nozzle height - mind blown, can you clarify

Hi folks. I've been having an issue with my 2nd layer not sticking to my first. I contacted the manufacturer (I don't want to name him in case there's just a simple misunderstanding, I don't want to bash him) and he blew my mind. He asked about nozzle height. I told him I use a feeler gauge. 0.104mm and set it to light drag. He said, at the home position the nozzle should be touching the glass because the nozzle will lift 0.Xmm to print my first layer. If I start with the nozzle 0.1mm above the glass, I'm actually adding 0.2mm to the mix. Is this true? Should my nozzle be touching the glass in the home position?
If so, how do you check your bed level? Up to this point, I've been putting the nozzle in the home position, then manually moving the head around the bed and using my feeler gauge to set level. If the nozzle is touching the glass, how do I know if the nozzle is just toughing, or applying force on the glass?
The printer is a reprap design.

For what I've seen, most people set the Z=0 position using a piece of plain paper. You run the nozzle down very close to the bed just to the point it is grabbing a piece of paper, but not pinning the paper down. You can usually feel it while moving the paper around.

I don't think that your man is correct in his advice. You must have a clearance between the nozzle and the glass to enable the head to move without scratching the glass or wearing the nozzle. I normally just use the thickness of an ordinary sheet of printer paper which is a little under 0.1mm.

Thinking about your problem. I don't normally use a different layer thickness for the first layer and it normally ends up well squeezed onto my print surface. If this is the case with yours then the filament is filling all the gap between the print surface and the nozzle tip. When the nozzle raises for the second layer it must therefore be the right height if all the settings are correct.

Firstly, measure accurately from the bed to a known fixed point on the carriage. Use the printer manual controls to raise the head by a set amount (say 100 mm). Repeat the measurement and see if the difference from the first is exactly 100 mm. If the Z steps per mm are out then the head may be raising by an amount more than the layer thickness so the filament will not fill the gap and stick. This test should allow you to check this.

Next check the filament diameter set in your slicer. Some slicers default to a non-standard value larger than 1.75mm. The result of this is under extrusion which again tends to cause the layers not to stick. If you have a caliper measure the actual diameter of the filament at intervals over about 1m and set the diameter in the slicer to the average of these values (stick a label with the value on the filament reel for future reference).

The layer thickness set in the slicer should not excede 2/3 of the nozzle diameter or the new layer will not be pressed firmly enough against the previous one.

Quote
PDBeal
For what I've seen, most people set the Z=0 position using a piece of plain paper. You run the nozzle down very close to the bed just to the point it is grabbing a piece of paper, but not pinning the paper down. You can usually feel it while moving the paper around.

OK, that's what I'm doing, but using a metal feeler gauge. Sounds like there might have been some miscommunication between he and I.
To be clear that I understand, when I initialize a print the printer goes to home and everything heats up. From there, the printer does NOT lift 0.Xmm and start the first layer. The first layer starts on the home z height, correct?

No that's not correct, the nozzle will move to first layer height been 0.1, 0.2, 0.3 or whatever layer height you selected in the slicer. Only when homing your printer the nozzle will go to Z0 and that's only if you do G28 X0Y0Z0. With ABL you can do G28 X0Y0 and then G29, the printer will probe on the points selected and then will move to first layer height from slicer.

So if you use a 0.1 blade from your gauge and then set your first layer at 0.3 your first layer will actually print at 0.4, you can correct this issue with a Z offset in the slicer to compensate it or if you use ABL you can correct it with the probe offset.

Doing it manually as you are you can home Z and then lift it 0.1, measure all the points you want in the bed with the 0.1 blade and once calibrated your home point for Z will be 0, all printers do X and Y first so your nozzle will never drag on the bed.

Edited 2 time(s). Last edit at 03/21/2017 07:24PM by ggherbaz.

The sheet of paper is for thermal expansion. If you have your bed and nozzel at print temperature the nozzel should touch the bed when homed from what I've read. If you want to level it so it is dead on when touching the glass, manually control the nozzel to .1mm off the bed when both are at print temperature and use a feeler gauge while it is at .1 instead of at 0. I would check the steps per mm of travel personally if the second layer wouldn't stick to the first as it may be moving more than intended. If after calibrating the nozzel height at .1mm it puts pressure on the bed when homing it is moving too much and may need adjusted in the firmware. Hope there is something useful in here for you.

Edited 2 time(s). Last edit at 03/21/2017 10:18PM by lockezio.

2nd layer not sticking is unusual.
Have you calibrated the Z axis. Request and 100mm move in Z and measure it.
Make sure cooling fans are off.

Thanks folks. You've given me food for thought. Tonight I'll calibrate z steps and set home position to true zero and see what happens.
Here's what doesn't make sense to me. I've never had this problem before. 1st layer ALWAYS sticks to the glass, layers always lay down nice. But, the largest thing I've printed (flat surface area-speaking) is a phone case or a light switch plate. Those are much smaller than the 5.75" squared piece I'm printing now, and they have holes in the print that break up those layers.
I'll report back. Thanks again.

Going to larger parts will show up problems with a non-flat bed. Mine is horribly bowed (glass clipped to a pcb heater) and it used to drive me nuts trying to print large parts. The only way I could do it was to compromise on the Z0 setting so that it was too close in the middle, but still too far at the corners, and then print the first layer extra thick to try and fill in the difference. The right answer is to replace the bed with something actually flat, but in the interim I added a simple Z probe and enabled distortion compensation in repetier firmware. The difference in ease of printing is night and day.