3D printer - Piezo Z probing

[Dr_Ju_ju]

Try some glue-stick on the bed surface....

[Gina]

I have used that but generally find that PLA sticks alright to clean glass if the settings are right.

[tekkydave]

What temperature are you using. I generally print PLA at 210 or even 220. Seems to make it 'stickier'. Bed is 70 for first layer and 60 after that.

I haven't had any issues if the glass is clean and nozzle at correct height. PLA seems to benefit from being well squished to the bed.

HTH

 

[Gina]

First layer 240°C others 200°C.  Bed I estimate as about 60°C from touch.  Bed heater thermistor says 140°C but there's 2mm of aluminium and 4mm of glass above that.  I think the bed temperature is probably the trouble - too much heat loss in the glass but the thermistor embedded in the bed heater could be wildly out I guess.

EDIT...  Just checked the temperature rating of the heater I'm using (Keenovo) and it's 260°C so I can increase the bed temperature considerably before getting near the limit.

Edited March 16, 2019 by Gina

[tekkydave]

Do you have the thermistor set up correctly in config.g? It shouldn't be that far out. I have 6mm bed & 3mm glass and it reads within a few degrees difference top/bottom.

From my config.g

M305 P0 T100000 B3950 C0 R4700            ; Set thermistor + ADC parameters for heater 0

You have a different make heater to me so they may differ, but most of the silicone heaters are fairly similar.

Also have you run a PID tune on the Bed? (M303 H0 P1 S60)

Another thing you could try is printing all layers at same extruder temperature. Maybe some relative shrinkage going on and popping the print off. 

[Gina]

Used the "qwik-fix" of Pritt Stick to get a print.

AFAICT the Keenovo  uses the same thermistor.

I haven't run PID tune on the bed - I'll try that.

Relative shrinkage could be the problem.

The underside of the bed heater is open and I've felt that - it's a lot hotter than the glass top surface but can't be 140°C or it would have burnt my fingers.  I can touch it but only briefly so guess something like 80-90°C.  I'm thinking of adding a layer of polyurethane foam (or maybe ultra high temperature insulation) and boxing it in with thin plywood.  It's helping to heat my living room ATM! ?

[tekkydave]

That's very strange. How thick is your Ali plate? I can't feel any difference between the top of the glass & surface of the heater below. Mine is completely open too. My heater is 750W 220V.

Also, what type of glass? I'm using borosilicate.

Edited March 16, 2019 by tekkydave

[Gina]

The ali plate is 2mm thick and the glass plate 4mm (float glass).  Heater is 400mm square 220v 1200W.  This is a BIG printer!  2ft cube box.

[Dr_Ju_ju]

If you want to insulate the bottom, use a cork layer between the foam & bed. This ensures that the foam doesn't get too hot, and start emitting some noxious substances....

[Gina]

Looked up PID in the Duet web page and see that it isn't suitable for mains voltage SSR controlled heating.

[tekkydave]

Works fine on mine. I use a SSR.

[tekkydave]

37 minutes ago, Gina said:

Looked up PID in the Duet web page and see that it isn't suitable for mains voltage SSR controlled heating.

Can you post a link to that?

[Gina]

Link  What I was going by is that SSRs use "bang bang" control - on for a bit, off for a bit.  My understanding is that PID uses PWM.  Maybe I'm wrong.  "It's not unusual" ?

[tekkydave]

Where does it say don't use PID for mains SSR control?

[Dr_Ju_ju]

I agree with Dave, I have an SSR driving my 6mm ally plate, no problems, quickly getting to & keeping temperature.

In fact its a little too fast in the initial heating, I've had to program in a small delay to ensure heat has spread evenly across the bed.

[Gina]

OK I stand corrected ?  But is there any advantage of PID considering the very long time constant of the bed.  My setup maintains the bed temperature within ±1°C, I would have thought that was quite adequate.

[Gina]

Well...  Back on topic...  I had to take the extruder apart to sort out a filament jam and now I have to set everything up again.  The tension on the upper screws needs getting just right and the sensor trigger level likewise.  This seems to be more trouble than setting up Z height to get the right nozzle to bed gap.  Now I'm wondering if piezo probing is such a good idea after all, at least for my Concorde printer.  This is the first printer that has achieved a level bed straight off and for which Mesh Grid Probing is unnecessary so Z homing is all that's needed but that still needs some sort of probe.

