Archive for September 23rd, 2019

bCNC Probe Camera Calibration

I’m sure I’ll do this again some time …

Focus the camera at whatever distance needed to clear the longest tooling you’ll use or, at least, some convenient distance from the platform. You must touch off Z=0 at the surface before using bCNC’s probe camera alignment, because it will move the camera to the preset focus distance.

Align the camera’s optical axis perpendicular to the table by making it stare into a mirror flat on the platform, then tweaking the camera angles until the crosshair centers on the reflected lens image. This isn’t dead centered, but it’s pretty close:

CNC 3018-Pro - bCNC Probe Camera - collimation - detail
CNC 3018-Pro – bCNC Probe Camera – collimation – detail

The camera will be focused on the mirror, not the reflection, as you can tell by the in-focus crud on the mirror. Whenever you focus the lens, you’ll probably move the optical axis, so do the best you can with the fuzzy image.

You can adjust small misalignments with the Haircross (seems backwards to me) Offset values.

A cheap camera’s lens barrel may not be aligned with its optical axis, giving the lens a jaunty tilt when it’s correctly set up:

CNC 3018-Pro - Engraving - taped
CNC 3018-Pro – Engraving – taped

With the camera focus set correctly, calibrate the camera Offset from the tool (a.k.a. Spindle) axis:

  • Put a pointy tool at XY=0
  • Touch off Z=0 on a stack of masking tape
  • Put a dent in the tape with the bit
  • Move to the camera’s focused Z level
  • Make the dent more conspicuous with a Sharpie, as needed
  • Register the spindle location
  • Jog to center the crosshair on the dent
  • Register the camera location

Calibrate the Crosshair ring diameter thusly:

  • Put an object with a known size on the platform
  • Touch off Z=0 at its surface
  • Move to the camera’s focused Z level
  • Set the Crosshair diameter equal to the known object size
  • Adjust the Scale value to make the Crosshair overlay reality

For example, calibrating the diameter to 10 mm against a shop scale:

CNC 3018-Pro Probe Camera - scale factor - detail
CNC 3018-Pro Probe Camera – scale factor – detail

At 10 mm above the CD, setting the camera’s resolution to 11.5 pixel/mm:

CNC 3018-Pro - bCNC probe camera - settings
CNC 3018-Pro – bCNC probe camera – settings

Makes the outer circle exactly 15.0 mm in diameter to match the CD hub ring ID:

CNC 3018-Pro - bCNC probe camera - red-blue CD target
CNC 3018-Pro – bCNC probe camera – red-blue CD target

I doubt anybody can find the pixel/mm value from first principles, so you must work backwards from an object’s actual size.

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