Posted 12/17/15

In my last post on this subject I described my (ultimately successful) efforts to level my new glass print bed and disable Printrbot’s ‘auto leveling’ feature so I could get decent prints everywhere on my print bed.

During this project I had been posting results and questions on the ‘PrintrbotTalk’ forum (see this post), and one responder suggested the use of a cheap dial indicator from Harbor Freight to level the bed, independent of Printrbot’s Z-axis probe.  After some initial missteps I was able to get one  (item #623 on Harbor Freight’s website) and figure out a way to mount it on the extruder carriage. The following photos show the mounting arrangement, using the handy mounting tab on the back of the dial indicator.

Dial indicator mounted to the extruder carriage. Note ground-down portion of the carriage .

1/4″ by 1/4″ bushing fit nicely inside the 1/4″ I.D. mounting hole.

Dial indicator mounting tab and mounting screw/bushing

Dial indicator mounted, with carriage height adjusted to achieve a zero reading

With this arrangement, I was able to simply move the carriage around by hand, noting the needle excursions from zero.  Then painter’s tape was added to the low side (or removed from the high side) to minimize the differential across the printing area.  A 1mm height difference equates to about 40 small divisions on the indicator, and after shimming I was able to achieve a height differential of    +/- 5 small divisions (about 0.125mm) across the print area.

After getting the glass plate all shimmed up, I decided that because I was no longer using the ‘auto-leveling’ (actually more like ’tilt correction’) feature, I could now remove the copper foil layer from beneath the painter’s tape –  BIG MISTAKE!!  On the very first test print after doing this, I realized to my horror that the Printrbot still needed to find the Z-axis ‘home’ position, and the only way to do that was with the Z-axis metal-sensing probe.  Fortunately I was able to pull the power plug before the extruder tip had a chance to shatter my nice new glass plate!

After beating myself up for a while over such a bonehead move, I realized I had just two choices; I could laboriously replace the copper foil layer (one quarter-inch strip at a time), or I would have to find some way of replacing the metal-sensing probe with something else.  I  did not want to go through the agony of replacing the foil layer, so I was left (I thought) with option 2.  Someone on PrinterbotTalk had mentioned a mechanical switch replacement for the Z-axis probe, and  said there was at least one Thingiverse design for a bracket that mounted to one of the vertical carriage posts.  I took a look at this and decided I could adapt it for one of the normally-open pushbutton switches I had in my electronics parts bins.  After some further Googling, I realized that the replacement project might be a bit more involved than I originally thought, as there was an issue with later rev motherboards requiring a pullup resistor and some extra wiring to make the mechanical switch idea work.

Then I had an uncharacteristically brilliant idea, if I do say so myself.  Rather than removing the Z-axis probe and replacing it with a mechanical switch mounted on the vertical slide assembly, why not combine the two ideas and simply mount the Z-axis probe itself on the vertical slide assembly?  Then I get the best of both worlds – I don’t have to screw with the wiring at all, and the metal carriage base is perfect for the Z-axis probe  to sense – voila!

Looking around a bit, I found that if I mounted the probe on the rear vertical slide post bushing, it would have a clear shot at a nice, flat open spot on the carriage base.  All I needed was a right-angle bracket to attach the probe to the bushing.  A few minutes in TinkerCad produced a printable design, and after a few minutes more I had the bracket printed up (I had to fake the Z-home a bit to get the bracket printed, but who’s counting).  I super-glued the bracket to the slide bushing as shown in the following photo, and simply adjusted the height of the probe in the bracket so the extruder just ‘grabbed’ a sheet of printer paper when the Z axis was ‘homed’.

Z-axis probe relocated to the rear vertical post bushing

OK, so now I have a really cool glass print bed, with no ugly copper foil layer, and the Printerbot is no longer trying to murder my  glass plate.   I’ve only done a couple of test prints so far, mostly to figure out what M212 offset is required now with all the changes.  However, I  am looking forward to consistent prints with the new setup.

Stay tuned!

Frank

Posted 11/25/2015

In my last episode of “The Perils of Pauline” (aka Printrbot Simple Metal Glass Print Bed Issues), I described a set of measurements and print tests that ultimately led exactly nowhere, except maybe to the conclusion that Printrbot’s vaunted ‘Auto-leveling’ (more accurately ‘bed tilt correction’) wasn’t all that effective, and might even be part of the problem rather than part of the solution.

So, in this episode, I decided  to manually level the glass print bed with painter’s tape shims, to see if I could get better prints that way.  To do this, I used the existing Z-axis probe, the G29 ‘auto-level’ command, and the G30 ‘Z-axis probe here’ command.  The G29 command tells the Printrbot to probe the Z-axis at three pre-configured points ( (10, 150), (10,10) and (150,10) in my case), and the G30 command tells it to probe the Z-axis wherever it happens to be (I used this to probe the bed at (150,10) – the rear right corner).  Between these two commands, I could measure the print bed surface height at all 4 corners and adjust accordingly.

