Calibration – WYISYG, Don’t use a 20mm Cube for accuracy!

img_7315https://www.thingiverse.com/thing:1278865

 

Lots of basic printing suggested that the front to back measurement was slightly off…so, finally I’ve decided to calibrate the printer….

By, printing a calibration cube! I’ll do it a little differently and record just a bit more information…I’ll measure 3 points for each axis, giving 9 points for the cube….

  • X = 20.09 Bottom, 20.06 Middle, 20.10 Top…Max/Min Diff = 0.04
    • Average = 20.083…Face = 20.06…
  • Y = 20.16 Bottom, 20.07 Middle, 20.12 Top…Max/Min Diff = 0.09
    • Average = 20.117….Face = 20.14…
  • Z = 20.08 Front, 20.12 Middle, 20.18 Back….Max/Min Diff = 0.10
    • Average = 20.126  (X at the front)
  • Z = 20.1 Front, 20.0 Middle, 20.02 Back….Max/Min Diff = 0.10
    • .Average = 20.04 (Y at the front)

 

So, if we just measure the face, as requested in the instructions, we’d get different readings compared to measuring the average (more accurate?) of 3 readings.

  • X Face = 20.06, X Average = 20.083 ….Difference = 0.023
  • Y Face = 20.14, Y average = 20.117….Difference = 0.023
  • Z Height – difference of 0.1 between front and back! with X at the front
  • Z Height – Difference of .08 between front and back with Y at the front!

 

My thinking………

3D printers will print items with slight bulges and convex / concave surfaces….it’s their nature, the bottom of the item , that first layer will always be slightly wider than the one above it….That’ll throw off the X and Y ‘bottom’ measurements…as can be seen

Looking at the Difference between the largest and smallest measurements for each axis, we have Just a 0.1 milimeter maximum on the Z axis (need to set Z layer initial height better!)

Y axis, we have a difference of 0.1mm

X axis just 0.04mm…..

 

or, in other words,

So, how should we measure the cube!? how should we interpret the measurements to calibrate the printer – Instructions say ‘the face’ – I don’t think that’s enough….

Personally, I don’t now think that the 20mm cube is sufficient to calibrate the printer…at 20mm per side and 0.1mm difference between measurements for that side…..at 200mm, there would be 1mm difference in measurements….at 300mm (my maximum bed width) there would be a 1.5mm difference…….

or, in other words, me, badly measuring my 20mm cube to be somewhere between 20 and 20.1mm and using those measurements to calibrate my printer right now….

When I print something 300mm deep it’s guarenteed to be up to 1.5mm ‘out’

If we take that same ability to measure to a 0.1mm accuracy and work backwards…..to the cube and how it should measure…

300mm wide print – measured to be 300.1mm……….

in 300mm you can fit 15 off 20mm cubes

300.1 divided by 15 = 20.007mm

NONE of my measurements were close to that….

 

Summary, DONT use a 20mm cube to calibrate when you’re close to 20mm on your print….use something significantly larger!

DO use a 20mm cube for your ‘first print’ just to make sure you’re in the right ballpark!

 

Good luck

 

 

 

 

 

 

Generic cube Instructions extract from thingverse, no changes made to these instructions from gmyort posted on Feb 10, 2018

  1. This printed cube should be ideally 20.00mm X 20.00mm X 20.00mm
  2. After you successfully print your XYZ calibration cube, measure it.
    To measure “X”, measure from left to right along the face of the “X” (width). Record that value. For me it was 20.00.
    To measure “Y”, measure from left to right along the face of the “Y” (depth). Record the value. For me it was was 20.28.
    To measure the “Z”, measure from the bottom (blank) up to the “Z” face (height). Record the value. For me it was 19.93.
  3. If you have not already, you will need to download a program to interface directly with your printer in order to adjust your XYZ steps/mm. For my MonoPrice Select Mini, I connect via USB and use pronterface. Lots of helpful videos and guides on the internet on how to use this program.
    (1) launch Pronterface.exe
    (2) Connect your printer via USB to your PC.
    (3) Click the “connect” button (on your PC in the pronterface interface) and it should connect your printer to the interface (you should see confirmation on the RH box.
    (4) On the command line (lower RH text box), type “M503”
    (5) Look for “M92 ….” this will show you your current X, Y, Z, and E steps/mm
    For example, mine was set “M92 X93.00 Y93.00 Z1097.5 E97.00”
    6) I then used the ratio formula to determine my ‘should be’ steps/mm.
    New X = (20/20) 93.00 = 93.00 (no adjustment needed)
    New Y = (20/20.28) 
    93.00 = 91.72
    New Z = (20/19.93) * 1097.5 = 1101.35
  4. To change the M92 values in pronterface you will use the M92 Gcode. In my case, I typed M92 X93.00 Y91.72 Z1101.35
  5. To confirm the values, type M503 and check M92
  6. If it is correct, type M500 to save the values to the EEPROM
  7. Disconnect from printer in pronterface (click “disconnect” button) and power your printer off for a couple of seconds.
  8. Power on your printer and click “connect” in pronterface to reconnect.
  9. Type M503 command and verify the M92 values are perminently changed.
  10. Reprint cube and see how well this worked.
  11. My new cube measured X=20.00mm, Y=20.02, Z=20.00

Author: Bleugh

Early Fourties, Wife, two kids in primary school. Both of us work full time...5 years now we’ve been Back in Blighty after a decade away...It's a new country for everyone! Still finding time to tinker!

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