 358C.

[www.geeetech.com]

Quote
Roberts_Clif
From my own thermister open circuit and displaying -14C, would say you have an shorted thermister this should read 358C.

[www.geeetech.com]

turns out that the high temp reading had something to do with the way the thermistor was screwed into the hotend block. When I started to unscrew the grub screw holding the thermistor in, it staraight away started reading room temp. I just screwed it back in, but left it a bit looser than it was before, and it's been working fine since.

Could it have been from being screwed in too tight? Does that make sense?

What is perplexing is that I haven't touched it since getting the printer working a couple of weeks ago, and have completed multiple prints without this issue.

I have had this problem myself just recently. It appear that they may be scrimping on the insulating material to save costs.

Was planing to buy some 3mm standoffs or button head screws and High temp Epoxy, Adhesive or Putty and glue the thermisters into the standoffs.
They have many different types ranging to 350C to 1000C used mainly for exhaust repair and high temp gaskets.

short time solution problem wrap the retaining washer with high temp tape then poke a hole through the tape for the screw.

Edited 3 time(s). Last edit at 09/03/2017 07:38AM by Roberts_Clif.

Hi Roberts_Clif,
It seems to be working OK now, however if I get the issue again I might try your solution...Thanks.