From previous measurements I knew that the bed tilted upwards from the rear to the front, and from left to right.  Therefore I started out by moving the single layer tape shim from the front edge, and adding a single layer shim along the left edge.  After three or four iterations, I wound up with two tape layers on the left and rear edges, and the resulting probe measurements were within 0.2mm everywhere. It’s hard to get much closer than that, due to variations in the probe results.

Initial painter’s tape shim layout

After getting the bed as level as I could, I ran another set of test prints.  As shown in the photo below, all 5 positions printed successfully.  I also noticed that the Z-axis worm gear moved noticeably less during print operations, providing an additional indication that the print bed was in fact pretty darned level.

All positions printed successfully, although the front right (Position 5) print was very lightly attached

When I removed the test prints from the bed, I noticed that the near-right (Position 5) print was less well attached than the others, and in fact the attachment quality improved as I went from Position 5 to 4 and then to 3.  Positions 3, 2, and 1 (all in the same vertical line) seemed to all be nicely attached.  So, the actual print results indicate that the extruder tip is getting farther away from the print bed as it progresses to the right, but the Z-axis probe measurements indicate the exact opposite situation – the print bed actually gets  higher as it goes from left to right!  How can this be?  Well, based on everything I have learned to date, I now strongly suspect that the answer is that the tilt correction algorithm is correcting the wrong way somehow.  Maybe the fact that the Printrbot’s ‘home’ position is at (Xmin, Ymax) rather than (Xmin, Ymin) is causing a sign error somewhere, and this is causing the Printrbot to correct the Z-axis  up for movements in the positive X and negative Y directions  when it should be correcting it  down.  This theory tracks with what I have experienced so far, as it would imply that the closer I can get the plate to absolute level, the smaller the error  would be, ultimately going to zero error for a perfectly flat plate. Of course, if I’m right, I should be able to somehow track down and correct this sign issue, and therefore convert what is now a force for evil into a force for good – arggghhhh!!!!

Late addition:  After posting this to the ‘Printrbot Talk’ forum, I got a note from ‘Retiree Jay’ to the effect that I could disable the tilt correction function entirely by omitting the ‘G29’ command from the startup script, leaving only the ‘G28 X0 Y0 Z0’ command. So, I did this and ran another series of test prints as shown in the following photo.  As you can see, all prints were successful, even the added Position at the right rear of the print bed.  And even better, Retiree Jay was correct in that the Z-axis motor did not run at all (up or down) during prints of a specific layer (since I printed only one layer for each position, this means that the Z-axis motor didn’t run at all during the entire print series.  This pretty much proves that my new glass plate  is flat across the entire print area; “Tilt Correction?  We don’t need no stinkin Tilt Correction!” ;-))

Print series with tilt correction disabled. “Tilt Correction? We don’t need no stinkin Tilt Correction!”

Stay tuned,

Frank

Posted 11/24/2015

A few days ago I posted a long account of my attempt to add a glass plate printing surface to my Printrbot Simple Metal, to address printing problems due to  non-planar warping issues with the original print bed.  As described in the post, I was able to satisfy myself that the glass plate was much flatter than the original bed, but I still couldn’t get consistent prints except in a very small area – printing the same part at the same Z-axis offset resulted in either a print that wouldn’t adhere, or gouges in the painter’s tape.  My conclusion at the time was that maybe the vaunted ‘auto-leveling’ feature (actually just 3-point tilt correction) has been inadvertently disabled in my firmware version, or maybe it wasn’t ever functional in the first place.

So, in this post I describe my efforts to manually level the glass plate print bed.  Based on the results from the previous post, I know that the bed must slope downward from the back to the front, as the print was more or less OK at the back, but became separated from the bed as the print positions progressed toward the front.  So, I put down a layer of painter’s tape underneath the glass plate at the front edge of the original print bed, and then ran the same series of prints, starting at the rear right corner as before.

The following two photos show the painter’s tape shim installation

Painter’s tape shim installation

Painter’s tape shim installation

With this setup, I started by printing the 20mm cal cube at the extreme left rear corner of the print area, what I’m calling ‘Position 1’.  The first print failed partway through when the piece detached from the bed, indicating the Z-axis offset was too high, at Z = -1.1.  This was interesting all by itself, as Z = -1.1 was the offset I used for the entire first experiment a few days ago; I speculate that the addition of the painter’s tape shim may have lowered the plate very slightly at the back corner.  I changed the offset to Z = -1.2, and this gave me a good Position 1 print, as shown below.

Position 1 print with Z = -1.1. Note the detachment

Position 1 print with Z = -1.2

Then I moved on to Position 2 (still at extreme left edge, but midway from back to front) and tried again.  As shown in the following photo, I got a good print here as well.