[Gina]

Rejecting defeat, I carefully adjusted everything so that the piezo triggered from nozzle and not from Z drive vibration.  I did this cold.  Then tried a print run and found I needed to reduce the hotend fan to 40% to avoid triggering.  Maybe if I can rubber mount the fan I can have it running faster.  Maybe piezo probing isn't dead on the Concorde after all...

[Gina]

The saga continues...  Had the printer merrily doing a print run (a part for my ASC) when I went to bed and this morning there was an adequate test part sitting on the print bed having cooled down and released from the bed.  So this morning I designed another part and attempted to print it on Concorde, expecting the same results as last night.  HUH!!!  Guess I'm an optimist!!  No way was this stupid printer going to behave itself!

[Gina]

This is taking too long - I want a printer that works!  I want to progress my other projects not fool around with 3D printers (even if 3D printers do interest me).  Makes me wonder if I should simply add a microswitch Z endstop and calibrate that.

[Gina]

Wanted to use the Concorde to print test parts for my ASC so commented out all the homing and probing from the start gcode and set the nozzle height by hand.  It's now printing.

[Gina]

Until the hotend jammed!  But that's another matter.

I've found why the bed thermistor is showing low temperature - the glue has come unstuck and the middle of the heater is hanging down a mm or two in the centre where the thermistor is.  So the thermistor is no longer in contact with the aluminium plate!

As for the piezoelectric Z probing - it was an interesting idea and no doubt ideal for printers that suffer from bed level problems and where it could be implemented reliably such as on my Mini printer, where it's working well.  BUT... It has turned out that Concorde doesn't need it and it's more trouble than it's worth.  I now plan to fit a standard micro-switch endstop for the Z axis on Concorde and calibrate the nozzle height with a sheet of paper.  This should only need doing once per multiple print run.

Edited March 17, 2019 by Gina

[Gina]

I would like to thank everyone who has commented and given me ideas on this.  All this may not be wasted as I may apply it to another 3D printer.  Whether this will be my Giant printer or something else I'm thinking about.  The Concorde has turned out to be a big printer with the same printing area as the Giant but half the height.  I don't really think there's much if anything I want to print that's over the 350mm build height of the Concorde.  I think the Giant was really just to prove I could do it without much of a practical purpose.  I'm thinking of dismantling it and recovering the large space it's currently occupying.

It might make more sense to build a printer in between the Mini and the Concorde if I feel the need for three printers.  Currently the Mini is equipped with a 0.25mm nozzle and works well for small print jobs.  Print volume is 200mm x 200mm x 250mm.  Concorde print volume is 400mm x 400mm x about 350mm.  The large 400mm square bed heater is rated at 1200W and really overkill for medium sized prints.  I still have the print bed from my dismantled Titan printer which is 300mm x 300mm with a 750W mains powered heater.  An intermediate sized printer could be an option for the future.  I had intended the Concorde to be a replacement for the Titan but it would seem to be more a Giant replacement!

Edited March 17, 2019 by Gina

[Gina]

Been looking at the bed heater on my Concorde printer.  Removed the glass plate and turned the aluminium plate plus heater pad over. There is a bulge in the middle about 6" diameter which I couldn't press out.  So I took a pair of scissors and lifted an edge followed by working my steel rule between pad and ali sheet until it reached the bubble.  That released the air (or whatever) and I was able to flatten the centre a bit, though not totally. 

I checked the Giant printer bed which also uses the same type of heater pad and aluminium plate (though this one is 5mm thick) and that showed a similar bulge.  Then I checked the bed from the Titan printer which has a smaller heater pad stuck directly to glass and that was quite flat.  Seems these pads don't stick to aluminium as well as glass.

With the bed heater upside down and the heater pad visible I thought I'd try heating it up and see what affect differential thermal expansion might do.  It went flat when hot and the aluminium sheet got a lot hotter than it did with the bed the right way up.  Looks like I need to do that same as I have with the Mini printer and press the heater pad with polyurethane foam but I would need to keep the temperature below around 100°C as the foam is only rated to 105°C.

After letting the bed cool down there was a slight separation of the heater pad from the ali but nothing like as bad as before.

 

Edited March 18, 2019 by Gina

[Gina]

The bed heater is much improved by releasing the bubble. 

The problem now Is that I want to use a standard endstop for the Z axis same as X and Y but I can't see how to do that in Duet.  All sorts of different probes but no mention of standard micro-switch type endstop module.  Anyone know how to do this?