Auto Bed Leveling and Leveling Print Bed
I just finished installing a BL-Touch (BLT) probe and decided to test it a bit... I am certain there are several probe technologies that work great. Not trying to sell anyone on one bed leveling tool or another, just want to share my experience with the hardware I have.
I wanted to
1) Measure the accuracy of the BLT
2) See how out of plane my be is
3) See how to easy it is to use the BLT to auto-level the bed, and
4) See if I can figure out how to use the BLT to level the bed
Couple of important points:
For those new or considering an FT-5, remember the bed size is large, and leveling this bed is harder than leveling a smaller bed...
I used RC-8 of Marlin as I read the software has improved probing routines and a few bug fixes specific to probing, so if you are using the software that FT recommends for the printer and hosts on their Google site... well, you know. YMMV.
Installing the BLT was fairly easy. It was wierd using a 3D printer to make a part for itself. Thanks @Walter for the STL file. I modified it a bit to suit my particular setup.
I was able to use the BL-Touch to
* Help me find binding in a Z-Axis support rod.
* The BL-Touch and Z-Axis movement was repeatable to about 0.015mm with STD of about 0.005mm
* Use the Auto Bed Leveling feature
* Quickly and accurately level the bed - (as opposed to ABL)
So, just how accurate is the BL-Touch?
Marlin has a routine to check the repeatability of the probe -- M48. From the Marlin Documentation:
M48: Z probe repeatability measurement function.
Usage M48 P# X# Y# V# E L# S Where P, X, Y, V, E, S, and L are all optional
P = Number of sampled points (4-50, default 10)
X = Sample X position
Y = Sample Y position
V = Verbose level (0-4, default=1)
E = Engage Z probe for each reading
L = Number of legs of movement before probe
S = Star (with P points to the star)
I ran this (P=10) at the center and at each edge of the print bed to see if there were differences. Keep in mind that because the bed moves, this is the repeatability of the SYSTEM, not just the probe. That is, there is variability in the probe accuracy plus variability in returning the Z Axis to the probe position. The measurements below represent values for the measured (probed) "Z-axis" as returned by Marlin software.
1) Reading the range at (Max X, Max Y) -- the last column, confirms my suspicion that this end of the bed is sticking a bit. Good catch! After adjusting this edge of the bed, the Range went down to 0.02mm - similar to other measurements.
2) The range of readings is about 0.015mm (after fixing binding, that data not shown).
3) My bed leveling ability sucks. I didn't need a probe to tell me this, however!
ABL is fairly easy - meaning you don't have to do the math. You do have to enable it in the software -- you then must home the printer (G28) first. Then level the printer (G29 P3<3 by 3 grid> V4 T ).
From this point on, when you move from one X,Y point to another, the Z axis moves just a bit to keep a constant distance from the hot end tip to the build surface. Note this does not set the Probe tip to hot end tip offset. That is a separate and necessary step.
Using Auto-Bed Leveling to Level the Bed
The only thing better than ABL is a level bed to begin with. Why? Every time the Z axis needs to move to keep the nozzle a certain height off the bed (that is what ABL does), there is a chance for a visible step in the printed part.
Here is how I used ABL to help level my bed. Note that I did not try to get it perfectly flat. I have 2 pieces of mirror glass on the stock bed.
1) Using Pronterface, home the printer (G28)
2) Tighten 4 corner bed leveling screws to a point where each can be either tightened or loosened - adjusted either way.
3) Loosen the leveling screws along the sides so they were just "snug"
4) Do a G29 as above. Select the parameters so G29 only measures the 4 corners that is where the adjustments are...
Here is the Pronterface output:
Look at the lines that start with "Bed X:" highlighted.
Note most corners are at about Z = ~1mm. Note near X0 Y0, the bed is out a little more than 1.5 mm (red arrow). Since the adjusting screws are M3 hardware, they move up or down 0.5 mm per turn of the adjusting nut. Since Z is high near (0,0) I tightened up the screw about 1.5 turns. Here is the "After adjustment" reading:
I could get this closer, but you get the idea.
Then I snugged up the bed adjusting screws in the middle of each side to help carry the load.
Hope this helps.