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z-axis motion goes too far up

From the Forum there are a couple of threads where users have found that the objective protrudes above the stage when the microscope is first assembled.

https://openflexure.discourse.group/t/first-time-user-staging-problems/2177 https://openflexure.discourse.group/t/my-lens-is-too-high/2137

(photos in both threads)

There are two issues related to those reports. One is in the instructions for initial set-up after building (#402). A second potential issue here is in the hardware.

Playing around with a separate z-axis, I find that the ‘up’ motion actually goes quite a bit further than the design range. The total motion in z should be about 4mm, 2mm below centre and 2mm above. When I drive the z gear by hand clockwise it raises the actuator column until it hits the inside of the top of the actuator housing after 4 and a bit turns. At that point the optics dovetail has moved about 2mm down. When I drive the z gear by hand anti-clockwise it lowers the actuator column. The gear keeps turning beyond the 4 turns needed for 2mm upwards movement of the optics dovetail. The band tension begins to feel weaker, but the actuator keeps on moving until the bottom of the actuator column hits the inside of the foot. At that point the optics dovetail has moved about 5mm up.

As the maximum upwards travel of the z-axis is set by the foot cut-out, it seems to be too big for the intended travel range.

From microscope_parameters.scad the travel is z_actuator_travel(params) = z_lever_length(params)*flex_a(); which calculates to 2.925mm. In libfeet.scad the end stop inside the foot is set to h-travel-0.5 (line 232 at e0ed3a5b), which adds an extra 0.5mm clearance to give 3.4mm potential travel. However in the built z-foot the actual distance in the STL comes out as 3.75mm.
image
This is because the stop is shifted vertically down in z, while the actuator motion is at 24.22 degrees. This increases the effective length by 1/cos(24.22).

Beyond that issue, the angle of the stop is horizontal when the foot is placed on the angled actuator, which places it parallel to the lever when in the central neutral position. When the the actuator is actually touching the stop it is at a different angle, so it meets the stop only at the outside edge where the bottom of the actuator column is. Because the column remains vertical the bottom of the actuator is not in line with the lever, this allows the actuator to move even further.

The stop in the foot could be set to the angle of the displaced actuator, which would make the stop operate as planned and also make the foot stronger.

In x and y the actuator is vertical, so the issue does not occur. The levers are longer, making xy_actuator_travel(params) 5.14mm and the actual depth in the foot is 5.64mm.

  • Correct the depth of the stop in feet by cos(actuator angle)
  • Consider placing the stop in the feet at an angle parallel to the lever at the end of the travel.
  • Consider whether the 0.5 mm clearance is appropriate.
  • Match the cut out in the wall of the foot to the final position of the stop.
Edited by William Wadsworth