Height difference between hot and cold bed

[Johncoffee]

After doing my Rostock Max V2 calibration using a depth gauge (as shown here: http://forum.seemecnc.com/viewtopic.php?f=36&t=10027) I checked it with a cold 23°C bed temperature. The gauge indication on this Youtube video was 0.60mm (just as a indicated value - no real zeroing was done) for all 4 positions (C;X;Y;Z). When heating the bed, the indicated value changed to 0.8mm (also for all 4 positions very accurate). In my case it meant that the nozzle was closer to the bed - e.g. the bed lifted 'somehow' - or the gauge was lower. I thought that this is strange - so I decided to measure with a cold be again. The value went back to 0.60mm - again very accurate for all positions.

I can't image that the distance changes, but still it seems to be the case. Since I only print with 70-80°C heated bed, this is no issue for me.

But anyway - do you guys have different settings for Z=0 at a cold / hot bed?

Thanks for any reply.
*John*

[AlanZ]

Yes, the z height of the bed will change as it heats up. It expands and is constrained at the perimeter by the screws.
I bring the bed up to temp and let it stabilize before taking measurements.

[IMBoring25]

Since materials expand with temperature (generally, with some exceptions, such as ice vs. water when a phase change is involved), they do tend to move around based on temperature. Unless you're doing a cold-bed print, the zeroing on a cold bed is irrelevant. That's why making adjustments with everything at temperature is the standard procedure.

[mhackney]

Read my posts on this - the bed is constrained in 6 places around the perimeter. Most materials expand when heated to these temperatures. That expansion needs to be accommodated in some way, in this case by a bowing movement. I posted last week about this and the benefit of using 3 oval holes to attache the heat bed/aluminum heat dissipator. This allows expansion/contraction radially. I do this on all my deltas and can calibrate with a cold bed with no problems.

[DeltaCon]

I agree with mhackney on the movement of material and that you should calibrate under working conditions. And I think the oval holes is a great idea. Michael is full of great Ideas
But I think the "bowing movement" is contradicting the fact that johncoffee is reading equal 0.2mm offsets on xyz AND center.
And is the stiffness of the boroglass really less that that of the construction that is carrying it? I cannot imagine that.

I just found a culprit that I think has been throwing off my calibrations for some time... the bedheat led is not level with the surface of the onyx... It pushes up the glass a tiny bit!

[mhackney]

DeltaCon - my assessment is that the Snowflake and Onxy are bowing because they are constrained. The borosilicate remains flat but is clipped to the snowflake/onyx. the bowing pushes up on the plate and in this case raising the glass reasonably equally. The glass can actually help constrain movement and push it out to the periphery. This is how I designed my original aluminum heat dissipator. But its better to just let things move in a controlled direction.

[DeltaCon]

Yes I see your point now.
When the clips are not strong enough the glass will be lifted by the bowing of the onyx and/or snowflake.
And the clips most likely ARE not strong enough indeed.

[Johncoffee]

Thanks to all suggestions - especially mhakney for the oval hole suggestion. Will definitely look into this!

[Johncoffee]

Read my posts on this - the bed is constrained in 6 places around the perimeter. Most materials expand when heated to these temperatures. That expansion needs to be accommodated in some way, in this case by a bowing movement. I posted last week about this and the benefit of using 3 oval holes to attache the heat bed/aluminum heat dissipator. This allows expansion/contraction radially. I do this on all my deltas and can calibrate with a cold bed with no problems.

mhackney - thanks for your suggestion. Would you mind posting a link to your modification? I was looking for all your post - and am not sure whether you refer to this here?
http://forum.seemecnc.com/viewtopic.php ... ing#p73843

Also Jason128 suggested using some silicon pads on the 3 tower locations to have the glass 'floating' without constrains.

Thanks - *John*

[DeltaCon]

Jason128 suggested using some silicon pads on the 3 tower locations to have the glass 'floating' without constrains.

That would undoubtedly introduce problems getting your bed up to higher temps...

[Johncoffee]

Why that DeltaCon? The silicon for sure withstands higher temperatures than the bed ever would have. (Like never above 120°C). This would be no issue for silicon - what problems do you think would occur?

[jason128]

to be clear- i used liquid RTV/ silicone sealant and just squished it down on the onyx. there definitely is a gap between the bed and glass, but is seems to work fine.
measuring with a thermocouple, it seems no worse as far as cooling towards the edges than it was before. Anyway - rather deal with that than the pain of calbrating on a curved bed