Position 2 print with Z = -1.2

From there I moved on to Position 3 (extreme front left  corner of the print area), and Position 4 (front edge, midway from left to right).  As shown in the following photo, I got a very good print at Position 3, but the Position 4  print detached immediately.

Positions 3 and 4. Position 3 printed OK, but Position 4 detached immediately

So, it appears at the moment like I have the plate leveled in the front-to-back direction, but not in the left-to-right direction.  Next I’m going to try adding a layer of tape from front to back at the extreme right edge and see what this does.  Note in all this that Printrbot’s ‘Auto-level’ feature  should already be compensating, but it appears to be AWOL.

With  one layer of tape added, I was still getting very poor results in the X (left to right) direction, so I added a second layer of tape.  After this, I was able to get all 5 prints to stick, although as the photo below shows,  the last two were ‘iffy’

With two layers of tape front to back at the extreme right edge

So, I added a third layer of tape, and ran another print series, and got almost identical results!  How can this be?  I’ve added three layers of tape, and I’m still not able to print in the near right-hand corner – this just doesn’t make any sense.

OK, the only possible way it could make any sense at all, is if the Printrbot ’tilt-correction’ algorithm  IS enabled, but not functioning correctly, either due to the algorithm itself, or due to incorrect Z-axis sensor readings from the probe.  So  did another test print, but this time I concentrated solely on the Z-axis servo worm gear.  At first I simply put my fingers on the worm gear so I could feel if there was any Z-axis movement during lateral head moves, and sure-nuff, there was some!  Then I put a painter’s tape ‘flag’ on the worm gear and I can easily see Z-axis movement during even small lateral head moves.  The clear indication is that the tilt-correction algorithm is working, but for prints near the right-hand edge of the print bed, the correction is wrong.  I wonder if the errors are due to scaling based on inaccurate print bed dimensions in the printer setup dialogs; this would seem inconceivable, but what do I know?

On a recent print at the near right edge, with 3 layers of tape, the probe measurements were:

(10,143,1.46),  (10,10,1.79),  (143,10,2.32)

with the tape removed, the measurements are

(10,143,1.35),  (10,10,1.59),  (143,10,1.94).

This seems to indicate that positive Z is ‘up’ and that lower positive Z values indicate a surface that is actually closer to the bottom of the printrbot.  This, in turn, should mean that I could achieve what I want by putting tape under the  left edge, not the right, even though the print performance indicated just the opposite! Oh, I have a headache!

with the right-edge 3-layer tape shim removed entirely, I re-ran the original test print series.  Instead of the mess I got the first time, printing at all 5 positions succeeded as shown in the following photo – huh?????

All three tape shim layers removed from the right edge

OK, so this just should  not be happening!  When I started this post, with no shimming at all, I could print to position 1, 2 & 3 (along the left edge, from rear to front), but not positions 4 or 5 (along the front edge, from left to right).  This behavior is what led me to shim along the right edge, but although I thought I was making the situation better, increasing the shim height from 2 to 3 layers  didn’t  help at all, and now I find that with  no shim layers I get really nice prints at all 5 positions – what gives?

Thinking back through everything I did this evening, I realized there was one thing I did that could maybe have affected the situation – I changed the printer extents via the Repetier ‘Printer Settings’ dialog.  This  should not affect tilt correction calculations at all, since the Printrbot already knows the absolute coordinates for all three Z-axis probe points, so why would it care (or even  know) what extents have been set in Repetier?

As a test of this wild and zany idea, I set the Y-Max value in Repetier  to 350, basically twice what it should be, and set up for another print series.  When the probe results came in, they were:

(10,143,-1.06),  (10,10,-0.85),  (143,10,-0.5) – whoa!  How can that possibly be?  Clearly there is some coupling from the values set in Repetier’s Printer Settings dialog, but why?

For  position 2, the Z-axis probe reports (10,143,1.42),  (10,10,1.56),  (143,10,1.90)

– Oh, now I have an even  bigger headache!  How can this be?  Negative values one time, and positive values the next – with exactly the same hardware and software configuration (including the same – bogus – Y Max value)??

