Taking note of the findings after rough tramming of Z-axis on the Movable Gantry CNC Machine, I decided that the Movable Gantry Z-axis drive mechanism needed a major makeover as there were too many inaccuracies. These inaccuracies came about because firstly, I was using MDF material, and secondly, I was drilling and cutting the parts by hand and somehow it was not accurate enough.
Since I had already made a Movable Bed CNC Machine which I considered sufficiently accurate, I decided to cut the parts for the Movable Gantry CNC Machine using this former machine – letting the machine do the work that it was suited for. This time I would be using plywood, which would be harder than MDF, and would not distort or split when I screwed pieces together.
All the dimensions in the component part designs were drawn in Autocad, which I think was the standard when it comes to engineering design. When translated into G-code and loaded into LinuxCNC machine driving software it was accurate to the nearest 0.0001 mm. In addition, I have calibrated the Movable Bed CNC Machine to be as accurate as I possibly could, with its limitations. So, I felt that using the machine would be much better for fabricating parts rather than relying on the old fashion method of hand cutting and eyeballing.
For the movable gantry cnc machine, it was rather easy to notice and to adjust the X and Y axes so that they become perpendicular to each other. The movements of the tool would be rather large and would be measurable with an ordinary ruler. The resulting cuts on the work piece would definitely show up when these axes were not perpendicular to each other.
However, with the Z axis, it was not so simple. During the cutting operation, the Z-axis movement could be very small, perhaps from 0.0 mm to -0.2 mm. If the Z-axis movement were not exactly vertical, it would not show up conspicuously. In addition, the Z-axis was supposed to be vertical all around and this was rather difficult to judge with the naked eye.
So I had been putting off tramming the Z-axis until now. As the other issues that had cropped up for the movable gantry cnc machine began to be solved, I felt it was time that I looked into tramming the Z-axis. The benefit of it was that the holes that I cut would become more right angled and the parts that I fabricate would become more accurate. In other words, the cnc machine would be able to machine other component parts for other machines or fixtures more accurately.
Tramming means adjusting the mill head so that it becomes perpendicular to the mill table’s X and Y axis. In order to know whether the tool was exactly vertical to the work piece, a extension arm was fabricated so that one end could be clamped to the tool, magnifying the rotating movement, while the other end would have a marking device for indication purpose. In this case, I had used a pencil. Essentially the extension arm acted like a large compass with the center of rotation at the tool center.
The purpose of the extension arm was to determine where the high and low areas of the work piece (waste board) were relative to the tool shank. After determining these areas, the Z-axis would have to be adjusted accordingly to even up the heights for all the areas. Prior to this, the waste board would have gone through a session of facing to make it as flat as possible. It was considered alright to face the waste board before tramming because the resulting cuts would show up as overlapping parallel paths with slight slopes but still pretty much even throughout.
While trying to adjust the Z-axis to make it vertical in all the side directions, it dawned on me that the movable gantry machine needed quite a major modification. The correcting movement to bring the Z-axis to vertical along the X-axis was simply too much.
The round bars for the Y-axis were not placed vertically to each other. This automatically meant that the Z-axis would always be tilted along the Y-axis movement. I tried to add some shims to the linear bearings, but unfortunately, this also affected the track bearing position and its free movement.
Finally, I narrowed down to only 2 options – I could either re-drill the holes that were holding the round bars or I could add a wedge to make the Z-axis vertical. I chose the latter, since it was relatively easier to do. The former option would entail more disturbances to the machine alignment, and would certainly be more time consuming.
Thus, while the machine was still functional, I decided not to use the results of the tramming to adjust the Z-axis, because it was too difficult, but instead to fabricate a completely new Z-axis and an accurate wedge so that the machine would become a better constructed one. Only when this was ready again would I do re-tramming to make the final adjustments.