Bed probe for a heated chamber.

[Mark Napier]

In my printer the head to bed clearance shrinks as the bed and chamber heat up.  I don't see why because the bed/table is "hanging" from the screws just a few mm down from the gantry so there shouldn't be much expansion going on.  But I have to watch the 1st layer and keep bumping up the microsteps to keep that layer going.  Is OK after that but it fails a lot.  (head banging noises)

Also means that I can't really set the distance with the top off so the paper gauge method is worthless.  I could heat soak the bed and chamber and then probe to set the height and compensation.  So I need a bed probe that won't crap out when it's hot.

I'm going to give this thing a try:

RemoteHT bed level probe, high temperature probe – HighTemp3D

[Dan Newman]

On 7/8/21 8:06 AM, Mark Napier wrote:
> In my printer the head to bed clearance shrinks as the bed and chamber
> heat up.  I don't see why because the bed/table is "hanging" from the
> screws just a few mm down from the gantry so there shouldn't be much
> expansion going on.  But I have to watch the 1st layer and keep bumping
> up the microsteps to keep that layer going.  Is OK after that but it
> fails a lot.  (head banging noises)
>
> Also means that I can't really set the distance with the top off so the
> paper gauge method is worthless.  I could heat soak the bed and chamber
> and then probe to set the height and compensation.  So I need a bed
> probe that won't crap out when it's hot.

You can check to see if the heated bed is crowning. That's fairly common
if the heating element (e.g., pcb with serpentine traces) is rigidly
attached rather than secured with oversized holes in the PCB/substrate
and fasteners that allow for expansion and contraction of the
PCB/substrate in the XY plane. When too rigidly attached, all the PCB
can do is crown along the Z axis.

To check, you can use a good straight edge when cool (possibly shining a
light from behind it). Then let things heat up and check again.

Dan

[Mark Napier]

Hey Dan,

The bed is 1/4" aluminum MIC plate with glass on top of it.  Heat is conducted with white heat sink squares to the glass.  On the bottom is an AC silicone pad heater.  Under that is a thin layer of fiberglass held up with a sheet of luan and small screws into the bed on the edges.  It insulates plus it also holds the heating pad in place.   The bed is held above the table by 3 countersunk screws and springs.  The bed level and height above the lift table is adjustable with 3 nuts on the underside.

I don't see much evidence of the glass distorting with heat.  But maybe once the probe is working the duet mesh grid height map may tell a different story.

Mark

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[LukeH]

I also have a thick aluminium plate with thick glass on top, and when it is hot, it is still flat (see pic).

It is great that the bed doesn’t warp, but all metals expand when they get warmer, which includes the bed and the nozzle, and so your gap is always going to be less than when it is cold. Level the bed when both the bed and nozzle are at printing temperature - the gap at room temperature is meaningless.

BTW, if you are after a levelling sensor that can operate in a heated chamber, you might consider a piezo sensor, since it would be possible to only have the bi-metallic ring part of the sensor in the chamber and mount the electronics outside the chamber. Since it is a signal generated by physical bending of the metal ring, it will be less sensitive to temperature variation than most of the other options.