Some updates on the Movable Gantry CNC Machine:
At the bed board frame, I found that the holes that I drilled for putting in the X-axis round rods were not positioned as accurately as I had wanted. The rods was supposed to be parallel to each other, but they were not.
To adjust the rods into their correct positions, I did some precise measurements between the rods at both ends, remove the rods, filed off some portions of the location holes, put the rods back, measured again a few times until both the round rods were parallel to one another.
To prevent the rods from moving out of place, I filled the rough holes with epoxy resin and let the latter harden around the rods.
I had to make sure that the rods would not be glued in place and that I could remove the rods later on. I used masking tape to act as a barrier between the epoxy resin and the rods.
Once the round rods were reasonably parallel, I worked on the gantry. Here again, there were some flaws.
It was not so easy to cut the mdf boards exactly. I did some fine finishing by eyeballing and hand filing, so that the gantry would fit into position squarely.
The gantry was supposed to move in the x-direction supported by ball bearings. To prevent sideways lateral movements, I had designed it to run on linear bearings along the parallel round rods.
The reasoning behind this was because I did not have a fully supported round rod-linear bearing assembly. I reckon the weight of the cnc machine structure, motors, spindle and other components will cause the round rod to flex and the machine would not run true. The ball bearings were added in to take up the weights.
My strategy was to use the floor as the reference point. The bed board side wooden edge, being quite reasonable accurately cut would serve as the extension of the reference point. As far as I could see, the dimensions of the bed board and frames were quite accurately cut.
The moving gantry would have to travel along the bed board side edge and still maintain the accuracy of the reference point. So when adjusting the gantry, I made sure that the ball bearings were touching the bed board side edge, and the other cuts were referenced to this height.
Because of this reasoning, the linear bearings were positioned in place after the ball bearings were in place.
Next, in order to make the gantry assembly more rigid, I had screwed in a gantry bottom plate just slightly above the ball bearings. Cutting this plate was a bit tricky. It had to be exact. I spoilt the first plate by cutting about 3 mm smaller. There was no play to accommodate this because the location of the gantry was exactly as the distance between the parallel round rods when running on the linear bearings. Luckily, I had another mdf piece which I could cut later to the exact dimensions.
My next tasks would be to fabricate the Z-axis holder so that linear bearings attached to this would be able to move along the Y-axis. Following the same design as the previous movable bed CNC machine, I would be making this structure in the form of a box for rigidity. This time, I had access to some 12.7 mm (1/2″) MDF boards, so I would be using these for fabrication.
After the z-axis holder had been installed with the linear bearings and ball bearings, I would try fitting out the assembly. From then, I should be able to adjust the position of the linear round rods and the horizontal cross beam of the gantry, by cutting/filing and enlarging holes if needed. As before, once I consider the assembly reasonably accurate in terms of measured distances and angles, I plan to glue them in place using epoxy resin.