For position 3, I get:  (10,143,1.46),  (10,10,1.72),  (143,10,2.03) – print was fully successful

For position 4, I get:  (10,143,1.44),  (10,10,1.70),  (143,10,2.02) – print was successful, but marginal

For position 5, I get:  (10,143,1.44),  (10,10,1.71),  (143,10,2.02) – print fully successful

All three tape shim layers removed from the right edge

OK, I give up – I’m officially baffled.  Everything I have done has had either no effect, or an effect opposite to what I expected.  Then returning the experimental setup  to its baseline condition  did not  restore baseline results – WTFO@!!@#$%^&*(

Frank

Posted 11/19/2015

A week or so ago I posted an account of my attempt to add a glass plate to my Printrbot Simple Metal print bed.  I thought I had a great idea – place patches of copper foil tape on top of the glass plate at the three points where the Z-axis probe sensor checks for print bed tilt (it’s called auto-leveling, but it is actually more like auto-tilt correction), and everything should be groovy.  Unfortunately, what I thought would happen and what  actually happened were two different things.  It appeared that either tilt correction wasn’t being applied at all, or it  was  being applied, but in the wrong direction (which doesn’t make any sense either,  because I didn’t change anything except adding the glass plate).

So, I decided to make another run at the problem.  My new approach was three-fold; first, I took some time to thoroughly investigate the flatness of the original Printrbot print bed, and the flatness of the glass plate.  Second, I  decided to cover the entire glass plate with copper foil, not just the small areas used by the Z-axis sensor.  Third, I figured out a way (the ‘G30’ command) to take Z-axis sensor reading at multiple spots on the print plate, rather than just the 3 points used in normal printing, so I could really determine how well the Z-axis sensor itself behaved.

Printrbot print bed and glass plate flatness investigation:

I used two different techniques to evaluate the flatness of both the original Printerbot print bed and the glass plate addition.  The first technique was to place a light source so that it illuminated the bed from the side, and look for light leakage under a straight-edge placed on the bed.  The second technique was to use a piece of printer paper (approx 0.09 mm thickness) to see if there were any areas where the paper would slide freely under the straight edge.  The photos below show the result for both the original print bed and the glass plate.  The first image shows the setup, with an LED flashlight placed so it shines light parallel to the surface, with a steel straight-edge blocking the beam except where the bed is warped.  Subsequent photos show warped areas.

Flashlight shining along surface of print bed, with steel straight-edge blocking light except where the bed is warped.

Next I did the same thing with the glass plate, as shown in the following images.

The glass plate is clearly  much flatter than the original print bed.  I verified this using the paper technique – I was able to easily slide a piece of 0.09 mm paper under the straight-edge on the original print bed, but not on the glass plate.  So, as far as flatness is concerned, the glass plate is a clear winner.

Z-Axis Probe Measurements

Since I was now convinced that the glass plate was much flatter than the original print bed, this issue could not be the reason I was having so much trouble trying to get reliable prints at different points on the print bed.  So, I turned my attention to the only other possible factor, the Z-axis probe itself.  If it was somehow producing aberrant readings, maybe that would explain why the Z-axis offset required for printing at the near right corner was wildly inappropriate for the same print in the back left corner.  To acquire the data I made use of the G-code ‘G30’ command, which causes a Z-axis probe operation at the current X/Y location.  I recorded Z-axis probe readings at 12 locations on the glass plate, and plotted the results using Excel’s ‘Surface’ plot feature

Z-axis probe sensor readings for glass plate

11/20/15 Glass Plate Z-axis probe sensor results

As can be seen in both the data and the surface plot,  the glass plate is pretty flat – with only a 0.42 mm deviation from one extreme from the other.

The same set of measurements were performed on the original Printrbot metal print bed, with the following results:

11/20/15 Original print bed Z-axis probe results

11/20/15 Original print bed Z-axis probe results

The data shows that the original print bed is pretty flat too, even though the illumination experiment showed that it exhibits significant warping.  I suspect that the probe measurement locations were too widely spaced to capture the low spots.  In any case, all that can be said for sure at this point is that both surfaces (original print bed and glass plate) are pretty flat, and both exhibit some tilt, with Z-axis probe measurements consistently rising from the left rear corner (0,153) to the front right corner (130,3).

Print Trials:

After convincing myself that the glass plate was indeed flat, and that any residual tilt should be well within the Printrbot’s correction range, I decided to do a series of test prints – starting at the rear left corner and moving toward the near right corner.  For the first print, I adjusted the Z-offset for optimum printing – not too close, not too far away.  Then I kept this same Z-offset for all the other prints.  The following photos tell the story.

20mm cal cube printed at the rear left corner (0,153)

20mm cal cube printed at (0,103)

First try at printing 20mm cal cube at the third position (0,53). Note the blob attached to the extruder!

2nd try at printing 20mm cal cube at the third position (0,53). Note the raised corner

20mm cal cube printed attempt at the near right corner.  Part is separated from the bed and ‘blobbed’ to extruder and

20mm cal cube printed at the rear edge, midway toward the right

20mm cal cube print attempt at the rear right corner.  Part has separated from bed and is ‘blobbed’ to extruder.

So, at this point I’m pretty sure of three things:

1. The glass plate print bed is pretty darned flat
2. The Z-axis probe seems to be working correctly
3. The Printrbot tilt correction feature  does not appear to be working properly.

Stay tuned,

Frank