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Calibrate and Tram Z – Movable Gantry CNC Machine 19

Now that all the major modifications had been made to the movable gantry cnc machine, the final part was to calibrate and tram the z-axis again. As a recall, the work being done as major modifications were:

    • The z-axis housing was converted from MDF to plywood because the former was not able to hold its shape after being screwed. Several cracks develop on the body and the screw threads were not holding very well. With the housing being machined out of plywood, the location of the holes for parallel rods, motor, linear bearings, leadscrew nut and other ball bearings were more accurate.
    • By adding a tapered backing plate to the z-axis housing, the z-axis was made able to travel vertically as it was meant to do. Without the tapered backing plate, the z-axis was travelling at a slight angle from vertical. This was an inherent problem of the horizontal y-axis which was not addressed during its fabrication. A decision was made to use the tapered backing plate as a temporary measure until a permanent solution could be made. A permanent solution would involve fabricating a complete set of gantry side plates to replace the existing ones.
    • Converting the y-axis from belt driven to leadscrew driven. This was to solve some problems which occurred due to insufficient space left between two faces of the timing belt, adjustment of tension, and also at that time some loosening of bolts holding the idler bearings. However, converting to leadscrew drive also presented its own challenges:
      • Leadscrew diameter was uniform throughout its length. One end had to be machined or ground to size to fit a stepper motor flexible coupling.
      • Leadscrew was slight bent. The leadscrew had to be bent back to as near perfectly straight as possible.
      • The position of the leadscrew had to be determined so that it is parallel to the round rods of the y-axis. The position of the leadscrew in relation to its end bearings and leadscrew nut should be placed so as not to create additional unbalanced forces on the motor flexible coupling and cause whipping effects.

To calibrate the z-axis, a vernier caliper was slotted into a groove on a piece of plywood so that it could be held vertical. A piece of steel bar was fixed at the z-axis so that it would be able to press and move against the vernier caliper. When the z-axis was commanded to moved down a certain fixed distance at the computer, this would cause the former to correspondingly move downward physically. Adjustments on the configuration of the z-axis made compared with the physical movements after several attempts was sufficiently accurate to ensure that whatever movement on the computer would translate into the same physical movements.

To tram the z-axis, the same tramming procedure as described in Tramming the Z-axis was used. Previously, the tramming was not successful because the whole z-axis housing was not truly vertical. This time however, because of the addition of the tapered backing plate, the inherent fault had been corrected and the tramming was found acceptable.

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