Z-height of single layer wide box too high ... how to fix ?

[jimc]

watch this quick video the guys at hyrel did. start watching at 1:35. this will apply to what we are talking about here.


https://www.youtube.com/watch?v=Fg9omXlYR-Q

[sthone]

On an off note... Can you adjust the Z height in S3D while it's printing with the jog controls? (Kind of like what they are doing with the onscreen buttons in this video.) I currently just pop the K knob to accomplish this and wonder if the jog controls would be more precise if they worked that way.

-Steve

[Dale Reed]

Steve,

Looking at the G-code a few posts back in this topic, I don't think that would work. Notice the line that says "G1 Z5 F1200" -- this tells me that S3D is generating code relative to an "absolute" (zero-based) Z reference, and not relative from layer to layer. So if the jog command slipped into the command stream (and considering the command buffering at the printer, who knows when it would actually execute!), I think its effect would be quite temporary -- once another Z move (like the "G1 Z5 F1200") took place, the offset from the jog command would be (effectively) nulled.

Turning the Z knob, on the other hand, introduces a Z offset that the firmware has no clue took place -- as far as it knows, the "G1 Z5 F1200" command took the Z axis to 5.0000 mm above the bed based on the count of steps it has sent to the Z motor. There is no other position feedback (it's not a servo-based system), so it has no way to know an offset occurred.

My opinion and not tested.
Dale

[Mind]

OK, so I am having a hell of a problem getting the bed level. I feel like I am just going round and round in circles

So I ordered a dial indicator. It should be here tomorrow (yay for amazon prime!). Then I can get the bed properly level beyond any doubt and work on offset/etc.

[jsc]

Your offset explains why your last layer is at 5.1mm. You can indeed mess with the offset to adjust your level, but I think fixing it mechanically is the right way to go. As I may have mentioned elsewhere, messing with the first layer height setting doesn't do what you might expect wrt extrusion/layer heights. As Ed says, when you're getting down to low layer heights, you need absolute precision.

As a first step, just take the measured error that you have in your cube and raise your level by that amount. You can estimate how much each turn of the screw raises the bed by how much, I believe Ed has done that calculation somewhere on his blog. Also take the opportunity to reset your offset to 0.

[ednisley]

when you're getting down to low layer heights, you need absolute precision.

And that is why I think there's a good case for locating Z=0 exactly on the platform: you then adjust the first layer from a known reference and read the actual nozzle height from the G-Code to see what's supposed to be happening.

each turn of the screw raises the bed by how much

The three platform screws are 3x0.5 mm, so (for all practical purposes) when you turn the hex driver 1/6 turn the platform directly over the screw moves 0.1 mm. If you're measuring the platform height in a corner, then each 1/6 turn at the screw moves the far corner much more than 0.1 mm, so tweak gently...

[jsc]

Ed, would you happen to have the numbers for the z adjustment screw handy as well?

[ednisley]

the numbers for the z adjustment screw

That went back into the metric stockpile when I put the Z switch on the gantry where it directly senses the platform position... which works a whole lot better, but that's a whole 'nother story:

http://softsolder.com/2013/11/04/makerg ... ht-switch/

The doc says it's an M4 screw, so it's probably 0.7 mm per turn: figure a smidge more than 0.1 mm per 1/6 turn.